Note: Descriptions are shown in the official language in which they were submitted.
21~5~3~
-,
Title: A llethod and ~ ..AI.- for Oil ~ell Sti 1At;
FIEI D OF THE
This invention relates ~n~rAl ly to the field of
5 extraction of hydrorArhon~, such as oil, ga~ and
cnn-l~n~At~, from ul.de. yLuul~d reservoirs. More
particularly, this invention relate~ to the stimulation
and ~nhr-- L of production or ecv. Ly of such
hy~lrornrhnn~ from ~uch reservoirs.
0 Rar------ OF THE lh .
Much of our current energy needs are met through
use of hydr~cArhon~, such as oil, natur~l gas, and
cnn~n~ates, which are Leccv~:~d from naturally occl~rrin~
deposits or reservoirs. Typically, such II~ILU~ Are
15 in a liquid or gas phase in the reservoir. Liquid
IIYdL~ I.V. ~ are often pLuduced by pumping them from the
reservolr to storage tanks or a flow line connected to the
wellhead. The pumping or "lifting" costs include capital
costs, such as the pump, the prime mover (i.e., motor),
20 the rods and the tubing, and operating costs, such as
labour, royalties, taxes, and electricity. Because some
of these costs are fixed, a certain production rate is
required to make such Lecv.~Ly e-~r rAlly fe~Rihle. If
the revenue generated by selling the Lecuv~:~ed
25 hy~lrnrArhon~ is less than the lifting cost3 to 80 recover
them, then the well may be t _ rily closed up or
p~ n l ly ghut in. In some ca~es wells may be Ltop_.~ed
when new terhnoloy-y becomes available, and in other cases
the well may be eopelled if energy prices rise, once again
30 3raking production and ~ ;v~._Ly ~ irAl ly attractive.
Alternatively, a E ly shut-in well would be plugged
with c:olluL-~L~ and ~h~n-lnn~d alto~e~h~r.
- 2 -
Typically, an oil well will be shut in or
Ah~n~ir.nPd when only 20-50 percent of the total oil in the
reservoir ifi LeC:G.._L~_d, because it becomes l7ne~ to
rontlnllP to operate the- well. This UIlL~:Cu.~ ed oil has
5 been re~ o~n i -~d as a lost ~ ~ne ~ e in the p_st and thus
there lave been many te~ hnlq~o~ p v~o~ to stl lAt~
production rates and cù \~r~ l ly ~n--reAce the ul~cimate
L~CCI._L,2' of oil from reservoirs.
There are a n~mber of reasons why oil and gas
10 well productivity may decline over time. For example,
productivity riPclinPs if 1) there is insllfflr1Pnt pressure
differential between the well and the reservoir, 2) the
flow between the reservoir and the well is ob.,L.u~ed, or
3 ) the mobility of the oil is restricted due to relative
15 r ~ ty effects. ConvpntlnnAl production practice,
such as wAtPrfl-~o~ g~ gas re-in~ection and the like, is
~ffective for r-int~lnirg reservoir ~ to u. __ -
the first problem. Many different r~ can result in
to the flow of fluid hydrocarbon from the
20 reservoir to the wellbore. For example, there may be
precipitation of mineral scales, such as calcite,
anhydrite or the like, in the formation, the p~LLoration
tunnels ( located at the bottom of the well ) or the
wel 1bore. There may be mobile inorganic fines, such as
25 clay or sand, which are carried by the flow of the fluid
being Le V._L~d into narrow pore throats thereby ~ kln~
them. There may be clay mlnPrAl~ which swell under the
lnfll~Pn~e of L.~cuve~Ly and which t - f ~ result in flow
path restrictions and a flow reduction. There may be an
30 alt~rat~ of the sat~ratll~n of a particular phase of the
well. For example, in a low r- -hil lty reservoir with
a very low w~ter content, damage can be caused if ~ater
contacta the reservoir. The damage occurs as a reductio-
~in the relative r~ -h~lity (i.e., mobility) of the oil
35 phase.
~ 5~35
-- 3 --
It is believed that one of the ma~or flow
o~_L~ucLions which results in rJr~cl inin~ productivity i~
the ~c__ l Ation in the reservoir at or ad~acent to the
well of solid phase wax. This wax may be due to either an
5 ~:c_ l ~tion of mobile waxy- solids with b-~h~e ~
plugging or narrowing of the pore throats in the reservoir
rock or precipitation of sOlid wax due to temperature,
es~uLe~ or composition changes in the hydrr,~rh~mR beinq
~c~v~ed. Such changes might occur ~t any point between
10 the reservoir and the storagQ tanks on the surface.
lol,au~ , because the wax is associated with the oil
phase, any r l Ation of sûlid phase wax in the well
tends to selectively damage the mobility of the oil phase
and thus reduce the production of oil from the well.
Nany methods have been developed and pl,~,~osed to
stimulate the production of oil in wells to increas~
profitability and extend the ultimate ~Cr~V~LY. One
common and relatively successful te~hniTl~ is referred to
as hydraulic fL~c.Lu~:. In this technique, a high pressure
20 fluid is used to Ll _Lul~ the rock formation, thus
creating a channel which penetrates into the reservoir.
The LL~_Lule is subsequently propped open using a ~rAnl~lAr
material, such as sand. The fracture bypasses hydraulic
restrictions to the inflow of oil into the well by
25 creating a new open channel and also by ~YroRin~ a large
surface area of the reservoir rock to the channel, thereby
greatly increasing productivity of the formation
n~ the bottom of the well. However, this
terhniqu~ is sub~ect to failure if the ~ pa.lL is not
30 s~lrc~ f~ll ly carried into the new L- _LuLes made in rock
formation. Further, it can be difficult to control the
LL _Lu- lng process and if the L. ~_Lule s~rr;~r~n~:~l ly is
e~Lel ded ~eyond the oil zone into a gas or water zone,
then the well may become une - ' r to oper~te .
35 Hydraulic LL.. _Lu~lng can t~ ~ ly improve the
- 2l~a3~
-- 4 --
productivity of wells which have a productivity decline
due to an ac_ 1 Ation of solid wax. ~lowever, such
t~rhn i ~ does not rentove the existing wax damage or
change the basic wax damage chAn;P~; it merely bypasses
5 existing wax damage . Thu~, productivity of a LL _ LULe~d
well will often decline at ~ high rate due to the
~. _ 1 Ation of uax damage in the LL~ACLUL~: channel .
S~ t refracturing of the reservoir may provide an
in productivity, but again productivity will
10 decline over time. s~ refracturing thereafter
typically does not provide sufficient productivity
1nrr~PQ~ to be r-~- 'r. Such LLa_LuLlng may thus
provide a short-term method of increasing production from
a well, but because it does not address the wax
15 ~ 1 Ation problem, the problem usually re-asserts
itself, resulting eventually in a 1088 of effectiveness
f or the f rncturing method .
other treatments to stimulate wells include
peLL~/L~ing the casing of the well with shaped charges to
20 provide rh~nn~l~ or peLLoLItion tunnels through which the
f luids can f low . Again this t~rhn ~ qu~ provides a short
term i ~.~. which m~y byp~ss, but does not remove,
ac.;l 1 Ations of wax, nor, prevent the further
Al lAtion of wax.
l~atrix acidization, in which an acid is pu~tped
into a reservoir to dissolve forr-tinn rock and
precipit~ted scnles c~m also 8~ l 1 At~ production in
uells. However, for wells having solid wax damage, matrix
acidization may not work effectively, as solid wax is
~nRoll-hle in acid. Because acidization ig lnh~rf~ntly
prone to create rh~nn~l Fl along the path of " least
re~istance", the acid often bypaB8e8 the low r~ h;l~ty
wax damaged oil zone and instead 3?eneLLaLes directly into
~ water zone at the bottom of the reservoir. Thus w~x
deposits can limit the success of acidization stimulation,
3 ~
even preventing effective removal of any dissolvable rock
or precipitation which are wnx coated.
Another t~rhniql~ for sti l~tin~ production is
thermal st; lAt~D. In the case of thermal sti lAtinn~
5 oil, water or steam heated above grade may be pumped to
the bottom of the well to try to stimul~te production from
the Leco.~L~ area. However, it has been found very
~iiffi~..lt to transfer the heat by steam, wnter or oil to
the bottom of the well by reason of the thermal losses
10 which take place as the hot medium is being I ,~ poLLed
down the well bore. (Society of Petroleum ~n~in~rs~ Paper
No. CIN/SPE 90-57 OPTIMIZING HO~ OILING/WATERING JOBS TO
MTNTMT~R FOR15ATION DAMAGE by John N~nni~r and Gina
1 igl~r of N iger Rn~in~rin~ Inc. )
For example, in the ~hot oiling te~hniT~
crude oil, solvent or water is heated above grade to a
typical t~ _~ LuLe of 100-125C and then pumped into the
well. Usunlly the heated fluid is pumped into the annulus
between the tubing and the casing . D~nA i n~ on the
20 particular situation, some fluid will ~c.. 1 Ate in the
annulus, some fluid will flow into the reservoir, and some
f luid will f low back up the tubing and out of the well .
Heat from the "hot oil" is lost through the casing to the
rock sll~ro~nA;n~ the well. Heat is also lost in counter-
25 current heat ~YrhAn~ with the f luid which circulatesupwards out of the tubing. I. ~ at the
bottom of the well show that the bottom hole t~ _ LUL~
drop~ during the treatment and excessive volumes of hot
fluid do not si~nifirAntly raise the bottom hole
30 t _ aLuL~. Typically, the heated fluid will lose its
e_cess t~ - in th~ top 300-400 m section of the
well dl~e to heat losses to the casing and the counter-
current heat ~Y~han~e d~13crihed above. Due to the
geothermal ~r~Ail~nt~ by the time the Nhot fluid" reaches
35 the production zone at bottom of the well, it is likely
- ~15~3~
-- 6 --
cooler than the casing and thus actually absorbs heat from
the casing and the rock D~.Luu..ding the well. Thus for
~ost Arrlirati~.nR (for wells deeper than 300 m), the "hot
fluid ~ arrive~ at the bottom of the well at A t~ ~
5 below the reservoir lLure. Because the bottom hole
t~ LuLe decreases during treatment, waxy solids are
likely to precipitate from the crude oil and be filtered
out in the pores of the reservoir in the ~GUIJ . ..~.y zone as
the fluid flows into the ~GCU~_Ly zone. Thus, although
10 the "hot oil" terhniq~lo removes the wax deposits near the
wellhe~d, it often cauges an Al 1 Ation of the waxy
~olids in the peLroL~Lion tunnels and reservoir
~uLLuunding the well. Thus, the Ar~l irAtir~n of heat to
the well by pumping "hot oil ~ into the well through the
15 annulus is inadequate to remove w~xy deposits in the
formation and in fact usually leads to even greater
formation damage. The hot watering technique ~Yr~r~c~nr~
- -rAhle heat losses and causes additional fr,rr^t i r~n
damage (e.g., by increasing the water saturation around
20 the well, precipitation of inorganic scales, etc.), 80 hot
watering is not an effective terhniqu~ for removing
f ormation damage due to wax .
Another method of thermal stimulation i8
rl~R~d in CAnAriiAn Patent 1,182,392, dated February 12,
25 1985 in the name of Richardson et al. (see also U.S.
patent 4,219,083) which ~i~c1 r~8~8 a nitrogen g~s
7~n~r~A~tirn system to produce a heat spike in a water-based
brine solution. In thL~ method, the salt water solution
~ GLyùcs a ~' irAl re~ction to release heat, t~gQtl--
30 with nitrogen gas, aR it is being delivered down the well,thereby avoiding some of the heat losses a_sociated with
l-A-~ ,LLing a hot fluid down the well as r~ ed above
for the "hot oilll techniqu~; the salt water ~olution only
become_ hot when it iD some way down the well. The _alt
35 water Rolution may then be shut in for a period of about
24 hours to ~llow the heat carried by the solution to melt
5~35
-- 7 --
waY located in the ~c~, . LL~ zone . The ~ lo~;ure not~s
that wax solvents may be flushed down the well prior to or
after the in~ection of the heat-rroAI~ in~ salt wAter
~I-lutinn.
However, there are several inherent
di~ndvantages to the method disclosed in patent 1,182,392.
Firstly, the wax is not soluble in the salt w~ter
~>]llt~- n~ 80 even if the heat developed is sufficient to
~elt the solid wax deposits, two separate liquid phases
will occur (i.e. ~ liquid ~ L~C~L~ phase ~no]~A~n~
liquid wax and crude oil and a liquid aqueous phase
including - ~on water and salt water solution). If
the water saturation is high in order to get a 4i~ni~Ant
rise then the relative r- -hil ity of the
liquid hydrocarbon phase will be very low as: _ ' to
the water and the mobility of the 11~ ILucaLL-JJ~ phase
Cl~ntA 1n1n~ the wax will be o~,, LLU~; Led. Thus, the water-
based fluid cannot effectively carry the melted wax out of
the reservoir. E:ven if solvent is present in the
formation, either by mean~ of a pre-treatment flush, or ~
post-treatment flush, the salt water solution and nitrogen
gas pL~duced by the reaction will together greatly impede
the solvent from coming into contact with any such melted
wax, greatly reducing the treatment 8 effectiveness.
Past studies have shown the effect of water
8At.... r~t~.n on relative r ~h~ 1 ity (B.C. Craft and M.F.
Hawkins Applied Reservoir ~n~ina~rin~ Prentice-Hall,
195~) . The relative r --hi 1 ~ty curves (i.e. dat~) for
a particular reservoir allow the flow rate of oil or water
through rock pores to be calculated as a function of fluid
sAt~rr.t~ nd ~La8~uLe: drop. For example, on page 357
Figure ?.1 shows that if the water saturation exceeds
0.85, then the ~ ~nin~ 0.15 volume fr~ctlon of oil will
not be mobile. Fig. 7.2 of this ~feL..~ce also shows that
35 an increase in the water s_turation of ~ust 0.35 ~lo<-raA~9~3
2~ 35
the relative E -hi 1 ity (or mobility) of the oil phase
by 100 fold. ~hus, if salt water solution is scrl~ez~d
into the fnrr-t~nn~ the saturation of the water is
~n~r~ARed and the relative E -hil~ty of the oil/melted
5 wax phase will be greatly reduced. If the water s~Lu~Led
f~r~-t~r~n i~ ~-.1.R~ ly contacted with a solvent, the
solvent will tend to channel due to the r~lAt~nnQhi~
between relative E -hility and fluid sat~rat~-~n
rl~Qrr~ hed above . Thus, the solvent cannot ef fectively
10 contact or ' i 1; ~ the melted wax. Thus, contacting the
formatlon with an aclueous based heating fluid to be
followed by a solvent is unlikely to effectively remove
the wax from the pores of the reservoir rock.
FUrt~ e ~ water can be rl gi n~ to some reservoirs as
15 it can cause clay swelling or fines ~ tion.
What is desired th~LeLo~e is a method for
removing the Ar l~tionR of solid wax from the fluid
pARsA. _yD which _ Re the well to remove ~ ~ q
to the flow of licluid hydrocArhonR being pLuduced from the
20 reservoir to enable increased licluid hydrocarbon
production rates. Preferably, such a method would be
in~ nRive to use and would be capable of being used
without a great deal of inconvenience or alteration to the
well itself. Preferably, the treatment would physically
25 remove any solid wax, and would be effective every time it
w~s used. The method also would preferably not introduce
any water - based licluids into the formation to ~void
reducing relative ~ --h;l~ty~ and hence mobility of the
licluid l~dL~JC~ . Such method would also avoid he~t
30 losses associated with LL~I..s~uLLing ~ fluid from a cold
~ocat~nn (i.e., the wellhead) to a warmer zone (i.e., the
- h~le production zone), which could lead to a decrease
in the bcLL -1~. t~ ~ and cause wax precipitation
and L lAt~on~ resulting in formation damage.
35 swlKaRy OF 'rHE ~
According to one aspect of the present
-- 215503S
g
invention, there i8 provided a well treating process to
remove solid wax from fluid p~a~ ays betw~en the well
and a DuLLou..ding u-~delyLo~l~d reservoir, said process
in~:
selecting a solvent which is g~n~r~lly miscible
with melted wax,
pùmping said solvent down the well at ambient
t' _ ~r
heating said solvent below grade in the well at
a posit~nn ad~acent to the wax to ~e treated to minimi~
heat losses from said solvent during ~ fi~ L Lation of
said solvent to the wax to be treated,
contacting said heated solvent with the solid
wax to be removed to ~ ~ 1 i 7~ said wax without reducing
~:he relative E- -h11ity of the wax/solvent phase, and
removing said solvent and said ~ i 1 i 7~el wax
from said fluid pA~sa~ yD .
According to another aspect of the present
~ nvention there is disclosed a method of stimulating an
oil well by removing solid wax deposits from a LLe~i
area, said method ~ n~S
placing an electrical heater ad~acent the area
to be treated, supplying power to said heater to cause a
relea~e of heat while simult~n~ol~ly passing a solvent
past the electrical heater to directly heat said solvent
to a t- _ _Lu-e above the naturally oCc~rrin~ tre~tment
area t~ - tULe~, but below the t- ~ at which
unacceptable solvent degradation occurs, pasDsing the
heated Dolvent into the treatment area to co~tact the
heated solvent with the solid wax deposits to be treated
to 11i7'~ the wax and to form a liquid phase ~ ~in~
oil, wax and solvent and then removing said liquid phase
~ont~nin~ said 'ili7gd wax from the treatment area,
without lowering th~ mobility (i.em, relative
35 r -hil~ty) of the oil/wax/solvent phase within the
'Te,' ': area.
21~5~3~
10 --
According to another aspect of the present
invention there i8 ~liQ~ln8ed an electrical heater for
heating fluidg, c, Qin~
~ means for attArhin~ the he~ter to a source of
5 electrical power; and
a resistive electric heating element means, said
heating element means having a hydraulic p ~3UL~ drop
there across of 20 mPa or less for a flowrate of 1 m~/day;
a heat transfer area greater than 10m2 per lm3 of
10 heater; and
an electrical resistance greater th~m or equal
to 1 ohm and less than or equal to 200 ohms.
31RIEF UIS~;WC1~.L_ OF ~
Reference will hereinafter be made by way of
15 example only to the attached f igures which illustrate a
p ef~L ad ~ of the pre~ent invention and in
which:
Fig . 1 is a graph depicting the relat iOnQh i r
between solvent volume requirement to dissolve a downhole
20 wax deposit ( in m~ ~olvent/kg of wax) against treatment
t~ , Lu~e in degrees Celsius;
Fig. 2 is a preferred ~ i of the
invention;
Fig. 3 is a close up view of a _, of the
25preferred: ' ~ of Figure 2;
Fig. 4 is a cross-sectional view along line 5-5
of Fig. 3;
Fig. 5 is schematic of a part of a pL~cLL~d
circuit;
Fig. 6 is a ~ Ailed view of a , - ' of Fig.
3;
Fig. 7 is a cross-sectional view through the
~- , of Fig. 6; and
Fig. 8 is a circuit diagram of the preferred
3~ power circuit.
2155~3~
11
nRTATr.12n ~ _ OF THE o A
Up untll the present, the composition and
801-lh~ 1 ~ ty of wax has not been well understood.
~rypically~ wax has been treated as a single _ _ ~L, ' and
5 its sol-lh~ 1 i ty has been assumed to be a weak function of
temperature . However, the normal pA rA f f ~ n ~ ( N_PA rA f ~1 n~ )
which precipit~te to form wax rl~posit~ in U~Lde~y~usld
hy~lroc~rhnT~ reservoirs include species from C20 H"2 to Cqo
H182 and higher. As -- ' ;1 -i earlier, the wax deposits are
10 associated with the oil or c ~ te in the reservoir and
typically contain between 30 and 90 percent of the
associated liquid hydrocarhon. When a wax d~posit
precipitates from an oil or con~ nQate~ the composition of
a particular wax deposit appears to depend both on th~
15 amount of each of the N-paraffins dissolved in the liquid
hydrocarbon and the solllhil1ty of each of the N-paraffins
in such liquid hydrocarbon . The ~ol--hi 1 i ty of a
particular N-paraffin in a particular crude or conA~n~r,te
i8 related to the carbon number of the paraf f in and the
~0 t~ Lu~ ~: and the 801 llhl 1 i ty r~ Le. of the liquid
1.~1. uc~rLu~l. Thus, as the oil t , t: changes, the
composition of the wax depofiits changes. The solid wax
which precipitates nnd A- l Ates downhole at high
temperature tends to include higher l-~c~lAr weight
25 rArAffin~ and have higher melting points. ~see OPTINIZING
HOT OILING/w~rRl~ JOBS TO rl l ~ l M 1'~1' F~RM~ DANAGE by
John ~ r and Gina Nenniger of 1~ r Rn~i n~ r~ r~
Inc. ) ~ L, because these wax deposits occur
n~ ral ly at elevated t , _Lu ~B in crude oils and
30 con~i~n~rlt~ it i8 obvious that thes~ deposits contain
highly ~ n~ h1 e p~r~ffins.
One of the t~hn i ~i ~ which has been used by
industry to treat wells to remove wax deposits is to
employ solvents; a solvent is pumped or "s~J~?z~n into
35 the f~r~tior~ to dissolve the wax. When the well is put
- ~155~3~
- 12 -
back into production the solvent carrying the dissolved
wax is then pumped out of the well. Although thi~
~erhniqup has been fLe~ Lly used, the composition of the
wax deposit has g~nf~r~l ly not been known, and 80 the
5 gr,lllhility of the reservoir wax in the solvent is not
known either . Fig . 1 shows a 801llh~ 1 ~ ty curve of the
volume of a typical solvent required to dissolve 1
ki lr of a typical wax deposit as a function of
~. For a reservoir t~, of 40 C, more
10 than 2 m~ of solvent are required to dissolve ~ust 1
ki loqrAm of wax. In general, excessive volumes of solvent
are required to remove wax damage at reservoir
t LuLæ.
However, Fig. 1 also shows that if the solvent
15 can be heated to 70 C, then only two litres of solvent are
required per kg of wax deposit. Although dif ferent
solvents are slightly more or less effective, the effect
of temperature ( i . e . the slope of the curve in Fig . 1~ is
similar for many different solvents. Thus, one surprising
20 result $8 that the application t~ ,~~ LuLæ of the solvent
is 80 critical in det~rm1nin~ the effectiveness and
llqefl-l nPgs of any such solvent treatment . However, what
remains is how to effectively heat the solvent to achieve
the desired effective and useful result, namely, the
25 r~ 7~tion and removal of a gi~nif~ nt amount of the
Al 1 ~t.~d wax deposits. In this context it will be
appreciated that si~nifirS~nt means 8llff~ n~ removal of
wax to --P-lr~hly increase production rates or flow rates
through the treated area. In this context, to heat the
30 solvent, means that the solvent has had its , _ ~.Lu. el
raised above the naturally o~C--rri r ~ L _ ~ of the
reservoir.
Arcrrtl~ nq to the present invention there is
disclosed an ~ L~IL48 ~nd a method in which a solvent is
35 beated directly ad~acent to the LLe~ area. Several
21~5035
different sources of energy could be used to raise the
t~ of the solvent at the bottom of the well
(e.g., exothermic rh~mirAl reaction, electrical heating,
radioactive decay). However, electrical heating i8
S rr~f~rAhl ~ due to safety, control, r~1 i Ahi 1 i ty and cost
considerations. The use of electrical energy avolds
certain ~rnhl i nhc-r~nt in the heating the solvent via
rh~mir~ll reaction. Firstly, it avoids the I .,.--~vLLation
of bazardous rh~mirAl~ such as oYi~7;~rs and fuels.
10 Secondly, it avoids the ~7iff~rulties associated with
initiating ignition and controlling the rhc-mirAl reaction,
such a~ the rate of the rhPmirAl reaction and the hazards
AR80ciAted with any 1- lete reactions, such as resldual
explosive mixtures of gas or cnrro8inn. Electrical
15 heating also avoids formation damage due to the nYi~tinl7
of any aqueous species present. An example of this
problem would be the oxidation of Fe~ to Fe~ and a
8llh~e~lu l- precipitation of Fe(OH)3. Lastly, any p_rtial
oxidation of ~ L~C~ t-OI~R in a rh~mirA7 reaction heating
20 system can produce gums, tars or asphaltene-like material
which could plug the pores of the formation and create
even worse formation damage than the 801 i~ifi~d wax.
The generation of heat by dissipation of
electricnl power can occur by several means. For example,
25 ~ductive, resistive, dielectric and microwave
technologies can be used to generate heat from electrical
power. Of these, a resistive heater r7~crrihed herein is
preferred due to its compact size, Ql _lirity~ r~liAhility
end ease of control.
Fig. 2 shows a schematic diagram of a preferred
~i of the invention. The f~T~i ~ shown consists
of a number of ~ _ Q. A truck 2 is shown resting on
a surface grade 4. An oil well i8 ~hown schematically and
oversized generally as 6 with an outer casing 8 forming an
35 ~nnulus 10 around a tubing string 12. The tobing string
- 215S~3~
14 -
12 E~,~__L~tes through a formation 14 to a ~_U._Ly zone
15 .
At the bottom of the tubing string 12 i8 an
opening 16 which allows fluid i-~at~nn between the
5 tubing string 12 and the annulus 10. ~ ~ pelLoLe~LionR
18 are provided in the outer casing 8 at the Lecu~,_Ly zone
15. $he peLrn~ti~n~ 18 allow fluid c ~ration between
~he annulus 10 and the ~O~LY zone of the forr-ti~ 15.
Also shown above grade are an electrical
0 9~ sr ~n~l~rat~rl 8~ irAl ly at box 20 which has
power outlet cord - ~irg electrical Cùlldu~:~OI 22. The
g~n~r~Ator 20 i8 preferably of a portable diesel electric
type, although in situations where the well 6 has an
adequnte supply of electrical power, the y~ LntOI 20 may
15 be replaced by a conventional electrical power grid hook-
up, along with ~,upLiate transformers, rectiflers nnd
controllers. ~pf~ t on the application, it may be
advantAgeou~ to convert the alternating current (AC) power
to direct current (DC) as more power can be carrled by a
20 given conductor 22 in DC operation and inductive COurl i n~
between the cor,du~ Lol 22 and the tubing 12 is also
avoided .
The next _ L iB a wire line assembly,
which ~ nr l ~ a winch 2 6 which raises and lowers the
25 conductor 22 within the tubing 12. The winch 26 is
operated by a gas or electric motor or the like. The
insulated COI~UULUL 22 passes around the winch 26 and
through a lllhr~Ator 28. The lubricator 28 facilitntes the
passage of the insulated cond~.Lol 22 into and out of the
30 ~r~ l 1 hP~d of the tubing 12 . The lubricator 28 is also
adapted to provide a ~ DULe seal around the cables a~
required. The winch 26, l-lhr~ ator 28 nnd electrical
g~n~rAtor 20 will be f~ Ar to those skilled in the art.
C~ e~l .Fu~l ~y they are not described in any further datail
5035
_ 15 --
` ~
herein .
The electrical l,UI~dU~:LOLD 22 are preferablr in
the fûrm of inQul;-ted electrical cables. Where the depth
of the well is such that the strength of insulated cable
5 is irA~ Jtte, such cables could bQ r~plA~ l or ~L~ ed
onto the sucker rods ~not 5hown) whieh are usually used in
the ~ell to raise and lower the pump. If the sucker rods
were used as a co~.lu ;LDL, they would have to be
electrically i QO1 ~t~ to prevent contact with the
10 production tubing. The electrical power would then be
transmitted downhole through the sucker rods. A further
alternative would be to use the tubing 12 itself as a part
of the electrical circuit as jC~Q.-r;h~ in more detail
below. However, this alternative would also recluire
15 a~ccJpLlate electrical isolation.
At the bottom end of co~.lu~LoI 22 is shown a set
of ~ars 27 and a resistive heater 30 which are shown in
more detail in Pigure 3. The ~lrs 27 are slidably
connected to the conductor 22 and can be used to supply a
20 sudden impulse (~erk) to the heater 30 and thus free the
same in the event it becomes _tuck downhole. A contactor
32 is also shown which is lt; 1; - i when the tubing 12 is
used as a .;UJ~duu LOI to return the eurrent back to the
wellhead and to the ~J.~ l c~i 20 thereby completing the
25 electrical circuit. Thus, the contactor 32 may be
required to provide a good electrical contact between the
tubing 12 and the heate~r 30. Alternatively, the où~ .cLoc
22 could allow the current to return to the 9~ ~.c,t~,r 20
via a return insulated electric~l power line.
The internal ~LLU~;LU~e: of the resistive heater
30 is ~hown schematically in Figures 3 and 4. The heater
30 is attached to the ~ars 27 by a ~o -rl; r~ 42 . The
heater 30 has a slightly enlarged CiL. r ..ice 44 to seal
against the pump seating nipple at the bottom of the
- 215~035
-- 16 --
tubing (shown in Figure 2 as 29) to prevent solvent from
bypa~sing around the outside of the heater 30. The heater
30 has fluid p~Q8P3~ _yD or holes 43 in a threaded endcap
46 at ~he top to allow solvent to flow into the heater
5 body 30. The solvent then flows through holes 47 in an
upper distributor 48, through a packed bed 50 in a manner
as h~r~ After ll~rrihed, through holes 51 in a lower
distributor 52 and out of holes 53 in a threaded endcap 54
at the bottom of the heater 30.
Figure 4 shows the heater 30 in cross-section
through line 5-5 of Fig. 3. A "+o channel member 56
B.,rAr~tP.R the packed bed 50 into 4 channel 8-,
lAh~lled A, B, C and D. Also shown are inner liners 58,
which may be , ~sed by set screws 60 threaded through
15 an outer heater shell 62. The set screws 60 may be used
to compress the packed bed 50. Such ~ sion
facilitates electrical contact between ad~acent packing
~1 ~ as ~c~rihed in more detail below. The set
&crews 60 are located at regular intervals along the
20 length of the heater.
The electrical circuit through the packed bed
50 is shown schematically in Figure 5. To prevent
~lectrical short circuits the packed bed 50 and
distributors 48 and 52 are electrically isolated from the
25 ~+~ channel 56 and the inner liner 58 by an ~n~ Atin~
coating r^t~rlAl 64, such as a rubber, plastic or plasma
sprayed ceramic. The upper distr~hut~r of channel segment
A is connected to the power input from the CUnlU~ ~OL 22.
The current then f lows to the bottom of channel A of the
30 packed bed 50 and then through a co~ LoL to the bottom
of channel B. The electrical current then flow~ up channei
B to tha dLstributor at the top of channel B. The current
then flows through a ~o~cLor to the top of channel C.
~he electrical current then flows down channel C to the
35 distributor at the bottom of channel C, through a
2l~sa3s
-- 17 --
or to the bottom of channel D, up channel D to the
distrlhut~r at the top of channel D. This distr~hu~or is
in electrical contact to the header body 62 through a
c~ e_Lo and the current is eLuL~.~d to the wellhead ~,nd
5 the g~ LaLO~ 20 through the tubing 12 or else a second
~on~u~LoL 22 to complete the electrical circuit.
The lower distr~h~ltor 52 i8 shown in more detail
in figures 6 and 7. Figure 6 is a plan view of the lower
distrihllt~r 52 showing a contact plate 80 which acts as an
10 ~l~rtrir~l connector between channel fi-_ ' 9L D and C. The
contact plate 82 acts as an electrical connector between
channel se _ ,~ A and B. The contact plate 80 is; ~nl Ated
from the contact plate 82 by an insulating material 83. As
shown in Figure 7 the contact plate 80 is DU~O' Led on the
15 insulatir~g material 83, which, in turn, iB supported on a
backing plate 84.
. .
It will now be appreciated how the preferred
electrical circuit of the present invention is configured.
The electrical power is supplied by a variable vol~age
2 0 d rect current ( DC ) power supply . DC power has several
aevantages over alt~rnA~in~ current (AC), as ionc-d
before. The electric power is ~urrl ~l~d by a direct current
variable voltaqe 200 kW portable diesel electric power
gen~rAtor. The volt~Lge i~ controlled either manually or
25 aut --ir;llly on the basis of a ' Lu ~ meaDu ~ : in
the heater, and the maximum current is limited to 150 amps
to avoid ov~rh~atin~ conductor(s) 22. Figure 8 shows tb,e
electrical circuit ~c~ ~r~l ly~ ~nrlu~l~n~ the reslstance
69 of c.,...lucLol 22 on the ~L~ limb of the circuit an,d
30 resistances 70, 71, 72 and 73 caused by the packed bed
channel segments A, B, C and D respectively. The
resistance 74 of the return limb of the conductor 22 is
.. ~
also shown. A connection to ground is shown as 75. The
t~ aLu~e controller 61 is also shown connected between
the generator 20 ~,nd a - Lu. c sensing meaD,s such a3
5~3~
_ 18 --
a tl , 1Q or the like, shown as 90. It will be
ArprPc ;i~t~d by those skilled in the art that the
t LULeS sensor 90 can -~ irate with the t~ ~ æ
c~ntrol l~r via several different means including signal
5 wires bundled with Cv~l~u. Lu~ 22.
It will also be appreciated by those skilled in
the art that, in certain instances there may be no tubing
12 within the casing 8. In such ci ~ances, the cnsing
itself may be used as a return co.,du~LùL in the same
10 maer as ~ rrihed above for the tubing. In this case a
packer could be used to provide a hydraulic seal between
the casing nnd the heater to force the solvent through the
heater 30 and into the ~cuv.: ~ 20ne 15 of the reservoir.
The proper packing 50 for the present invention
15 is quite important. In the preferred ~ the
packing 50 is comprised of a plurality of 8rh~r1ri~l balls.
preferred length for the heater 30 is 6 m. However, the
length can vary fl~p~n~ ~ n~ on the amount of electrical
power available and allowable p.~ ~ssu~e7 drop. A preferred
20 outer ~ r for the heater is that of the outer
di~meter of the p~-imp, 80 the heater can then be raised and
lowered onto the pump seating nipple and sealed to
minimi~o fluid bypass around the outside of the heater. A
preferred inner ili Ler for the heater 30 is 4.0 cm.
25 ~owever, the inside diam~ter can vary to suit the inner
diameter of the tubing in a particular well.
In a typical oilwell, the tubing 12 has a 73 mm
outsr ~li; L tOD) and a 55 mm inner ~i~ LæL (ID). In a
~L~:~æLL~d ~ of the present invention, power is
30 s~rpl i~d by a 200 kW portable diesel electrical gen~tor.
The heat i~hsorhed by the solvent as it passes through the
heater is calculated arcr~rrl~n~ to the following equations
Q = ~T~ t - Ts jn) Cp, Den, F~
where s
- 21~5~35
-- 19 --
Q is the power dissipated in the heater (watts)
Ts O"t is the solvent t~ _ Lule leaving the heater (C)
T~ in iB the solvent t~ t _ ~nt~-r;n~ the heater (C)
Cp, is the heat cnpncity of the solvent ( typicnlly about
5 2000 J/kg C for liquid hydlrocArhonr~ )
Den, is the density of the solvent (typic~lly about 900
kg/m~ f or a heavy ~ff' ~ ~ -te )
F, is the solvent f lowrate in m3/second
Thus, for a given power or heat transfer rate,
10 higher solvent f lowrates will result in lower heater
outlet t- _ Lu~ _B. Alternatively, a high heater outlet
t~ can be obtained at a lower power by re-~ n~
the solvent flowr~te. Figure 1 shows thnt the required
solvent volume decreases by three orders of magnitude for
lS a 30 C t~ Lure rise. Thus a small t~ ~ _ rise can
provide a substantial benefit in terms of reducing solvent
volume requirement. However, ~8 the hot solvent is
pl A~ed into the pores in the reservoir formation or
rock matrix, the hot solvent will cool down and th~ rock
20 and ~ interstitial fluids will be heated. A large
iraction of the cost (up to 509~) of the stimulation
.-rihec1 herein is due to the cost of the solvent
injected downhole. Thus, it is desirable to heat the
solvent to the maximum feA~ihl~ tF', _Lu- e which avoidQ
25 solvent degradation and deleterious effects in the
reservoir, such as mineral t~ l..Lo- -~ nQ. In this ~nnner
a maxi~um amount of heat or thermal energy is carried by
a minimum volume of solvent.
When the above formula is ~pplied to a heater 30
30 having an output power of 150 kW, and a desired
t~ _ Lu e7 rise in the solvent of 200 degrees S yields a
solvent flow rate of 0.42 litres per second or 25 litres
per minute or 1.5 m3 per hour. As i;Qc~Qsed above, higher
or lower t~ Lul~ 8 and lower or higher flowrates will
35 be ~ opliate for different solvents.
- 2155~3~
-- 20 --
The he~t g~n~rAt i on rate within the resistive
heater at steady state, is equal to the heat flux from the
heater to the solvent ~8 defined in the following formulas
Q= Ht A ~T
5 Where s
Ht is the heat transfer coefficient between the solvent
and the heater ( W/m2C )
A is the surface area of resistive heater in contact with
the solvent ( m2 )
10 8T is the local t-, Lul~ dir~eL~ e between the
solvent and the heater element (C)
Thus, for a desir~d solvent exit t- , LULe
from the heater of 230C, (for an ~ l ,n~ e t Lu-~ of
30 C and ~ heat ris~ of 200 C across the he~ter) the
15 maximum t~ Lule: in the heater will occur in the heater
element at the outlet and will be 230 + ~T degrees
c-~nti~ra~i~. Thus, a resistive heater design which has a
large surface area (A) and a high heat transfer
coefficient (Ht) will oper.~te ~t ~ lower _- ~ for a
given power and thus reduce solvent degradation.
The ~ uLa drop for a flow of 0.42
l$tre/second can be estimated by the Burke-Plummer
eSrl~tinn (R.B. Bird, W.E. Stewart, and E.N. T~i~htfoot~
ILa~ .olL F` ~, John Wiley and Sons, pg 200, 1960)
ôP/L = (1.75/Db"~) Dens v2 (l-~ 3
where s
~P/L is the ~L~8~U e drop per length (Pa/m)
Db LI is the ball rli ~r (.003175 m)
Den, is the fluid density (900 kg/m~)
3D V $8 the solvent approach velocity (0.42 m/s)
is the void fr~ction (~.4 for spheres)
Thus, for ~ b~ll size of 3.175 mm ~ bed length
of 6 m, and flowrate of 1.5 m3/hr the pres~ure drop across
21~5~5
-- 21 --
the heater is about 5 NPa (750 psi), which is well within
the pl~suLe limitations of the tubing and lllhriratnr.
The ball size of 3.175 mm was convenient; larger balls
provide less ~L~:815UL~ drop and less heat transfer surface
5 for a given heater volume while small balls result in more
~ . P.~ .~ drop and more heat tr~n~fPr surface for a given
bed volume. A bed length of 6 meters is convenient
however the length could vary from 1 m to 20 m ~rc~n iin~
on the particular application. The p~-~c.,u. ~ drop of 5
10 NPa, for a flowrate of 1.5 m~/hr i8 convenient however, any
nfi~lrAtion with a p ~suLe drop less than 20 mPa for a
f 1. .._ te greater than 1 m~/day is acceptable.
The electrical resistance of most metals iB too
low to achieve any ~i~n i f i - ant heating without excessively
15 long heating .91 ~. However, in a packed bed
rt~nfig~rat jcn, a high electrical resistance arises due to
the limited contact are~ between ad~acent sphPri- Al balls.
The resistance of the packed bed is sensitive to a number
of factors, inrl~fiin~ the amount of -~ 99ion on the
20 bed, the surface preparation and finish of the balls, the
ball size, the type of metal and the maximum power applied
to the bed. It is preferred to use s~hPrir Al packing
P~ because the resistance will not depend on the
packing orientation and the sphere to sphere contact area
25 (i.e. the resistance) will be quite uniform thL~u~ uL the
bed. The accepted resistivity of Carpenter st~inl~g
steel type 440C is reported to be 6x10-7 Qm. The
resistivity of a packed bed of 3.175 mm balls made from
the 440C steel was ~~~UrPd at 1. 6xlO-~ f2m at 45 W/cc or
30 more than two orders of magnitude higher. Thus, the
resistance of a cyl in~ri~-Al packed bed 6 m long with an
inner diameter of 4 cm is 0.76 n. ~rhprefore in _ well
1000 meters deep, the resistance of both legs of the
co~ Lor 22 will be 2.052 for 1~4 AWG copper or 1.33~2 for
35 $2 AWG copper is 80 large compared to the heater
resistance that up to 70 9~ of the power would be
` ` . 2155~35
_ 22 --
dissipated in the power trAnP~ sion rather than in the
heater. However, by dividing the bed into 4 8~_ ~ and
connecting the F-_ ~ in series as ~ c~Psed above, the
heater 30 resistance is increased by more than an order of
5 magnitude due to the reduced cross sectional area of each
segment, as well as .~y the longer current path through the
bed. In this manner the heater resistance is increased to
lOn and the power tr~n~ inn losses are reduced to less
than 17 %. Although a lOn heater resistance i8
10 convenient, a heater resistance as low as ln could be used
in the present design. Higher heater resistanceg m7n7m77e
the power trAnP-iL7sinn losse~ but rec~uire higher voltage~.
~he maximum heater resistance (at 150 kW) should be less
than 200~ due to the breakdown of the electrical
15 insulation at high voltages.
Prom the foregoing it will be appreciated that
the +- channel cnnf7~7~rAt7nn for the packed bed is not
~ nt~Al For example, an alternative r^t~ri~l for the
srh~rjt i~l packing element could be used directly without
20 the "+" channel, provided it provides a packed bed
resistivity of 2x10-3 S2m. Also, it will be appreciated
that the ecluations set out herein can be r-n; r~ 1 Ated to
change any of the parameters, such as length, power,
packing element size and the like, which could yield
25 similar cnnf ~7lrAtions .
An additional benef it of the packed bed
cnnfi~r~ticn arises due to the multiple electrical
contacts between balls in the bed. For example each ball
could be in electrical cont~ct with up to 12 ad~acent
30 balls. Thus, many p~rall~l electrical paths occur within
the packed bed due to the multiplicity of electrical
contacts. Because there ~re 80 many altern~te pathways for
the current within a given channel segment, the packed bed
heater is not prone to the burnout and catastrophic
35 fai lure problem usually associated with electrical
21550~5
-- 23 --
resistance heaters.
It has been oLsc:Lved that the above ~ Y~rihed
henter c~nfi~r~tion is self-reg~lAtin~ in that it appears
to avoid excessive hot spot formation and catastrophic
5 burn out within the ~L~LeLLed power range. The ~L~f~L.~d
c~nfi~lration i8 a heater with uniform grh.~r1rAl
conducting el sl placed in a packed bed conFi~rAtion.
Thus each ball or conducting element is in contact with up
to t*elve other conducting ~ r~n-l i n~ on whether
10 the conducting el~ment is in th~ middle of the bed or At
a perimeter. The contact point between spheres is very
small in cross-sectional area due to the ~:uLvntu e of the
surface of the balls. Thus, the current flowing through
the bed meet6 with ~i~nif~r Ant electrical resistance as it
15 passes through each contact point. This resistance, in
turn, PL~nIUCe5 heat at each contact point.
When a prototype heater was tested it was
observed that the bed resistance is a function of the
power per unit volume. Thus, increases in power per unit
20 volume tend to decrense absolute resistance.
It was also obseL~ d that the packed bed behaves
as a ~ , ~ ~ electrical resistor. For example, at
50W/cc, with various bed dimensions, the electrical
resistance of the bed is inversely proportional to the
25 cross-sectional area and directly proportional to length.
Thi8 result demonstr~tes that the electrical current does
not channel through the bed. This result is important
because electric~l rhAnn~llin~ would create hot spots and
lead to fluid r~ rZI~IAtir~n. IIOL~ L/ the bed is not prone
30 to catastrophic burnout because of the multiplicity of
current pathways.
It will be appreciated that the foregoing
description relates to conducting ~ which zlre
- 2155~35
-- 24 --
uniform size spheres, prefPrAhly of s~A;nlPRs steel.
However, other packed bed cnnfisr~r~tfnn~, ;
spheres of different sizes, conducting el~ R of
different shapes, or Inr~ r~ conducting Pl~ ~ of
S different materials of the same or different sizes or
~hapes may also be used. It is believed that the
important point is to keep the bed in ~ - -; n-l ~ the
contact points small between ad~acent Pl - ~, and to
provide a plurality of alternate current pathways to allo~
10 the heater to ,.~ n~ an ~ l ;hri~lm which prevents local
hot spot heating and the attendant burnout that may be
associ~ted therewith.
In the ~.eL~ .ed method, the use of this heater
conf iguration allows the solvent to be displaced through
15 a self reqn 1 ~t i ng heater which prevents catastrophic
burnout of the heating element and avoids hot spot
formation, and, additionally, prevents degradation of the
solvent ~o be heated. This i~ important because solvent
degradation could produce solid Ly~ c,.lu~Ls such as coke
20 which could plug the fluid rh~nn~lR in both the heater bed
and in the oil reservoir.
Thus for 150 kW of power dissipated in the
heater, the required current will be lSOA and the voltage
required at the wellhead will be 1200V. The choice of 440C
25 8t~inlP~R was convenient in this application. However,
many alternate materials can be substituted, ;n~ llla;r~
~etals, alloys, ceramic composite materials,
~ cnn~ tor8~ m;nPrAl R and graphite. With an
alternative materi~l it may not be n~cess~ry to divide the
30 bed into sections to achieve a practical heater
resistance .
The surface area of the heater element is
calculated by multiplying the total number of balls in the
bed by the Rurface area of a ball.
21~503~
- 25 -
Surface Area= (Volk,d ~ ) /Volb.~ db l~2
(1.5 1l L ID2) (1-~)/ db l,
=8.5 m2
The heat LLtl~aLeL co~ff~ nt is calculated
5 using Eckert's correlation for packed beds pgs 411, 412 in
T. ~ JUL L F ~ .
a~ llOOm2/m3
Go = 300 kg/m2s
~1 ~ .001 kg/ms
10 ~- 1 for spheres
Re=Go/(a 11 ~)= 272.
~H=. 61 Re~ ~1~ = .061
but ~H = {Ht/(Cp, Go)}(Cp, Il/k)2/3
k ~ thermal conductivity of solvent ( .12W/m C )
15 Therefore Ht ~ 5,000 W/m2 C
~herefore ~T = Q/Ht A = 150,000/5000x8.5 = 4 C
,h.~._f~ the maximum t~ _ _LUL~ = 230 + 4 = 234 C.
The heat transfer coeffirient in the packed bed
is about 10 times better than for other conf ~ g~ration8
20 ~uch as heated tubes. In addition, the packed bed has a
large surf~ce area per unit volume ~1100 m2/m3), 80 the
heater is compact and has very high surface power rates t2
W/cm2) with very small temperature gradients (4 C) between
the heater and the solvent. Heat transfer surface areas
25 of 10 m2 per m3 of heater volume are a lower limit of
practical application. r~Pnorally it is ~ irAh]e to have
as large ~ heat trlmsfer area per unit heater volume ~8
practical .
The averJ~ge r~ nre time of solvent in the
30 heater ( the void volume divided by the f l~ L~te ) ~ s 7
~3econds. Thus the solvent heats up at ~ r~te of 30
C/second a8 it passes through the heater. The low he~ter
element, LULI: and the short contact times in the
packed bed are both highly desirable features to avoid
3 5
-- 26 --
solvent degradation.
A small scale heater was built and tested. A
resistivity of 1.6xlO-~ &, was ~- _ .d at 45 W/cc with AC
power with 3.175 mm rilrr~nt~r 440C stAinleAA balls at 20
5 C. This data indicates that a heater with the preferred
cnnf~g~r~tlon ~I~Acrih~d herein could possibly operate ~p
to 340 kW with a resistance of 12n. This result 18 more
than adequate for the ~ ft~L~d design, as slightly higher
resistivities require higher voltages and less ~ ge.
10 Thus, either smaller col~du~;LoLD 22 can be used or
nlternatively less power is lost in ~ ARion.
It may now be appreciated how the method of the
present invention may be employed. Prior to emp~oying the
preferred method the pump needs to be removed from the
15 well 6. This is usually AC_ _liA~t~d by "killing" the
well Nith a fluid to prevent ~ n~ d production of
hydrscArhnn~ while the well 6 is open to the a _,~h~re to
remove the pump. It is rr~?f~rAhle that the well be killed
with an oil or solvent rather than water. However, if the
20 well has been killed with water, then the water should be
displaced out of the well by circ~llA~inq oil or solvent
down the annulus and back up the tubing. Once the water
in the well has been ilAplRred~ a mutual solvent is
preferably pumped into the tubing to further ii~plA~e
25 water away from the e.~v~Ly zone D -.v ~lin~ the
1 lhnre. A mutual solvent is a liquid which is partially
soluble in both oil and water. Such a liquid is EGMBE
(ethylene glycol -'_Lyl ether) or ~A~I.Lvl,A~l/toluene.
Such a mutual Dolvent would have several b~n~fi-~iAl
30 effects, as will be now ArrreciAted. For example, the
mutual solvent will increase the ~ ^hil ~ty of the
solvent or oil by increasing the degree of saturation of
the oil phase relative to the water phase. This mutual
solvent will assist in bringing 8~ solvent
35 applications into greater contact with the wax to be
- ~15~03~
- 27 -
treated. By increasing the degree of sat~rAtin-~ of the
solvent, such a pretr~; will ~l180 facilitate the
removal or ~ pl~- of the oil/solvent/wax phase from
1:he formation ~uL uu..ding the well.
The next step in the pL~ft:LL~d method i8 for the
electrical cable 22 wlth the ~ars 27, resistive heater 30,
and contactor assembly 32, to be lowered to the
a~lu~Liate depth within the tubing 12 through the
1 ~lhr; r~trr 28 . The solvent truck 2 then begins to pump
solvent into the well 6 ~t the desired rate by means of a
pump 3~. As shown in Fig. 2, a hose 34 passes through the
lubricator 28 down into the tubing 12 and has a nozzle 36.
It will be appreciated by those skilled in the art that
the nozzle 36 may be placed at any desired 1~ r,atirr~ within
the tubing 12 and in fact, it may be sufficient merely to
connect the nozzle 36 to an appropriate orifice on the
wellhead and simply pump the solvent directly down through
the tubing 12 . Alternatively it may be des irable to
connect the hose 34 directly to the heater ~e.g., if the
tubing is completely blocked with wax) in order to pump
solvent directly to the heater. The solvent then makes
its way down the tube as in~lirAted by arrow 40 where it
F~nrollnt~r~ the registive heater 30. The generator 20 i8
started and electrical power is then transmitted through
2s electrical c_ble 22 and through the tubing 12 to the
heater 30. As the solvent is pumped down the tubing 12,
with the valve on the annulus 10 closed, it passes through
the heater 30, out the bottom orifice 16 of the tubing 12,
through the pc . L~L~Lions 18, in the casing 8 and into the
L~UV~Ly zone of the f ~-~n 15. In some cases it ~ay be
nF~r~RsAry to seal the annulus 10 to prevent the solvent
irom circulating up. In addition it may be .i.,~ i r~hle to
use a packer, gelled 1,~ rl.~ or non co~ l~n~ihl~ gas
to reduce heat losses due to convection in the annulus.
35 When the solvent is almost all completely
- ~155035
- 28 -
AiF~plArP~A into the fnrr~tinn/ the power is switched off.
The conductor 22 and the hs~ter 30 and hose 34, may then
be removed from the well and the well may be put back into
production. Alternatively, the hot solvent may be left to
5 soak for a period of time before the well is put back into
production .
In this context solvent refers to any fluid
s~hich has an external phase mi~:rihl~ in all proportions
with wax at the melting point of the wax. Preferred
10 solvents include crude oil and cnnA~n~atQ~ refinery
distillats and L- Le cuts (nAr~th~nirl rAr~ffin~rr or
i~romatic hydrocArhon~2), toluens, xylene, disgel, gA~olin~
naptha, mineral oils, chlnrinA~ed hydro~rhnn~, carbon
A~ lrhiA~ and the like. MiQcihility is desirable to
15 avoid relativs ~ -hil~ty rrnhl~ as A~crih~d above.
Tn the ca8e where the solvent could be considsrsd as an
l~ion (s.g., a crude oil c077~A~nin~ ~ small proportion
of ~. u~uced water), then the continuous phase o~ the
solvent is mi~rihl~ with the melted wax at the trsatment
20 t~ tu.~ and p ~su.e.
Ths flow rats of the solvent is dF-t~rmi --' by
the pump capacity and pL~s8u~ drop across the heater, as
well as ths dssirsd solvsnt t~ ~ Lu~ : rise for the
available powsr supply. The depth of heat p~l.eL~aLion
25 into the formation will depend upon the total volume of
solvent in~ected and the solvent t - e. The optimum
distance that the heated solvent is in~ected into the
reservoir will depsnd on ths amount and dspth of wax
damage, as well as the porosity of the rock and will vary
30 from well to well.
Ths volums of solvsnt used ~C~OrAin~ to ths
prsssnt lnvention will also vary, A~r~nA~n~ upon ths
-tit~n being treated. For exampls, if ths wax dsposits
or f ormation damags are prsssnt at a largs distancs away
- 2155035
- 29 -
from the wellbore, then a larger volume of hot solvent
will be ne~RA~y. The treatment typically will require
1-30 m~ of solvent per metre of formation being treated.
The removal of wax q lAt~on~ from the for~^tinn, or
5 even from the wellbore rods and tubing will enhance
productiYlty of the well. Such wax removal will alfio
enhance other types of well treatment activities,
increasing the effectivenes~ of a C-~_Lu,.: tre t, an
acid 8~ t i nn and the like . It will also be
10 appreciated by those skilled in the art that additives
could be included in the solvent to enhance v.~rious
properties. For ex_mple, these additives can include a
number of chemicals, such as surfactants, dispersants,
viscosity control additives, natural solvents, crystal
15 - i f i ~rR, inhibitors and the like .
As can be ArpreciAt~d from Pig. 1, increasing
the t~ ~ of the solvent 30 C increases the wax
carrying capacity of the solvent by 1000 fold. This
L L ~ rise in turn increases the ef fectiveness of
20 the well tre~ltment and reduces the volume of liquid
required. If less liquid is required, then less time is
ed to pump the solvent carrying the dissolved wax
out of the well, the wax iB less likely to cool down and
rerre~iritate in the formation rock and the
25 oil/gas/con~ ro-te production and profitability can resume
more quickly. By using a miscible heated and effective
solvent, the removal of wax from pores and mi~u~or~s at
the reservoir or production level can be a_ liRh~d. In
the reservoir, an additional benefit of the hot solvent is
30 due to mlnimi~in~ the g~s ~nd water saturation~ and thus
maintaining the highest feasible mobility or relative
?h~ y for the oil/solvent/wax phase.
The solvent is pumped or f lows through the
resistive heating apparatus and is heated. For
35 convenience and ~ d r~liAhility, there may be
2~ ~5035
- 30 -
t~ e, pressure and flow monitoring in~L~. ~t i- n
and co ltrol devices also i r~ rir~d in the heater.
It will be appreciated that this invention
teaches the removal of wax deposits from oil, gas and
5 -nn~ n~lAte reservoirs and production systems by the use of
a wax solvent which has been heated to greatly reduce the
volume of solvent req~lired to dissolve the solid wax. The
preferred method contacts the wax with a heated solvent
without raising the ~i~t~rat~o~ of the water phAse and
10 re~ rin~ the mobility of the oil/solvent/wax phase. The
solvent is heated near the w~x to be treated to avoid the
~ 1088 o heat (or solvent 1uid t~ ) as
rihed for hot oiling.
It can now be appreciated more clearly what the
15 f~7;1in~ of the prior watsr-based heat-pro~ in~ m~thods
are. In fact, it is not 80 important to apply heat to the
wax to be removed, as was previously taught. It Ls much
more important and effective to have a treatment which
heats the solvent, and then contacts the hot solvent with
20 the solid phase wax to ~ the wax and facilitate the
removal of the dissolved/melted wax from the formation
before the solid phase reasserts itself. The removal of
the liquid hydrocnrbon ph~se ti.e., the oil/solvent/wax
phase) from the rock will be severely ob~L .luLed by the
25 ~LcgG Ict: of the water and the gcs phases due to the
r~lative L -~ility effects in multiphase (i.e., water,
hydrocarbon liquid, gas) flow. In other words,
7ntro~i~lr~n~ water into a f~rr-7t~ 7 has the very
~7n~7.~ir~7h7,R result of preventing the oil/solvent/wax phnse
30 from being mobile through the formation. The higher the
w~ter content, the lower the ~ ty ~f the
oil/solvent/wax phase. Thi~ effect is ~1 im~n~te~7, in the
present invention because no water is used.
It will be appreciated by those skilled in the
- 21~3~
- 31 -
art that the foregoing description is by way of example
only, and that many variations are ro~s~hl~ within th~
broad scope of the claims. Some va~iAtinn~ have been
c~!~2gPd above and others will be al/pAr~ to those
5 skilled in the art. Further, it will be appreciated that
while reference has been mâde to treatment of the e:Cuv~Ly
zone ., -. . o~ Ain~ a well, the method and t~ Ltll~Ufi
~r~ ;n~ to the present invention will be equally useful
in removing wax damage in production 8y8tem8, i nrll-~l i n~
10 the tubing, the rods, the annulus, the wellhead, flow
lines, p~re~ -, storage tanks and the like. In short,
the heated liquid solvent can easily reach any wax
deposits in any fluid based LL~ ' ' system. It will
also be appreciated that this invention may be usefully
15 used to treat high water cut wells, or wells with water
coning prohl~ -, which have selective damage to the oil
siatur~ted zone due to wax. It will also be appreciated
that this invention may be usefully used to treat high g~s
cut wells, or wells with excessive gas production, which
20 have selective damage to the oil s~tuL~Led zone due to
wax. In both water coning and high GOR (Gas Oil Ratio)
problem wells, increasing the ~ hility of the oil zone
by removing wax deposits can increase the production rate
of oil and inrrP~ the ultimate L~CUVe:Ly of the oil from
~!i the rel~nr~
.
