Note: Descriptions are shown in the official language in which they were submitted.
HIGHLY EFFICIENT ACID-DISTRIBUTING METHOD CONSIDERING SPATIO-
TEMPORAL DISTRIBUTION CHARACTERISTICS OF ACID CORROSION
CRACK CONDUCTIVITY
TECHNICAL FIELD OF THE INVENTION
The invention belongs to the field of oil-gas field development, and
specifically relates to a highly
efficient acid-distributing method considering spatio-temporal distribution
characteristics of acid corrosion
crack conductivity.
DESCRIPTION OF THE PRIOR ART
In recent years, domestic researchers have attached great importance to
exploring an unconventional
reservoir stratum such as high-temperature deep carbonate rocks, and industry
participants have laid great
stress on improving the exploitation efficiency of unconventional oil-gas
resources and the recovery
efficiency of a reservoir stratum for a long time. Acid fracturing is one of
the important methods used in the
petroleum industry so as to increase the single well production and the
recovery efficiency of an oil-gas
reservoir, and it greatly contributes to increasing the production and tapping
the potentials for the old oil-
gas fields and exploring and developing new oil-gas fields. Among them, the
acid corrosion crack
conductivity is an important factor that determines the construction effect.
At present, there are many methods to improve the acid corrosion crack
conductivity. Some adopt a
means of injecting acid with variable viscosity and displaced volume, which
forms uneven etching while
increasing the penetration distance of cracks, and even an independent means
of injecting acid is adopted
for acid liquor with different viscosity so as to uniformly distribute the
acid and optimize the effect of acid
fracturing. Some adopt a process of multistagedly and alternately injecting
acid, which raises the length of
acid corrosion cracks and the density of remote cracks by setting appropriate
process parameters, as well as
increases the probability of leading to the reservoir strata around the well.
There are still some methods to
repeat acid fracturing combined with a specific process for the construction
effect after one acid fracturing,
which optimize the conductivity and stabilization time of acid corrosion
cracks, while increasing the
distance of cracks' extension and leading to the reservoir strata on both
sides of manmade cracks. However,
in these acid-distributing methods, although considering the structural
parameters of manmade cracks, the
distribution of crack networks in a reservoir, and the influence of
permeability damage of acid corrosion
cracks on the acid corrosion conductivity, they failed to effectively extend
the development and production
cycle for oil-gas fields, and limited the degree of reservoir development.
Therefore, the invention proposes
a highly efficient acid-distributing method considering spatio-temporal
distribution characteristics of acid
corrosion crack conductivity, which is of great significance for achieving
high-efficiency, low-cost and
stable development of oil-gas reservoirs.
SUMMARY OF THE INVENTION
The objectives of the invention are to provide a highly efficient acid-
distributing method considering
spatio-temporal distribution characteristics of acid corrosion crack
conductivity, which optimizes the acid
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Date Recue/Date Received 2021-09-24
corrosion crack conductivity based on expansion to the boundary that the acid
fracturing spreads to , and
improves the effect of supplying diversion networks with oil-gas for a long
time. The method can
effectively improve the degree of developing a reservoir stratum, overcome the
shortcomings and
deficiencies of the prior arts, and have broad market application prospects.
In order to achieve the above technical objectives, the invention adopts the
following technical
solutions.
A highly efficient acid-distributing method considering spatio-temporal
distribution characteristics of
acid corrosion crack conductivity sequentially includes the following steps:
Si. analyzing the structural feature, lithological characteristics, porosity
type, physical characteristics
and pressure-temperature characteristics of a carbonate reservoir stratum,
based on seismic data and drilling
data in blocks, so as to determine whether the following conditions exist:
a. the reservoir stratum being adjacent to the aquifer;
b. the crack expansion during the fracturing process being easy to cause the
through-strata
phenomenon, with small barrier stress difference between barriers;
c. the reservoir stratum having strong heterogeneity;
taking the measure to control crack height according to S2 if they exist but
do not meet the conditions
for acid fracturing construction, carrying out the acid fracturing
construction according to S3 if they do not
exist and meet the conditions for acid fracturing construction.
S2. taking the measure to control crack height that includes the following
processes:
adopting artificial gel barrier technology controlling crack height to the
target layer, injecting water-
based fracturing fluid into a stratum with carrying the gel particles, which
are cemented with each other to
form a gel barrier, thereby blunting the crack tip and increasing the
resistance value of the upper and lower
ends of the crack, further controlling the extension of crack height so as to
provide the requirement to
optimize the width and penetration distance of the acid corrosion crack(Peng
Yu. research on high-tech
technology and model of acid fracturing controlling cracks [DJ. Southwest
Petroleum University, 2014).
In the measure to control crack height adopted by the invention, the pumped
displaced volume is
generally selected to be set at 3-7m3/min, similar to artificial barrier
technology for sandstone reservoirs.
Under construction with the same addition amount, the toughness provided by
the gel barrier when
resisting extension of the crack height is more than 5 times the toughness
provided by the conventional
artificial barrier, so under precondition of effectively controlling the crack
height, the addition amount of
gels is set to 20% of that of ceramcite. The optimal addition amount of
ceramcite can be deduced from the
theoretical model based on the crack extension criterion, and then determined
by Newton Algorithm (Yi
Xiangyi, Wang Daobing, Hou Yanhong, Zhou Fujian, Liu Xiongfei, Li Xiuhui. a
method for determining the
optimal addition amount of artificial barrier agent for controlling fracture
height[Jj.Journal of Oil and
Gas Technology,2012, 34(06):145-147+ 170). At the same time, in order to
reduce the influence of acid-
rock reaction on the temperature field where the gel particles are situated in
the later acid fracturing stage,
the mineralization range of the water-based fracturing fluid is set to be 0-
100g/L, according to the
characteristic that the solidification time of the gel barrier increases with
an increase in the mineralization,
during the construction process, the deposition and cementation time of gels
are controlled by adjusting
mineralization, thereby getting the formation of the gel barrier under
control.
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Date Recue/Date Received 2021-09-24
S3. carrying out the acid fracturing construction, by means of pumping with
isochronous five-stage
deceasing displaced volume, which includes the following processes:
Stage 1: pumping preflush fluid to press out the cracks and decreasing the
formation temperature,
wherein the pumped displaced volume at this stage is equal to the displaced
volume of the conventional
acid fracturing;
Stage 2: pumping gel acid, etching the crack wall more evenly and improving
the crack conductivity
with use of the characteristics that 11+ is slowly released, wherein the
pumped displaced volume at this
stage is 80% of the displaced volume of the conventional acid fracturing;
Stage 3: firstly pumping gel acid to supplement the II+ concentration in the
crack, and then pumping
non-reactive fluid so as to push the acid to etch the crack wall at the far
end of a well, wherein the usage
ratio of the gel acid and the non-reactive fluid at this stage is 4:1, and the
pumped displaced volume at this
stage is 60% of the displaced volume of the conventional acid fracturing;
Stage 4: using a combination of the gel acid and the non-reactive fluid, as
the same as the pumping
procedure in Stage 3, wherein the usage ratio of both liquid is 4:1, and the
pumped displaced volume at this
stage is 40% of the displaced volume of the conventional acid fracturing;
Stage 5: pumping conventional acid, definitely improving the conductivity of
the crack near the well
end with use of the characteristics that the acid-rock reaction is faster in
speed, wherein the pumped
displaced volume at this stage is 20% of the displaced volume of the
conventional acid fracturing.
Where, the displaced volume of the conventional acid fracturing is 4-16m3/min,
and the displaced
volume of pumping an acid-liquid combination is calculated based on this. The
preflush fluid system is
hydroxypropyl guar gum (0.5%) + fungicide + clay stabilizer + demulsifier + pH
regulator (Cai Daqing,
Zhou Hao. acid fracturing technology applied into carbonate reservoirs in Tahe
Oilfield[Jj. Drilling and
Production Technology, 2004(03):44-46+62+3). The gel acid system is HC1 (20%)
+ gel agent (0.8%) +
corrosion inhibitor (4%)+ demulsification and cleanup additive (2%)+ iron ion
stabilizer (2%) (Xu
Xingjuan, Fu Yueyong, Yang Jinling, Jia Hongzhan, Wang Yunyun, Cui Fuyuan, Li
Wenjie, Yang Bin.
research and field application of gel acid system for acidization at 180
C[I1Petrochemical Industry
Application, 2018,37(07) :11-15). The non-reactive fluid is conventional water-
based fracturing fluid or
formation water. The conventional acid is HC1(20%)+KMS-6 (2.0%)+BD1-2 (1%)
+BD1-3 (1%) + BD1-5
(1%) (Li Nianyin. Evaluation of acid fracturing effect of high temperature
deep well in fractured oil-gas
reservoirs in Tarim lunan buried hill [DJ. Southwest Petroleum University,
2006). The total liquid volume
of acid fracturing construction is 300-500m3, which is determined in
combination with the crack length to
be fractured and the maximum value allowable to ground equipment. The usage
amount of preflush fluid is
33% of the total liquid volume, the usage amount of the gel acid is 53% of the
total liquid volume, and the
usage amount of the water-based fracturing fluid or the formation water is 7%
of the acid-liquid
combination. The usage amount of the conventional acid is 7% of the total
liquid volume.
S4. after fracturing and acidizing, in order to prevent the low-permeability
layer from getting effective
modification, adopting the construction temporarily hindering acidization
after completing the acid
injection, thereby optimizing the spatial distribution characteristics of the
acid corrosion crack conductivity,
simultaneously, omitting the acidization operation to unblock before the
construction temporarily hindering
acidization, and reducing the amount of acid fluid and construction time.
Considering the on-site
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Date Recue/Date Received 2021-09-24
construction cost and the effectiveness of the construction temporarily
hindering acidization, the present
adopts a three-stage process temporarily hindering acidization. The pumping
procedures in each stage of
the process temporarily hindering acidization are the same, that is, a certain
amount of temporary plugging
fluid is injected to effectively plug the high-permeability area and increase
the acid intake in the low-
permeability area during the subsequent acidization process. Cellulose (6mm)
is used as temporary
plugging material in the temporary plugging fluid, then gel acid is injected
to fully dissolve the cellulose in
the stratum and regain the acid corrosion conductivity of the high-
permeability area.
Where, the gel acid is HC1 (10%) + gel agent (0.8%) + corrosion inhibitor (4%)
+ demulsification and
cleanup additive (2%) + iron ion stabilizer (2%). Cellulose (6mm) is used as
temporary plugging material
in the temporary plugging fluid, in which the fiber concentration is 1%-2%,
and the cellulose can be
completely degraded when the 11+ concentration and temperature are high. The
displaced volume of
temporary plugging liquid is 1-3m3/min, the displaced volume of acid fluid is
2-7m3/min, and the strength
of cellulose is 1.5-2.5kg/m. The specific value of cellulose consumption is
determined in combination with
the length of the horizontal section and the maximum value allowable to ground
equipment. The acid
injection time after temporary plugging at each stage is roughly equivalent to
the fiber dissolution time, so
the usage amount of the acid fluid is calculated based on this, and temporary
plugging at next stage will be
performed immediately after completing the acid injection. In addition, in
order to lower the ambient
temperature and isolate the acid fluid from the fibers, high-viscosity
fracturing fluid in sections is injected
at beginning and ending of the temporary plugging fluid, respectively, and the
usage amount for each
injection is 50% of the temporary plugging fluid.
S5. At the production stage of oil-gas wells, because the gradual decrease of
formation pressure and
the increase of crack closure pressure make the acid corrosion crack
conductivity gradually decrease with
the production time, resulting in a decline in the productivity of the oil-gas
wells, slowly injecting a small
amount of acid into the reservoir with low displaced volume to clean the well
when the formation pressure
drops to 70% of the original formation pressure, after cleaning the well,
injecting an acid-liquid
combination at the pressure slightly higher than that to open the cracks, by
monitoring a large number of
indoor simulation tests and on-site production data. The acid-liquid
combination flows in the closed cracks
and dissolves the walls of the cracks to form grooves, which optimizes the
distribution of the acid corrosion
conductivity affected by the production time, and regains the crack
conductivity to more than 84% of the
initial conductivity after closed acidization construction. The acid-liquid
combination is self-generating
acid + gel acid + hydrochloric acid, and the usage amount of the three acids
is 40%, 35% and 25% of the
total amount of the acid-liquid combination, respectively. The three acids are
sorted into the self-generating
acid, the gel acid and the hydrochloric acid according to the acid-rock
reaction time from high to low.
Therefore, injecting into the stratum in this order can effectively improve
the conductivity of the far-end,
middle part and near-end of the cracks.
Where, the self-generating acid is synthesized by reacting a carbonyl compound
having high
polymerization degree and a chlorine-containing organic ammonium salt by 1:1
(Wang Yang, Yuan Qingyun,
Li Li. deep penetration and acid fracturing technology for self-generating
acid in carbonate reservoirs in
Tahe OilfieldO.Petroleum Drilling Technology,2016,44(05):90-93). The gel acid
is the same as S3. The
mass concentration of the hydrochloric acid is 20%. The displaced volume of
injecting the acid-liquid
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Date Recue/Date Received 2021-09-24
combination is 1-4m3/min, and the total usage amount of the acid-liquid
combination is 25% of the usage
amount of the acid with the same type and concentration under conventional
acid fracturing technology.
The acid used for cleaning wells is hydrochloric acid with a mass
concentration of 10%, the displaced
volume is 1.1-2.3m3/min, and the usage amount is 10-15% of the total amount of
the acid-liquid
combination.
Compared with the prior art, the invention has the following beneficial
effects:
The invention has the characteristics such as high acid-liquid utilization
efficiency, strong acid
fracturing timeliness, long effective stable production time, and remarkable
production-increasing effect,
and is conducive to the stable and efficient development of high-temperature
deep carbonate reservoirs.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG.1 shows the acid fracturing production curve of Well X.
FIG.2 is a schematic diagram of the effective closed pressure distribution in
the acid corrosion cracks
of Well Y when the stratum stress drops to 70%.
FIG.3 shows the influence of the effective closed stress of Well Y on the acid
corrosion conductivity.
FIG.4 is a schematic diagram of the effective closed pressure distribution in
the acid corrosion
fractures after closure acidization of Well Y
FIG.5 shows the acid fracturing production curve of Well Y.
DETAILED DESCRIPTION OF THE INVENTION
The invention will be further described below based on the drawings and
examples to help to
understand it for a person skilled in the art. However, a person skilled in
the art should note that the scope
of the invention is not limited to specific implementation methods, as long as
changes encountered are
define by the appended claims and within the scope of the spirit and scope of
the invention, they are all
claimed by the invention.
Example 1
The reservoir in the well region where Well X is located has good homogeneity,
with a thickness of
25-28m, but the water near the reservoir gathers and the barrier stress is
low. In order to avoid leading to
the lower water during the acid fracturing process on the basis of leading to
the oil-gas storage space at the
far-end of the well, the water-based fracturing fluid used to carry gel
particles with a density slightly higher
than that of the fracturing fluid is injected into the stratum, and the
mineralization is controlled to 80g/L
during the injection process to ensure that the gel does not prematurely
coagulate. After the gel particles
reach the target position, the mineralization is adjusted to 10g/L, promoting
the gel particles to absorb
water and swell so as to form a high-strength gel barrier and increase the
threshold of the stress through
strata. The volume of the water-based guar gum fracturing fluid used in this
process is 312.6m3, the volume
of the gel particles is 7.6m3, and the displaced volume is 4.5m3/min. The
construction scale, the liquid ratio
and the displaced volume in construction are optimized for parameters to
further improve the spatial
distribution of acid corrosion conductivity, according to the characteristics
of reservoir development, and
finally make the residual acid flow back and put it into production. During
this acid fracturing process, the
volume of the fluid injected into the stratum amounts to 412m3, the volume of
the preflush fluid is 136m3,
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Date Recue/Date Received 2021-09-24
the volume of the conventional water-based fracturing fluid is 29m3,the volume
of the gel acid is 218m3,
the volume of the conventional acid is 29m3, and displaced volume of pumping
the preflush fluid at the first
stage is 6.1m3/min. After adopting the technical solution according to
invention, the spatial distribution
characteristics of the acid corrosion conductivity is better, and the maximum
crack height does not exceed
20m. Compared with the production well adopting the conventional acid
fracturing, the effective crack
length is increased from 65m to 93m. FIG.1 shows the acid fracturing
production curve of Well X. It can be
seen from the figure that the initial output of Well X is 48t/d, and the
average output within 250 days after
the construction is 28.3t/d. The maximum daily output for the other production
wells close to Well X had is
18.6t/d after design and construction according to the traditional method, and
the average output within 160
days is 3.2t/d due to premature water breakthrough after construction. It can
be seen that the invention
effectively controls the crack height, increases the penetration distance of
cracks, optimizes the spatial
distribution characteristics of acid corrosion conductivity, and significantly
improves the capacity of
production and stable production time of production wells.
Example 2
Well Y is a horizontal development well with a horizontal section length of
450m. The well
completion method is barefoot well completion. The horizontal well section has
relatively complex
lithological characteristics, various types of reservoirs unevenly
distributed, and many different types of gas
reservoirs. The permeability span ranges from 3.2 to 21.6mD, and the
heterogeneity is strong. The well has
undergone one acid fracturing at the early stage, and the initial daily oil
production is 39.4m3/d, which
drops to 1.8m3/d one month later. In order to improve the development level of
the reservoir and ensure the
production efficiency, a three-stage process temporarily hindering acidization
is taken to the well section,
regaining the crack conductivity, meanwhile, improving the production-
increasing effect of acid on the
low-permeability area, and optimizing the spatial distribution characteristics
of the acid corrosion
conductivity. The cellulose (6mm) used in the construction amounts to 600kg,
added with 150kg, 200kg
and 250kg, respectively, by 3 times. The respective liquid volume used for
process temporarily hindering
acidization at each stage is: gel acid (45m3) with hydrochloric acid
concentration (10%), fracturing fluid
(20m3), and temporary plugging liquid (20m3), among them, the displaced volume
in construction of
temporary plugging liquid is 1m3/min, and the displaced volume in construction
of gel acid and fracturing
liquid is 3m3/min. In addition, before a process temporarily hindering
acidization, gel acid (40m3) is
injected by displaced volume of 2m3/min to clean the well to ensure that the
subsequent process
temporarily hindering acidization smoothly undergoes; at the same time, in
order to ensure the full
dissolution of cellulose, after the process temporarily hindering acidization
at the third stage is ended, gel
acid (45m3) is injected for ending by displaced volume of 3m3/min. The total
liquid volume for this
construction is 340m3. After process temporarily hindering acidization for 10
minutes, residual acid flows
back. After finishing the residual acid flowback, Well Y is put into
production and on-site production data is
monitored. When the formation pressure drops to 70% of the original formation
pressure, a closed
acidization process is taken for the reservoir. At this time, the closure
pressure distribution in the acid
corrosion crack is shown in FIG.2. It can be seen from the figure that the
maximum converted closure
pressure in the crack can reach 39.21MPa. FIG.3 shows the influence of the
effective closed stress of Y
well on the acid corrosion conductivity. Combining FIG.3, it can be found that
the maximum decrease of
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Date Recue/Date Received 2021-09-24
the acid corrosion conductivity is 56.7%. The closed acidification
construction is divided into two parts:
first, injecting hydrochloric acid (10%) by displaced volume of 0.8m3/min
under pressure of 40MPa for
acid pickling; next sequentially injecting self-generating acid, gel acid, and
hydrochloric acid with a mass
concentration of 25% into the stratum for closed acidification by displaced
volume of 2.1m3/min under
pressure of 52MPa. In construction, the volume of the hydrochloric acid with a
mass concentration of 10%
is 16m3, the volume of the hydrochloric acid with a mass concentration of 20%
is 60m3, the volume of gel
acid is 70m3, and the volume of self-generating acid is 80m3, so the total
liquid volume is 226m3. The
effective closure pressure distribution of acid corrosion cracks after
completion is shown in FIG.4. From
FIG.4, it can be seen that the converted fracture closure pressure has dropped
by more than 52.3%, and the
overall acid corrosion conductivity has been improved and regained to 93.21%.
At the same time, the acid-
liquid combination according to this technical solution is used for
acidization, so that the spatial
distribution of the acid corrosion conductivity is further optimized. FIG.5
shows the acid fracturing
production curve of Well Y. It can be seen from FIG.5 that the initial oil
production of Well Y is 105.6t/d
after acid fracturing, and the average production within 150 days is 45.6t/d.
In addition, after production for
90 days, the daily oil production of Well Y can still be stable at about 40t/d
for a long time, which fully
reflects the optimization effect of this method on the timeliness of acid
corrosion conductivity. For other
production wells close to Well Y, after adopting conventional production-
increasing measures, the initial oil
production can reach 62t/d, but the daily oil production declines rapidly, and
stabilizes at around 4.3t/d after
90 days, and the average production within 150 days is 12.6t/d.
The invention increases the volume of the oil-gas reserves where can be led by
the production well by
temporarily plugging the high-permeability area and definitely carrying out
acid fracturing to the low-
permeability area, and greatly improves the transportation capacity of the
crack to oil-gas resources. At the
same time, the invention takes into account the timeliness of the acid erosion
conductivity, and carries out
corresponding design and construction for it, which significantly optimizes
the spatio-temporal distribution
of the acid erosion conductivity, achieves high-efficiency acid-distributing
to reservoirs, improves the
development of reservoirs, makes the production well long-term stable and high-
yield, and has important
reference significance for on-site construction.
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Date Recue/Date Received 2021-09-24
