Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR RECOVERING OIL FROM AN UNDERGROUND
FORMATION
The invention relates to a method for recovering oil from an
underground formation. In particular, the invention relates to a
method for recovering oil from an underground formation that
consists for at least part thereof of permeable blocks of formation
material with a pore space substantially consisting of capillary
channels formed by interconnectcd pores, which channels at least
partially contain oil. Such blocks are surrounded by a fluid
filled fracture network which is either of natural or of artificial
origin. This type of formation is referred to hereinafter as a
fractured format;on.
Only a small amount of oil that is originally present in such
fractured formations can be recovered therefrom. Displacement
techniques wherein a displacing or drive medium is passed through
the formation between an injection well and a production well to
displace oil towards the production well and which techniques are
often found to be effective in homogeneous formations, fail when
applied in fractured formations (as defined above) since the
injected medium will sweep through the fractures only and bypass
the pore spaces of the individual blocks. In attempts to recover
oil from formations of this type by such displacement techniques,
large amounts of oil will be left in the formations, since the
particular combination of low permeable blocks wherein the oil is
trapped by capillary forces, and the relatively high permeable
passages around thes~blocks defy the displacement techniques that
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are attrac-tive for increasing the recoverable amounts of oil frorn
the homogeneous type of underground oil-containing forma-tions.
More specifically, the present invention is related to the
recovery of oil from fractured formations wherein the oil is
substantially being retained in the pore space of the blocks by
capillary channels having oil-wet walls. The Asmari limestone
reservoirs in Iran consist of this type of formation. ~hese
reservoirs comprise sedimentary beds of carbonate cut by a
substantially orthogonal pattern of fractures or fissures, and
further comprise subordinate oblique sets of fractures. Lateral as
well as vertical conductivity of fluids through the reservoirs is
extremely high on account of this fracture network.
In at least part of a reservoir of this type, a horizontally
extending oil layer is present in the fracture network. Water is
often present below this oil layer, and in some areas gas is found
in the upper part of the network. Oil is further present in at
least part of the blocks that are surrounded by the fracture ne-t-
work, this oil being trapped in the porespace of the blocks, which
porespace substantially consists of capillary channels having
oil-wet walls. These oil-containing blocks may be present in the
region of the above-referred oil layer, but also in the water
layer and/or in the gas layer if such layers are present below and
above the oil layer, respectively. It will be appreciated that the
presence of these blocks in one or both of these latter layers
largely depends on the oil-recovery techniques that have been
applied previously in the particular formation or reservoir.
In recovering oil from this type of reservoir, production
wells are drilled into such reservoirs, said wells communicating
with the oil-containing part of the fracture network. Oil is
recovered from the fracture network via the wells and depending on
the underground conditions, the gas cap above the oil layer expands
downwards (or a downwardly expanding gas cap is formed) and/or
the bottom water zone below the oil layer grows in size in upward
direction (provided that the reservoir communicates with a water
reservoir of sufficiently high pressure). Oil recovery rates will
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initially be high as a result of the relatively high conductivity
of the fracture network. After the oil in the fracture network has
been recovered,large volumes of oil are still present in the pore
space of the blocks. This oil, however, is for the greater part
thereof trapped in the pore space of the blocks by capillary forces,
and will thus remain in the pore space of the individual blocks.
The oil recovery rates will consequently decline to values that
are considerably lower than the ini-tial stage of the recovery
period. As observed already above, the application of secondary
recovery techniques making use of displacement fluids or drive
fluids with the object to increase the oil recovery rates will
be of no avail due to the presence of the fracture network.
Apart from the fractured oil-containing reservoirs consisting
of oil-wet rock for the greater part thereof, there exist
fractured reservoirs that consist of water-wet rock. In recovering
oil from such formations, use is being made in prior art processes
of the so-called imbibition technique, wherein water is being
supplied to the formation blocks containing the water-wet capillary
channels, which channels will then imbibe part of this water,
thereby drivirg out oil from the pore space.
Oil recovery by water imbibition in naturally fractured
water-wet formation has already been described as early as 1952 in
the Oil and Gas Journal Vol. 51, No. 28 (pg. 264, "Water-imbibition
displacement ----- can it release reluctant Sprayberry oil?" by
E.R. Brownscombe and A.B. Dyes).
U.S.A. patent specification 2,792,894 (Graham, McCardell,
Osoba and Richardson) describes the application of an imbibition
technique in a water-wet oil-containing formation comprising porous
rock sections adjacent channels of higher permeability than
the porous rock sections. A surface active agent is included
in the water that is being supplied to the formation in order to
be imbibed in the water-wet porous rock section, after these porous
rock sections have imbibed water to substantially the saturation
point. This allows additional quantities of oil to be produced
from the channels by displacement of oil by the water from the
blocks into the fracture network.
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USA patent specifica-tion 3,490,527 (Cook and Dimon) describes
an oil recovery process by imbibition of water in a tight, naturaly
fractured, partially depleted reservoir. Firs-t, the pressure in
the reservoir is restored to i-ts original value by injection of
water to recompress and redissolve the f'ree gas phase. The rate of
this repressuring is critical and must be slow enough to avoid
pushing the oil back from the fracture face into the interior of
the rock matrix. Production of the reservoir by water imbibition
is continued while maintaining the pressure in the reservoir at a
high level.
USA patent specification 3,498,378 (Stone, Graham and Blackwel~
describes the recovery of oil from a naturally fractured water wet
formation wherein water is cyclically supplied to the formation
thereby increasing the formation pressure, and oil is cyclically
produced therefrom by reducing the reservoir pressure, and a
surfactant is added to the cyclically injected water to render the
exterior portion of at least a portion of the matrix blocks oil-
wet.
Finally, USA patent specification 3,548,941 (Graham and
Ortloff) describes a cyclic pressure flooding technique (also
indicated as "pressure pulse" flooding) for recovering oil from a
water-wet, naturally fractured matrix reservoir by injecting a
solution containing a surface active agent. This agent is meant to
further increase the water wettability of the rock matrix and to
reduce the oil/water interfacial tension. It is imperative that the
matrix should remain water-wet so as to preserve the imbibition
property of the matrix.
None of the prior art techni~ues gives a solution for the
problems tha-t are faced when recovering oil from fractured
reservoirs with matrix blocks comprising oil trapped in capillary
channels formed by interconnected pores having oil-wet walls. It
will be appreciated that water present in the fractures cannot
enter these porespaces since the walls of the capillary channels
are oil-wet, and that none of the imbibition techni~ues as
described in the prior art is suitable for use in oil-wet fractured
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reservoirs having blocks of low permeability.
Objec-t of the invention is an oil recovery process for this
particular type of fractured formation, whereby oil trapped in the
capillary channels of the porespace of the blocks substantially
consisting of oil-wet rock can for an appreciable part thereof be
recovered from this formation.
The method according to the invention for recovering oil from
an underground formation consisting for at least part thereof of
permeable blocks substantially consisting of oil-wet formation
material with a porespace substantially consisting of capillary
ehannels at least partially containing oil, said blocks being
surrounded by a fluid-filled fracture network, includes the
sequential steps of:
a) compounding an aqueous solution of a surfactant, having the
ability to decrease the surface tension between water and the
oil present in the blocks;
b) supplying said solution through at least one injection well to
the fracture network sueh that the fluids in the fracture
network above the level of the aqueous solution are displaced
in a substantial vertical direction by the aqueous solution,
thereby raising the hydrostatic head in the fraetures surrounding
the individual blocks that are being submerged in the aqueous
solution and simultaneously decreasing the surface tension in
the oil/water interfaces in the capillary channels of said
blocks as a result whereof oil is being driven from said
passages into the fractures surrounding said blocks;
e) allowing the oil that is driven from the bloeks to colleet in
the fraetures on top of the rising level of the aqueous
solution; and
d) recovering oil via at least one production well penetrating the
formation and communicating with the fraeture network at a level
where oil is being assembled on top of the aqueous solution
present in the fraetures.
The invention will be deseribed by way of example in more
detail with reference to the drawing, wherein
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Figure 1 shows schematically a vertical section over the Asmari
formation in Iran;
Figure 2 shows schematically detail II of Figure 1 on a larger
scale than the scale of Figure 1; and
Figure 3 shows schematically sect;on III-III of Figure 2 on a
larger scale than the scale of Figure 2.
The oil-containing formation 1 (shown in Figure 1) is a highly
fractured formation consisting of sedirnentary beds of carbonate.
These beds are cut by an orthogonal pattern of fractures (not shown).
A plurality of wells have been drilled through the formation 2
and into the oil-containing formation 1. Only two of these wells have
been shown. The wells are completed in the normal manner and no
detailed description thereof is required. The well 3 is adapted for
supplying fluids to the formation 1, whereas the well 4 is adapted
for transporting fluids from the formation 1 to the surf'ace of the
earth. The injection well 3 communicates with the fracture network
of the formation 1 at a level above the water level 5. The production
well ~ communicates with the fracture network of this formation at a
higher level than the well 3.
It will be appreciated that the formation 1 shown in Figure 1
has been subjected to an oil recovery treatment whereby the oil
present in the fracture network has been recovered therefrom, thereby
leaving gas (such as hydrocarbon gas) in the upper part of the
fracture network, and water in the lower part thereof. The water
level has not risen during this oil recovery process, and the
formation part above the gas/water interface 5 in the fracture
network contains oil trapped in the tight reservoir rock thereof.
The recovery of oil from the fracture network via the well 3
has been effected by gas cap expansion and/or solution gas drive. It
will be understood that a small amount of oil continues to drain
from the blocks into the fracture ne-twork. This oil is being
collected on top of the water and will be recovered later on
simultaneously with the oil that is being recovered from the tight
formation rock by means of the method according to the invention.
Figure 2 shows detail II of Figure 1 on a larger scale. The
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blocks 6 fit rather closely together, and have a pattern 7 of
frac-tures extending therebetween, these fractures communicating
with one ano-ther thereby forming a network of comm-micating
passagew3ys that encloses the blocks 6. The ~racture network
extends through the formation 1 and communicates with the wells
(such as wells 3 and 4) that penetrate this formation.
More details of the way in which the blocks 6 are bounded
by fractures 8 of the network are shown in Figure 3 of the
drawing. In Figure 3 , the width of the fractures 8 is somewhat
exaggerated.
Each block 6 consists of carbonate rock of low permeability.
The porespace of the blocks substantially consists of capillary
channels extending in all directions, the majority of these
channels having oil-wet walls. Oil is trapped within these
channels by capillary forces, which are sufficiently strong to
keep the porespace of the blocks filled with oil over the total
height or approximately the total height thereof, even in those
blocks, such as block 6A, that are surrounded by fractures 8
containing gas.
It will be appreciated that by raising the water level 5 in
the fractures 8 surrounding the block 6A will not result in an
imbibition of water into the block 6A as described in the prior
art processes since the oil-wet walls of the capillary channels
of the block 6A prevent the water from entering the channel.
In the method according to the invention, an aqueous
solution is supplied to the fracture network 7 via the well 3, this
solution containing a surface active agent having the ability to
decrease the surface tension in the interface of the oil present
in the blocks 6, 6A and the water present in the fractures 8. The
solution after being injected into the fracture network, will be
distributed at a relatively low pressure gradient over the gas/
water interface 5, and the water level in the fracture network 7
will rise, thereby submerging the block 6A, as well as all other
blocks that are situated above the water level 5 shown in Figure 2.
The injected water having the surface active agent dissolved
therein displaces -the eas from the oil/gas interfaces at the
entrar.ces to the capillary channels of the blocks -that are being
submerged. The oil in the channels is then contacted by water at
an extremely low interfacial tension, and at an increasing pressure
since the water level is rising in the fractures surrounding the
blocks that arebeing submerged. The difference in density between
the water outside each block 6 that is being submerged and the oil
in the capillary channels thereof will then - since there exists
an extremely small interfacial tension at the gas/oil interfaces
at the entrances to the capillary channels - force water to enter
the oil-wet capillary channels and displace the oil into the
fracture network either directly from the top of the block that is
being submerged or from any other capillary channels that debouch
in the side walls of the relevant block.
It will be appreciated that the interfacial tension in the
gas/oil interfaces at all entrances to the capillary channels will
be reduced to a relatively small value when a block is fully
submerged. In this situation, even the oil that has been trapped
in capillary channels of extremely small dimension that prevented
earlier displacement of the oi~, will now be displaced from these
latter channels under influence of the difference in density
between the water in the fractures surrounding the block, and the
oil present in those extremely small capillary channels.
The oil that is displaced from the capillary channels of the
formation blocks that are being submerged in the aqueous solution
of the surface active agent, is collected in the fracture network
and will float on top of the rising waterlevel. When a sufficient
volume of oil is present, the pumps (not shown) in the recovery
well 4 are started and oil will be transported through this well
to the surface, where it is temporarily stored in a suitable
container (not shown) awaiting further transport to another
location, such as a refinery.
The wells 3 and 4 may periodically be opened to higher
levels of the formation 1, when the oil has been recovered from
the lower ~ones of this formation. Hereby, the loss of surface
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active agent by adsorption to the rock will be minimized, and the
wa-ter cut of the fluids recovered via the production well will
be kept within an acceptable limit.
Surface active agents suitable for use in the method of the
present invention are to be selected on the basis of their
ability to decrease the oil/water interfacial tension to the
desired degree under formation conditions, such as the formation
temperature, the salinity of the connate water present in the
formation and of the injection water used to prepare the surfactant
solution, and the properties of the oil in the formation.
~epending on these formation conditions suitable surfactants
can be selected from various classes of surfactants such as
anionic surfactants, nonionic surfactants and cationic surfactants.
Examples of suitable surfactants are surfactants of the alkyl
aryl sulphonate type, the alkyl aryl ethoxy sulphonate type,
the alkyl ethoxy sulpha-te type, the alkyl aryl ethoxylate type,
the alkyl ethoxylate type, the alkyl- (or alkylaryl) quaternary
anmoniun type, the betaine type surfactants and mixtures
thereof. It will be appreciated that this list of surface active
agents is not exhaustive and that the types indicated are given
by way of example only. Application of the invention is not
limited to a particular surface active agent since any surfactant
having the ability to decrease the oil/water interfacial tension
in -the formation to be treated by the present method will be
useful for the purpose.
Application of the invention is not restricted to fractured
formations comprising oil-containing blocks substantially consis-
ting of oil-wet rock, and having an oil/water distribution in the
fracture network as shown in Figure 1 of the drawings. The present
process may be applied with equally good results in the above type
of formation, wherein an oil layer is present between the water
zone and the gas zone in the fracture network. Surfactant
containing water is then injected at preferably not too large
distance below this oil layer, and the water zone is expanded
upwards, thereby lifting the oil layer, which grows in volume by
the oil that is being driven from the blocks that become submerged
in the water zone. The oil is recovered via one or more production
wells.
In another formation of the above-mentioned type, water may
be absent in the situation in which the present process is
initiated. Surfactant-containing water is then supplied to the
fracture network, preferably at a evel close to the bottom of
the formation in the initial stage of the method, and later on
to a level that is (just) below the highest level of the water in
the fracture network. The rising level of the solution subsequently
submerges the blocks thereby displacing the oil therefrom, which
oil is collected on top of the solution present in the fracture
network. This oil (as well as the oil that may originally be
present in the fracture network) is subsequently recovered through
one or more production wells.
In case no gas is present in the fracture network, the space
within this network will contain oil and/or water. If only water
is present, the water is removed from the network (e.g. by
injecting gas) until the waterlevel is just below the level of
those blocks that have oil trapped in the capillary porespace
thereof. Subsequently, the process according to the invention is
carried out in the manner as described with reference to Figure 1
of the drawing. If only oil is present, the production well 4 is
opened to the fracture network at as high a level as possible,
and surfactant solution is injected into the fracture network at
the lowest possible level of the assembly of oil-containing blocks.
If oil and water (and no gas) are present in the *racture
network, and some of the oil-containing blocks are submerged in the
water, steps are taken to lower the oil/water interface in the
fracture network (such as by the injection of gas) to a level
below the assembly of oil-containing blocks. Subsequently, the
process of the invention is initiated by supplying surface active
agent containing water to a level (just) below the oil/water
interface, and recovering o;l via production wells that communicate
with the fracture network at a higher level (e.g. the highest
possible level).
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If oil and water (and no gas) are present in the fracture ne-twork,
and the oil/water interface is at the lower boundary of the oil-
containing blocks, the process according to the invention can be
applied straigh-t away by injecting water containing the surface
active agent in the neighbourhood of this interface (and preferably
just below this interface), and recovering oil via production wells
that communicate with the fracture network at a level above the
said injection level of the water.
The injection of the surfactant solution may be periodically
interrupted. Oil is then recovered from the fracture network (where
it has been collected) during the periods of the injection of the
solution, as well as during the p~eriods that this injection has
been interrupted. To reduce water cut, the levels at which the
production wells communicate with the fracture network are
periodically raised.
The injection wells preferably communicate with the fracture
network at levels at which the injected solution is relatively
close to the upper level of the rising water level in the fracture
network. This is to reduce loss of surfactants to a minimum. It is
further desirable to reduce contact between the injected solution
and an oil layer present in the fracture network to a minimum,
such contact taking place when the injected solution flows from
the well to the body of water present in the fracture network.
Su~marizing it is observed that the process according to the
present invention makes use of an aqueous solution suitable for
reducing the surface tension between oil and water, which solution
is supplied to the fracture network of a fractured formation
substantially consisting of oil-wet rock: a) for submerging the
oil-containing formation blocks in the solution to increase the
hydrostatic head around the oil-containing blocks and b) for
decreasing the surface tension in the oil/water interfaces at the
entrances to the capillary porespaces of the blocks that are being
submerged, thereby forcing water into the capillary porespaces of
the blocks that are being submerged, to displace the oil from these
spaces into the fracture network.
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