GB2062063A - Method for recovering oil from an underground formation - Google Patents

Abstract

Oil is recovered from an oil-wet underground formation consisting of permeable blocks surrounded by a fracture network by supplying to the fracture network an aqueous solution of a surfactant suitable to decrease the surface tension between water and oil.

Description

SPECIFICATION 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 interconnected 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 formation.

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 these blocks defy the displacement techniques that are attractive for increasing the recoverable amounts of oil from the homogeneous type of underground oil-containing formations.

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. These reservoirs comprise sedimentary beds of carbonate cut by a substantially orthogonal pattern of fractures or fissures, and further comprise subordinate oblique set 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 network, 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 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 initial 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 driving 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; "Waterimbibition 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.

U.S.A. patent specification 3,490,527 (Cook and Dimon) describes an oil recovery process by imbibition of water in a tight, naturally fractured, partially depleted reservoir. First, the pressure in the reservoir is restored to its original value by injection of water to recompress and redissolve the free 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 maintainingthe pressure in the reservoir at a high level.

U.S.A. patent specification 3,498,378 (Stone, Graham and Blackwell) described 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, U.S.A. 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 waterwet, 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 techniques gives a solution for the problems that 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 techniques as described in the prior art is suitable for use in oilwet fractured reservoirs having blocks of low permeability.

Object 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 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 channels 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 such 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 fractures 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; c) allowing the oil that is driven from the blocks to collect in the fractures 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 fracture network at a level where oil is being assembled on top of the aqueous solution present in the fractures.

The invention will be described by way of example in more detail with reference to the drawing, wherein Figure 1 shows schematically a vertical section over the Asmari formation in Iran; Figure 2 shows schematically detail li of Figure 1 on a larger scale than the scale of Figure 1; and Figure 3 shows schematically section Ill-Ill 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 sedimentary 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 surface 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 4 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 network.

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 blocks 6 fit rather closely together, and have a pattern 7 of fractures extending therebetween, these fractures communicating with one another thereby forming a network of communicaring passageways that encloses the blocks 6. The fracture 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 te keept 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 gas from the oil/gas interfaces at the entrances 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 are being 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 oil, 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 water level.

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 zones of this formation. Hereby, the loss of surface active agent by adsorption to the rock will be minimized, and the water 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. Depending 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 sulphate type, the alkyl aryl ethoxylate type, the alkyl ethoxylate type, the alkyl- (or alkylaryl) quaternary ammonium 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 consisting 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. Surfactantcontaining water is then supplied to the fracture network, preferably at a level 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 oii-containing blocks.

If oil and water (and no gas) are present in the fracture 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 oil via production wells that communicate with the fracture network at a higher level (e.g. the highest possible level).If oil and water (and no gas) are present in the fracture network, and the oil/water interface is at the lower boundary of the oil containing blocks, the process according to the invention can be applied straight 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 periods 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.

Summarizing 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 oilwet 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.

Claims (5)

1. Method 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 channels at least partially containing oil, said blocks being surrounded by a fluidfilled fracture network, the method including 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 such 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 fractures 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; c) allowing the oil that is driven from'the blocks to collect in the fractures 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 fracture network at a level where oil is being assembled on top of the aqueous solution present in the fractures.

2. The method according to claim 1, wherein the fracture network substantially contains liquid that is at least for the greater part thereof replaced by gas prior to carrying out the steps a, b, c and d.

3. The method of claim 1 or 2, wherein the aqueous solution is introduced into the fracture network via an injection well that communicates with the fracture network at a level that is relatively close below the rising level of the aqueous solution present in the fracture network.

4. A method for recovering oil from an underground formation consisting for at least part thereof of blocks of relatively low permeability surrounded by a fracture network of relatively high permeability, the walls of the porespace of the blocks being substantially oil-wet, and at least the porespace of the blocks substantially containing oil, the method including the sequential steps of:: a) introducing into the fracture network an aqueous solution of a surfactant adapted for lowering the interfacial tension in the interfaces of the water and the oil present in the porespace of the blocks; b) allowing the aqueous solution to displace any fluids other than water in the fractures in a substantially vertical direction; c) submerging the individual blocks of formation material by the rising liquid level of the aqueous solution of surfactant, whereby the surfactant reduces the oil/water interfacial tension in the pore space of each individual block of formation material and oil is displaced from the pore space of the blocks by the aqueous solution under influence of the density difference existing between the fluids present in the pore space of the individual blocks and the aqueous solution in the fractures; ; d) allowing oil to assemble on top of the aqueous solution in the fractures; and e) recovering oil via at least one production well penetrating the formation and communicating with the fracture network at a level where oil is being assembled on top of aqueous solution.

5. The method for recovering oil from an underground formation substantially as described in the specification with references to Figures 1-3 of the drawing.