RU2777004C1 - Method for intensification of hydrocarbon inflows from clay-containing complex oil-producing rocks - Google Patents

Abstract

FIELD: oil industry.

SUBSTANCE: invention relates to the oil industry, namely to intra-contour flooding of reservoirs and maintenance of reservoir pressure in the development of complex oil deposits with a clay-containing collector for low-rate wells. A method for the intensification of hydrocarbon inflows from clay-containing complex collectors of oil-producing rocks includes pumping of fresh water into injection wells. Pumping of fresh water is performed into an open interval of uncovered rock or into a perforated interval of a cased well in a volume corresponding to a volume of an empty space. Then, the well is held until the dissolution of salts being in rock and the formation of additional emptiness in the occurrence of heat balance between liquid in the well and a reservoir system. At the same time, before and after pumping of fresh water and reservoir test, hydrodynamic and geophysical researches of wells are carried out. Upon results of researches, pick-up and return profiles are determined.

EFFECT: increase in an oil recovery coefficient of low-rate wells in the development of complex oil deposits with a clay-containing collector due to the formation of an additional system of cracks in low-permeable collectors and the connection of a wellbore of injection wells to permeable rock lenses, provision of ecological safety.

1 cl, 2 dwg

Description

The invention relates to the oil industry, namely to intra-contour waterflooding of reservoirs and maintaining reservoir pressure in the development of complex oil deposits with a clay-containing reservoir, for low-rate wells.

There is a known method for enhancing oil recovery, including the regulation of the permeability of water-conducting channels of the formation through injection wells by pumping a slug of reagents that provide in-situ sedimentation, which is carried out first from the marginal and waterfowl zones of the formation and continues successively covering zones with higher absolute marks of the roof of the formation (RF Patent No. 2291958 , published on January 20, 2007).

The disadvantage of this method is the difficulty of regulating the process of sedimentation in the reservoir and reducing the injectivity of injection wells, leading to an increase in repression on the reservoir.

There is a known method for enhancing oil recovery, including injection into the formation of a composition containing a low-clay solution with the addition of magnesium sulfate (Patent No. 2425967, publ. 10.08.2011), due to which the regulation of the permeability of the oil deposit is achieved, including with a clay-containing reservoir.

The disadvantage of this method is the simultaneous decrease in permeability in high and low permeability parts of the formation, which will lead to a decrease in the oil recovery factor.

Closest to the claimed method is a method of developing an oil deposit using intra-loop flooding with formation water or water having a salinity equal to formation water (Surgachev M.L. Secondary and tertiary methods for enhanced oil recovery - M.: Nedra, 1985, p. 308). This method is widely used in deposits with containing reservoirs, since the injection of mineralized water into the reservoir does not lead to changes in the porous structure of the rock matrix and does not reduce the permeability of the reservoir.

The disadvantage of this method is its low efficiency due to the filtration flows of injected water during the development of deposits that are heterogeneous in section, and therefore, washed zones are formed in the highly permeable parts of the reservoir, through which the injected water is filtered, and less permeable areas remain unaffected. At the same time, a significant part of hydrocarbon reserves remains unextracted.

The technical result consists in increasing the oil recovery factor in low-rate wells due to the formation of an additional system of fractures in low-permeability reservoirs and connecting the injection wellbore with permeable rock lenses, as well as reducing economic costs and ensuring environmental safety.

The stated technical result is achieved by the fact that in the method of intensification of hydrocarbon inflows from clay-containing complex reservoirs of oil source rocks, including the injection of fresh water into injection wells, the injection of fresh water is carried out into an open interval of the exposed rock or into a perforated interval of a cased well in a volume corresponding to the volume of void space, the well is maintained before the dissolution of salts in the rock and the formation of additional voids when thermal equilibrium occurs between the fluid in the well and the reservoir system, while before and after fresh water injection and reservoir testing, hydrodynamic and geophysical studies are carried out, the results of which determine the injectivity and return profiles.

A comparative analysis of the essential features of the proposed solution and the prototype allows us to conclude that the claimed invention meets the criterion of "novelty".

The claimed invention meets the criterion of "inventive step", as it clearly does not follow from the prior art.

The proposed method is characterized by drawings, where:

in fig. 1 shows the allocation of water-receiving intervals in wells. No. 558 of the Salym field according to thermometry;

in fig. 2 shows the performance of the well. No. 558 of the Salym field.

Core studies were carried out for rocks with a high content of organic matter from fractured-porous-cavernous reservoirs. With prolonged exposure to fresh water, the core samples of these rocks lengthened to a maximum of 1.3-1.5 meters. The core "swollen" and horizontal cracks were clearly visible in it with the release of white and yellow salts. A similar picture stands out in many other areas.

During storage of the core, salts are released on its surface and cracks are formed, mostly horizontal. There is practically no free water in reservoir rocks with a high content of organic matter, and the amount of residual water is on average 10 percent. In the central laboratory, water extracts were made from clay-bearing rocks of the Bazhenov formation of the Salymskoye deposit. Hurricane values of mineralization of chlorine and sulfur salts up to 400-500 and more mg/l have been obtained. The conducted studies of core collections on the impact of fresh water at a pressure of 5-10 MPa showed an increase in voidness and permeability, especially of rocks with a high content of organic matter (kerogen), up to their complete destruction into individual flakes.

Thus, the conducted studies indicated the possibility of formation of additional fracture capacity in hydrophobic clayey rocks when exposed to water at a pressure significantly lower than rock stress, which is explained by the release of energy of inhomogeneous rock stress (dilatant properties).

The performed studies testify to the effectiveness of additional field experiments on water injection into clay-bearing rocks with a high content of organic matter.

The main solved problems of water injection at high pressure are:

- intensification of inflow;

- creation of a wide fluid-dynamic connection in the reservoir within a radius of up to several kilometers;

- disbanding of both technogenic and natural filtration barriers formed in the fractured system of the formation.

Thus, at a reservoir temperature of more than 90°C and a pressure of 40 MPa, fluids in the thin pore space of a matrix with an openness of tens and a few hundreds of angstroms have critical properties that can also contain pore water residues. When the fluid exits the pore space into fractures, water and parts of hydrocarbon gas pass into a free state with the formation of oil-gas viscous emulsions in the fracture system, which prevent oil filtration and, with a certain decrease in pressure drop, completely blocks its movement along the fractures.

The method is carried out as follows. A well is drilled with a bottom hole several meters below the bottom of the formation under study. The well is cased to the top of the formation, leaving the studied (exposed) interval of exposed rocks open. When rocks are prone to collapse, the wellbore is cased with a production casing, cemented and opened by perforation with deep penetrating charges. In an open or perforated interval, fresh water is pumped in a volume of 2-30 thousand m ³ , for example, from overlying water-saturated reservoirs or from surface reservoirs with a cementing unit or special pumps used in hydraulic fracturing. At the same time, water is injected into the perforated interval at a higher discharge pressure. The volume of the injected liquid is determined based on the voidness of the rocks - collectors, the thickness and radius of drainage.

In the open hole, a hollow complex of well logging (GWL) is carried out before and after exposure, including flow metering, debitometry, highly sensitive thermometry (HCT), gamma ray logging (GK), thermal neutron logging (TNT), lateral logging (BK). The well is kept for several days (20-30 hours) until thermal equilibrium occurs between the fluid in the well and the reservoir system. Conduct tests and hydrodynamic studies. Each time in the process of testing, debitometry, VChT, GK, tubing and BC are carried out, injectivity and return profiles are determined, a conclusion is prepared and the well is subsequently put into operation, conducting observational field studies (flow rate, productivity, pressure, temperature) and well logging.

Targeted work to test the impact of fresh water on clay-containing rocks was carried out at the Salym field within the pilot area of the Bazhenov formation deposits. With the beginning of the experiment, a test injection of Cenomanian water was carried out in well 558. 2.3 thousand m ^{3 of} water were pumped and a well logging complex was carried out in order to identify the injectivity interval (Fig. 1). In the process of water injection, the pressure at the mouth was 23-30 MPa, and the injectivity was about 370 m ³ /day.

Before the start of water injection, the reservoir pressure in well 558 in the fractured system was 13.6 MPa and the well worked in the accumulation mode with a flow rate of less than 0.5 t/day. During the initial (before the impact) operation, the well showed itself as a typical low-rate one, for which the predicted value of the total oil recovery for the entire period of operation in natural mode was estimated at 3-5 thousand tons. During the re-development (after water injection) of a well for oil production, its flow rate averaged 115 tons per day. At the same time, more than 42 thousand tons of oil were taken during the year.

The reason for the increase in well production was the formation of a wide network of fractures in low-permeability intervals of clay-bearing rocks of the Bazhenov formation and the connection of the wellbore with permeable lenses of rocks represented by fractured or fractured-cavernous reservoirs. A significant role in this was played by the dissolution of salts included in the rock (fluids in it) with fresh water. However, such a connection could only occur with high-permeability intervals of the suite containing not the main volume of oil produced by the well, since this would have resulted in a noticeable increase in reservoir pressure. The initial formation pressure in well section 558 was about 45 MPa, and in the case of a wellbore connection with a high-permeability insulated lens, the formation pressure noticeably increases.

After washing and deepening the bottom hole, water was re-injected. The well injectivity was 570-600 m ³ /day. at a wellhead pressure of 15.3-15.8 MPa. In the second cycle, 20.5 thousand m ³ /water were pumped.

According to the results of field studies in the surrounding wells 550, 556, 557, an increase in flow rates by 2-3 times was observed, the appearance of water and an increase in reservoir pressure to 16.5 MPa in well 558. Moreover, well 557 was watered 15 days after injection. These surrounding wells did not have a hydrodynamic connection with well 558 before water injection. Reservoir pressure in well 550 at the date of injection was 22.6 MPa, in the well 556-32.6 MPa, and in the well 557-34.0 MPa.

After development, well 558 first blew oil and water, and then switched to a steady flow of clean oil with a low water content. The well flow rate for the first four months averaged 177 tons/day. The cumulative oil withdrawal before the flowing stopped exceeded 127 thousand tons, and the cumulative water withdrawal was estimated at 15-16 thousand m ³ . Well performance is shown in Fig. 2.

Thus, the appearance of water in the surrounding wells in the absence of a hydrodynamic connection between them and well 558 before the start of water injection and the presence of significant differences in reservoir pressures can only be explained by the formation of an additional system of fractures. The additionally formed system of fractures led to the resumption of well operation and a sharp expansion of the reservoir drainage zone.

Test water injection was used in wells 105 and 106 of the Salym field. The essence of the field experiment was to create a wide system of fractures in low-permeability and impermeable intervals of the Bazhenov formation, as well as to displace residual oil from the highly permeable fractured-cavernous interlayer P ² ₄ .

Well 106, characterized by the absence of highly permeable reservoirs, was injected with 1.4 thousand m ^{3 of} Cenomanian water at a wellhead pressure of 20-22 MPa. The well injectivity was 200-400 m ³ /day.

This experiment once again proved the possibility of water absorption in the low-permeability interval of clay-bearing rocks.

About 200 thousand m ^{3 of} Cenomanian water with an injectivity of 200-350 m ³ /day were pumped into well 105, characterized by the presence of a highly permeable layer at the base of the Bazhenov formation, at the time of the end of the experiment. The water-receiving interval was mainly a highly permeable R ² ₄ interlayer with a thickness of up to 2 m.

Since the start of the experiment, formation pressure was regularly monitored in the wells in the northern part of the pilot area. Reservoir pressure in high-rate wells, operating mainly due to the high-permeability interval, tends to increase after the start of water injection into well 105. Reservoir pressure in high-rate wells increased by an average of 8 MPa relative to the beginning of the experiment. In low-rate wells, there were no noticeable changes in reservoir pressure after water injection.

The method can be applied at various stages of exploration and development of source rock deposits:

- at the pre-drilling stage, by creating a wide fluid-dynamic connection in the reservoir;

- at the stage of well development - intensification of their inflow and expansion of the reservoir drainage zone;

- at the stage of intensive reduction of reservoir pressure in the fracture system with the formation of natural filtration blockades in them;

- at the final stage of development - water displacement of residual oil and gas from the fractured system in the zone of reservoir drainage by wells.

Thus, the effectiveness of the fresh water injection method for the purpose of intensifying hydrocarbon inflows from source rock reservoirs has been shown. The goal is achieved with a significant reduction in economic costs, environmentally safe and effective from the standpoint of commercial geology.

Claims (1)

A method for intensifying hydrocarbon inflows from clay-containing complex oil source rocks, including the injection of fresh water into injection wells, characterized in that fresh water is injected into an open interval of the exposed rock or into a perforated interval of a cased well in a volume corresponding to the volume of the void space, the well is maintained until the salts are dissolved located in the rock, and the formation of additional voids when thermal equilibrium occurs between the fluid in the well and the reservoir system, while before and after fresh water injection and reservoir testing, hydrodynamic and geophysical studies of wells are carried out, the results of which determine the injectivity and return profiles.

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