RU2101474C1 - Method of developing oil deposit in carbonate reservoirs of fissured type - Google Patents

Abstract

FIELD: oil production industry. SUBSTANCE: this can be used in development of carbonate beds with subvertical fissures. According to method, drilled are producing and injection wells, product is recovered through producing wells at determined values of bed pressure, displacing agent is injected in cyclic mode, acid solution is injected between cycles of injecting displacing agent, and levelling of displacing front is effected by injection of polymeric solution. After starting watering of producing wells and after fringing of acid solution, injected is fringing of polymeric solution at pressures which exceed opening pressure of vertical fissures. Carbon dioxide created at injection of acid solution reduces viscosity and surface tension of oil. Thickening of solution by polymeric systems leads to levelling of displacement front and in result, range of acting upon bed widens. EFFECT: high efficiency. 1 cl, 3 dwg

Description

 The invention relates to the field of oil field development and may find application in the development of carbonate formations with subvertical cracks.

A known method of developing an oil field, involving the drilling of production and injection wells, drilling, massive acid treatment of wells and water flooding [1]
The disadvantage of this method is that in conditions of fractured reservoirs, injected fluids are filtered only through highly conductive channels of the reservoir.

The closest technical solution to the proposed is the method described in [2]
According to this method, the field:
- drill a grid of production and injection wells and put them into operation;
- conduct oil production to reduce reservoir pressure to a value of 5-6 MPa;
- a working agent is pumped into injection wells, as the edges of a hydrochloric acid solution are used, followed by injection of the displacing fluid.

 To align the displacement front, polymer systems are pumped.

 A significant disadvantage of this method is that the injected polymer solution, continuously dispersed between the blocks of the fractured reservoir, “seals” individual blocks, mainly low permeability, which makes it difficult to filter oil from the latter. This reduces the coverage of formations by water flooding and oil recovery from them.

 The aim of the invention is to increase oil recovery by increasing the coverage of formations by water flooding.

 This goal is achieved by the described method, including drilling production and injection wells, production selection through production wells at certain reservoir pressures, cyclic injection of the displacing agent, injection of the acid solution between the cycles of injection of the displacing agent, and alignment of the displacement front by injection of the polymer solution.

 What is new is that after the start of flooding of producing wells, after the rim of the acid solution, the rim of the polymer solution is pumped at pressures exceeding the opening pressure of vertical cracks.

 In FIG. 1 shows a section of a carbonate reservoir of a fracture type drilled by production and injection wells; in FIG. 2 is the same as in FIG. 1: a process for injecting a polymer solution into an injection well 1 at injection pressures exceeding the opening pressure of vertical cracks; in FIG. 3 is the same as in FIG. 1: the process of oil selection and injection after injection of the polymer solution.

 The inventive method is carried out in the following sequence.

 The field, represented by a carbonate reservoir of a fracture type, is drilled with a grid of injection 1 and production 2 wells, its arrangement is carried out and the wells are put into operation.

 Produce products from wells. With a decrease in reservoir pressure to 5-6 MPa, the well is transferred under cyclic injection of a displacing agent. The first agent injection cycle is performed.

Then, hydrochloric acid solution and water are pumped into well No. 1 (FIG. 1)
In the process of drilling and production, wells are investigated, formation parameters are determined, and well production rates are measured. Measure formation temperature and pressure. The crack opening pressure is determined.

 As production is selected, reservoir pressure decreases to a value corresponding to the release of free gas as a result of the reaction of the acid with the rock. Vertical cracks that open at the initial reservoir pressure close after taking a certain amount of fluid and reducing the pressure in the reservoir.

The reaction of hydrochloric acid with the main differences of the carbonate reservoir is as follows:
CaCO ₃ + 2HCl CaCl ₂ + H ₂ O + CO ₂
CaMg (CO ₃ ) ₂ + 4HCl CaCl ₂ + MgCl ₂ + 2H ₂ O + 2CO ₂
The reaction products of hydrochloric acid with carbonates CaCl ₂ , MgCl ₂ are soluble in water, and the released gas CO ₂ is in a gaseous state at certain values of reservoir pressure and temperature.

 The required amount of hydrochloric acid is calculated taking into account the concentration of the acid used and the volume of the necessary gas rim to displace oil from low-permeability zones of the reservoir.

During operation of the reservoir, the reservoir pressure in it is maintained at a value such that the CO ₂ is in a gaseous state.

 To determine these values, studies are carried out to determine the phase state of carbon dioxide depending on pressure and temperature, i.e. for reservoir conditions.

 After the injection of hydrochloric acid injections of the estimated volume, water is cyclically pumped, compensating for the selection of products from production wells. It is known that the cycling scheme is selected depending on the specific geological and physical conditions of the object of the water flooding system and the development stage.

 Due to the lower filtration resistances of the formation system, hydrochloric acid and water primarily displace oil from fractures and the highly permeable part of the reservoir.

 After the injection of hydrochloric acid and water, the next portion of hydrochloric acid is injected.

When injecting injections, solutions of hydrochloric acid of 10-25% concentration are used. The proportion of solutions of this concentration varies from 1050 to 1120 kg / m ³ . In the interaction of 1 ton of hydrochloric acid of 10-25% concentration with rock carbonates, 61-153 kg of carbon dioxide are released, heat, the effect of which improves the permeability and porosity of the rock. The resulting carbon dioxide lowers the viscosity and surface tension of oil, which are favorable factors for the displacement of oil from formations.

 When hydrochloric acid is injected as a result of its reaction with a rock of cracks, the released gas occupies the roofing part of the cracks and formation.

 Gas formation in the formation during the interaction of acid with the rock without lowering the reservoir pressure is also possible with increasing temperature.

 If you do not plug the cracks, then the working agents will advance to the producing wells without performing useful work.

 Therefore, after exposure to an acid solution on all blocks of the carbonate reservoir, including low permeability, and this will happen when the displacing agent approaches the production wells, the polymer solution is injected.

 The beginning of flooding of producing wells is a signal for oil displacement from fractures and treatment of blocks with acid. Hydrochloric acid must react with the rock and neutralize.

 The injected polymer composition must have a high binding, wetting and adhesive ability. The composition should have adjustable properties and go into a stable hydrogel in reservoir conditions.

 To isolate subvertical pore cracks throughout the profile, the concentration of the polymer solution is selected so that the specific gravity is less than the specific gravity of the injected and produced water. The polymer solution occupies the entire section of the cracks and does not allow gas and fluids to break through to production wells.

The conditions under consideration correspond to aqueous solutions of cellulose ethers or cross-linked polymer compositions (polyacrylamide + chromium acetate, etc.)
It is known that the filtration properties of carbonate reservoirs are largely due to the presence of fracturing. Vertical cracks are kept open when the reservoir pressure of the fluid filling the pores and reservoir cracks exceeds the lateral pressure.

 To open cracks in the reservoir, the pressure at the bottom of injection wells must exceed the lateral rock pressure, the magnitude of the lateral rock pressure is determined experimentally or by calculation and depends on the depth of the reservoir, type and condition of the reservoir.

 The polymer solution is injected at a pressure at the bottom of injection wells that exceeds the opening pressure of vertical cracks. The polymer solution is pushed by the displacing agent to the required distance from the injection well. The polymer solution forms “plugs” on the main fluid-conducting channels at a specific location. The location of the plug is adjustable in space and time. Due to destruction, pushing to the producing well, injection of a polymer solution destabilizer (for example, acid, bleach), this “plug” can be eliminated.

 The required plug installation distance depends on many factors. In the initial stage, the operation of the “plug” field can be installed at a distance of 1/3 of the distance from the injection well of the design grid of wells. With a significant water cut of the produced products of producing wells, the “plug” is pushed past the middle of the distance between the wells. By adjusting the discharge pressure, the “plug” can be set to the required distance. After depressurization in the formation, the “plug” is in a stable state in a certain place and, due to its good adhesive ability, blocks highly conductive channels. The heterogeneity of the production facility is reduced. With a decrease in layer-by-layer and zonal heterogeneity, the coefficient of formation coverage by both impact and water flooding increases. To effectively use the working agent during their breakthroughs, production wells are temporarily stopped during the injection of reagents.

An example of a specific implementation. The oil reservoir in the fractured carbonate reservoirs (Fig. 1) is drilled with one injection 1 and one producing 2 wells. The distance between the wells is 400 m. The wells have been drilled by drilling a carbonate formation at a depth of 1200 m with an oil-bearing thickness of 15 m. The balance reserves calculated by the volumetric method are 145 thousand tons. According to well survey data, the formation is fractured. The opening pressure of vertical cracks is 60-80% of rock pressure. At the mouth of injection wells, when injecting a displacing agent with a specific gravity of 1000 kg / m ^3, it is necessary to create an injection pressure of (7.4-13.9) MPa so that vertical cracks of various types are opened. It is known that, at lower injection pressures, cracks open in the more permeable parts of the formation. With increasing injection pressure, cracks begin to open in less permeable parts of the formation and in more remote areas of the highly permeable part of the formation.

 To increase the oil recovery coefficient, 2,000 tons of hydrochloric acid were cyclically pumped into injection well 1.

 After the commissioning of well 2, the extraction of oil about 4% of the initial balance reserves and the implementation of two cycles of hydrochloric acid injection, the injected input appeared in the production well. An increased rate of increase in water cut was observed.

To limit the flow of water with a simultaneous increase in the coverage of the blocks by exposure to the injection well, a polymer solution was injected. A polymer solution in a volume of 1000 m ^{3 of} 0.5% concentration, having a specific gravity of 950 kg / m ³ , was pumped at an injection pressure of 13.0 MPa and the specific gravity of 1000 kg / m ³ pumped with water injected at a distance of 200 m (Fig. 2) from the well (calculated by the average rate of fluid filtration in the fractures). After injection, the wells were put into operation (figure 3).

 The oil content in the production of well N 2 increased, and the rate of increase in water cut slowed down.

 The calculations showed that the calculated layer-by-layer heterogeneity decreased by 3 units, the coverage of formations by the impact increased from 60 to 67%, which made it possible to increase the oil recovery coefficient by 7% (see table).

Claims (1)

 A method of developing an oil field in fractured carbonate reservoirs, including drilling production and injection wells, selecting products through production wells at certain reservoir pressures, cyclic injection of the displacing agent, injection of the acid solution between the cycles of injection of the displacing agent, alignment of the displacement front by injection of a polymer solution, characterized the fact that after the start of flooding of producing wells, following the rim of the acid solution, the rim is pumped olimernogo solution at pressures exceeding the pressure of the disclosure of the vertical cracks.

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