RU2339799C1 - Method for development of massive oil deposit - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: invention refers to oil extracting industry, particularly to methods of development of massive oil deposit with injection of water-gas mixture. Method includes injection of water-gas mixture into a deposit; at that mixture has been prepared at the head or in the well in the process of injection, adding of surface active substances into water, changing the degree of aeration of water-gas mixture and changing of injecting pressure. According to the invention the density of water-gas mixture at input to a bed and bottom pressure in a pressure well, medium at interval of injecting is maintained in accordance with analytical expression. The density of water-gas mixture and bottom pressure are from time to time changed with consideration to analytical expression.

EFFECT: invention facilitates increasing ratio of oil extraction.

3 cl, 2 dwg

Description

The invention relates to the oil industry, to methods for developing an oil reservoir using methods for increasing the oil recovery coefficient (CIN), in particular, by injecting a water-gas mixture into the formation.

Known methods for the development of oil deposits with injection into the reservoir of a water-gas mixture described in patents No. 2060378, No. 2088752, No. 2269646, No. 93003931, No. 94007734. A gas-gas mixture is prepared either on the surface or in the wellbore by mixing water and hydrocarbon or other gas, at a given concentration of gas in the C * mixture, under conditions at the bottom of the injection well.

Closest to the proposed is the method according to patent No. 2088752, according to which the injection of water and gas into injection wells is carried out in the form of a water-gas mixture formed during the ejection. The volume of suction gas is maintained at a level such that the viscosity of the water-gas mixture formed under reservoir conditions is equal to the viscosity of the reservoir oil. After the front displaces the production wells, the degree of aeration of the water-gas mixture is increased by 2-5 times to a decrease in the injectivity of injection wells by 1.5-2.5 times. Then, the injection pressure is increased 1.1-1.5 times until the initial injectivity is restored. A mixture of two surfactants — anionic type and nonionic type — with a concentration of 0.1-1% at a ratio of 1: 1-1: 3, is added to the injected water.

A significant drawback of this patent is that during its implementation, negative phenomena are possible due to the outflow of the water-gas mixture into the water-saturated volume of rocks underlying the massive oil deposit, as well as the breakthrough of the gas-gas mixture into the upper part of the deposit, saturated with oil. There are no restrictions on the density of the water-gas mixture injected into the reservoir - ρ _s , and the limits of the permissible pressure on the bottom of the injection well - R _s are not indicated. This is a significant disadvantage of the known method.

The importance of taking into account the restrictions as applied to the first parameter - the density of the water-gas mixture will be shown in the following example. Figure 1 presents the calculated graphs of the change in the density of the water-gas mixture ρ _s versus pressure at the density used for the preparation of the mixture of water ρ _in = 1150 kg / m ³ and gas, under standard conditions, ρ _g = 0.8 kg / m ³ and various values the concentration of the mixture at the entrance to the reservoir - at a pressure at the bottom of the well P * = 26 MPa and the concentration of gas in the mixture at the bottom of the injection well C * = 0.05-0.3. As can be seen from the graphs, the density of the water-gas mixture at a pressure of P _s > 5 MPa varies from 400 kg / m ³ (at C * = 0.3 and P _s = 5 MPa) to 1130 kg / m ³ (at C * = 0 , 05 and P _c = 30 MPa). If, for example, the density of oil in reservoir conditions is ρ _{n. pl} = 700 kg / m ³ and the density of water: formation or injected into the reservoir before injection of the gas-gas mixture ρ _{in. pl} = 1000 kg / m ³ , then at a density of the gas mixture ρ _c <700 kg / m ^{3 the} mixture will “float” in oil, and at ρ _c > 1000 kg / m ^{3 it will} “sink” in produced water.

Both cases are not advantageous from the point of view of increasing CIN. In the first case, breakthroughs of the injected water-gas mixture through separate highly permeable channels upward will be observed, as a result of which there will be a low value of the coefficient of coverage by oil displacement by the injected water-gas mixture. In the second case, especially when the process is implemented at the initial stage of development, there will be no effect from the injection of the water-gas mixture at all, or the effect will be minimal, since the mixture will tend to go down under the influence of gravitational forces and may ultimately occupy a volume below the initial oil-water level contact (KSS), i.e. outside the oil reservoir. For this reason, a layer of a gas-gas mixture — a liquid with gas bubbles — will not be formed at the interface between oil and produced water, which will increase the oil displacement coefficient in front of the water front. The final condition for the first of the considered parameters has the form

Condition (1) allows the water-gas mixture to move upward (“pop up”) only within the waterlogged volume of the reservoir between the initial and current positions of the OWC and not go down due to gravitational forces.

Restrictions on the second parameter - bottomhole pressure in the injection well during the injection of the water-gas mixture P _s into it - is due to the following. As is known (Lysenko V.D., Graifer V.I. Rational development of oil fields. M., Nedra, 2005, pp. 28-31), the injection pressure should be higher than the initial pressure of saturation of the reservoir oil with gas P _{us. beg} . This is explained by the fact that the increase in oil recovery factor due to the injection of a water-gas mixture, along with other reasons, is associated with additional saturation of the oil contained in the voids of the rock with gas, in particular hydrocarbon gas, and the resulting increase in the volume of reservoir oil. This is expressed in the growth of the volumetric coefficient of oil b _n after its contact with gas bubbles at a pressure higher than the initial saturation pressure of the reservoir oil with gas. If the water-gas mixture is injected with a pressure P _c lower than P _{us. Initially} , then the increase in the volume of reservoir oil will not occur, and, of course, the oil recovery factor will be either minimal or not at all. Therefore, a condition for the effective use of injecting a water-gas mixture is to maintain the pressure at the bottom of the injection well higher than the initial saturation pressure of the formation oil with gas. However, in this case, if the sum of the bottomhole pressure in the injection well and the pressure created by the weight of the column of the water-gas mixture h from the injection interval to the depth of the initial OWC - g · ρ _c · h - exceeds the current pressure at the depth of the initial _OWC R _tVNA possible outflow of a part of the volume of the injected water-gas mixture, and with it the displaced oil, beyond the initial contours of the oil reservoir, into the water-saturated volume located below the initial oil-water contact.

In order to prevent the outflow of the injected water-gas mixture beyond the oil reservoir — below the depth of the initial OWC, if the interval for injecting the mixture in the interval located above the depth of the initial OWC by h is proposed to maintain the pressure at the bottom of the injection well from the condition

where g is the acceleration of gravity;

h is the vertical distance, height, from the middle of the injection interval to the initial OWC;

ρ _c is the weighted average density of the gas mixture along the height h.

Improving the effectiveness of the proposed method is carried out by periodically changing over time the density of the injected water-gas mixture and bottomhole pressure within the limits given in expressions (1) and (2).

The greatest coverage of the volume of a massive deposit by displacement, as is known (Lysenko V.D., Graifer V.I. Rational development of oil fields. M., Nedra, 2005, pp. 55-60) is achieved by forcing a displacing agent into the lower part of the deposit, provided that here is a layer with increased permeability of the rock.

In the absence of a highly permeable interval in the lower part of the reservoir, it is proposed to conduct a directed horizontal high-penetrating hydraulic fracturing at the depth of the initial VOC or to drill one or more horizontal boreholes or horizontal bends from an existing vertical well in this interval, and then pump the gas-gas mixture into the reservoir.

The water-gas mixture entering the formed horizontal fracture will propagate radially from the bottom of the well, covering a large area. Simultaneously with the movement in the radial direction, the gas-gas mixture will also move in the vertical direction - due to its lower density in comparison with the density of the formation water. As a result of this, a large volume of rock can be covered with a water-gas mixture - from the initial to the current position of the OWC.

Figure 2 presents a diagram of the process with the location within the initial VNC horizontal trunk (1). The water-gas mixture emerging from the perforation holes of the well due to the difference in the densities of the produced water and the gas-water mixture is located by the layer (2) above the horizontal wellbore and then gradually moves upward to the surface of the current position of the oil-water supply. At the same time, a large volume of rock is covered by displacement.

An increase in the coverage of a reservoir by oil displacement by a water-gas mixture can also be achieved by gradually closing the upper part of the opened reservoir interval with disconnecting devices. In this case, the water-gas mixture is initially injected into the entire opened interval of the deposit, and then, after a certain time, the injection interval is reduced, cutting off the upper part of the opened interval of the deposit. Due to the transfer of injection to the lower part of the reservoir, a larger volume of the reservoir is covered by displacement, the breakouts of the displacing agents to production wells located near the injection are reduced.

Claims (3)

1. A method of developing a massive oil reservoir, which involves increasing the oil recovery by injecting a gas-water mixture prepared at the wellhead or in a well during ejection, adding surfactants to the water, changing the degree of aeration of the gas-gas mixture and pumping pressure, characterized in that the density water-gas mixture ρ _with at the entrance to the reservoir support from the condition: ρ _npl <ρ _with <ρ _int , where ρ _npl and ρ _vpl - respectively, the density of oil and water in reservoir conditions, and the bottomhole pressure in the injection well, the average in the injection interval, P _s from the condition: P _start <P _with <(P _tVNAnach -g · ρ _c · h), where R _tvnaknach - current reservoir pressure in the reservoir at the depth of the initial oil-water contact - VNK; g is the acceleration of gravity; h is the vertical distance is the height from the middle of the interval of the injection of the gas-water mixture to the initial OWC; P _for - the initial pressure of oil saturation with hydrocarbon gas; ρ _s is the weighted average density of the gas mixture along the height h, and a periodic change in time of the density of the water-gas mixture and bottomhole pressure is carried out in the above ranges. 2. The method according to claim 1, characterized in that the water-gas mixture is injected into the reservoir through a horizontal crack formed as a result of directional high-penetrating hydraulic fracturing conducted at the depth of the initial oil hole, or through horizontal shafts drilled at the depth of the initial oil hole, or through horizontal branches from available vertical well. 3. The method according to claim 1, characterized in that the injection of the water-gas mixture is initially carried out in the entire opened interval of the deposit, and then after some time the injection interval is gradually reduced by shutting off the upper part of the opened interval by the disconnecting devices.

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