RU2365741C1 - Method for oil pool development - Google Patents

Abstract

FIELD: oil and gas production.

SUBSTANCE: invention is related to oil production industry. Method includes cyclic injection of water into watered payout bed, sampling and testing of reservoir fluid samples, measurements of bed pressure and saturation of bug holes with fluids by means of downhole logging methods. In every cycle in phase of bed pressure rise, water injection to bed is carried out until bed pressure reaches limit - maximum value - not exceeding rated pressure values, when gas contained in bug holes is completely dissolved in oil, which is identified in compliance with mathematical expression. In phase of bed pressure decrease - limit - minimum value - its value is established not lower than the pressure, at which gas saturation of bug holes equals critical value that corresponds to beginning of filtration of gas in rock, which released from oil available in bugholes. If gas saturation of bug holes goes below specified minimum value, prior to another cycle of water injection in pool, rated volume of gas or water-gas mixture is injected.

EFFECT: improved oil recovery of bed by water injection.

2 cl, 2 dwg, 2 tbl

Description

The invention relates to the oil industry, to methods for developing an oil reservoir using methods of increasing oil recovery, in particular by pumping water into the reservoir in a cyclic mode.

Known methods for the development of oil deposits with injection into the reservoir of water in a cyclic mode, published in the works: "Cyclic impact on heterogeneous oil reservoirs", Sharbatova IN, Surguchev ML - M .: Nedra, 1988; "Improving the efficiency of oil displacement from heterogeneous reservoirs by unsteady water-gas exposure", DL Alekseev, IV Vladimirov, RV Vafin - NTZ “Interval. Advanced oil and gas technologies. ”- Samara, No. 2, 2007, - p. 5-10, and in patents: RF No. 2266396, cl. 7 Е21В 43/20, publ. 2005.12.20; RF number 2299979, class 7 Е21В 43/20, publ. 2007.05.27; AS No. 193402, cl. 5a, 41, IPC E21b. In the process of cyclic water injection and fluid production, reservoir pressure is measured, reservoir oil samples are sampled and studied, and geophysical surveys of the reservoir intervals opened by wells are performed: they determine the saturation of rock voids with oil and gas.

Closest to the proposed one is the method described in AS No. 193402. According to this method, when developing oil fields using periodic flooding in oil-saturated zones, the elastic margin is increased by creating a free gas phase in them by periodically lowering the reservoir pressure below the saturation pressure or cyclic gas injection into these zones. This increases the coverage of the formation by water flooding. It is proposed to reduce reservoir pressure to a value of the order of 15-20% below the saturation pressure. After that, the mode of water injection and oil extraction is changed so that the pressure in the reservoir rises to the initial level. Subsequently, the cycles are repeated in the sequence described.

The disadvantage of this prototype method of cyclic water injection is that when it is carried out in a reservoir under the conditions of a dissolved gas, when setting pressure limits for a cycle: maximum at the end of the reservoir pressure increase phase and minimal at the end of the reservoir decrease phase pressure, the features of changes in reservoir pressure due to the presence of oil, water and gas in the voids of the rock are not taken into account, which affects the energy intensity of the cyclic water injection process. In particular, in the phase of increasing pressure, the rational limits of its increase are not justified - it is only indicated that it should "rise to the initial level." And this is not always advisable, since the initial pressure level may exceed the value of the current pressure of oil saturation with gas, the gas will completely dissolve in the oil and only one liquid will compress for some time: oil and water, which, as will be shown below, leads to increased energy costs for compressing the water injected into the reservoir . As a result of this, during the cyclic impact, the rational use of the energy expended for pumping water into the reservoir and, ultimately, for increasing the recoverable oil reserves is not ensured. In the phase of reducing the reservoir pressure, it is not indicated what determines the magnitude of the pressure reduction "of the order of 15-20% below the saturation pressure."

The claimed invention is aimed at solving the following technical problem: increasing the efficiency of developing an irrigated oil reservoir with a reservoir pressure below the initial pressure of oil saturation with gas, into which water is cycled to increase the oil recovery factor - oil recovery factor.

The goal is achieved by reducing the energy consumption for water injection into the reservoir by establishing in each cycle during water injection the maximum values of reservoir pressure: maximum - maximum and maximum - minimum. The method of developing an oil reservoir includes cyclic injection of water into an irrigated productive formation, taking and studying reservoir fluid samples, measuring reservoir pressure, temperature and fluid saturation by reservoir and geophysical methods, and provides for water injection into the formation in each cycle in the phase of increasing reservoir pressure until the reservoir pressure reaches the maximum - maximum - not higher than the calculated pressure values at which complete dissolution in oil occurs gas, in the voids, Rpl max ≤ Pnas, and in the phase of reservoir pressure decrease - maximum - minimum, the value of which is set not lower than the pressure at which the gas saturation of the rock becomes critical, Rpl min ≥ Rpl cr, corresponding to the beginning of filtration in the rock of gas released from oil in the voids; if the gas saturation of the rock voids decreases below a predetermined minimum value, the calculated volume of gas or gas mixture is pumped into the reservoir before the next cycle of water injection, after which the water is continued to be pumped in a cyclic mode.

The invention is based on the principle of retaining in the voids of the rock part of the volume of water that periodically penetrates into them with increasing reservoir pressure and then is displaced from there by expanding fluids with a subsequent decrease in reservoir pressure.

The increase in oil displacement coefficients with water during cyclic exposure to a waterlogged oil reservoir depends on the product of the volume of water entering the voids present in the oil-saturated rock: pores, cracks, cavities, and its retention coefficient in voids. With the same difference between the maximum and minimum reservoir pressures achieved during one cycle, the volume of introduced water and, consequently, the increase in the displacement coefficient will be greater when the compressibility of the fluids in the voids of the rock is higher. This is possible under conditions when the composition of compressible fluids in the voids of the rock contains free gas, the compressibility of which is much higher than the compressibility of the liquid phase: water and oil. Therefore, for a more efficient implementation of the process and optimal use of the energy of water injected into the formation during cyclic impact on the formation, it is suggested that during the phase of increasing the reservoir pressure, it should not be exceeded above the pressure necessary for complete dissolution of the gas that was in the voids at the beginning of the cycles, and during the phase reduction of reservoir pressure - at the end of cycles - to prevent a decrease in reservoir pressure below a level at which the gas saturation of the voids of the rock can exceed the limit value, and which begins the filtration of gas in the rock of a given formation filled with a three-phase medium, thereby preventing the breakthrough of gas from low-permeability rock volumes into high-permeability layers or into cracks.

The proposed technical solution is confirmed by calculations to assess the effectiveness of oil displacement by water at various values of the maximum reservoir pressure at the end of this cycle, Rpl max. The ratio of the growth of the displacement coefficient to the maximum reservoir pressure achieved during this cycle is determined, DKv / Rpl max. The maximum value of this ratio corresponds to the minimum specific energy consumption for pumping water into the formation for oil extraction in one cycle during cyclic treatment of the formation, since the energy consumption consumed by the water injection pumps is proportional to the product of the water flow and the pressure drop created by the pumps, which depends on Rpl max : the more Rpl max, the higher the pressure required at the mouth of the injection wells, and therefore at the outlet of the water-injection pumps.

The increase in the displacement coefficient per cycle DKv is estimated by the approximate formula (1), which was not taken into account when calculating, because of the small size, the compressibility of water and the compressibility of the rock - only the compressibility of hydrocarbons: gas and oil, which most strongly affects the process, was taken into account. The technology of stimulation of the reservoir was considered when at the beginning of the cycle the reservoir pressure is equal to the minimum Rpl min, and during the water injection in the first phase of this cycle it increases and reaches the maximum Rpl max value, at which the gas contained in the voids in the oil is partially or completely dissolved in the oil beginning of the cycle. After this, the reservoir pressure in the second phase of the cycle is reduced to a minimum Rpl min, due to the properties of the rock and the fluids saturating them, at which the gas saturation of the rock voids does not exceed the critical value, critical pressure, corresponding to the beginning of gas filtration in the rock saturated with oil, water and gas

Here:

DКв - an increase in the coefficient of oil displacement by water per cycle;

bn (Rpl min), bn (Rpl max) - volumetric oil coefficients, respectively, with reservoir pressure values Rpl min and Rpl max;

Sн (0) - oil saturation of rock voids at the beginning of the cycle; for the examples considered in the calculations it is taken equal to 0.259;

Sg (Rpl min), Sg (Rpl max) - gas saturation of rock voids, respectively, with reservoir pressure values Rpl min and Rpl max;

Ky is the coefficient of retention of introduced water in voids; for the examples considered in the calculations it is taken equal to 0.3;

Sн (beginning) - the initial oil saturation of the voids of the rock with oil when calculating the geological reserves of oil in the reservoir; for the examples considered in the calculations, it is taken equal to 0.7.

Included in the formula (1), the value of Sg (Pmelt max) with complete dissolution of the gas in oil is 0, and with partial dissolution it is determined by the formula:

Here

Tst, Rst - respectively, temperature and pressure under standard conditions;

Z (RPL min), Z (RPL max) - gas compressibility coefficient, respectively, at reservoir pressure Rpl min and Rpl max and reservoir temperature;

G (Rpl min), G (Rpl max) - gas solubility coefficient in oil at reservoir pressure Rpl min and Rpl max and reservoir temperature, respectively;

Mp - reservoir temperature.

The pressure required for the complete dissolution of the gas in oil contained in the voids at the beginning of the cycle at Rpl min was determined by the formula:

Here

Rnas is the pressure at which gas contained in voids at the beginning of a cycle is completely dissolved in oil;

G (Pnas) is the solubility coefficient of gas in oil at reservoir temperature and Rnas pressure.

As examples in Tables 1 and 2, the results of calculations of DKv values are presented, as well as the DKv / Rpl max ratios for a given Rpl min = 10 MPa and various Rpl max for two values of gas saturation of voids at the beginning of the cycle: 0.1 (Table 1) and 0.06 (table 2). In the calculations, the experimental dependences of the parameters bn, Z, and G were taken, obtained in the study of reservoir oil of one of the fields of the Lower Volga region in the form of polynomials

According to experimental data, the coefficients in formulas (4), (5), (6) and (7) are equal: C ₁ = 0.0258; C ₂ = 0,0007; q ₁ = 5.85; q ₂ = 18; A ₁ = 1.124; A ₂ = 0.0127; A ₃ = 0.015; A ₄ = 0.0023 (pressure dimension - MPa).

Table 1 Rpl max, MPa G, 1 / MPa bn DK DKv / Rpl max, 1 / MPa Sн (0) Z Sg 10 7.65 1,251 0.259 0.812 0.1 eleven 7,486 1,263 0.0081 0,00074 0.801 0,078 12 7.35 1,276 0.0145 0.00121 0.791 0,060 13 7,235 1,289 0.0196 0,00151 0.783 0,046 fourteen 7,136 1,301 0,0237 0.0017 0.776 0,034 fifteen 7.05 1,314 0,0271 0.00180 0.770 0,023 16 6,975 1,327 0,0298 0,00186 0.766 0.015 17 6,909 1,340 0,0320 0,00188 0.764 0.007 eighteen 6.85 1,352 0,0338 0,00188 0.762 0 19 6,797 1,350 0,0340 0,00179 0.762 0 twenty 6.75 1,348 0,0342 0,00171 0.764 0 21 6,707 1,346 0,0345 0,00164 0.767 0 22 6,668 1,343 0,0347 0,00157 0.771 0

table 2 Rpl max, MPa G, 1 / MPa bn DK DKv / Rpl max, 1 / MPa Sн (0) Z Sg 10 7.65 1,251 0.259 0.812 0.06 eleven 7,486 1.2637 0.0063 0,00059 0,8009 0,042 12 7.35 1.2764 0.0113 0,00094 0.7912 0,028 13 7,235 1,2891 0.0152 0.00117 0.7829 0.0167 fourteen 7,136 1,3018 0.0183 0.00131 0.776 0.007 14.8 7,066 1,312 0,0203 0,00137 0.7715 0 fifteen 7.05 1.3115 0,0203 0.00136 0.7705 0 16 6,975 1.3092 0,0206 0.00128 0.7664 0 17 6,909 1.3069 0,0208 0.00122 0.7637 0 eighteen 6.85 1.3046 0.0210 0.00116 0.7624 0 19 6,797 1.3023 0.0212 0.00111 0.7625 0 twenty 6.75 1.3 0.0214 0,00107 0.764 0

The calculated dependences of DKv and the ratio of DKv / Rpl max on Rpl max for the above examples are presented in Fig. 1 and Fig. 2, which show that the values of the DKv / Rpl max complex acquire the highest values at Rpl max equal to the saturation pressure, which has different values, depending on the gas saturation of the rock voids at the beginning of the cycle, Sg (Ppl min). When Sg (Rpl min) = 0.1 Pnac = 18 MPa; at Sg (Rpl min) = 0.06 Pnas = 14.8 MPa. From the graphs shown in figure 2, it can be seen that the same values of the ratio DKv / Rpl max can be obtained at different values of Rpl max. So, for example, in the variant with gas saturation at the beginning of the cycle Sg (Rpl min) = 0.1, the ratio DKv / Rpl max = 0.0018 is obtained at Rpl max equal to 15 MPa, which is lower than Rnas = 18 MPa, or at 19 MPa, higher than saturation pressure; in the variant with Sg (Rpl min) = 0.06, the ratio DKv / Rpl max = 0.0012 is obtained at Rpl max equal to 13.2 MPa and 17.2 MPa. The data presented indicate that it is advisable to carry out the process of cyclic water injection at Rpl max ≤ Pnas, which allows one to obtain the same specific increase in the displacement coefficient, and, consequently, additionally produced oil at lower energy costs.

When implementing the indicated technical solution, at the beginning of each cycle of pumping water into the reservoir, the field and geophysical methods determine the current values of reservoir pressure, temperature, rock saturation with oil and gas and use the data obtained in calculating the growth of displacement coefficients at various Rpl max values and choosing the maximum reservoir pressure during the next cycle.

If the gas saturation of the rock at the beginning of the cycle is lower than the set value, it is suggested that before the start of water injection, the calculated volume of gas or water-gas mixture is injected into the formation, which ensures the achievement of the required value of Sg at the beginning of the cycle - before water is injected into the formation.

Monitoring at all stages of water-gas exposure is carried out by taking samples of reservoir oil and gas and determining their physical parameters in PVT laboratory units, as well as conducting measurements by reservoir geophysical methods of reservoir pressure, temperature, and rock saturation with oil and gas. The information obtained is quickly, at the beginning of each cycle, used in the calculations according to formulas (1) - (7) of technological parameters during the cyclic injection of water into the reservoir.

Claims (2)

1. A method of developing an oil reservoir, including cyclic injection of water into an irrigated productive formation, sampling and studying reservoir fluid samples, measurements of reservoir pressure, temperature and fluid saturation by field geophysical methods, characterized in that in each cycle in the phase of increasing reservoir pressure water is injected into the reservoir until the reservoir pressure reaches the maximum - maximum - not higher than the calculated pressure values at which complete dissolution in oil occurs contained in gas voids, Rpl max ≤ Rnas, determined in accordance with the formula

,
where Rnas is the pressure at which, during the injection of water into the reservoir, the gas contained in the voids of the rock at the beginning of the cycle is completely dissolved in oil;
RPL min - reservoir pressure at the beginning of the cycle;
bn (Rpl min) is the volumetric coefficient of oil at a value of reservoir pressure P PL min;
Sн (0) - oil saturation of rock voids at the beginning of the cycle;
Sg (Rpl min) - gas saturation of the voids of the rock at a value of reservoir pressure Rpl min;
Tst, Rst - respectively, temperature and pressure under standard conditions; Tst = 293 K; Pst = 0.1013 MPa;
Z (Ppl min) - gas compressibility coefficient at reservoir pressure Rpl min and reservoir temperature;
G (Ppl min), G (P us) - the solubility coefficient of gas in oil, respectively, at reservoir pressure Rpl min and Rnas and reservoir temperature;
Mp - reservoir temperature
and in the phase of lowering the reservoir pressure - the limiting - the minimum, the value of which is set not lower than the pressure at which the gas saturation of the rock becomes critical, Rpl min ≥ Rpl cr, corresponding to the onset of filtration in the rock of gas released from the oil in the voids of the rock. 2. The method according to claim 1, characterized in that if the gas saturation of the voids of the rock decreases below a predetermined minimum value, the estimated volume of gas or gas mixture is pumped into the reservoir before the next cycle of water injection, and then the water is continued to be pumped in a cyclic mode.

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