Method for reinjection of produced water from the natural energy production phase of an oil field into the ground

Abstract

The present application provides a kind of method for backfilling oilfield natural energy exploitation stage produced sewage to underground, comprising: step 1, measuring the annual sewage output of oilfield in natural energy exploitation stage;Step 2, screening underground sewage backfilling layer;Step 3, measuring the sewage capacity of sewage backfilling layer;Step 4, implementing test injection, and evaluating the injection of backfilling layer;Step 5, measuring the minimum water absorption of single well of backfilling well;Step 6, measuring the minimum number of backfilling wells required to complete the sewage backfilling task;Step 7, measuring the static water absorption capacity of single well of backfilling well;Step 8, measuring the single well injection life;Step 9, measuring the cumulative injection volume of backfilling well injection life;Step 10, re-measuring the number of backfilling wells, and deploying sewage backfilling scheme accordingly.The method is economical, advanced in technology, simple to operate, convenient to implement, and achieves the purpose of protecting the fragile ecological environment of desert surface and promoting the green and sustainable development of oilfield.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oilfield produced water treatment, in particular to a method for backfilling oilfield produced water in the natural energy exploitation stage to the underground. BACKGROUND

[0002] During the oilfield exploitation process, with the production of underground oil, gas and water fluids, the underground depletion becomes larger and larger, and the formation pressure becomes lower and lower, and the oil well production decreases. In order to make up for the problem of insufficient formation energy, water injection is generally used to supplement the formation energy during the oilfield development process. However, for oilfields with sufficient natural energy, the oilfield is developed in the early stage by using natural energy, and it is not necessary to deploy water injection wells to supplement the formation energy.

[0003] The amount of oilfield produced water is small in the early stage of development, and the method for treating oilfield produced water on site in the desert area is to build several dry ponds near the joint station, and to discharge the oilfield produced water into the dry ponds to solve the problem of oilfield produced water by natural evaporation. With the increase of development intensity, the amount of oilfield produced water increases continuously, and if the natural evaporation method is continued to treat the produced water, more and more evaporation ponds will be built, which will bring a fatal threat to the fragile ecological environment in the desert area, so a new way of treating oilfield produced water must be sought.

[0004] According to the actual situation of the desert area, either a sewage treatment plant is built to discharge the treated oilfield sewage to the surface after reaching the standard, or the sewage is backfilled and injected into the underground non-production reservoir layer, and then backfilled into the production layer to supplement the formation energy when the formation pressure drops in the middle and late stages of development. After analysis and demonstration, from the perspectives of environmental protection and economy, the scheme of backfilling and injecting the sewage into the underground non-production reservoir layer in the natural energy exploitation stage is superior.

[0005] In the Chinese patent application with application number 201910427491.0, a method for backfilling oilfield produced water in the natural energy exploitation stage to the underground is involved, which comprises the following steps: step 1, measuring the annual amount of oilfield produced water in the natural energy exploitation stage; step 2, screening the underground sewage backfill layer; step 3, measuring the sewage capacity of the sewage backfill layer; step 4, developing a test injection to evaluate the sewage backfill of the backfill layer; step 5, measuring the single-well water absorption capacity of the backfill well; step 6, measuring the single-well water absorption amount and the minimum number of backfill wells required to complete the sewage backfill task; and step 7, determining the ground water injection technology suitable for the sewage backfill. In the application patent, there are three imperfections. First, the life of the backfill well is not measured, second, the cumulative water absorption amount of the backfill well during the life is not measured, and third, the number of sewage backfill wells is not re-measured according to the cumulative water absorption amount of the backfill well during the life.

[0006] In the Chinese patent application with the application number: 201010511439.2, a method for treating oilfield produced water is disclosed, which comprises the steps of air flotation, biochemical treatment, filtration, ultrafiltration, reverse osmosis, etc. The oilfield produced water is subjected to biochemical treatment, which can effectively reduce the content of organic matter, oil and oligomers in the wastewater that can cause membrane pollution. After the biochemical treatment, the effluent is subjected to double-filter and disc-filter to remove part of the suspended solids, and then enters the ultrafiltration device. The ultrafiltration adopts forced circulation and cross-flow filtration mode, and is designed to perform regular backwashing and chemical backwashing. The ultrafiltration effluent enters the reverse osmosis device, which is designed to perform regular low-pressure positive flushing and regular sterilization. The reverse osmosis product water is reused as high-quality water for thermal recovery boiler. The concentrated water from the reverse osmosis is treated by recharging. The air flotation, biochemical treatment, filtration and special membrane operation mode can effectively prolong the cleaning cycle and service life of the ultrafiltration membrane and the reverse osmosis membrane.

[0007] In the Chinese patent application with the application number: CN201710079809.1, a method for resource utilization of oilfield produced water is disclosed, which comprises the following steps: (1) pumping the oilfield produced water into a gravity oil removal tank to separate and remove macromolecular oil droplets, water and solid suspensions; (2) then passing through a vortex-cave air flotation machine, and the bubbles in the water carry out the oil droplets in the water; (3) the water from the vortex-cave air flotation machine is sent to a dissolved air flotation machine to remove emulsified oil and dissolved oil in the water again; (4) then sent to an immobilized microorganism treatment tank, and the effluent enters an intermediate tank I; (5) then pumped into an MBR device; (6) the effluent from the MBR device is filtered through a security filter after passing through an intermediate tank II, and then directly enters a reverse osmosis device or an electrodialysis device for treatment.

[0008] The above prior art has great differences from the present application and cannot solve the technical problems we want to solve. Therefore, we have invented a new method for recharging the produced wastewater in the natural energy exploitation stage of an oilfield to the underground. SUMMARY

[0009] The purpose of the present application is to provide a method for recharging the produced wastewater in the natural energy exploitation stage of an oilfield to the underground, which effectively solves the problem of treating the produced wastewater in the natural energy exploitation stage of an oilfield in a desert area.

[0010] The purpose of the present application can be achieved by the following technical measures: a method for recharging the produced wastewater in the natural energy exploitation stage of an oilfield to the underground, which comprises the following steps:

[0011] Step 1: calculating the annual wastewater production of the oilfield in the natural energy exploitation stage;

[0012] Step 2: screening the underground wastewater recharging layer;

[0013] Step 3: calculating the wastewater capacity of the wastewater recharging layer;

[0014] Step 4, implement the test injection, and evaluate the injection layer by water injection;

[0015] Step 5, calculate the minimum water absorption of a single injection well;

[0016] Step 6, calculate the minimum number of injection wells required to complete the sewage injection task;

[0017] Step 7, calculate the static water absorption capacity of a single injection well;

[0018] Step 8, calculate the single well injection life;

[0019] Step 9, calculate the cumulative injection volume of the injection well within the injection life;

[0020] Step 10, recalculate the number of injection wells and deploy the sewage injection scheme accordingly.

[0021] The purpose of the application can also be achieved by the following technical measures:

[0022] In step 1, according to the current comprehensive water content and water content rising law of the oilfield, combined with the development prospect plan of the oil and gas field, the annual sewage output of the oilfield in the natural energy exploitation stage is calculated.

[0023] In step 2, the selection of the sewage injection layer follows the following principles: shallow burial, large reservoir capacity, high permeability, and fluid in the reservoir is no longer developed and utilized.

[0024] In step 3, first, calculate the maximum average rising pressure of the injection layer; second, calculate the volume of the sewage injection layer sand body; and finally, calculate the total amount of sewage that the sewage injection layer can accommodate.

[0025] In step 3, when calculating the maximum average rising pressure of the injection layer, the sewage injection adopts general water injection rather than layered water injection, and the formation pressure near the bottom of the injection well is the highest. The highest formation pressure is calculated according to the following formula:

[0026] P 升 =P wh +P h —P i

[0027] In the formula, P 升 is the maximum rising pressure of the formation near the bottom of the injection well, MPa; P wh is the wellhead pressure, MPa; P h is the static water column pressure of the wellbore, MPa; P i is the original formation pressure, MPa;

[0028] Here, the static water column pressure of the wellbore is: P h =h×ρ

[0029] Where h is the depth of water injection string; p is the density of recharging sewage,

[0030] Initial formation pressure:

[0031] P i = dP x h

[0032] The maximum rising pressure near the bottom of recharging well at the end of water injection is:

[0033] P 升 = P wh + P h - P i

[0034] The rising pressure of the farthest formation of water injection well is 0 MPa, and the maximum average rising pressure of recharging layer is

[0035] P 平均上升 = (P 升 + 0) / 2.

[0036] In step 3, the formula for calculating the volume of sewage recharging layer sand body is:

[0037] V = s x h x η

[0038] Where s is the area of reservoir sand body, m 3 ; h is the thickness of reservoir sand body, m; η is the sand ratio, dimensionless.

[0039] In step 3, the formula for calculating the total amount of sewage that can be accommodated by the sewage recharging layer is:

[0040] N = V x ΔP x C t x Φ

[0041] Where V is the total volume of reservoir rock, m 3 ; ΔP is the maximum average rising pressure of recharging layer; C t is the comprehensive compression coefficient of reservoir rock and formation water; Φ is the porosity, dimensionless.

[0042] In step 5, the minimum water absorption of single well is calculated according to the following formula:

[0043] N = π x r 2 x h x η x ΔP x C x Φ

[0044] Where: π is the circular constant, dimensionless; r is the swept radius of injected water, m; h is the thickness of reservoir sand body, m; η is the sand ratio, dimensionless; ΔP is the average rising pressure of reservoir, MPa; C is the pore compression coefficient of reservoir rock, MPa -1 ; Φ is the porosity of reservoir rock, dimensionless.

[0045] In step 6, the minimum number of wells required for sewage recharge is calculated according to the minimum water absorption of single well:

[0046] n = Q / q

[0047] Where: Q is the total amount of injected sewage, m 3 ; q is the sewage recharge of single well, m 3 .

[0048] In step 7, the water absorption capacity of single well is calculated according to the following formula:

[0049] q = ΔP x Jw x H

[0050] Where: ΔP is the bottom hole injection pressure difference, MPa; q is the water absorption capacity of single well, m 3 / d; Jw is the water absorption index per meter of formation, m 3 / m.MPa.d; H is the water absorption thickness of formation, m.

[0051] Where, the bottom hole injection pressure is calculated according to the following formula:

[0052] ΔP = P wh +h x ρ / 100 - P f -P 启 -P i

[0053] Where: P wh is the wellhead injection pressure, MPa; P h is the wellbore static water column pressure, MPa; P f is the wellbore friction, MPa; P 启 is the injection starting pressure, MPa; P i is the original formation pressure of recharge layer, MPa; h is the injection string depth, m; ρ is the sewage density, g / cm 3 ;

[0054] Wellbore friction P f is calculated according to the following empirical formula:

[0055] P f = 1.086 x 10 -13 λLq 2 / d 5

[0056] Where: λ is the friction coefficient of injection well; d is the inner diameter of tubing; L is the injection pipe depth; q is the daily injection of single well.

[0057] In step 8, sewage recharge can be divided into two processes: the first process is constant yield injection process, and the second process is constant wellhead pressure injection process; corresponding to the two injection processes, the solutions of the equations are respectively:

[0058]

[0059]

[0060] where r is the radius of the cylindrical formation centered on the injection well, m; t is the time from the beginning of injection, s; h is the water-swept thickness of the formation, m; q is the injection rate of a single well, m / s; μ is the viscosity of the injected water, mPa.s; k is the relative permeability of water, dimensionless; τ is the constant pressure injection time, s; -E 3 (-r i (-r 2 / 4ψt), is the power integral function, dimensionless; P i is the original formation pressure, MPa; P(r,t) is the pressure at a distance of r from the injection well at time t, MPa;

[0061] Both of the above equations do not have an analytical solution, and the numerical solution of the equations is obtained by establishing a geological model and using a numerical simulation method.

[0062] In step 10, the number of sewage injection wells is calculated according to the following formula:

[0063] n=Q / qi

[0064] where Q is the total amount of injected sewage, m 3 ; qi is the single well sewage injection amount calculated by numerical simulation, m 3 .

[0065] The method for injecting sewage produced in the oilfield natural energy exploitation stage into the underground in the application protects the ground ecological system by injecting sewage produced in the natural energy exploitation process of the oilfield in the desert area into the non-production reservoir layer underground. The method is economical and saves resources, advanced in technology, simple to operate, convenient to implement, and achieves the purposes of protecting the fragile ecological environment of the desert surface and promoting the green and sustainable development of the oilfield. If the oilfield sewage injection scheme is deployed by using the method of the Chinese patent application with application number 201910427491.0, the number of wells is 36% more than that of the present method, which will cause about 10% of the equipment capacity redundancy and about 4% of the investment waste. The application patent method was applied to the Tarim desert oilfield 20 years ago, and the calculation method is based on a static algorithm, not the dynamic method used in the present application. To calculate the injection well life, the seepage mechanics equation needs to be solved. Since the seepage mechanics equation has no analytical solution, only a numerical solution can be obtained. The numerical solution can only be obtained by using a numerical simulation method. At that time, numerical simulation technology was mainly used abroad, and the application range was very small in China. Most oil reservoir engineers did not have access to numerical simulation technology, let alone master and apply the technology. Therefore, the static method has some imperfections. The present application solves the above technical problems. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 This is a graph showing the change in average formation pressure over time in a specific embodiment of the present invention.

[0067] Figure 2 This is a graph showing the change in water absorption capacity of a recharge well over time in a specific embodiment of the present invention;

[0068] Figure 3 This is a graph showing the cumulative water absorption of a recharge well over time in a specific embodiment of the present invention.

[0069] Figure 4 This is a flowchart of a specific embodiment of the method for reinjecting wastewater produced during the natural energy extraction stage of an oilfield into the ground, according to the present invention. Detailed Implementation

[0070] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0071] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0072] like Figure 4 As shown, Figure 4 This is a flowchart illustrating a method for reinjecting wastewater produced during the natural energy extraction stage of an oil field back into the ground, according to the present invention. The method includes the following steps:

[0073] Step 101: Calculate the annual wastewater production of the oilfield during the natural energy extraction stage;

[0074] Step 102: Screening underground sewage recharge layers;

[0075] Step 103: Calculate the amount of sewage that the sewage recharge layer can hold;

[0076] Step 104: Conduct a trial injection and evaluate the water injection in the reinjection layer;

[0077] Step 105: Calculate the minimum water absorption of a single reinjection well;

[0078] Step 106: Calculate the minimum number of reinjection wells required to complete the sewage reinjection task;

[0079] Step 107, calculating the single-well static water absorption capacity of the recharging well;

[0080] Step 108, calculating the single-well water injection life span;

[0081] Step 109, calculating the cumulative water injection amount in the water injection life span of the recharging well.

[0082] Step 110, re-calculating the number of recharging wells and deploying the sewage recharging scheme accordingly.

[0083] The following are several specific embodiments of the application.

[0084] Embodiment 1:

[0085] In the specific embodiment 1 of the application, the method for recharging the sewage produced in the natural energy exploitation stage of an oilfield to the underground of the application comprises the following steps:

[0086] In step 1, according to the current comprehensive water content and water content rising law of the Tarim oilfield in the Taklamakan Desert area, combined with the development prospect plan of the oil and gas field, the annual sewage output of the oilfield in the natural energy exploitation stage is calculated. The calculation results are as shown in Table 1:

[0087] Table 1 Annual sewage output of the oilfield

[0088] Year Year n Year n+1 Year n+2 Year n+3 Year n+4 Year n+5 Annual sewage production (10 4 m 3 )]]> 147 170 201 200 190 180 Daily sewage quantity (m 3 )]]> 4027 4657 5506 5479 5205 4931

[0089] In the middle and late stages of oilfield development, according to the development planning design, water injection needs to be implemented. After treatment, most of the sewage produced by the oilfield will be injected into the production layer. Therefore, only the sewage amount in the 6 years before water injection needs to be considered when recharging the sewage produced in the natural energy exploitation stage of the oilfield to the non-production layer. The total sewage output of the oilfield in 6 years is 1088×10 4 m 3 . The sewage recharging layer must be able to accommodate at least 1088×10 4 m 3 of water.

[0090] This is an indispensable step for the sewage recharging scheme. Only when the total amount of water that needs to be recharged is clear, can the recharging layer be selected with a clear goal in mind. Otherwise, the selected recharging layer may not be able to accommodate the amount of sewage that needs to be recharged, and the sewage recharging goal cannot be achieved.

[0091] In step 2, the selection of the sewage recharging layer follows the following principles: shallow burial depth of the reservoir (but the burial depth requirement is greater than 1000 meters to avoid pollution of underground water), large reservoir capacity, high permeability, and the fluid in the reservoir is no longer developed and utilized.

[0092] According to the above principles, the strata of Tarim Oilfield in the Taklimakan Desert region are selected from top to bottom. The Quaternary near the surface is a set of sand and clay layers with a thickness of about 50m, which cannot be used for sewage recharge. The strata of the Kuche Group and the upper sand body of the Kangcun Group between 1300m and 2600m do not contain oil and are non-production layers. The strata of the Kuche Group and the upper sand body of the Kangcun Group are well developed and stably distributed in the whole region, with an area of more than 100 square kilometers. The sand bodies are staggered and superimposed on the vertical and horizontal directions, forming a large-scale connected body with large thickness and strong reservoir capacity. The porosity of the two sets of sand bodies is high, and the reservoir properties are good. At the same time, the two sets of strata are high salinity water layers, and the formation water has no industrial exploitation value, which is a suitable sewage recharge layer.

[0093] The thickness of the strata of the Kuche Group and the upper sand body of the Kangcun Group is more than 770m, the porosity of the strata sand body is 22%-30%, and the average porosity is 26%. The permeability is (200-1500) x 10 -3 μm 2 , which fully shows that the strata of the Kuche Group and the upper strata of the Kangcun Group are suitable for sewage recharge.

[0094] In step 3, the sewage capacity of the sewage recharge layer is calculated.

[0095] First, the maximum average rising pressure of the recharge layer is calculated.

[0096] The sewage recharge adopts general water injection instead of layered water injection. The formation pressure near the bottom of the recharge well is the highest, and the highest formation pressure is calculated by the following formula:

[0097] P 升 = P wh + P h — P i

[0098] In the formula, P 升 is the maximum rising pressure of the formation near the bottom of the recharge well, MPa; P wh is the wellhead pressure of the injection well, MPa; P h is the static water column pressure of the wellbore, MPa; P i is the original formation pressure, MPa.

[0099] Here, the static water column pressure of the wellbore is: P h = h x p

[0100] In the formula, h is the depth of the water injection string, taking an average value of 1700m; p is the density of the recharge sewage, which is 1.154g / cm 3 , and the static water column pressure at the bottom is:

[0101] P h = h x p = 1700 x 1.154 = 19.6 MPa

[0102] The formation pressure gradient of Kuqia group is dP = 1.0375 MPa / 100 m, so the original formation pressure is:

[0103] P i = dP x h = 1.0375 x 1700 = 17.6 MPa;

[0104] The injection pump pressure P wh is taken as 16 MPa, the horizontal pressure loss is 1 MPa, and the friction of the injection wellbore at the end of injection can be ignored, so the maximum formation uplift pressure near the bottom of the injection well at the end of injection is:

[0105] P 升 = P wh + P h - P i = (16-1) + 19.6-17.6 = 17 MPa

[0106] The uplift pressure of the formation at the farthest end of the injection well is taken as 0 MPa, so the maximum average uplift pressure of the injection layer is

[0107] P 平均上升 = (17+0) / 2 = 8.5 MPa

[0108] For the purpose of insurance in the sewage injection project, 5 MPa is taken as the subsequent calculation parameter.

[0109] In the present application, the injection pump pressure is taken as 16 MPa, instead of (30-35) MPa used in most oilfields, mainly considering the cost and safety problems of high-pressure injection pipe network. Compared with the injection pipe network with pressure resistance of 16 MPa, the injection pipe network with pressure resistance of (30-35) MPa has much higher investment and operation cost. Through repeated trial calculation for multiple rounds, the injection pump pressure of 16 MPa can fully meet the needs of sewage injection in Tarim Oilfield.

[0110] Secondly, the volume of the sewage injection layer sand body is calculated.

[0111] The sand body in the upper part of the formation of Kuqia group and the formation of Kangcun group in Tarim Oilfield is relatively developed and stable in distribution, with an area of up to hundreds of square kilometers. A sand body with an area of 100 km 2 is taken for preliminary calculation. The sand body ratio is 0.75, so the volume of the sand body is:

[0112] V = 100 x 10 6 x 770 x 0.75 = 5.8 x 10 10 m 3

[0113] Finally, the total amount of sewage that can be accommodated by the sewage injection layer is calculated according to the following formula:

[0114] N = V x ΔP x C t x Φ

[0115] = 5.8 x 10 10 m 3 x 5 MPa x 4.4 x 10 -4 MPa -1 x 0.26

[0116] = 3317 x 10 4 m 3

[0117] where V is the total volume of the reservoir rock, m 3 ; ΔP is the maximum average rising pressure of the recharge layer, which is 5 MPa in this case; C t is the comprehensive compression coefficient of the reservoir rock and the formation water, which is 4.4 x 10 -4 MPa -1 in this case; and Φ is the porosity, dimensionless, which is 0.26 in this case.

[0118] The total amount of sewage that can be accommodated by the sewage recharge layer is 3317 x 10 4 m 3 , which is much larger than the cumulative output of sewage in the natural energy production stage of 6 years, i.e. 1088 x 10 4 m 3 . In fact, the reservoir of 30 square kilometers can accommodate 1088 x 10 4 m 3 of output sewage, and the screened sewage recharge layer can fully meet the requirements.

[0119] In step 4, the perforation test injection is performed on the well section of 1562-1610 m of the lower part of the Jurassic Kuche Group in the injection well. The pressure test result shows that the formation pressure of the Kuche Group is very close to the hydrostatic column pressure, and the pressure gradient is 1.04 MPa / 100 m. The test injection result shows that the water absorption index of the formation per meter is 3.2 m 3 / m﹒MPa ﹒d, and the water injection starting pressure is 1.8 MPa. The formation water absorption capacity is strong, and the water injection starting pressure is low, which is suitable for the sewage recharge layer.

[0120] In step 5, the minimum water absorption amount of a single well of the recharge well is measured and calculated.

[0121] If the sewage overflow into the upper and lower adjacent layers through the cracks during the recharge process is not considered, the sewage amount that can be absorbed by a single well is the total amount of sewage that can be accommodated by the reservoir controlled by the single well, which is the volume of the part that can be compressed by water and rock pores in the range that can be reached by water in the recharge formation centered on the injection well.

[0122] In the calculation, the influence range of sewage recharge is referred to the influence range of oil-water well in water injection development oil field, and 1000 meters is considered comprehensively; the formation rising pressure is the average value of the bottom hole pressure and the pressure at the farthest end of water injection, 5 MPa. The minimum water absorbing capacity of single well is calculated according to the following formula:

[0123] N = π × r 2 × h × η × ΔP × C × Φ

[0124] = π × 1000 m × 1000 m × 770 m × 0.75 × 5 MPa × 4.4 × 10 -4 MPa -1 × 0.26

[0125] = 103.7 × 10 4 m 3

[0126] In the formula, π is the circular constant, dimensionless; r is the injection water influence radius, m; h is the reservoir sand thickness, m; η is the sand ratio, dimensionless; ΔP is the average rising pressure of reservoir, MPa; C is the reservoir rock pore compression coefficient, MPa -1 ; Φ is the reservoir rock porosity, dimensionless.

[0127] In fact, due to long water injection time and good reservoir properties, the injected water can reach further places, and the maximum sewage quantity that can be absorbed by single well is greater than 103.7 × 10 4 m 3 .

[0128] In step 6, the minimum well number required for sewage recharge is calculated.

[0129] According to the minimum water absorbing capacity of single well, the minimum well number required for sewage recharge can be calculated:

[0130] n = Q / q

[0131] = 1088 / 103.7 = 10.5 ≈ 11 (m

[0132] In the formula, Q is the total injected sewage, m 3 ; q is the sewage quantity of single well, m 3 ;

[0133] 11 water injection wells can complete the sewage recharge task in the natural energy development stage of Tarim oil field.

[0134] In step 7, the water absorbing capacity of single sewage recharge well is calculated.

[0135] The water absorbing capacity of single well is calculated according to the following formula:

[0136] q = ΔP × Jw × H

[0137] wherein ΔP is the injection pressure difference at the well bottom, MPa; q is the water absorption capacity of a single well, m 3 / d; Jw is the water absorption index per meter of the formation, m 3 / m.MPa.d; H is the water absorption thickness of the formation, m.

[0138] wherein the injection pressure at the well bottom is calculated according to the following formula:

[0139] ΔP = P wh +h x p / 100 - P f -P 启 -P i

[0140] wherein P wh is the injection pressure at the well head, MPa; P h is the static water column pressure of the well bore, MPa; P f is the friction of the well bore, MPa; P 启 is the injection starting pressure, MPa; P i is the original formation pressure of the recharge layer, MPa. H is the injection depth of the water injection string, m; p is the density of the sewage, g / cm 3

[0141] The friction of the well bore P f is calculated according to the following empirical formula:

[0142] P f = 1.086 x 10 -13 λLq 2 / d 5

[0143] wherein λ is the friction coefficient of the water injection well, which is taken as 0.03 according to experience; d is the inner diameter of the oil pipe, which is taken as 0.076 m for the full well injection using 3-1 / 2 smooth pipe string; L is the injection depth of the water injection pipe, which is taken as 1700 m, the average middle depth of the recharge layer; q is the daily water injection volume of a single well (m 3 / d).

[0144] For safety, the water absorption index per meter of the formation of the Kuche group is taken as 80% of the test injection result, i.e. 2.5 m 3 / m.MPa.d; the sewage recharge initially breaks the formation by 60 m. The injection depth of the water injection string is taken as the average value of 1700 m. The pressure loss of the surface pipe string from the water injection pump to the well head is taken as 1 MPa according to experience. According to this, the water absorption capacity of a single well, the friction of the well bore, the injection pressure at the well head, and the initial minimum pump pressure corresponding to different injection pressure differences are calculated as shown in Table 2:

[0145] Table 2: Parameters corresponding to different daily injection volumes of a single well

[0146] Wellbore friction, MPa 0.1 0.2 0.3 0.5 0.8 1.1 1.4 1.8 2.2 2.6 3.1 Initial wellhead pressure, MPa 3.8 4.6 5.4 6.3 7.2 8.1 9.1 10.2 11.3 12.4 13.5 Injection pressure differential, MPa 1.3 2.0 2.7 3.3 4.0 4.7 5.3 6.0 6.7 7.3 8.0 Single-well water injectivity, m 3 / d]]> 200 300 400 500 600 700 800 900 1000 1100 1200 Minimum pump pressure, MPa 4.8 5.6 6.4 7.3 8.2 9.1 10.1 11.2 12.3 13.4 14.5

[0147] From Table 2, it can be seen that the higher the pump pressure, the greater the single well water absorption capacity, and the greater the wellbore friction. The greater the wellbore friction, the more power provided by the pump will be wasted on the wellbore. At the same time, the higher the pump pressure, the investment and operating cost of the water injection pipeline network will increase exponentially. Through calculation and demonstration, it is proved that the sewage recharge system using 16 MPa water injection pump can fully meet the sewage recharge requirements. Considering comprehensively, the single well injection allocation is 1000 m 3 / d, and the water injection pump specification is 16 MPa plunger pump.

[0148] In step 8, the injection life of the recharge well is calculated.

[0149] During the sewage recharge process, the formation water and rock will be continuously compressed, and the formation pressure will continuously rise. The sewage recharge process belongs to the unstable seepage process of an elastic compressible liquid.

[0150] The initial injection of the injection well is 1000 m 3 / d constant injection, in order to maintain this constant injection, a constant injection pressure difference must be maintained; in order to maintain a constant injection pressure difference, the bottom hole flowing pressure needs to be continuously increased accordingly; the increase of the bottom hole flowing pressure can only be achieved by continuously increasing the injection pump pressure. Since the pump pressure cannot be increased indefinitely, its maximum value is 16 MPa. When the injection pump pressure reaches the maximum value, with the continuous rise of the formation pressure, the injection pressure difference will continuously decrease, and the single well water absorption capacity will also continuously decrease accordingly. When the single well water absorption capacity is less than 100 m 3 / d, the recharge well loses its recharge value, and it can be considered that the injection life of the recharge well ends.

[0151] Sewage recharge can be divided into two processes: the first process is constant yield injection, and the second process is constant wellhead pressure injection. Corresponding to these two injection processes, the solutions of the equations are respectively:

[0152]

[0153]

[0154] In the formula, r is the radius of the cylindrical formation centered on the injection well, m; t is the time from the start of injection, s; h is the formation water absorption thickness, m; q is the single well injection, m 3 / s; μ is the viscosity of the injected water, mPa.s; k is the relative permeability of water, dimensionless; τ is the constant pressure injection time, s; -E i (-r 2 / 4Ψt), is a power integral function, dimensionless; P i is the original formation pressure, MPa; P(r, t) is the pressure at a distance of r from the injection well at time t, MPa.

[0155] Both of the above equations have no analytical solution, and the vertical pipe flow table of the wellbore is obtained by establishing a geological model and using the VFPi module of the Eclipse numerical simulator, and then the numerical solution of the equation is obtained by using the Eclipse numerical simulator.

[0156] The corresponding geological parameters and engineering parameters are substituted into the numerical simulator, and the results are that the reservoir pressure rises linearly in the early stage and slowly in the later stage Figure 1 , and the initial injection rate is 1000 m 3 / d, and the injection can last for 3.6 years. At the end of the 6th year, the single-well injection rate drops to 105 m 3 / d Figure 2 . The single-well injection life of the recharge well is greater than 6 years, which meets the requirement of 6-year sewage recharge.

[0157] In step 9, the cumulative injection volume of the sewage recharge well during the injection life is re-established.

[0158] The numerical simulation results show that the cumulative injection of a single well during the injection life of the recharge well is 1.64 million m Figure 3 , which is greater than the minimum water absorption of 103.7 million m Figure 1 3 of the recharge well calculated by the static method in step 5. This shows that the average formation pressure rise parameter taken in the static method is quite different from the injection simulation pressure rise. Therefore, the cumulative injection of a single well of 1.64 million m

[0159] In step 10, the number of recharge wells is re-calculated, and the sewage recharge scheme is deployed accordingly.

[0160] The number of sewage recharge wells is calculated according to the following formula:

[0161] n = Q / qi

[0162] = 1088 / 164 = 6.6 ≈ 7

[0163] Where: Q is the total injected sewage, m 3 ; qi is the single-well recharge sewage volume calculated by the numerical simulator, m 3 ;

[0164] Finally, the sewage recharge scheme uses a well spacing of 1000 meters, and a total of 7 injection wells are deployed, which can complete the entire sewage recharge task during the natural energy production stage. The number of sewage recharge wells established in the final scheme is 4 less than that calculated by the static method of reservoir engineering alone.

[0165] The static method of reservoir engineering can only consider the static parameters of reservoir engineering when calculating the number of injection wells, and cannot consider the dynamic changes. The numerical simulation of reservoirs not only considers the static conditions of the reservoirs, but also reflects the dynamic changes of the reservoirs. Therefore, the final sewage injection scheme is also deployed according to the results of the numerical simulation of 7 injection wells.

[0166] The economic benefits of sewage injection and sewage treatment after reaching the discharge standard are analyzed and compared.

[0167] Sewage injection requires 7 injection wells, most of which can use abandoned old wells, and a small part of which is new drilling wells. When the scheme is designed, there are 6 old wells that can be used for sewage injection. With the increasing development of oilfields, more and more abandoned exploration wells and development wells without production potential can be used for sewage injection. To be on the safe side, 1 new well is used for investment estimation. The average drilling depth of a single well is 1700m, the drilling investment of a single well is 1000yuan / m, the pipe string and perforation of a single well is 2000000yuan, and the sewage injection operation cost is 0.5yuan / m 3 , so the total static investment of sewage injection is 9140000 yuan.

[0168] Sewage treatment after reaching the discharge standard requires the construction of a sewage treatment plant. The one-time investment of the sewage treatment plant is 30000000 yuan, the treatment operation cost is 1 yuan / m3, and the total static investment of the sewage treatment plant is 40880000 yuan. The total static investment required for sewage treatment after reaching the discharge standard is 31740000 yuan more than that for sewage injection.

[0169] The benefit of sewage injection is better than that of sewage treatment after reaching the discharge standard.

[0170] The sewage injection scheme is implemented in the Tarim Oilfield in the Taklimakan Desert area. 7 injection wells are deployed during the peak period of sewage production, with a daily injection capacity of 7000m 3 / d. In the later stage of sewage injection, the single-well injection capacity of the injection wells decreases to 600m 3 / d, which cannot meet the requirements of sewage injection capacity. Through screening and selection, 2 development wells without production potential are selected as sewage injection wells, with an average single-well injection capacity of 1000m 3 / d. The sewage injection demand is met. When the oilfield development enters the water injection stage to supplement the formation energy, most of the sewage can be injected into the production layer. The excess sewage without outlet is still treated by sewage injection into the non-production layer.

[0171] Finally, it should be noted that the above only describes the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art will appreciate that modifications can be made to the technical solutions described in the foregoing embodiments, or some of the technical features thereof can be replaced equivalently, without departing from the spirit and principle of the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

[0172] All that is not described in the specification is known to those skilled in the art.

Claims

1. A method of re-injecting produced water from the natural energy production phase of an oil field into the ground, characterised in that, The method for recycling the produced water in the oilfield natural energy exploitation stage to the underground comprises the following steps: Step 1, measuring the annual produced water output of the oilfield in the natural energy exploitation stage; Step 2, screening the underground produced water recycling layer; Step 3, measuring the water capacity of the produced water recycling layer; Step 4, implementing a test injection to evaluate the water injection of the recycling layer; Step 5, measuring the minimum water absorption capacity of a single well of the recycling well; Step 6, measuring the minimum number of recycling wells required to complete the produced water recycling task; Step 7, measuring the static water absorption capacity of a single well of the recycling well; Step 8, measuring the single well water injection life; Step 9, measuring the cumulative water injection amount of the recycling well in the water injection life; Step 10, re-measuring the number of recycling wells and deploying the produced water recycling scheme accordingly; In step 8, the produced water recycling is divided into two processes: the former is a constant-yield water injection process, and the latter is a constant-wellhead pressure water injection process; corresponding to the two water injection processes, the equations are as follows: In the formula, r is the radius of the cylindrical formation centered on the injection well, in meters; t is the time from the start of water injection, in seconds; h is the formation water absorption thickness, in meters; and q is the water injection rate per well, in cubic meters. 3 / s; μ is the viscosity of the injected water, mPa·s; k is the relative permeability of water, dimensionless; τ is the constant pressure injection time, s; -E i (-r 2 / 4ψt), for Power integral function, dimensionless; P i P(r,t) represents the original formation pressure, in MPa; P(r,t) represents the pressure at time t at a distance r from the injection well, in MPa. Both of the above two equations have no analytical solution, and the numerical solution of the equation is obtained by establishing a geological model and using a numerical simulation method; In step 10, the number of produced water recycling wells is measured according to the following formula: n = Q / qi Where: Q is the total amount of injected sewage, m 3 ; qi is the single well recharge sewage amount calculated by numerical simulation, m 3 .

2. The method of re-injecting oilfield natural energy production phase produced water into the subsurface according to claim 1, characterized in that, In step 1, the annual produced water output of the oilfield in the natural energy exploitation stage is measured according to the current comprehensive water cut and water cut rising law of the oilfield, combined with the oil and gas field development prospect plan.

3. The method of re-injecting oilfield natural energy production phase produced water into the subsurface according to claim 1, characterized in that, In step 2, the screening of the produced water recycling layer follows the following principles: shallow burial, large reservoir capacity, high permeability, and the fluid in the reservoir is no longer developed and utilized.

4. The method of re-injecting oilfield natural energy production phase produced water into the subsurface according to claim 1, characterized in that, In step 3, firstly, the maximum average rising pressure of the recycling layer is measured; secondly, the volume of the sand body of the produced water recycling layer is measured; and finally, the total amount of the produced water that can be contained by the produced water recycling layer is measured.

5. The method of re-injecting oilfield natural energy production phase produced water into the subsurface according to claim 4, characterized in that, In step 3, when measuring the maximum average rising pressure of the recycling layer, the produced water recycling adopts general water injection rather than layered water injection, the formation pressure near the bottom of the recycling layer is the highest, and the highest formation pressure is calculated according to the following formula: P 升 = P wh + P h — P i wherein P 升 is the maximum uphole pressure of the formation near the injection well bottom, MPa; P wh is the injection wellhead pressure, MPa; P h is the static water column pressure of the wellbore, MPa; P i is the original formation pressure, MPa; Here, the wellbore hydrostatic pressure: P h = h x p In the formula, h is the depth of the water injection string; ρ is the density of the recycling produced water, The original formation pressure is: P i = dP x h The maximum rising pressure of the formation near the bottom of the recycling well at the end of water injection is: P 升 = P wh + P h - P i The rising pressure of the farthest end of the water injection well is 0 MPa, and the maximum average rising pressure of the recycling layer is P 平均上升 = (P 升 + 0) / 2.

6. The method of re-injecting oilfield natural energy production phase produced water into the subsurface according to claim 4, wherein, In step 3, the formula for measuring the volume of the sand body of the produced water recycling layer is: V = s × h × η where s is the sand body area, m 3 h is the sand body thickness, m; and η is the sand fraction, dimensionless.

7. The method of re-injecting oilfield natural energy production phase produced water into the ground according to claim 4, characterized in that, In step 3, the formula for measuring the total amount of the produced water that can be contained by the produced water recycling layer is: N = V x ΔP x C t x Φ In the formula, V—the total volume of the reservoir rock; ΔP—the maximum average upward pressure of the recharge layer, which is 5 MPa here; C t —The combined compressibility coefficient of the reservoir rocks and formation water, here 4.4 × 10⁻⁶. -4 MPa -1 Φ—porosity, dimensionless.

8. The method of re-injecting oilfield natural energy production phase produced water into the ground according to claim 1, characterized in that, In step 5, the minimum water absorption capacity of a single well is calculated according to the following formula: N = π x r 2 x h x η x ΔP x C x Φ where: π is the circle constant, dimensionless; r is the swept radius of injected water, m; h is the sand body thickness of the reservoir, m; η is the sand ratio, dimensionless; ΔΡ is the average rising pressure of the reservoir, MPa; C is the pore compression coefficient of the reservoir rock, MPa; and Φ is the porosity of the reservoir rock, dimensionless. -1 where: π is the circle constant, dimensionless; r is the swept radius of injected water, m; h is the sand body thickness of the reservoir, m; η is the sand ratio, dimensionless; ΔΡ is the average rising pressure of the reservoir, MPa; C is the pore compression coefficient of the reservoir rock, MPa; and Φ is the porosity of the reservoir rock, dimensionless.

9. The method of re-injecting oilfield natural energy production phase produced water into the subsurface according to claim 1, wherein, In step 6, according to the minimum water absorption capacity of a single well, the minimum number of wells required for produced water recycling is measured: n = Q / q Where: Q is the total amount of injected sewage, m 3 ; q is the single well recharge sewage, m 3 .

10. The method of re-injecting oilfield natural energy production phase produced water into the subsurface according to claim 1, wherein, In step 7, the water absorption capacity of a single well is calculated according to the following formula: q = ΔP × Jw × H wherein: ΔP is the injection pressure difference at the well bottom, MPa; q is the water absorption capacity of a single well, m 3 / d; Jw is the water absorption index per meter of the formation, m 3 / m.MPa.d; H is the water absorption thickness of the formation, m; In which, the bottom water injection pressure is calculated according to the following formula: ΔP = P wh + h x p / 100 - P f - P 启 - P i where: P wh Pw is the wellhead injection pressure, MPa; P h Pw is the wellbore hydrostatic pressure, MPa; P f Pw is the wellbore friction, MPa; P 启 Pw is the injection start-up pressure, MPa; P i Pw is the original formation pressure of the recharge zone, MPa; h is the depth of the water injection string, m; p is the density of the contaminated water, g / cm 3 ; Wellbore friction P f Calculated from the following empirical equation: P f = 1.086 x 10 -13 λLq 2 / d 5 In the formula: λ is the friction coefficient of the water injection well; d is the inner diameter of the oil pipe; L is the depth of the water injection pipe; q is the daily water injection amount of a single well.

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