Induced earthquake evaluation method for hydraulic fracturing activated faults

Abstract

An induced earthquake evaluation method for hydraulic fracturing activated faults includes obtaining an induced earthquake evaluation unit, cumulative injection equivalent energy, induced earthquake probability of fault type, induced earthquake probability of fault distance, hydraulic fracturing fluid diffusion capacity in fractured formation, new effective fault stress after fracturing, formation fluid pressure change rate and cumulative released equivalent energy. The probability of induced earthquake by hydraulic fracturing can be obtained. The induced earthquake possibility of each evaluation unit or single well under different stages of hydraulic fracturing, different construction injection methods and production conditions can be quantitatively evaluated. The method provides a quantitative result that can provide data support for optimizing hydraulic fracturing and reducing induced earthquake hazards.

Claims

1. An induced earthquake evaluation method for hydraulic fracturing activated faults, comprising: step S1, obtaining an induced earthquake evaluation unit for hydraulic fracturing activated faults according to a geological structure type, a fault distribution, a fault size, a work area size, an injection well distribution, a production well distribution, injection and production formations; wherein the geological structure type is one selected from anticline, syncline and a fault structure among a medium structure and following structure types; the fault distribution is obtained in relation to a relative location of hydraulic fracturing injection wells and production wells; the fault size is obtained by fault grade; the induced earthquake evaluation unit is not larger than an area of a working area, and is distributed in second and third grade faults of the working area; more than 90% of hydraulic fracturing fluid injection and production wells are located in the working area and the induced earthquake evaluation unit; the injection and production formations are the same or adjacent connected formation; step S2, obtaining a cumulative injection equivalent energy W of hydraulic fracturing fluid according to an injection method, an injection form, an injection rate and a cumulative injection amount of hydraulic fracturing fluid; wherein a formula for calculating the cumulative injection equivalent energy W is as follows: $W=.Math.W_i=.Math.\left(W_{\mathrm{ik}}+W_{\mathrm{ip}}\right)=.Math.\frac{1}{2}\left[\left(m_i\right){\left(\frac{?y}{?t}\right)}^2+{k_i\left(\frac{?y}{?x}\right)}^2\right]$ where W is the cumulative injection equivalent energy, W.sub.ik is a kinetic energy of an i injected fluid, W.sub.ip is an elastic potential energy of the i injected fluid, m.sub.i is a mass of the i injected fluid, k.sub.i is an elastic potential energy coefficient of the i injected fluid; wherein a cumulative injected equivalent energy for continuous injection is multiplied by a factor A, and a cumulative injected equivalent energy for simultaneous injection of multiple wells is multiplied by a factor B; and formulas for calculating the factors A and B are as follows: $\{\begin{array}{c}A=\frac{d}{2{\ln }^{d+1}}\\ B=\frac{n}{{\ln }^{3n}}\end{array}$ where d is a number of continuous injection days and n is a number of wells injected simultaneously; step S3, obtaining an induced earthquake probability P.sub.1 caused by fault type factors according to a fault type; and obtaining an induced earthquake probability P.sub.2 according to a distance between hydraulic fracturing well and fault; step S4, obtaining a diffusion capacity D of hydraulic fracturing fluid in a fractured formation according to rock porosity, a comprehensive permeability and a fracture development degree of hydraulic fracturing formation; and a formula for obtaining the diffusion capacity D is as follows: $D=\frac{M.Math.P_d.Math.K}{{?}_1\left(P_d+a^{2}M\right)}$ where $?=1-\frac{K_d}{K_q},P_d=K_d+\frac{4}{3{?}_d},M=\frac{1}{\left(\frac{?}{K_f}+\frac{?-?}{K_g}\right)},$ D is the diffusion capacity, K.sub.f is injected fluid volume modulus, K.sub.g is rock skeleton volume modulus, K.sub.d is rock mineral particle volume modulus, K is a formation rock permeability, ? is formation rock porosity, ?.sub.d is formation rock skeleton shear modulus, ?.sub.1 is injected fluid viscosity; step S5, obtaining an effective stress ? according to a fluid pressure and mechanical characteristics of faults; wherein hydraulic fracturing fluid diffuses over faults, weakening an effective stress on the faults; and the effective stress ? is obtained by the following formula:
?=?(?.sub.n?P)+?.sub.0 where ? is an internal friction factor of the faults, ?.sub.n is an vertical pressure of the faults, P is a fluid pressure at the faults, and ?.sub.0 is a fault polymerization strength; step S6, taking the induced earthquake evaluation unit as a whole, obtaining an average fluid pressure during injection and an original formation pressure before injection of all hydraulic fracturing injection wells, and obtaining a change rate ? of formation fluid pressure; step S7, in the induced earthquake evaluation unit, obtaining a cumulative released equivalent energy E by hydraulic fracturing according to cumulative fluid production time, a production method and a production well distribution after hydraulic fracturing; and a formula for calculating the cumulative released equivalent energy E is as follows: $E=.Math.E_i=.Math.C_1.Math.C_2.Math.C_3.Math.?\left(T\frac{?}{K_T}-P\right)dV$ where C.sub.1 is a production mode correction coefficient, C.sub.2 is a production speed correction coefficient, C.sub.3 is a production location correction coefficient, ? is a volume expansion coefficient, K.sub.T is an isothermal compression coefficient, T is a temperature, P is the fluid pressure; step S8, in the induced earthquake evaluation unit, obtaining an induced earthquake probability P according to the accumulative injection equivalent energy W, the induced earthquake probability P1 by the fault type, the induced earthquake probability P2 by the fault distance, the diffusion capacity D of hydraulic fracturing fluid, the effective stress ?, the change rate ? of formation fluid pressure, and the accumulative released equivalent energy E; wherein the induced earthquake probability P is obtained from the following formula:
P=f.sub.1(P.sub.1,P.sub.2,?).Math.f.sub.2(W?E,D,?) where $f_1\left(P_1,P_2,?\right)=?.Math.\left(P_1+P_2+P_1.Math.P_2\right),f_2\left(W-E,D,?\right)=\frac{A_{2}D}{?}{e}^{A_1\left(W-E\right)},$ A.sub.1 is an equivalent energy correction coefficient, A.sub.2 is a hydraulic fracturing fluid diffusion energy correction coefficient in a formation rock.

2. The induced earthquake evaluation method for hydraulic fracturing activated faults as claimed in claim 1, wherein the injection method of the hydraulic fracturing fluid in step S2 includes continuous injection and intermittent injection; and the injection form includes single-well injection and multi-well simultaneous injection.

3. The induced earthquake evaluation method for hydraulic fracturing activated faults as claimed in claim 1, wherein in step S3, the fault type factors are divided, according to the induced earthquake probability P.sub.1, into four categories: type III fault, type IV fault, type II fault and type I fault; an induced earthquake probability of the type III fault greater than an induced earthquake probability of the type IV fault greater than an induced earthquake probability of the type II fault greater than an induced earthquake probability of the type I fault; the distance between hydraulic fracturing well and fault is divided, according to the induced earthquake probability P.sub.2, into four categories: a distance less than 1 kilometer (km), a distance of 1-3 km, a distance of 3-10 km, and a distance greater than 10 km; an induced earthquake probability of the distance less than 1 km greater than an induced earthquake probability of the distance of 1-3 km greater than an induced earthquake probability of the distance of 3-10 km greater than an induced earthquake probability of the distance greater than 10 km.

4. The induced earthquake evaluation method for hydraulic fracturing activated faults as claimed in claim 1, wherein in step S5, the hydraulic fracturing fluid diffuses into the faults, a fluid pressure increases at the faults, a friction factor and a polymerization strength decrease, which results in an effective stress change.

5. The induced earthquake evaluation method for hydraulic fracturing activated faults as claimed in claim 1, wherein in step S7, the production method includes continuous production and intermittent production, a production speed includes rapid production after hydraulic fracturing injection and slow production after hydraulic fracturing; and production locations include production less than 3 km from faults and production greater than 3 km from faults.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram of induced earthquake by hydraulic fracturing activated faults,

(2) FIG. 2 is a schematic diagram of induced earthquake probability by different fault types in hydraulic fracturing in specific embodiments,

(3) FIG. 3 is a schematic diagram of the induced earthquake probability by the distance between hydraulic fracturing wells and faults in specific embodiments,

(4) In all the above figures, 1 is the hydraulic fracturing injection well, 2 is the injection fluid flow direction, 3 is the induced earthquake events, 4 is the fault strike, 5 is the fault, and 6 is the production well.

DETAILED DESCRIPTION OF EMBODIMENTS

(5) The technical scheme of the invention is further explained in detail in combination with the attached drawings and specific embodiments. It should be understood that the following embodiments are illustrative descriptions and interpretations of the invention only and should not be interpreted as limiting the scope of protection of the invention. All technologies realized based on the above contents of the invention are covered within the scope of protection intended by the invention.

(6) FIG. 1 provides a schematic diagram of hydraulic fracturing activated fault-induced earthquakes for embodiments of the invention. In this embodiment, the Wufeng-Longmaxi formation is selected as the evaluation unit (The Wufeng-Longmaxi Formation is one stratum). An induced earthquake evaluation method for hydraulic fracturing activated faults includes the steps S1 to S8.

(7) Step S1, according to the geological structure type, fault distribution, fault size, work area size, injection well distribution, production well distribution, injection and production formation, the induced earthquake evaluation unit of hydraulic fracturing activated fault is determined.

(8) As in this embodiment, the evaluation units are first divided according to the geological structure type of the work area. If the entire working area is a large anticlinal structure, then all evaluation units are within this anticlinal structure. Further, based on the fault distribution and fault size, according to the principle that the type IV faults are divided in the evaluation unit, the type I faults are divided outside the evaluation unit, the type II and III faults are mainly distributed outside the evaluation unit and a small number of faults are distributed inside the evaluation unit, the work area can be divided into three evaluation units. Further, the evaluation unit can be further refined according to the principle that 90% or more of the hydraulic fracturing fluid injection and production wells are distributed within the working area and the induced earthquake evaluation unit. Further, according to the principle that injection and production formation are the same or adjacent connected formation, the work area is divided into three induced earthquake evaluation units of hydraulic fracturing activated faults. As shown in Table 1:

(9) TABLE-US-00001 TABLE 1 Geological Injection and structure Fault Injection well Production well production NO Name type distribution Fault size distribution distribution formation 1 Evaluation Anticline The type III fault There are 93% of the All wells are Wufeng- unit 1 is distributed three faults wells were distributed Longmaxi outside the of the type located within the evaluation unit, III and five within the evaluation and the type IV faults of evaluation unit fault is distributed the type IV unit inside the evaluation unit 2 Evaluation Anticline The type III fault There are All wells 98% of the Wufeng- unit 2 is distributed three faults are wells were Longmaxi outside the of the type distributed located evaluation unit, III and within the within the and the type IV eight faults evaluation evaluation fault is distributed of the unit unit inside the type IV evaluation unit 3 Evaluation Anticline The type III fault There are 95% of the 99% of the Wufeng- unit 3 is distributed two faults wells were wells were Longmaxi outside the of the type located located evaluation unit, III and within the within the and the type IV four faults evaluation evaluation fault is distributed of the unit unit inside the type IV evaluation unit

(10) Step S2, the cumulative injection equivalent energy W of hydraulic fracturing fluid is obtained according to the injection method, injection form, injection rate and cumulative injection amount of hydraulic fracturing fluid.

(11) As in this embodiment, the hydraulic fracturing fluid in evaluation unit 1 was injected continuously with 65 wells. In the evaluation unit, three wells were continuously injected at the same time for a long time, with an average injection displacement of 18 m.sup.3/min for each well and a cumulative injection volume of 50,000-150,000 m.sup.3 per well. They have been continuously injected for 280 days. The hydraulic fracturing fluid in evaluation unit 2 was injected intermittently with 54 wells. Long-term intermittent injection of two wells at the same time, the average injection displacement of each well is 15 m.sup.3/min, and the cumulative injection volume of each well is 50,000-150,000 m.sup.3. They have been injected intermittently for 370 days. The hydraulic fracturing fluid in evaluation unit 3 was injected intermittently with 21 wells. Long-term intermittent injection of one well, the average injection displacement of per well is 15 m.sup.3/min, and the cumulative injection volume of a single well is 50,000-150,000 m.sup.3. They have been injected intermittently for 320 days. Formula 1 was used to obtain the cumulative injection equivalent energy W of the three evaluation units, as shown in Table 2.

(12) $\begin{array}{cc}W=.Math.W_i=.Math.\left(W_{ik}+W_{ip}\right)=.Math.\frac{1}{2}\left[\left(m_i\right){\left(\frac{?y}{?t}\right)}^2+{k_i\left(\frac{?y}{?x}\right)}^2\right]& \left(\mathrm{Formula}1\right)\end{array}$

(13) Where W is the cumulative injection equivalent energy, W.sub.ik is the kinetic energy of the i injected fluid, W.sub.ip is the elastic potential energy of the i injected fluid, m.sub.i is the mass of the i injected fluid, k.sub.i is the elastic potential energy coefficient of the i injected fluid.

(14) When calculating the cumulative injection equivalent energy, the cumulative injection equivalent energy of continuous injection is multiplied by factor A, and the cumulative injection equivalent energy of multiple wells injected simultaneously is multiplied by factor B. In this embodiment, the evaluation unit 1 is continuous multi-well simultaneous injection, which is multiplied by factor A and then factor B in calculating the cumulative injected equivalent energy. The evaluation unit 2 is intermittent multi-well simultaneous injection, multiplied by factor B when calculating cumulative injected equivalent energy. The evaluation unit 3 is an intermittent 1-well injection that does not require multiplication of factors A and B.

(15) $\begin{array}{cc}\{\begin{array}{c}A=\frac{d}{2{\ln }^{d+1}}\\ B=\frac{n}{{\ln }^{3n}}\end{array}& \left(\mathrm{Formula}2\right)\end{array}$

(16) Where d is the number of continuous injection days and n is the number of wells injected simultaneously.

(17) Step S3, the induced earthquake probability P.sub.1 caused by fault type factors is obtained according to a fault type. The induced earthquake probability P.sub.2 caused by distance factors of the hydraulic fracturing well and fault is determined according to the distance between hydraulic fracturing well and fault.

(18) As in this embodiment, the evaluation unit 1, unit 2 and unit 3, the distribution and size of adjacent faults are shown in Table 1. The trend of induced earthquake probability P.sub.1 caused by fault type factor is shown in FIG. 2, and the trend of induced earthquake probability P.sub.2 caused by hydraulic fracturing well distance factor is shown in FIG. 3. In the evaluation unit 1, there are three faults of the type III and five faults of the type IV. The type III fault is distributed outside the evaluation unit, and the type IV fault is distributed inside the evaluation unit. On the basis of the induced earthquake probability of each fault type, combined with the weight of the number and distribution proportion of faults, the weighted average method can be used to obtain the induced earthquake probability P.sub.1 of the fault type. In the evaluation unit 1, there are 65 hydraulically fractured wells. All hydraulically fracturing wells are spaced 1-3 km from the type IV fault and 3-10 km from the type III fault. Based on the probability of earthquake induced by each fault distance and combined with the weight of the number of wells of each distance type, the probability of earthquake induced by fault distance P.sub.2 can be obtained. Similarly, combined with Table 1, there are 54 hydraulically fractured wells in the evaluation unit 2. 46 of the wells are separated from the type IV fault by 1 to 3 km. There are 8 wells with a distance of less than 1 km from the type IV fault and 3-10 km from the type III fault. There are 21 hydraulically fractured wells in the evaluation unit 3. The distance from the type IV fault is 1-3 km, and the distance from the type III fault is 3-10 km. According to FIG. 2 and FIG. 3, the fault type and fault distance induced earthquake probabilities P.sub.1 and P.sub.2 of the three evaluation units are shown in Table 2.

(19) Step S4, according to rock porosity, comprehensive permeability and fracture development degree of hydraulic fracturing formation, the diffusion capacity D of hydraulic fracturing fluid in fractured formation (i.e., the hydraulic fracturing formation) is obtained.

(20) As in this embodiment, the average porosity is 4.5%, the average comprehensive permeability is 0.25 millidarcy (mD), and other fractures did not develop except those newly generated by hydraulic fracturing in the evaluation unit 1. The average porosity is 4.2%, the average comprehensive permeability is 0.21 mD, and other fractures did not develop except those newly generated by hydraulic fracturing in the evaluation unit 2. The average porosity is 5.6%, the average comprehensive permeability is 0.31 mD, and other fractures did not develop except those newly generated by hydraulic fracturing in the evaluation unit 3. Formula 3 is used to obtain the diffusion capacity D of the three evaluation units, as shown in Table 2.

(21) $\begin{array}{cc}D=\frac{M.Math.P_d.Math.K}{{?}_1\left(P_d+a^{2}M\right)}& \left(\mathrm{Formula}3\right)\end{array}$

(22) Where

(23) 0$?=1-\frac{K_d}{K_q},P_d=K_d+\frac{4}{3{?}_d},M=\frac{1}{\left(\frac{?}{K_f}+\frac{?-?}{K_g}\right)},$
D is the diffusion capacity, K.sub.f is the injected fluid volume modulus, K.sub.g is the rock skeleton volume modulus, K.sub.d is the rock mineral particle volume modulus, K is the formation rock permeability, ? is the formation rock porosity, ?.sub.d is the formation rock skeleton shear modulus, ?.sub.1 is the injected fluid viscosity.

(24) Step S5, the hydraulic fracturing fluid diffuses over the fault, weakening the effective stress on the fault. The new effective stress ? is determined according to the fluid pressure and the new mechanical characteristics of the fault.

(25) As in this embodiment, the fluids in the evaluation unit 1, unit 2 and unit 3 did not percolate into the type III fault, but only penetrated into the type IV fault. The new effective stress of the three evaluation units after hydraulic fracturing are obtained by using formula 4, as shown in Table 2.
?=?(?.sub.n?P)+?.sub.0(Formula 4)

(26) Where ? is the internal friction factor of the fault, ?.sub.n is the vertical pressure of the fault, P is the fluid pressure at the fault, and ?.sub.0 is the fault polymerization strength.

(27) Step S6, taking the induced earthquake evaluation unit as a whole, the average fluid pressure during injection and the original formation pressure before injection of all hydraulic fracturing injection wells are obtained, and the change rate of formation fluid pressure ? is obtained.

(28) As in this embodiment, the average injected fluid pressure of the evaluation unit 1, 2 and 3 is 105 MPa, 104 MPa and 95 MPa respectively, and the average original formation pressure of injected formation is 82 MPa, 85 MPa and 78 MPa respectively. The formation fluid pressure change rates of the three evaluation units after hydraulic fracturing are shown in Table 2.

(29) Step S7, in the induced earthquake evaluation unit, the cumulative released equivalent energy E by hydraulic fracturing is obtained according to the cumulative fluid production time, production mode and production well distribution after hydraulic fracturing.

(30) As in this embodiment, the producing wells in the evaluation unit 1, unit 2 and unit 3 are converted from injection wells after the completion of injection in the earlier stage. They are from injection wells to production wells. The wells in the three evaluation units are produced continuously and slowly. Production wells are located far from the fault (greater than 3 km).

(31) Produced fluids include subsurface gas and injected fluids. The cumulative released equivalent energy E of each of the three evaluation units is obtained by using formula 5, as shown in Table 2.

(32) $\begin{array}{cc}E=.Math.E_i=.Math.C_1.Math.C_2.Math.C_3.Math.?\left(T\frac{?}{K_T}-P\right)dV& \left(\mathrm{Formula}5\right)\end{array}$

(33) Where C.sub.1 is the production mode correction coefficient which is obtained through the work area experience chart query. C.sub.2 is the production speed correction coefficient which is obtained through the work area experience chart query. C.sub.3 is the production location correction coefficient which is obtained through the work area experience chart query. ? is the volume expansion coefficient, K.sub.T is the isothermal compression coefficient, T is the temperature, P is the fluid pressure.

(34) Step S8, finally, the formula 6 is used to calculate the induced earthquake probability based on the cumulative injection equivalent energy W, fault type induced earthquake probability P1, fault distance induced earthquake probability P2, hydraulic fracturing fluid diffusion capacity D, effective fault stress ?, formation fluid pressure change rate ?, and cumulative released equivalent energy E. The current probability P of induced earthquake by hydraulic fracturing activated fault of each of the three evaluation units is shown in Table 2.
P=f.sub.1(P.sub.1,P.sub.2,?).Math.f.sub.2(W?E,D,?)(Formula 6)
Where

(35) $f_1\left(P_1,P_2,?\right)=?.Math.\left(P_1+P_2+P_1.Math.P_2\right),f_2\left(W-E,D,?\right)=\frac{A_{2}D}{?}{e}^{A_1\left(W-E\right)},$
A.sub.1 is the equivalent energy correction coefficient, A.sub.2 is the hydraulic fracturing fluid diffusion energy correction coefficient in the formation rock.

(36) TABLE-US-00002 TABLE 2 Fault Effective Effective Formation Probability Cumulative Fault type distance fault fault fluid Cumulative of induced injection induced induced Fluid stress of stress of pressure released earthquake equivalent earthquake earthquake diffusion the type the type change equivalent by fault energy W probability probability capacity III fault IV fault rate energy E activation NO Name (MW) P.sub.1 P.sub.2 D ? (MPa) ? (MPa) ? (MW) P 1 Evaluation 85995 0.5324 0.3246 0.4134 44.346 26.432 1.28049 34806 0.3456 unit 1 2 Evaluation 71442 0.6542 0.8765 0.3873 48.537 18.543 1.22353 24085 0.7894 unit 2 3 Evaluation 27783 0.4562 0.3426 0.4567 45.345 24.601 1.21795 8690 0.4032 unit 3

(37) This embodiment is an embodiment of several evaluation units within a work area. A unit of work or a specific area, a well or several wells may also be evaluated individually in the implementation. The induced earthquake probability of hydraulic fracturing activated faults of different evaluation objects can be realized from step S2 to step S8 without step S1. In addition, the induced earthquake probability of dynamic hydraulic fracturing activated fault in different stages of the evaluation object can be obtained by conducting evaluation at different stages of hydraulic fracturing or production.

(38) The above is only the preferred embodiment of the invention, and the scope of protection of the invention is not limited to the above embodiment. For those skilled in the art, any modification, equivalent replacement, improvement, etc. made without deviating from the technical conception of the invention shall be included in the scope of protection of the invention.