METHOD FOR COUPLING HYDRAULIC FRACTURE NETWORK EXTENSION AND PRODUCTION PERFORMANCE OF HORIZONTAL WELL IN UNCONVENTIONAL OIL AND GAS RESERVOIR

Abstract

A method for coupling hydraulic fracture network extension and production performance of a horizontal well in an unconventional oil and gas reservoir includes: establishing a complex hydraulic fracture network model of a fractured horizontal well in an unconventional oil and gas reservoir based on a fracture extension theory; constructing three-dimensional three-phase mathematical models of seepage for the fractured horizontal well based on an embedded discrete fracture model; and constructing a fully implicit numerical calculation model by a finite difference method through three-dimensional orthogonal grids, and solving iteratively, thereby accurately predicting a production performance characteristic of the fractured horizontal well in the unconventional oil and gas reservoir. The method combines a fracture extension model with a production performance prediction model to realize the coupled simulation and prediction of the hydraulic fracture network extension and production performance of the horizontal well in the unconventional oil and gas reservoir.

Description

TECHNICAL FIELD

The present disclosure relates to the technical field of unconventional oil and gas reservoir exploitation, and in particular to a method for coupling hydraulic fracture network extension and production performance of a horizontal well in an unconventional oil and gas reservoir.

BACKGROUND

In China, the huge unconventional oil and gas reservoir resources have become the main areas of the country for increasing reserves and production at present and in the future. Compared with conventional oil and gas reservoirs, unconventional oil and gas reservoirs have more complex geological conditions with natural fractures, featuring low porosity and low permeability, and resulting in extremely low oil and gas production. Field practice shows that the techniques of horizontal wells with long sections and stimulated reservoir volume (SRV) fracturing are the main means for unconventional oil and gas reservoirs to obtain industrial productivity. The fluid with a higher pressure than the fracturing pressure is injected into the formation to create hydraulic fractures and open natural fractures, and a proppant is pumped to provide effective support for the fractures, so as to build an effective flow channel from the reservoir to the wellbore.

Therefore, the key to the accurate prediction of the production performance of the fractured horizontal well in the unconventional oil and gas reservoir lies in the accurate characterization of the hydraulic fracture network extension shape and the accurate prediction of the post-fracture production performance of the horizontal well with coupled complex flow laws. However, the existing hydraulic fracture network extension and gas well production performance simulation are independent of each other, making it hard to capture the mutual dynamic response of mechanics and flow, thereby resulting in the lack of an effective coupled simulation technique.

SUMMARY

In view of this, an objective of the present disclosure is to provide a method for a coupled simulation of hydraulic fracture network extension and production performance of a horizontal well in an unconventional oil and gas reservoir. The method of the present disclosure includes: establishing a complex hydraulic fracture network model of a fractured horizontal well in an unconventional oil and gas reservoir based on a fracture extension theory; constructing three-dimensional three-phase mathematical models of seepage for the fractured horizontal well based on an embedded discrete fracture model; and constructing a fully implicit numerical calculation model by a finite difference method through three-dimensional orthogonal grids, and solving iteratively, thereby accurately predicting a production performance characteristic of the fractured horizontal well in the unconventional oil and gas reservoir. The method of the present disclosure specifically includes the following steps:

S1: constructing, based on a displacement discontinuity method, a displacement discontinuity and stress relationship model of a fracture element and a fracture failure type criterion;

S2: constructing a numerical model for hydraulic fracture network extension of a horizontal well by comprehensively considering a reservoir's natural fracture distribution characteristic, and hydraulic fracture flow, extension and deformation, and acquiring, through iterative simultaneous solution, an extension shape and a spatial distribution characteristic of a hydraulic fracture network;

S3: generating a geological body of the horizontal well based on the extension shape and spatial distribution characteristic of the hydraulic fracture network, and performing spatial grid discretization by three-dimensional orthogonal grids;

S4: constructing, based on an embedded discrete fracture model, three-dimensional three-phase mathematical models of seepage for the horizontal well and a fully implicit numerical model based on a finite difference algorithm; and

S5: iteratively solving the constructed fully implicit numerical model, and predicting a post-fracture production performance characteristic of the horizontal well.

The present disclosure has the following beneficial effects:

1. By comprehensively considering the distribution characteristic of the natural fracture in the unconventional oil and gas reservoir, as well as the effects of proppant settlement and filtration of different components of the fracturing fluid during hydraulic fracturing, the present disclosure constructs a hydraulic fracture network extension model for the horizontal well, and realizes accurate prediction of the complex hydraulic fracture network extension shape.

2. Based on the extension characteristics of the hydraulic fracture, the present disclosure constructs the three-dimensional three-phase fully implicit numerical model of the fractured horizontal well by combining the finite difference method and three-dimensional orthogonal grids. The present disclosure realizes the coupled simulation of the hydraulic fracture network extension and production performance of the horizontal well in the unconventional oil and gas reservoir, and overcomes the shortcomings of the traditional independent hydraulic fracture network extension model and production performance prediction model.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method for coupling hydraulic fracture network extension and production performance of a horizontal well in an unconventional oil and gas reservoir;

FIG. 2 is a schematic view of a hydraulic fracture extension shape of the horizontal well considering a natural fracture distribution;

FIG. 3 is a schematic view of three-dimensional grid partition of the horizontal well and a fracture network;

FIG. 4 shows a production pressure distribution of the fractured horizontal well;

FIG. 5 shows a forecast curve of daily oil production and cumulative oil production;

FIG. 6 shows a forecast curve of daily water production and cumulative water production; and

FIG. 7 shows a forecast curve of daily gas production and cumulative gas production.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To describe the technical features, objectives and beneficial effects of the present disclosure more clearly, the technical solutions of the present disclosure are described in detail below, but it should not be construed that the protection scope of the present disclosure is limited thereto. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present disclosure without creative efforts should fall within the protection scope of the present disclosure.

The present disclosure is described in further detail below with reference to the drawings and embodiments.

(1) First, geomechanical parameters, natural fracture parameters and engineering parameters of the reservoir are input, and simulation is performed based on a fracture failure type criterion, to obtain the shape and spatial distribution characteristics of the hydraulic fracture network, as shown in FIG. 2.

The specific parameters used in this embodiment are shown in Table 1.

TABLE 1
Case calculation parameters
Parameter	Value	Unit
Minimum horizontal principal stress	40	MPa
Maximum horizontal principal stress	40.5	MPa
Young's modulus	30	GPa
Poisson's ratio	0.25	—
Fluid viscosity	9	cp
Proppant diameter	0.00015	m
Proppant density	2800	kg/m3
Fracture spacing	30	m
Type I cracking toughness of matrix rock	2	MPa · m1/2
Type II cracking toughness of matrix rock	4	MPa · m1/2
Initial aperture of natural fracture	0.2	1e−5 m
Closure aperture of natural fracture	1	1e−5 m
Compressibility of natural fracture	0.05	MPa−1
Friction angle of natural fracture	20	°

(2) Spatial grid partition is performed on the extension shape of the generated hydraulic fracture network by using three-dimensional orthogonal grids. The total calculation area has a volume is 400×200×20 m³, and the length of the horizontal well section in the calculation area is 200 m. A five-section multi-stage hydraulic fracture is created through hydraulic fracturing, as shown in FIG. 3.

(3) Based on the reservoir grid partition results are combined with the three-dimensional three-phase fully implicit numerical model of fractured horizontal well. The basic parameters of the model (Table 2), the pressure-volume-temperature (PVT) parameters (Table 3) of crude oil and natural gas, matrix permeability data (Table 4 and Table 5), and matrix capillary force data (Table 6) are brought to obtain the production performance data of the simulated well, and the post-fracture production performance characteristics of the horizontal well are predicted, as shown in FIGS. 4 to 7.

TABLE 2
Basic parameters of model
Parameter	Value	Parameter	Value
Matrix permeability,	0.001	Formation water	4 × 10−4
D		compressibility,
		MPa−1
Rock compressibility,	10−4	Formation water	0.0009
MPa−1		viscosity, Pa · s
Initial porosity,	0.1	Initial water saturation,	0.3
dimensionless		dimensionless
Hydraulic fracture	25	Initial oil saturation,	0.7
permeability, D		dimensionless
Initial oil-phase	20	Flowing bottom-hole	8
pressure, MPa		pressure, MPa
Initial volume factor of	1.01	Aperture of hydraulic	0.005
formation water,		fracture, m
dimensionless
Initial density of	1010	Aperture of natural	0.003
formation water,		fracture, m
kg/m3

TABLE 3
PVT parameters of crude oil and natural gas
	Crude oil
				Solution gas-oil	Natural gas
Pressure	Density	Viscosity	Volume factor	ratio	Density	Viscosity	Volume factor
(kPa)	(kg/m3)	(cP)	Dimensionless	Dimensionless	(kg/m3)	(cP)	Dimensionless
3000	660.13	1.17	1.1806	45.93	25.9351	0.012654	0.035502
4500	652.26	0.97	1.2104	57.63	38.6406	0.01315	0.023019
6000	644.92	0.85	1.2397	69.43	51.8383	0.013675	0.016821
7500	637.83	0.76	1.2698	81.65	65.6071	0.014254	0.013138
9000	630.87	0.69	1.3010	94.44	79.956	0.014907	0.010714
10500	623.96	0.64	1.3338	107.92	94.848	0.015649	0.009012
12000	617.07	0.58	1.3734	122.18	110.212	0.016493	0.007762
13500	617.83	0.58	1.3697	122.18	—	—	—
15000	619.82	0.60	1.3653	122.18	—	—	—
16500	621.74	0.61	1.3611	122.18	—	—	—
18000	623.58	0.63	1.3571	122.18	—	—	—
19500	625.34	0.64	1.3533	122.18	—	—	—
21000	627.04	0.66	1.3496	122.18	—	—	—

TABLE 4
Matrix oil-water permeability
sw	krw	kro
0.21	0.0000	1.0000
0.24	0.0074	0.8565
0.27	0.0209	0.7291
0.30	0.0385	0.6164
0.33	0.0592	0.5174
0.36	0.0827	0.4307
0.39	0.1088	0.3555
0.42	0.1371	0.2905
0.45	0.1674	0.2349
0.48	0.1998	0.1877
0.51	0.2340	0.1480
0.54	0.2700	0.1150
0.57	0.3076	0.0878
0.60	0.3469	0.0657
0.63	0.3876	0.0481
0.66	0.4299	0.0343
0.69	0.4736	0.0237
0.72	0.5187	0.0158
0.75	0.5651	0.0100
0.78	0.6129	0.0060
0.81	0.6619	0.0033
0.84	0.7121	0.0017
0.87	0.7636	0.0007
0.90	0.8163	0.0003

TABLE 5
Matrix oil-gas permeability
sg	krg	kro
0.04	0	1
0.08	0.01103	0.70778
0.12	0.02912	0.55844
0.16	0.05138	0.4454
0.20	0.07687	0.35562
0.24	0.10506	0.28302
0.28	0.13561	0.22392
0.32	0.16827	0.17574
0.36	0.20286	0.13656
0.40	0.23923	0.10485
0.44	0.27725	0.07938
0.48	0.31683	0.05912
0.52	0.35788	0.04319
0.56	0.40031	0.03084
0.60	0.44408	0.02143
0.64	0.48911	0.01442
0.68	0.53536	0.00933
0.72	0.58279	0.00574
0.76	0.63134	0.00332
0.79	0.67989	0.0009

TABLE 6
Matrix oil-water and gas-oil capillary force
	sw,	pcow,	1-sg,	pcgo,
	dimensionless	kPa	dimensionless	kPa
	0.2	8000	0.21	4760
	0.25	4300	0.26	2940
	0.3	3000	0.31	2220
	0.4	1780	0.41	1490
	0.5	1210	0.51	1040
	0.6	790	0.66	510
	0.7	430	0.76	270
	0.8	100	0.96	0
	0.9	0

The present disclosure is described above with reference to the preferred embodiments, but those skilled in the art should understand that these embodiments are only intended to describe the present disclosure, rather than to limit the scope of the present disclosure. Further improvements of the present disclosure made without departing from the principle of the present disclosure should also be deemed as falling within the protection scope of the present disclosure.

Claims

1. A method for coupling hydraulic fracture network extension and production performance of a horizontal well in an unconventional oil and gas reservoir, comprising the following steps: S1: constructing, based on a displacement discontinuity method, a displacement discontinuity and stress relationship model of a fracture element and a fracture failure type criterion;S2: constructing a numerical model for hydraulic fracture network extension of the horizontal well by comprehensively considering a reservoir's natural fracture distribution characteristic and hydraulic fracture flow, extension and deformation, and acquiring, through iterative simultaneous solution, an extension shape and a spatial distribution characteristic of a hydraulic fracture network;S3: generating a geological body of a fractured horizontal well based on the extension shape and the spatial distribution characteristic of the hydraulic fracture network, and performing spatial grid discretization by three-dimensional orthogonal grids;S4: constructing, based on an embedded discrete fracture model, three-dimensional three-phase mathematical models of seepage for the fractured horizontal well and a fully implicit numerical model based on a finite difference algorithm; andS5: iteratively solving the fully implicit numerical model, and predicting a post-fracture production performance characteristic of the horizontal well.

2. The method for coupling hydraulic fracture network extension and production performance of the horizontal well in the unconventional oil and gas reservoir according to claim 1, wherein step S1 comprises: S11: assuming a vertical height of a pseudo-three-dimensional fracture with a series of line segment elements horizontally to be a reservoir thickness, wherein when the fracture is subjected to an external load, relative sliding occurs between upper and lower surfaces of the fracture element; and defining variables to be calculated in each fracture element, namely a normal displacement Dn and a tangential displacement Ds, as displacement discontinuities: Ds=ux(x,0−)−ux(x,0+)  (1)=Dn=uy(x,0−)−uy(x,0+)  (2)wherein, ux(x,y), uy(x,y) denote surface displacements of the fracture element at a point (x,y) along an x-axis and a y-axis, respectively, m; and 0+ and 0− denote upper and lower wall surfaces of the fracture element along the y-axis, respectively;expressing a stress, a strain and a displacement of the fracture element at any point in space by the displacement discontinuities: ux=[2(1−v)f′y−yf′xx]+[−(1−2v)g′x−yg′xy]  (3)uy=[(1−2v)f′x−yf′xy]+[2(1−v)g′y−yg′yy]  (4)σxx=2G[2f′xy+yf′xyy]+2G[g′yy+yg′yyy]  (5)σyy=2G[−yf′xyy]+2G[g′yy−yg′yyy]  (6)τxy=2G[2f′yy+yf′yyy]+2G[−yg′xyy]  (7)wherein, σ(·) denotes a stress tensor of the fracture element, with a subscript xx denoting a stress perpendicular to a yz plane, and a subscript yy denoting a stress perpendicular to an xz plane; T denotes a shear stress tensor of the fracture element; G denotes a shear modulus of an elastic medium; v denotes a Poisson's ratio; y denotes a y-axis coordinate at any point; f′(·) and g′(·) denote derivations of integral functions f and g, respectively, with a subscript denoting an independent variable, for example:

3. The method for coupling hydraulic fracture network extension and production performance of the horizontal well in the unconventional oil and gas reservoir according to claim 1, wherein step S2 comprises: S21: generating a random point, a random azimuth and a random length by a random number generation method to model the natural fracture distribution characteristic, wherein a random point N (Nx,Ny) conforms to a uniform distribution on an interval [0,1]: Nx=rl×randk  (25)Ny=rw×randk+1  (26)wherein, rl denotes a reservoir length; rw denotes a reservoir width; and rand denotes a random number, with a subscript denoting a random number of times to generate the random number;a fracture azimuth and a fracture length are expressed as follows: θp=π×randk+2  (27)lp=Lmax×randk+3  (28)wherein, θp denotes the fracture azimuth; and lp denotes the random fracture length;S22: constructing a hydraulic fracture network extension model of the horizontal well by considering hydraulic fracture flow, extension and deformation, and constructing a mass conservation equation of a pure fracturing fluid component f by considering leakage:

4. The method for coupling hydraulic fracture network extension and production performance of the horizontal well in the unconventional oil and gas reservoir according to claim 1, wherein step S3 comprises: S31: generating a geological body based on an actual geological condition of a research area, a trajectory of the horizontal well, and distribution characteristics of the hydraulic fracture and the natural fracture; andS32: editing and importing data of the geological body, and generating a discrete model by three-dimensional orthogonal grids.

5. The method for coupling hydraulic fracture network extension and production performance of the horizontal well in the unconventional oil and gas reservoir according to claim 1, wherein step S4 comprises: S41: constructing, based on the embedded discrete fracture model, the three-dimensional three-phase mathematical models of seepage for the fractured horizontal well, wherein for a matrix system:

6. The method for coupling hydraulic fracture network extension and production performance of the horizontal well in the unconventional oil and gas reservoir according to claim 5, wherein in step S5, the step of iteratively solving the constructed fully implicit numerical model, and predicting the post-fracture production performance characteristic of the horizontal well comprises: assuming there are N fracture grids in M matrix grids, each grid being expressed by three equations of oil, gas and water and having three unknowns δpoi, δswi and δsgi, then constructing a fully implicit calculation matrix considering reservoir seepage and hydraulic fracture flow: E3(M+N)×3(M+N)×δX3(M+N)×1F3(M+N)×1  (53)wherein, E3(M+N)×3(M+N) denotes a coefficient matrix; δX3(M+N)×1 denotes an unknown vector; and F3(M+N)×1 denotes a constant vector;iteratively calculating for each time step until a satisfied accuracy of the unknown vector δX3(M+N)×1 thereby obtaining a pressure value and a saturation value at the (n+1)-th time step; and starting a cycle at a next time step; andfinally, outputting pressure and saturation distributions of the reservoir at each time step, and calculating a production performance of the fractured horizontal well according to a production formula, wherein since each fractured horizontal well comprises multiple hydraulic fractures, an output of the fractured horizontal well is a sum of flow rates of all fractures flowing into a wellbore; and a flow rate of each fracture flowing into the wellbore is:wherein,