METHOD FOR STIMULATING OIL AND GAS RESERVOIR VOLUME BY FORMING BRANCH FRACTURES IN MAIN FRACTURE

Abstract

The present invention provides a method for further forming a plurality of small fractures in a main fracture. If a radial main fracture is further fractured axially to form a plurality of branch fractures, the stimulated volume of the oil and gas reservoir will be achieved to greatly increase the yield of dense oil and gas. The main contribution of the present invention is to form three-dimensional fracture networks for any low-permeability oil reservoirs to achieve stimulated reservoir volume. The concept is that an original main fracture is further fractured to form a plurality of branch fractures (the branch fractures form included angles with the original main fracture), and the locations for forming the branch fractures in the main fracture and even the size and length of the branch fractures can be controlled artificially, so that the branch fractures can be formed by fracturing where needed.

Claims

1. A method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture, wherein explosives are detonated in the main fracture by pressurizing to form a plurality of branch fractures, which comprises a step of forming a plurality of branch fractures in the main fracture in stages and/or forming a plurality of branch fractures in the main fracture at once so that the main fracture is changed into a fracture network to improve the seepage of low-permeability oil and gas reservoirs.

2. The method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture according to claim 1, wherein forming a plurality of branch fractures in the main fracture in stages comprises steps of: adding fracturing fluid to the main fracture in stages after the completion of perforation; adding fracturing fluid containing pressure-controlled explosives in the first stage, wherein the pressure-controlled explosives are located on the top of the main fracture after the completion of the fracturing fluid leak-off, injecting conventional fracturing fluid at high pressure and detonating the explosives to form the branch fractures in the main fracture; adding the conventional fracturing fluid in the second stage to increase the length of the main fracture and the branch fractures formed in the first stage; in the third stage, repeating the operations of the first stage, and in the fourth stage, repeating the operations of the second stage, during which the location, number and length of the branch fractures are controlled according to the leak-off amount of the fracturing fluid into the formation and the amount of the pressure-controlled explosives.

3. The method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture according to claim 2, wherein forming a plurality of branch fractures in the main fracture in stages comprises: a first step of fracturing the main fracture: running a pipe string for perforation and fracturing operations, and lifting the pipe string out after the completion of the operations; a second step of injecting the fracturing fluid containing pressure-controlled explosives into the main fracture by a low-pressure pump; and running a dual-packer single-stick pipe string which comprises a top packer, a pressure guide sleeve and a bottom packer, wherein the fracturing fluid containing pressure-controlled explosives is injected after the top packer and the bottom packer are set, and the pressure-controlled explosives are located on the top of the main fracture after the completion of the fracturing fluid leak-off; a third step of injecting spacer fluid into the main fracture by a high-pressure pump and detonating the pressure-controlled explosives: injecting the spacer fluid into the main fracture by a high-pressure pump; and perforating the main fracture when the pressure in the main fracture reaches a detonating point of the pressure-controlled explosives and detonating the pressure-controlled explosives to form the branch fractures in the main fracture; a fourth step of continuing injecting the fracturing fluid into the main fracture by the high-pressure pump to further increase the length of the main fracture; after the completion of perforation in the branch fracture, injecting the fracturing fluid by the high-pressure pump to increase the length of the branch fractures; a fifth step of repeating the second step to the fourth step as needed, and determining how many branch fractures need to be formed in the main fracture according to the requirements of fracturing design, and repeating the second step to the fourth step; and a sixth step of discharging the explosives from the main fracture and the branch fractures.

4. The method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture according to claim 1, wherein forming a plurality of branch fractures in the main fracture at once comprises: after the completion of perforation, increasing the length of the main fracture by injecting fracturing fluid, then placing the explosives in the main fracture in advance, igniting the explosives in the main fracture by an igniter to form the branch fractures, injecting the fracturing fluid into the main fracture again after the initiation of detonation to increase the length of the branch fractures, and controlling the location and the length of the branch fractures according to the amount or concentration of the explosives injected.

5. The method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture according to claim 4, wherein forming a plurality of branch fractures in the main fracture at once comprises: a first step of fracturing the main fracture: running a pipe string for perforation and fracturing operations, and adding the fracturing fluid to increase the length of the main fracture after the completion of the operations; a second step of adding fracturing fluid containing pressure-controlled explosives: in this step, a method for adding pressure-controlled explosives in stages is used, when there is fracturing fluid containing pressure-controlled explosives at different depths of the main fracture, alternately injecting the fracturing fluid and the fracturing fluid containing pressure-controlled explosives by pumps at time intervals by calculating leak-off amount and discharge amount, and placing the explosives at destination locations; a third step of detonating the explosives by an electronic igniter: detonating explosives by an igniter after the explosives have been completely injected by the pumps, and re-perforating the main fracture to form the branch fractures; and a fourth step of fracturing the branch fractures: adding the fracturing fluid to the main fracture again, where, at this moment, the fracturing fluid permeates into the branch fractures to increase the length of the branch fractures, so that the main fracture and the branch fractures form a fracture network to improve the seepage of the oil and gas reservoir.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1-1 is a side view of main and branch fractures in a wellbore of a vertical well according to the present invention;

[0027] FIG. 1-2 is a top view of the main and branch fractures in the wellbore of the vertical well according to the present invention;

[0028] FIG. 2 is a side view of main and branch fractures in a wellbore of a horizontal well according to the present invention;

[0029] FIG. 3-1 is a schematic diagram of a top end of the main fracture in method 1 according to the present invention;

[0030] FIG. 3-2 is a schematic diagram in which the branch fractures are formed at the top end of the main fracture in the method 1 according to the present invention;

[0031] FIG. 3-3 is a schematic diagram in which the main and branch fractures are lengthened in the method 1 according to the present invention;

[0032] FIG. 3-4 is a schematic diagram in which pressure-controlled explosives are added for a second time in the method 1 according to the present invention;

[0033] FIG. 3-5 is a schematic diagram in which the branch fractures are formed for a second time in the method 1 according to the present invention;

[0034] FIG. 3-6 is a schematic diagram in which the main fracture is lengthened and the branch fractures are formed for a second time in the method 1 according to the present invention;

[0035] FIG. 4-1 is a schematic diagram in which a long main fracture is formed at once in method 2 according to the present invention;

[0036] FIG. 4-2 is a schematic diagram in which electronically detonated explosives are injected based on process requirements in the method 2 according to the present invention; and

[0037] FIG. 4-3 is a schematic diagram of forming a plurality of branch fractures at once in the method 2 according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0038] The present invention will be discussed in detail below with reference to the accompanying drawings.

[0039] The present invention mainly aims at transforming a single fracture formed when fracturing the low-permeability reservoirs into a three-dimensional fracture network, so that the original main fracture is further fractured to form branch fractures which form included angles with the main fracture so as to achieve the stimulated volume. The locations for forming the branch fractures in the main fracture are controlled by the leak-off amount and the injection time of the fracturing fluid free of pressure-controlled explosives by pumps, and the length of the branch fractures is controlled by the injection time of the fracturing fluid, so that the branch fractures can be formed by fracturing where needed.

[0040] The stimulation of oil and gas reservoir volume by forming branch fractures in a main fracture is suitable for vertical wells, directional wells and horizontal wells, and the fracturing method thereof is pipe string dragging fracturing. Its key point lies in leak-off amount and concentration of carried explosives in the stage of injecting the pressure-controlled explosives.

[0041] The pressure guide sleeve is placed at a perforation section by running the pipe string, the fracturing fluid containing pressure-controlled explosives is injected into the main fracture, and part of the fracturing fluid will leak into the formation according to different geological environments and different kinds of fracturing fluid,

[00001]$C=\frac{2.Math..Math.\left(1-{}^2\right).Math..Math.H_p^2.Math.}{3.Math.{\mathrm{EH}}_P.Math.\sqrt{t}}$

[0042] where fp=H/Hp, dimensionless; Hp is the thickness (m) of a fractured layer; H is the fracture height (m); p is the fitting pressure (MPa); is the Poisson's ratio of the formation rock, dimensionless; E is the elastic modulus (MPa) of the formation rock; t is the injection time (s) of the fracturing operation; and is the ratio of the average pressure in the fracture to the pressure in the well bottom, dimensionless. The leak-off amount of different kinds of fracturing fluid in different geological environments can be obtained by calculation.

[0043] The pressure-controlled explosives enter the main fracture along with the fracturing fluid and are finally placed on the top of the main fracture. Afterwards, spacer fluid is injected by a high-pressure pump to detonate the pressure-controlled explosives, and the fracture is perforated. Since the pressure-controlled explosives enter the main fracture along with the fracturing fluid, it only needs to determine the concentration of carried explosives or the like to control the density of the branch fractures. The flow conductivity

[00002]$F_0=\frac{K_1.Math.W_1}{K.Math.L_1}.Math.\left(1-D\right)$

of the main fracture is calculated first to determine the concentration of carried explosives, and the concentration of carried explosives is calculated by a Nolte linear slope formula

[00003]$c=c_{\max }.Math.{\left(\frac{t-t_0}{t_i-t_0}\right)}^{\left(1--0.05.Math.\text{/}.Math.\right)}$

according to the reservoir parameters and the fracturing scale.