Using horizontal directional drilling and liquid nitrogen cyclic freeze-thaw process to improve permeability in gas drainage

Abstract

A liquid nitrogen cyclic freeze-thaw permeability-improvement gas drainage method based on horizontal directional boreholes comprises: first constructing a main borehole at an intake roadway or a return roadway, a low-level roadway and a high-level roadway, after a drill bit reaches a pre-set target position of a coal bed, uniformly and directionally constructing a plurality of branch boreholes along a horizontal direction of the coal bed, injecting water to the coal bed, opening a valve, filling the main boreholes with liquid nitrogen, rapidly freezing the water injected into the branch boreholes and the periphery of the coal bed, and stopping injecting the nitrogen when an average temperature of a pre-permeability-improvement area monitored by a temperature measuring hole is lowered to 2 C. or lower.

Claims

1. A gas drainage method to improve permeability, based on horizontal directional boreholes and using liquid nitrogen cyclic freeze-thaw, comprising: a. constructing a main borehole in a target coal bed in an intake roadway or a return roadway of a recovery coal bed along a bedding of the coal bed, a penetrating layer of a low-level roadway or a penetrating layer of a high-level roadway; developing a thickness of the main borehole until it reaches a position 2 m to 10 m away from an upper edge of the coal bed, taking the main borehole as a center, and using a horizontal directional drilling machine to uniformly arrange a plurality of branch boreholes along the horizontal direction of the coal bed, wherein the plurality of branch boreholes have same angles and lengths of 3050 m; b. arranging a low-temperature-resistant steel pipe in the main borehole after the drilling machine is withdrawn, wherein a front portion of the low-temperature-resistant steel pipe is a floral pipe with a length at 1 m to 3 m, and the front portion of the floral pipe is sealed; wherein a pressure measuring port is provided on the low-temperature-resistant steel pipe, and a high-pressure gauge is connected to the pressure measuring port; c. injecting a prepared high-pressure borehole sealing material slurry into a gap between the low-temperature-resistant steel pipe and the main borehole by virtue of a grouting pump to perform the grouting hole sealing, wherein length H of a grouting hole sealing section is between 15 m to 25 m; d. symmetrically constructing two temperature measuring holes surrounding the low-temperature-resistant steel pipe, wherein a distance L from the centers of the two temperature measuring holes to the center of the main borehole is 30 m to 50 m, and an area between the two temperature measuring holes is a coal bed fracturing area; arranging a temperature sensor in each temperature measuring hole, connecting each temperature sensor to a digital display temperature instrument arranged outside of the temperature measuring holes by leading out a conducting wire, arranging a sensor casing pipe fixed by a temperature measuring hole sealing section at an inlet section of each temperature measuring hole, and monitoring the temperature in a borehole temperature measuring area in real time by pushing and pulling the temperature sensor forwards and backwards in the sensor casing pipe, wherein the arrangement length of the borehole temperature measuring area in the coal bed is 5 m to 10 m; e. injecting water into the low-temperature-resistant steel pipe via a rapid connector by utilizing a water injection device provided in the intake roadway or the return roadway, the injected water being divided by the floral pipe located at the end portion of the low-temperature-resistant steel pipe, entering from the branch boreholes, permeating to remain in a coal body, and continuously permeating and entering into natural micro coal-bed cracks; f. removing a water injection valve on the rapid connector after the injected water permeably flows for 2 to 3 hours in the coal mass, installing a liquid nitrogen valve, connecting the low-temperature-resistant steel pipe in the main borehole to a liquid nitrogen tank car provided in the intake roadway or the return roadway (6), opening the liquid nitrogen valve, filling the low-temperature-resistant steel pipe in the main borehole with liquid nitrogen, monitoring the temperature in the borehole temperature measuring area through the temperature measuring holes, measuring a temperature at each end in the borehole temperature measuring area, averaging the temperature measured at each end in the borehole temperature measuring area to obtain an average temperature, determining that the coal bed fracturing permeability improvement area is already at a frozen state, and closing the liquid nitrogen valve to stop injecting the nitrogen when the average temperature is no greater than 2 C., making the coal mass naturally thawed for 2 to 3 hours, and completing a freeze-thaw cycle of a phase changer fracturing unit; g. implementing a gas drainage borehole to the coal bed in the coal bed fracturing permeability improvement area between the two temperature measuring holes, and draining the gas according to a conventional method; and h. injecting water and filling in liquid nitrogen for multiple cycles to the coal bed through the low-temperature-resistant steel pipe and the branch boreholes, wherein the coal body reaches a coal mass stress fatigue limit under the alternative effect of freezing-thawing-freezing in multiple freeze-thaw cycles and is fractured.

2. The method according to claim 1, characterized in that: in the liquid nitrogen filling process, turning off the liquid nitrogen valve when the pressure of the liquid nitrogen in the low-temperature-resistant steel pipe is no less than 8 MPa, and opening the liquid nitrogen valve when the pressure of the liquid nitrogen in the low-temperature-resistant steel pipe is no more than 2 MPa, to continuously fill the liquid nitrogen.

3. The method according to claim 1, characterized in that: the number of branch boreholes with the same angle and with the length of 30 m to 50 m uniformly distributed and directionally constructed along the horizontal direction of the coal bed is between 4 to 8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of the coal-bed bedding directional borehole liquid nitrogen cyclic freeze-thaw permeability improvement gas drainage method.

(2) FIG. 2 is a schematic diagram of an A-A section of FIG. 1.

(3) FIG. 3 is a schematic diagram of layout and connection of the steel pipe in the main borehole in FIG. 1, FIG. 5 and FIG. 6.

(4) FIG. 4 is a schematic diagram of the temperature measuring hole of a B-B section in FIG. 2 and FIG. 7.

(5) FIG. 5 is a schematic diagram of the low-level roadway penetrating-layer upstream hole liquid nitrogen cyclic freeze-thaw permeability improvement gas drainage method.

(6) FIG. 6 is a schematic diagram of the high-level roadway penetrating-layer downstream hole liquid nitrogen cyclic freeze-thaw permeability improvement gas drainage method.

(7) FIG. 7 is a diagram of a C-C section and a D-D section of FIG. 5 and FIG. 6.

(8) Wherein: 1branch borehole, 2floral pipe, 3main borehole, 3-1low-temperature-resistant steel pipe, 4hole sealing section, 5rapid connector, 5-1water injection valve, 5-2liquid nitrogen valve, 6intake roadway or return roadway, 7coal bed, 8device unit, 8-1water injection device, 8-2liquid nitrogen tank car, 9temperature measuring hole, 9-1borehole temperature measuring area, 9-2temperature sensor, 9-3sensor moving casing pipe, 9-4temperature measuring hole sealing section, 9-5digital display temperature instrument, 10gob, 11low-level roadway, 12high-level roadway, 12high-pressure pressure gauge.

DETAILED DESCRIPTION

(9) Embodiments of the present invention are further described in detail in combination with the attached drawings:

(10) A liquid nitrogen cyclic freeze-thaw permeability-improvement gas drainage method based on the horizontal directional borehole comprises the steps as follows:

(11) a. constructing a main borehole 3 to a permeability-improvement drainage coal bed 7 in an intake roadway or a return roadway 6 of a recovery coal bed along a bedding of the coal bed, a penetrating layer of a low-level roadway or a penetrating layer of a high-level roadway, according to the thickness of the coal bed 7, when the main borehole 3 reaches a position 2 m to 10 m distanced to the upper edge of the coal bed 7, taking the main borehole 3 as a center, and uniformly and directionally constructing a plurality of branch boreholes 1 with the same angle and with a length of 30 m to 50 m along the horizontal direction of the coal bed 7 by adopting a horizontal directional drilling machine;

(12) b. arranging a low-temperature-resistant steel pipe 3-1 in the main borehole 3 after the drilling machine is withdrawn, wherein the front portion of the low-temperature-resistant steel pipe 3-1 is a floral pipe 2 with a length of 1 m to 3 m, and sealing the front portion of the floral pipe 2; forming a pressure measuring port on the low-temperature-resistant steel pipe 3-1, and connecting a high-pressure pressure gauge 13 at the pressure measuring port;

(13) c. injecting well-prepared high-pressure borehole sealing material slurry into a gap between the low-temperature-resistant steel pipe 3-1 and the main borehole 3 by virtue of a grouting pump to perform the grouting hole sealing, wherein the length H of a grouting hole sealing section 4 is 15 m to 25 m;

(14) d. symmetrically constructing two temperature measuring holes 9 at two sides of the low-temperature-resistant steel pipe 3-1, wherein a distance L from the centers of the two temperature measuring holes 9 to the center of the main borehole 3 is 30 m to 50 m, and an area between the two temperature measuring holes 9 is a coal bed fracturing permeability improvement area; arranging a temperature sensor 9-2 in each temperature measuring hole 9, connecting each temperature sensor 9-2 to a digital display temperature instrument 9-5 arranged outside a porthole by leading out a conducting wire, arranging a sensor casing pipe 9-3 fixed by a temperature measuring hole sealing section 9-4 at an inlet section of each temperature measuring hole 9, and monitoring the temperature in a borehole temperature measuring area 9-1 in real time by pushing and pulling the temperature sensor 9-2 forwards and backwards in the sensor casing pipe 9-3, wherein the arrangement length of the borehole temperature measuring area 9-1 in the coal bed 7 is 5 m to 10 m;

(15) e. injecting water into the low-temperature-resistant steel pipe 3-1 via a rapid connector 5 by utilizing a water injection device 8-1 provided in the intake roadway or the return roadway 6, the injected water being divided by the low-temperature-resistant steel pipe 3-1, entering from six branch boreholes 1, permeating to remain in the coal mass, and continuously permeating and entering micro coal-bed cracks;

(16) f. after the injected water permeably flows for 2 to 3 hours in the coal mass, removing a water injection valve 5-1 on the rapid connector 5, installing a liquid nitrogen valve 5-2, connecting the low-temperature-resistant steel pipe 3-1 in the main borehole 3 to a liquid nitrogen tank car 8-2 provided in the intake roadway or the return roadway 6, opening the liquid nitrogen valve 5-2, filling the low-temperature-resistant steel pipe 3-1 in the main borehole 3 with liquid nitrogen, wherein the liquid nitrogen is gasified and expanded to generate expansive pressure, a great amount of heat is absorbed in the gasification process of the liquid nitrogen, the water injected into the branch boreholes and the periphery of the coal bed is rapidly frozen, and free water in the cracks of the coal bed is gradually transformed from liquid to solid during the freezing process to have the phase change; monitoring the temperature in the borehole temperature measuring area 9-1 through the temperature measuring holes 9, when an average temperature at two ends in the borehole temperature measuring area 9-1 is lower than 2 C., determining that the coal bed fracturing permeability improvement area is already at a frozen state, closing the liquid nitrogen valve 5-2 to stop injecting the nitrogen, making the coal mass naturally thawed for 2 to 3 hours, and completing a freeze-thaw cycle of a phase changer fracturing unit; and under the collective action of the water phase change frost heaving force, the liquid nitrogen gasification expansive force and the microporous liquid flowing osmotic pressure, the macro cracks and the micro cracks of the coal mass are expanded and communicated to form a crack net, thereby improving the permeability of the coal bed;

(17) g. after the injection of the liquid nitrogen is ended, according to a conventional method, implementing the gas drainage borehole to the coal bed in the coal bed fracturing permeability improvement area between the two temperature measuring holes 9, and draining the gas; and

(18) h. in the gas drainage process, according to the variation of the gas drainage effect, injecting water and liquid nitrogen repeatedly for multiple times to the coal bed 7 through the low-temperature-resistant steel pipe 3-1 and the six branch boreholes 1, thereby realizing a purpose of improving the permeability of the coal bed surrounding the borehole and rapidly and effectively draining the gas; and the coal mass reaches a coal mass stress fatigue limit under the alternative effect of freezing-thawing-freezing in multiple freeze-thaw cycles and is fractured.

(19) In the liquid nitrogen filling process, when the pressure of the liquid nitrogen in the low-temperature-resistant steel pipe 3-1 is higher than 8 MPa, the liquid nitrogen valve 5-2 is closed, and when the pressure is lower than 2 MPa, the liquid nitrogen valve 5-2 is opened to continuously fill the liquid nitrogen.

Embodiment I

(20) As shown in FIG. 1, FIG. 2, FIG. 3 and FIG. 4, performing the directional borehole liquid nitrogen freeze-thaw permeability-improvement gas drainage and decompression to the bedding of the coal bed 7 comprises the steps: a main borehole 3 is first constructed to a permeability improvement drainage coal bed area in an intake roadway or return roadway 6 along the bedding of the coal bed, the penetrating layer of the low-level roadway or penetrating layer of the high-level roadway; according to the thickness of the coal bed 7, when the main borehole 3 reaches a position 2 m to 10 m distanced to the upper edge of the coal bed 7, and by taking the main borehole 3 as the center, six branch boreholes 1 with a length of 30 m to 50 m are directly constructed by adopting a guide function of a horizontal directional drilling machine at an interval of 60 degrees along the horizontal direction of the coal bed 7; after the drilling machine is withdrawn, the low-temperature-resistant steel pipe 3-1 is led into the main borehole 3, the front portion of the low-temperature-resistant steel pipe 3-1 is the floral pipe 2 with a length of 1 m to 3 m, the front portion of the floral pipe is sealed, thereby being convenient for transporting the medium water and liquid nitrogen in all directions; the low-temperature-resistant steel pipe 3-1 is provided with a pressure measuring port, and the pressure measuring port is connected with the high-pressure pressure gauge 13; the well-prepared high-pressure borehole sealing material slurry fills a gap between the low-temperature-resistant steel pipe 3-1 and the main borehole 3 through a grouting pump so as to implement the conventional high-pressure hole sealing, and the length H of the grouting hole sealing section 4 is 15 m to 25 m; two temperature measuring holes 9 are symmetrically formed at two sides of the low-temperature-resistant steel pipe 3-1, the distance from the centers of the two temperature measuring holes 9 to the center of the main borehole 30 m to 50 m, and an area between the two temperature measuring holes 9 is a coal-bed fracturing permeability improvement area; a temperature sensor 9-2 is arranged in each temperature measuring hole 9, and each temperature sensor 9-2 is connected with a digital display temperature instrument 9-5 disposed outside porthole by leading out a conducting wire; the inlet section of each temperature measuring hole 9 is provided with the sensor casing pipe 9-3 fixed by the temperature measuring hole sealing section 9-4; the temperature in the borehole temperature measuring area 9-1 is monitored in real time by pushing and pulling the temperature sensor 9-2 in the sensor casing pipe 9-3, and the length of the borehole temperature measuring area 9-1 in the coal bed 7 is 5 m to 10 m; water is injected into the low-temperature-resistant steel pipe 3-1 through the water injection device 8-1, the water injection pressure is controlled at 5 MPa to 10 MPa, after the water injection is ended, the main borehole water injection valve 5-1 is closed, and the injected water permeates the coal mass, remains in the coal mass along the six branch boreholes 1 and continuously flows into the micro cracks; water flows for 2 to 3 hours, the water injection valve 5-1 is removed, the low-temperature-resistant steel pipe 3-1 is connected with the liquid nitrogen tank car 8-2, the liquid nitrogen valve 5-2 is opened to fill the low-temperature-resistant steel pipe 3-1 with liquid nitrogen, the nitrogen injection pressure is controlled at 2 MPa to 8 MPa, when the average temperature of the borehole temperature measuring area 9-1 is monitored to be lower than 2 C. through the temperature sensor 9-2, the nitrogen injection is stopped, the coal mass is naturally thawed for 2 to 3 hours, and a freeze-thaw cycle of the phase change fracturing unit is completed; and in the process of filling the liquid nitrogen, when the pressure of the liquid nitrogen in the low-temperature-resistant steel pipe 3-1 is higher than 8 MPa, the liquid nitrogen valve 5-2 is closed, and when the pressure is lower than 2 MPa, the liquid nitrogen valve 5-2 is opened to continuously fill the liquid nitrogen. The conventional gas drainage borehole is implemented to the coal bed in the coal bed fracturing permeability improvement area so as to drain the gas. In the drainage process, according to the variation of the gas drainage effect, water and liquid nitrogen are repeatedly injected for multiple times into the coal bed, and the coal mass reaches a coal mass stress fatigue limit under the alternative effect of freezing-thawing-freezing in multiple freeze-thaw cycles and is fractured.

Embodiment II

(21) As shown in FIG. 5 and FIG. 7, performing the upstream directional borehole liquid nitrogen freeze-thaw permeability-improvement gas drainage and decompression for the penetrating layer of the low-level roadway 11 is basically the same with the embodiment I. The embodiment II differs from the embodiment I in that the freeze-thaw unit is implemented to the freeze-thaw permeability improvement area in the upper coal bed 7 from the penetrating layer of the low-level roadway 11, the depth of the main borehole penetrates through the rock layer to reach the coal bed 7, and according to the thickness of the coal bed, the main borehole shall penetrate into the coal bed for 10 m to 100 m. The remaining part is the same with the embodiment I and is omitted here.

(22) As shown in FIG. 6 and FIG. 7, performing the downstream directional borehole liquid nitrogen freeze-thaw permeability-improvement gas drainage and decompression for the penetrating layer of the high-level roadway 12 is basically the same with the embodiment I. The embodiment III differs from the embodiment I in that the freeze-thaw unit is implemented to the freeze-thaw permeability improvement area in the lower coal bed 7 from the penetrating layer of the high-level roadway 12, the depth of the main borehole penetrates through the rock layer to reach the coal bed 7, and according to the thickness of the coal bed, the main borehole shall penetrate into the coal bed for 10 m to 100 m. The remaining part is the same with the embodiment I and is omitted here.

Embodiment III

(23) As shown in FIG. 6 and FIG. 7, performing the downstream directional borehole liquid nitrogen freeze-thaw permeability-improvement gas drainage and decompression for the penetrating layer of the high-level roadway 12 is basically the same with the embodiment I.

(24) The embodiment III differs from the embodiment I in that the freeze-thaw unit is implemented to the freeze-thaw permeability improvement area in the lower coal bed 7 from the penetrating layer of the high-level roadway 12, the depth of the main borehole penetrates through the rock layer to reach the coal bed 7, and according to the thickness of the coal bed, the main borehole shall penetrate into the coal bed for 10 m to 100 m. The remaining part is the same with the embodiment I and is omitted here.