Inversion calculation method of coal-bed gas parameters of fast test while-drilling

Abstract

The present invention provides an inversion calculation method of coal-bed gas parameters of fast test while-drilling. The technical solution is that an inversion calculation method of coal-bed gas parameters of fast test while-drilling includes: during drilling in a coal bed, testing a gas flow and a gas concentration of an orifice in real time while-drilling, calculating drilling gas discharge amounts of the orifice, inversely calculating a coal-bed gas pressure at a drill bit based on borehole and coal-bed permeability parameters, and calculating a coal-bed gas content according to a gas content and gas pressure relational expression. The present invention has the beneficial effects that the present invention does not occupy the drilling and drill rod replacement time, is accurate, convenient, real-time and fast, and can test and calculate the coal-bed gas parameters of each section along the whole borehole length.

Claims

1. An inversion calculation method of coal-bed gas parameters of fast test while-drilling, comprising: during drilling in a coal bed, testing a gas flow and a gas concentration of an orifice in real time while-drilling, calculating a real-time drilling gas discharge amount and an average drilling gas discharge amount of the orifice, inversely calculating a coal-bed gas pressure at a drill bit based on borehole and coal-bed permeability parameters, and calculating a coal-bed gas content according to a gas content and gas pressure relational expression.

2. The inversion calculation method of coal-bed gas parameters of fast test while-drilling according to claim 1, wherein the inversion calculation method specifically comprises the following steps: a. during drilling of a coal-bed borehole or a coal-passing borehole, mounting a blowout prevention device or an orifice quick sealing device at an orifice section, and connecting a comprehensive gas parameter tester and a drainage pipeline connected with a drainage system to an extraction opening of the blowout prevention device or the orifice quick sealing device; b. connecting the drill bit to a drill rod, and starting to drill after the drill bit passes through the blowout prevention device or the orifice quick sealing device; c. during drilling of the coal bed, recording a coal appearing time and position, automatically recording the gas flow and the gas concentration by the comprehensive gas parameter tester, and calculating the real-time drilling gas discharge amount and the average drilling gas discharge amount; d. during drilling of the boreholes, automatically calculating a coal-bed gas pressure of a test section by formulated ground monitoring and analysis software according to input drilling parameters, the coal-bed permeability, and an average drilling gas flow, and calculating the coal-bed gas content according to a coal adsorption constant and environmental parameters; e. during drilling of a main borehole and branch boreholes by a directional drilling machine, automatically calculating the coal-bed gas pressure of the test section by the formulated ground monitoring and analysis software according to the input drilling parameters, an exposure time of each coal section, the coal-bed permeability, and the average drilling gas discharge amount, and calculating the coal-bed gas content according to the coal adsorption constant and the environmental parameters; f. predicting an outburst risk of each section of the coal bed according to parameters of the coal-bed gas pressure and the coal-bed gas content of each section; and g. in the process of drilling the coal-bed borehole or after the drilling ends, stopping drilling, closing a slag outlet, automatically recording a gas flow and a gas concentration within each time period by the comprehensive gas parameter tester, and calculating a natural gas discharge velocity of the borehole by the foiiiiulated ground monitoring and analysis software, automatically calculating a penetrability coefficient and a permeability of the coal bed at the section, and correcting the calculated coal-bed gas content or pressure parameter.

3. The inversion calculation method of coal-bed gas parameters of fast test while-drilling according to claim 2, wherein the inversion calculation method of the coal-bed gas pressure specifically comprises: in a drilling process of the drilling machine, recording in real time the gas flow and the gas concentration of the orifice and the real-time drilling gas discharge amount in the drilling process by the comprehensive gas parameter tester at the orifice, calculating the average drilling gas discharge amount, and inverting gas feature parameters of different positions of the coal bed according to the average drilling gas discharge amount, wherein a total amount of gas drained by the gas drainage system at the orifice is composed of three portions, comprising a gas amount released from a borehole wall newly formed in the coal-bed drilling process of the drilling machine, a gas amount released by drilling cuttings peeled off from the borehole wall, and a gas amount released from the borehole wall before a new borehole wall is formed; and the coal-bed gas pressure at the drill bit in the drilling process is: $\begin{array}{cc}\mathrm{pi}=\sqrt{\frac{\begin{array}{c}{Q}_{\mathrm{total}}-V_{\mathrm{drill}\mathrm{bit}}\Delta {\mathrm{tS}}_{\mathrm{section}}\gamma {\int }_0^{\frac{l_{\mathrm{rock}}+l_{\mathrm{coal}}}{v}}{Q}_0{e}^{-B_{1}t}\mathrm{dt}-\\ \underset{1}{\overset{n-1}{.Math.}}{\int }_0^{V_{\mathrm{drill}{\mathrm{bit}}^{\Delta t}}}{\int }_{t_{i-1}}^{t_n}{q}_i{e}^{-B_{2}t}\mathrm{dtdl}\end{array}}{-\frac{k}{2\mu p_n}{\int }_0^{V_{\mathrm{drill}{\mathrm{bit}}^{\Delta t}}}{\int }_{t_{i-1}}^{t_n}{e}^{-B_{2}t}\mathrm{dtdl}}},& \left(1\right)\end{array}$ wherein pi is the coal-bed gas pressure of a calculation point; Q.sub.total is the total gas discharge amount measured in a calculation section; t.sub.0 is the first coal appearing time; t.sub.1, t.sub.2, . . . , t.sub.n are selected time points for calculating the coal-bed gas parameters, Δt=t.sub.n−t.sub.n−1; Q.sub.0 is a drilling cutting gas discharge intensity at an initial exposure moment, m.sup.3/t.Math.min; β.sub.1 is a drilling cutting gas attenuation coefficient, min.sup.−1; v is a water flow velocity, m/s; V.sub.drillbit is a borehole drilling speed, m/s; l.sub.rock and l.sub.cool are a length of a formed rock borehole and a length of a formed coal-bed borehole, m; S.sub.section is a cross-sectional area of the borehole, m.sup.2; γ is a coal bulk density, kg/m.sup.3; q.sub.i is a gas discharge amount on a coal wall per unit area, m.sup.3/m.sup.2.Math.min; β.sub.2 is a borewall gas attenuation coefficient, min.sup.−1; k is the coal-bed permeability, m.sup.2; μ is a dynamic viscosity coefficient of gas, Pa.Math.s; p.sub.n is an absolute pressure of gas drainage Pa; R.sub.M is an effective influence radius around the borehole, m.

4. The inversion calculation method of coal-bed gas parameters of fast test while-drilling according to claim 3, wherein to calculate a gas pressure of an i.sup.thcoal hole section, gas pressures of the previous (i−1) coal hole sections are calculated at first; since the gas pressure of each branch hole is different, q.sub.i is also different; the gas pressure of any coal hole section be calculated according to the above formula (1), and q.sub.i is calculated according to the formula: $\begin{array}{cc}q=-\frac{k}{2\mu p_n}\frac{\partial p^2}{\partial x};& \left(2\right)\end{array}$ the drilling cutting gas attenuation coefficient β.sub.1 and the borehole wall gas attenuation coefficient β.sub.2 may be measured by experiments and field tests; and the coal-bed gas content X.sub.mi may be calculated through the gas content and gas pressure relational expression according to a coal-bed gas adsorption constant and the environmental parameters.

5. The inversion calculation method of coal-bed gas parameters of fast test while-drilling according to claim 1, wherein the gas flow and the gas concentration of the orifice of the borehole are tested in real time while-drilling; a real-time gas discharge amount of the orifice of the borehole is calculated by a comprehensive gas parameter tester and a drainage system, and then the average drilling gas discharge amount is calculated; and a time interval is time corresponding to a borehole drilling distance of 2 to 5 m.

6. The inversion calculation method of coal-bed gas parameters of fast test while-drilling according to claim 3, wherein in the step d and the step e, corresponding actually measured coal-bed permeability parameters are used for different drilling operations; and when no actually measured coal-bed permeability value is present, an original coal bed may use an original coal-bed permeability value of a coal bed in this region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of arrangement of boreholes subjected to directional drilling in an air way drilling field of a certain coal and gas outburst mine 12171 according to an embodiment of the present invention;

(2) FIG. 2 is a schematic diagram of distribution and change of coal-bed gas parameters in a lengthwise direction of a borehole and a comparison result with an actually measured value of a coal-bed gas content according to an embodiment of the present invention; and

(3) FIG. 3 is a schematic diagram of distribution and change of coal-bed gas parameters in a lengthwise direction of a borehole and a comparison result with an actually measured value of a coal-bed gas content according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) To clearly describe the technical characteristics of the present invention, the following describes the present invention through specific implementations.

(5) The present invention provides an inversion calculation method of coal-bed gas parameters of fast test while-drilling, specifically including: during drilling in a coal bed, a gas flow and a gas concentration of an orifice of a borehole are tested in real time while-drilling, a real-time drilling gas discharge amount and an average drilling gas discharge amount of the orifice are calculated, a coal-bed gas pressure at a drill bit is calculated based on borehole and coal-bed permeability parameters, and a coal-bed gas content is calculated according to a gas content and gas pressure relational expression.

(6) The method specifically includes the following steps.

(7) a. During drilling of a coal-bed borehole or a coal-passing borehole, a blowout prevention device or an orifice quick sealing device is mounted at an orifice section, and a comprehensive gas parameter tester and a drainage pipeline connected with a drainage system are connected to an extraction opening of the blowout prevention device or the orifice quick sealing device.

(8) b. The drill bit is connected to a drill rod, and drilling starts to be carried out after the drill bit passes through the blowout prevention device or the orifice quick sealing device.

(9) c. During drilling of the coal bed, a coal appearing time and position are recorded, and the comprehensive gas parameter tester automatically records the gas flow and the gas concentration, thus calculating the real-time drilling gas discharge amount and the average drilling gas discharge amount.

(10) d. During drilling of the boreholes, a coal-bed gas pressure of a test section of a drilled position of the drill bit is automatically calculated by formulated ground monitoring and analysis software according to input drilling parameters, the coal-bed permeability, and an average drilling gas flow, and the coal-bed gas content is calculated according to a coal adsorption constant and environmental parameters.

(11) e. During drilling of a main borehole and branch boreholes by a directional drilling machine, a coal-bed gas pressure of the test section is automatically calculated by the formulated ground monitoring and analysis calculation software according to the input drilling parameters, an exposure time of each coal section of each hole, the coal-bed permeability, and the average drilling gas flow, and the coal-bed gas content is calculated according to the coal adsorption constant and the environmental parameters.

(12) f. The outburst risk of each section of the coal bed is predicted according to the parameters of the coal-bed gas pressure and the coal-bed gas content.

(13) g. In the process of drilling the coal-bed borehole or after the drilling ends, drilling is stopped, a slag outlet is closed, the comprehensive gas parameter tester automatically records a gas flow and a gas concentration of the borehole within each time period, and the formulated ground monitoring and analysis software calculates a natural gas discharge velocity of the borehole, thus calculating a penetrability coefficient and a permeability of the coal bed at the section, and correcting the calculated coal-bed gas content or pressure parameter.

(14) The inversion calculation method of the coal-bed gas pressure specifically includes: in the drilling process of the drilling machine, the comprehensive gas parameter tester at the orifice records in real time the gas flow and the gas concentration of the orifice and the real-time drilling gas discharge amount in the drilling process, the average drilling gas discharge amount is calculated, and gas feature parameters of different positions of the coal bed are inverted according to the average drilling gas discharge amount. The total amount of gas drained by the gas drainage system at the orifice is composed of three portions, including a gas amount released from a borehole wall newly formed in the coal-bed drilling process of the drilling machine, a gas amount released by drilling cuttings peeled off from the borehole wall, and a gas amount released from the borehole wall before a new borehole wall is formed. The coal-bed gas pressure at the drill bit in the drilling process is:

(15) $\begin{array}{cc}\mathrm{pi}=\sqrt{\frac{\begin{array}{c}{Q}_{\mathrm{total}}-V_{\mathrm{drill}\mathrm{bit}}\Delta {\mathrm{tS}}_{\mathrm{section}}\gamma {\int }_0^{\frac{l_{\mathrm{rock}}+l_{\mathrm{coal}}}{v}}{Q}_0{e}^{-B_{1}t}\mathrm{dt}-\\ \underset{1}{\overset{n-1}{.Math.}}{\int }_0^{V_{\mathrm{drill}{\mathrm{bit}}^{\Delta t}}}{\int }_{t_{i-1}}^{t_n}{q}_i{e}^{-B_{2}t}\mathrm{dtdl}\end{array}}{-\frac{k}{2\mu p_n}{\int }_0^{V_{\mathrm{drill}{\mathrm{bit}}^{\Delta t}}}{\int }_{t_{i-1}}^{t_n}{e}^{-B_{2}t}\mathrm{dtdl}}}.& \left(5\right)\end{array}$
pi is the coal-bed gas pressure of a calculation point. Q.sub.total is the total gas discharge amount measured in a calculation section. t.sub.0 is the first coal appearing time. t.sub.1, t.sub.2, . . . , t.sub.n are selected time points for calculating the coal-bed gas parameters, Δt=t.sub.n−t.sub.n-1. Q.sub.0 is the drilling cutting gas discharge intensity at the initial exposure moment, m.sup.3/t.Math.min. β.sub.1 is a drilling cutting gas attenuation coefficient, min.sup.−1. v is a water flow velocity, m/s. V.sub.drillbit is a borehole drilling speed, m/s. l.sub.rock and l.sub.coal are the length of a formed rock borehole and the length of a formed coal-bed borehole, m. S.sub.section is a cross-sectional area of the borehole, m.sup.2. γ is a coal bulk density, kg/m.sup.3. q.sub.i is a gas discharge amount on a coal wall per unit area, m.sup.3/m.sup.2.Math.min. β.sub.2 is a borewall gas attenuation coefficient, min.sup.−1. k is the coal-bed permeability, m.sup.2. μ is a dynamic viscosity coefficient of gas, Pa.Math.s. p.sub.n is an absolute pressure of gas drainage Pa. R.sub.M is an effective influence radius around the borehole, m.
To calculate the gas pressure of an i.sup.th coal hole section, it is necessary to calculate the gas pressures of the previous (i−1) coal hole sections. Since the gas pressure of each branch hole is different, q.sub.i is also different. The gas pressure of any coal hole section may be calculated according to the above formula (5), and q.sub.i is calculated according to the formula:

(16) $\begin{array}{cc}q=-\frac{k}{2\mu p_n}\frac{\partial p^2}{\partial x}.& \left(6\right)\end{array}$

(17) The drilling cutting gas attenuation coefficient β.sub.1 and the borehole wall gas attenuation coefficient β.sub.2 may be measured by experiments and field tests.

(18) The coal-bed gas content X.sub.mi may be calculated through the gas content and gas pressure relational expression according to a coal-bed gas adsorption constant and the environmental parameters.

(19) The gas flow and the gas concentration of the orifice of the borehole are tested in real time while-drilling. The real-time gas discharge amount of the orifice of the borehole is calculated by the comprehensive gas parameter tester and the drainage system, and then the average drilling gas discharge amount is calculated. A time interval is time corresponding to a borehole drilling distance of 2 to 5 m.

(20) The step e specifically includes: the coal-bed gas parameters are respectively calculated from a coal appearing point section by section; during drilling of the main borehole and the branch boreholes by the directional drilling machine, the ground monitoring and analysis software automatically calculates the coal-bed gas pressure of the test section according to the input drilling parameters, the exposure time of each coal section of each hole, the coal-bed permeability, and the average drilling gas discharge amount, and the coal-bed gas content is calculated according to the coal adsorption constant and the environmental parameters.

(21) In the step d and the step e, corresponding actually measured coal-bed permeability parameters are used for different drilling operations. When no actually measured coal-bed permeability values are present, an original coal bed may use an original coal-bed permeability value of a coal bed in this region.

(22) A specific example of the test by using the inversion calculation method of the coal-bed gas parameters of fast test while-drilling is specifically as follows.

(23) Directional drilling is performed in an air way drilling field of a certain coal and gas outburst mine 12171. The drilling arrangement is shown in FIG. 1. The ZDY120000LD type crawler full-hydraulic tunnel drilling machine for a coal mine is used for drilling. Before drilling, a blowout prevention device is mounted at an orifice section, and a CGWZ-100 (C) pipeline laser comprehensive gas parameter tester and a drainage pipeline connected with the drainage system is connected to an extraction opening of the blowout prevention device. During the drilling, the coal appearing time and position are recorded. The CGWZ-100 (C) pipeline laser comprehensive gas parameter tester automatically records the gas flow and the gas concentration. The real-time drilling gas discharge amount and the average drilling gas discharge amount are calculated. The formulated ground monitoring and analysis software automatically calculates the coal-bed gas pressures and the coal-bed gas contents of the test sections of a borehole No. 3 and a borehole No. 4 at the hole depth of 100 m to 300 m according to the input drilling parameters, the coal-bed permeability, and the average drilling gas flow. The coal-bed gas contents reflect the distribution and changes of the coal-bed gas parameters in the lengthwise direction of the boreholes, and are compared with an actually measured value of the coal-bed gas content, as shown in FIG. 2 and FIG. 3. According to data comparison results, a difference between the coal-bed gas content tested while-drilling and the actually measured coal-bed gas content is 1.3% to 4.13%, which is less than 5%, and may fully meet the actual application needs on site. Under normal circumstances, a region with the coal-bed gas content that is greater than 8 m.sup.3/t or the gas pressure that is greater than 0.74 MPa is a coal-bed outburst danger region.

(24) The technical features of the present invention that are not described may be implemented by using the existing technology, and are not described herein again. Certainly, the foregoing descriptions are not intended to limit the present invention, and the present invention is not limited to the foregoing examples. Changes, modifications, additions or replacements made by a person of ordinary skill in the art within the essential scope of the present invention shall fall within the protection scope of the present invention.