Hydraulic fracturing theory for conditions of thermal stress
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Coal mine goaf drainage with surface boreholes is increasingly used in coal mines worldwide.However,poor performance of some of these surface gas boreholes is often experienced in terms of the gas concentration,flowrate and borehole stability.In order to improve performance,theoretical studies and field investigations were carried out to develop optimal surface borehole designs based on the experience from Huainan and Australia in recent years.Through the extensive numerical modelling of mining impact in the surrounding strata,and performance of various gas drainage configurations,and by applying the concept of the 3D annular-shaped overlying zone for optimal methane drainage,the trials using large diameter boreholes with diameters of 244.5 mm and 177.8 mm were designed and implemented.The trial results demonstrate that the large diameter boreholes perform significantly better than conventional boreholes in terms of total gas captured,longwall gas control effectiveness,and borehole stability.The total gas captured from a single large borehole can reach 3.167 million m3 with 1.7 million m3 of pure methane,and the borehole productive life is 106 days.The methane concentration in the longwall return gateroad is effectively controlled during the operation of the large boreholes.The trial results also show that the surface borehole located on the longwall return side performes better than that on the intake side.The total gas captured from the return side borehole is 33.7% more than that from the intake side and the methane concentration is 75.3% higher.
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Breakout
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Drilled boreholes generally are the only feasible means to access the subsurface for the<br>emplacement of downhole electrodes for most hole-hole and hole-surface resistivity<br>experiments. However, the very existence of the borehole itself creates the potential for<br>significant noise due to the inevitable conductivity contrast that develops between the borehole<br>walls and the formation. Borehole effects develop whenever a current source is placed in a<br>drilled borehole. Borehole geometries may range from nearly perfect cylinders to highly,<br>irregular, rugose holes in consolidated rock, to relatively minor, collapsed, disturbed zones in<br>caving sediments. Boreholes in non-caving formations generally are filled with artificial<br>materials to afford crucial, electrical continuity between downhole electrodes and the borehole<br>walls. Filled boreholes form cylindrically shaped heterogeneities that create significant noise<br>due to preferential current flow up and down the conductive columns. Selected conditions are<br>simulated with a finite difference model to illustrate the significance of borehole effects on holehole<br>and hole-surface mise-à-la-masse electrical potentials near a current electrode. Preliminary<br>modeled results of borehole effects for electrical resistance tomography (ERT) suggest the<br>existence of an inhomogeneity between the two boreholes which may cause misinterpretation of<br>the data, depending on the resistivity contrast of the borehole fill and earth material.
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Electrical Resistivity Tomography
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Drilling techniques of large diameter bend borehole which was the replacement of slant tunnel for gas discharge were researched in Yangquan.Total six test boreholes were drilled.The average deflecting intensity of bend borehole was up to 1.2°/m.The total depth of six boreholes was up to 378.55 m.The result comparison shown that the gas discharge effects were nearly the same by large diameter bend boreholes and tunnel.Therefore the possibility of using large diameter borehole to replace slant tunnel was possible.
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Borehole strain observation is an important mean in the study on the crustal movement,and plays an important role in earthquake precursor observation. This paper introduces characteristic of borehole strain observations, the latest research progress in borehole strain observation in developed countries such as Japan, the United states etc., provides nowadays the borehole strain observation equipments and their performances, and finally discusses China's supervision network of borehole deformation and experiment of deep borehole comprehensive observation system.
Strain (injury)
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ABSTRACT In the past, the doorstopper method has been used successfully for determining in situ tectonic stresses. Here an example is given about the application of the borehole slotter in the domain of tectonic stress determinations, which is an innovative strain relief technique without overcoring. Data are presented that test the reliability of the strain measurements by the borehole slotter under well‐known stress conditions, that indicate the reproducibility of borehole slotter strain data compared with those from doorstopper measurements, and that demonstrate the utility of the borehole slotter even for the determination of weak tectonic strains. In northern Switzerland the borehole slotter was used in a horizontal borehole under lithostatic loading conditions only, in a rock slope area with very heterogeneous stress conditions, and in a vertical borehole where tectonic stress should be dominant. The results show that the borehole slotter is a reliable technique, which allows precise determination of in situ strains. In addition, borehole slotter measurements are less time‐consuming and therefore less costly than most other stress‐measuring techniques—an important aspect in a future of reduced research budgets. Summary The borehole slotter is a strain relief technique, using a recoverable strain sensor that need not be glued to the rock surface and need not be overcored. It is quicker to use than most other stress or strain measuring techniques, especially in comparison to standard overcoring techniques (Fig. 14). The borehole slotter supplied convincing results with respect to principal stress orientations, in good agreement with: (i) principal stress orientations related to lithostatic stress conditions (Dittingen 2 site), (ii) palaeostress orientations during the main stage of Jura folding only 13 to 7 Ma (Laufen 1 site), (iii) the results of neighboring door‐stopper measurements (Dittingen 1 site). The borehole slotter tests do not supply stress magnitudes directly. Thus, the elastic properties of the rock mass have to be determined independently. Stress ratios from borehole dotter tests in Dittingen 2 are realistic. In conclusion, the borehole slotter is a fully developed probe to measure in situ strains in boreholes. It allows the reliable determination of a large number of data within a short time even in weakly stressed rocks. This makes the borehole slotter a powerful tool for the determination of contemporary regional tectonic stress fields.
Strain (injury)
Overburden pressure
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The borehole pressure relief is one of the pressure-relieving methods of rock burst prevention.The results of borehole pressure relief are under the impact of borehole depth,borehole diameter,borehole angle and its collocation.The paper makes numerical simulation study based on orthogonal test design and analyzes the simulation results with multi-parameter and equilibrium theory.And then the paper obtains the influence principles of borehole depth,borehole diameter and borehole angle on borehole pressure relief.The borehole with 133 mm diameter and the angle of up-borehole and low-borehole with 15 and 8 respectively have good effects.These results correspond with local mining condition.
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Measurement while drilling
Directional Drilling
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