The "Point’’ Type Directional Hydrofracture Mechanism And Its Engineering Application

Gas extraction not only can eliminate mine gas disasters and reduce greenhouse gas emission, but also allows the gas to be utilized as high-quality energy. However, the coal layers of most mine areas in our country are poor in air permeability,so that the air permeability improvement technology becomes a bottleneck problem. Large-scale fracturing equipment is difficult to be used in a limited space under the shaft, and the crack propagation direction as well as scope of the single drilling fracturing arrangement are uncontrollable; various damage type hydrofracture mechanisms of coal still need further research; and the gas capacity prediction technology of the coal layer after the fracture and extraction also cannot satisfy the demands. If smaU-scale equipment is combined with a new acturing process to master and control the propagation dir*ection and scope of the crack,then the large-area popularization and application of the hydrofracture 1:echnology under the shaft becomes possthle.In the article, a new idea of "point" type diiectional hydrofracture is put forward. The systemic,safe, reliable and economic "point" type directional fracture process is formed bynrstly changing 1:he fracture functioning "surface" into a fracture functioning "point" in a multi-point and multi-implementation mode, and on the premise of complying with tihe crack propagation discipline, adopting control holes and auxiliary fracture holes to guide the ciack to propagate Awards a preset direction and then dekrmining fracturing construction parameters. Also the supporting equipment such as small-sized "point" type hydrofracture high-pressure water pump, and special hole packer are developed.The article researches crack initiation conditions by using the fracture mechanics method according to different 过amage types of coal body structure and mechanical property.By numerical simulation and analysis methods, the article researches the rules of effect of crustal stress, initial fissures, coal-rock interface and other natural factors on the direction of crack propagation,The article also researches the "broken rock zone" effect of drilling hole and applies the control hole based thereon to hydroacture for guiding crack propagation. In addition,it applies the mechanic mechanism of gradual propagation of the crack under t;he condition of pore pressure of the permeable coal to the hydrofracturing measures of coal layer, realizes directional fracUire through the auxiliary ciack anddescribes the "point" type fracturing as well as the fUndamentals of hydraulic loose.In view of vadose coupling,the article takes 4# coating layer of Jixi Chengshan Mine as the example to perform numerical experiments respectively on 1:he "point" type directional hydrofracture of bedding holes and coal layer penetrating holes by utilizing the vadose-injure coupling function analysis system RFPi\2Dw software and RPPA SD-flow software,simulate the crack propagation and fradure surface space form under the action of three-dimensional sthess and water pressure in three-dimensional space, and ftirther analyzes the mechanism of action and effect of the control hole and auxiliary fracture hole. It reveals the whole process ineluding initiation, devel pment and cut-through of the fracture of the coal layer,and researches the evolution law of the shearing stress,the hydraulic pressure and the water flow distribution.Fracturing paramsters are designed according to 1:he natural conditions of the coal seam and surrounding rock to make 円eld 1;est on"point" type directional fracturing process and ancillary equipment, and comparison of the single-hole fracturing, multi-hole fracturing(with control drilling hole). Water pressure and flow curve are analyzed to understand that the fluctuations in water pressure at crack propagation sl;age are caused by alternating expansion of initial fractures with filled inner surface and secondary fractures with unfilled inner surface,the fractured cohesion of Cw,the leaching coefficient and effective pressure value after infiltration are obtained by calculating the amount of fluctuation. The field test results have verified l:he correcUiess of l;he design of the numerical simulation and process parameters through comparison.An experiment place is selecl;ed,the gas parameters of the coal layer and the crucial stress paramster are measured, and a steld test fbr the directional hydrofi*actui*e of the drilling holes along with the coal layer is performed by combining with the laneway structural design, the drilling hole arrangement and the fracture paramethrs to analyze the curve change rule of the fracturing pressur*e and the flow. Diversity of the fracture system of the coal as well as the irregular dirt bands in the coal layer to cause difference of the hydraulic pressure of the same test place is obtained; it is discovered that the hydraulic piessure ucthation of the fracture s1;age is caused by alternate propagation of fracUire in initial fissure with lied inner surface and secondary interstice with unlied inner surface.The correctness of the numerical simulation and the technical parameters design are verified;and after the water infiltration,the cohesion among the fractures is calc山ated from Cw,seepage loss coefficient and effective hydraulic pressure value. The comparative experiment with a normal fracture proves that the "point" type directional fracthre time is short,the water flow is low and the construction Cost is low.Gas-contaiiiing coal petrography fluid-structure interaction(RFPA-GAS) model is utilized to perform gas extraction and discharge simulation after fracturing, and based on the results of the simulation,two tiew productivity prediction methods, iweighted quadratic surface integral method and negative exponential function statistic method, are proposed.After establishment of mathematical model and preparation of computer software, the prediction of productivity resulting ftom "point" COntrol fracturing measures is made,indicating that the e对raction rate is enlarged greatly due t:o fracturing measures. ReW observations indicate that the drop coal and the gas gushing amount on the coal mining face are significantly decreased,and the number of gas overrun in the test area drops th zero,showing that fracturing the "point" type directional control fracturing measures can enhance gas productivity of the coal layer and can effectively prevent gas accidents.