Optimization Of Sealing Material And Sealing Length For Hydraulic Fracturing Borehole In Underground Coal Mines

In recent years, hydraulic fracturing technology has been initially applied and achieved good results for coal gas mining. Hydraulic fracturing borehole sealing plays a key role in hydraulic fracturing technology, which directly related to the subsequent fracturing measures. However, sealing material used in hydraulic fracturing borehole in underground coalmines have some shortages, such as shrinkage, poor sealing effect, high cost and unreasonable sealing length, which lead to the sealing adhesive quality on sealing material and fracturing tube. Besides, coal and rock is difficult to guarantee that borehole and the surrounding fissure zone leak, impacting the effect on increasing the permeability of outburst coal seam. Therefore, researching the optimization of the sealing material and sealing length for hydraulic fracturing borehole in underground coal mines has a significant meaning in improving the sealing quality and sealing efficiency, reducing production costs and avoiding sealing blindness. It will do some good for coal gas mining and coal mine production safety.By integrating theory with experiments in this paper, a mechanical model of hydraulic fracturing borehole sealing has been established. The influence of interfacial shear scale factor, mechanical properties of material, sealing parameter(The hole diameter and sealing length) on the maximum water pressure that can be bore by sealing material is analyzed. Through indoor laboratory experiments, a sealing material which is both high-performance and economical is optimized. Then, a computational model for hydraulic fracturing borehole sealing length was established. At last, the result verified by a site experiment. The main conclusions are stated as follows:① In accordance with the principle of hydraulic fracturing borehole sealing, a mechanical model for borehole sealing was established, which lead to the relationship between the maximum water pressure that can be bore by sealing material, interfacial shear scale factor, mechanical properties of material and sealing parameter. The maximum water pressure that can be bore by sealing material amplifies with the increase of the strength, elasticity modulus of sealing material and sealing length until a constant value is nearly attained when the sealing length reaches a certain point.② To meet the requirements for sealing material by hydraulic fracturing borehole sealing, a new material composed of cement, early strength water-reduced agent, polypropylene fiber is developed. By shrinkage rate and compressive strength tests, the optimal ratio of the new material optimized. High pressure seal performance test and synthetic indexes show the material are both high-performance and economical, making it ideal for hydraulic fracturing borehole sealing in underground coal mines. According to the theoretical results, the rational sealing length of the hydraulic fracturing borehole is analyzed. A computational model for rational sealing length was established under the conditions of sealing, extraction and constructions to guide the reasonable sealing length in the field.③ The sealing process of hydraulic fracturing in coal mine was developed and the rational sealing length was calculated by the computational model in Chongqing coalmine. Field experiment results show that the strength as well as the bearing capacity of the new sealing material met the requirements. Meanwhile, for the sealing length by the computational model, the sealing effect proves good without abnormalities, which verifies the engineering applicability of the computational model.