ZHANG Suian, CAO Lihu, TAN Yangjun, LIU Yan, WANG Wei, HUANG Zhaoxin
The successful development of coalbed methane in China is essential to national energy security, coal mining safety and the environment. Creating high conductivity fracture within coal matrix using hydrofracture has been regarded as one of the most effective technologies for coalbed methane development. As a complicated procedure, however, the ability to create fractures with high conductivity is significantly influenced by flowing back, smashing and embedding of the proppant, which can be well represented by the operations in the study area Qinshui basin. The proppant is easily embedded into the surface around the fracture during hydrofracture in highly matured soft coal in Qinshui basin. Hence, conductivity of the generated fracture is dramatically reduced as a result of narrow fracture with embedded proppant. Currently, studies on proppant embedding are mainly conducted in physical experiments, lacking the corresponding numerical models. To analyze the influence of closure pressure and Protodyakonov coefficient on proppant embedding in a numerical method, width formulas and embedded pressure formulas for coal were deduced based on Langmuir law. Experiments for examining the model have been conducted as well. The results show that there is a critical pressure during insertion of the proppant into the coal surface. The smaller the protodyakonov coefficient is, the lower the critical pressure becomes, and the easier the insertion of the proppant is. When the closure pressure was lower than 30 MPa, and only half or even less of the proppant was inserted into the coal, relatively high similarity was obtained between the calculated results from the numerical model and the experimental results. Thereby, the deduced width formulas and embedded pressure formulas are valuable in the hydrofracture design for the high rank coal in Qinshui Basin.
