Adjustment of fracture network geometry during temporary plugging and diverting fracturing in deep coalbed methane reservoirs: An experimental study

Abstract

Deep coalbed methane reservoirs generally exhibit characteristics such as extremely low permeability, significant heterogeneity, high in situ stress, and dense geological discontinuities. Notably, these geological discontinuities cleats, bedding planes, and natural fractures, as mechanically weak planes, significantly contribute to the creation of extremely complex and tortuous hydraulic fracture (HF) networks near the wellbore, but impede the propagation of HFs to the far-field region. This will lead to insufficient stimulated reservoir volume, thereby limiting the CBM production. Under this background, a series of physical simulation experiments of temporary plugging and diverting fracturing (TPDF) were carried out on large-size coal blocks under true triaxial stress conditions. Combining high-energy industrial computed tomography scanning technology, first, the morphology of fracture propagation of the sample before TPDF is divided into two fracture propagation modes. Then, TPDF experiments were conducted to analyze the behavior of fracture propagation under different modes. Finally, a mode of TPDF tailored for adjusting the HF network geometry in deep CBM reservoirs was explored innovatively. The effects of concentration and particle size of the temporary plugging agent (TPA) on pressure increment, plugging location and fracture diversion behavior during TPDF were examined in particular. Experimental results indicate that optimizing the concentration and particle-size of TPA based on the resulting fracture geometry is crucial for adjusting the fracture network geometry (simplifying the growth behavior of HF near the wellbore while increasing fracture complexity in the far-field region) during conventional fracturing (before using the TPA). When a complex fracture network is created under the condition of formation with high-dense natural fractures (NFs) near the wellbore region, using small-particle-size TPA (e.g., 70/140 mesh) is optimal for adjusting the fracture geometry, as it can effectively plug the NFs and allow them to continue extending toward the far-field region. Meanwhile, a higher concentration of TPA is beneficial for plugging the interval of HF closer to the wellbore, and then causing the creation of complex fracture networks. When a long single HF is generated under the condition of a formation with low-dense NFs, using the medium-particle-size TPA (e.g., 40/70 mesh) is optimal for enhancing the fracture complexity near the wellbore region. Using TPA of excessively large particle sizes (e.g., 20/40 mesh) tends to plug the HFs at their entrances, causing HFs to be reinitiated from the unstimulated segment of the wellbore. This study can provide crucial theoretical guidance for optimizing the scheme design of TPDF in deep CBM reservoirs.