Pore Structure Sensitivity to External Fluids for Coal Reservoir and Fracturing Initiation/Propagation Mechanism by Using the T-1 similar to T-2 NMR Spectrum and Physical Simulation Experiment

Fracture initiation expansion and external fluid sensitivity evaluation have become the main focus of the coal reservoir fracturing effect. Therefore, a study on fracture propagation of hydraulic fracturing and the coupling mechanism between the fracturing fluid and pore structure of coal reservoirs need to be studied. In this paper, fracture conductivity and external fluid sensitivity evaluation were carried out. First, an automatic reservoir fracturing simulation tester and a large-scale computed tomography scanner are used to compare the fracture distribution and calcite content and evaluate the fracturing effects of different fracturing fluids and construction parameters. Second, the variation of porosity and permeability of coal before and after the injection of different external fluids are measured by an overlying pressure porosimeter, high-pressure mercury injection, and two-dimensional nuclear magnetic resonance. The results are as follows. (1) The T-1 similar to T-2 spectra of dried and saturated water samples indicate adsorbed water and mobile water in the adsorption pore, while mobile water mainly exists in the seepage pore and fracture. Different occurrence states of water have different T-1 similar to T-2 spectrum ranges. (2) The experiments of external fluid injection show that different external fluids have different effects on the porosity of the adsorption pore, seepage pore, and fracture. High-viscosity slick water and the acidizing fluid harm the seepage pore porosity, while low-viscosity tricky water and active water improve it. Fracture porosity is not sensitive to the guanidine gel-breaking fluid. (3) Experiments of fracturing initiation show that acid fracturing can reduce the fracture pressure and cumulative injection by 6.9 and 33.9%, respectively. The reaction of the acidizing fluid with carbonate mineral components can significantly increase the acid dissolution ratio and porosity of calcite, which forms the complex fractures and contributes to the increase of fracturing reconstruction volume.