Experimental study on influence of simulated frac hits on shale gas mobility

This study aims to reveal the microscopic migration process of frac hit fluid after invading the matrix and its differential influencing mechanisms on fluid mobility in pores of different scales. Core samples from the main production layer of marine deep formation in Dingshan of southeastern Sichuan were taken as the research objects. Combined with nuclear magnetic resonance (NMR) technology, systematic physical simulation experiments of depletion exploration under conditions of no frac hit, low-pressure (10 MPa) frac hit, and high-pressure (20 MPa) frac hit were conducted to compare and analyze the overall recovery degree and pore-scale mobilization characteristics under different conditions. The results showed that under conditions of no frac hit, pores in the 10 to 100 nm range were the primary contributing range to the total gas production (contributing 40% to 60%), while pores in the 100 nm to 1 μm range had the highest gas mobilization efficiency (80% to 100%), but their low pore volume fraction limited their contribution to the total gas production. Continuously increasing the production pressure difference was crucial for effectively mobilizing gas in micro-nano pores (1 to 100 nm). Frac hit fluid invasion significantly damaged the ultimate recovery factor ofgas reservoirs (reduction of 4.1% to 32.1%). The higher the simulated bottom-hole flowing pressure (representing more abundant reservoir energy) when frac hit occurred, the more severe the damage (the reduction under 20 MPa frac hit was 3 to 4 times that of 10 MPa frac hit) and the earlier the damage effect appeared. The decline in recovery factor primarily stemmed from impaired gas mobility within the < 1 μm pore system, and the dominant damaged pore range was controlled by the reservoir's microscopic pore structure and pore-throat connectivity. This study, for the first time, quantitatively reveals the damage mechanisms and extent of damage caused by frac hits on shale gas mobilization capacity at the pore scale, aiming to provide a basis for accurately assessing frac hit risks in the field, optimizing the timing and parameters of infill well fracturing, formulating differentiated frac hit prevention and control measures, and developing recovery plans for the production capacity of old wells.