Experimental study on unstable imbibition characteristics of fracturing fluids at high pressures and temperatures in the tight continental reservoir

Abstract

Enhanced oil recovery by imbibition (IEOR) is the key technology to effectively develop tight reservoirs with “three lows” characteristics. It can be valuable for practical development to investigate the process of fracturing fluid imbibition in steady and unsteady states and quantitatively assess the impact of fracturing fluid imbibition in various pores under various circumstances. By combining NMR technology with indoor physical simulation experiments, this paper quantitatively evaluates the differences in imbibition characteristics of various pores in different conditions under high temperature and high pressure. Further research is done into how the imbibition effect is affected by permeability, pore size, imbibition duration, steady-state, and unsteady-state (pressure change). The results indicate that micropore (0.1–1 ms) imbibition efficiency is the fastest at the early stages, though it initially tends to be stable. Throughout the process of unsteady imbibition, it typically becomes stable at 48 h. However, in the steady-state imbibition process, the stabilization time lags far behind and tends to be stable at 108 h. Additionally, the mesopore (1–10 ms) has a faster imbibition rate, which is the main contributor in the middle and late stages of imbibition. As well, imbibition occurs mainly in nanopores (0.01–0.1 ms) and micropores (0.1–1 ms), followed by mesopores (1–10 ms), and finally macropores (>10 ms). For permeability <0.1 × 10−3 μm2, micropores (0.1–1 ms) are the main contributors. For the permeability of 0.1–1 × 10−3 μm2, micropores (0.1–1 ms) and mesopores (1–10 ms) are the main contributors. And for permeability >1 × 10−3 μm2, the mesopores (1–10 ms) make up the majority of the contribution. In addition, Imbibition productivity is positively associated with permeability and differential pressure in both steady and unsteady imbibition. As differential pressure and permeability increase, imbibition occurs more effectively.