Effect of pressures on imbibition in shales using the Nuclear Magnetic Resonance

Abstract:

Imbibition is one of the main mechanisms after the process of hydraulic fracturing and plays a significant role in hydrocarbon recovery in shale gas reservoirs. Spontaneous imbibition, normally carried out under atmospheric pressure conditions, is a common method studying imbibition. However, in-situ imbibition process is affected by pressures. Therefore, we conduct two kinds of imbibition experiments in two shale samples of Sichuan Basin using the nuclear magnetic resonance (NMR). One is performed increasing the differential pressure, ∆p(=p_in-p_out, where p_in is the inlet pore pressure and p_out is the outlet pore pressure, here equals to atmospheric pressure), from 0.25 to 1MPa at constant confining pressure p_c=8MPa; Another one is run decreasing p_c from 8 to 0 MPa at constant ∆p=0MPa. The results show that the peak values of T₂ curves gradually increase and the two wings of T₂ curves gradually expand with the increase of ∆p at constant p_c. At constant ∆p, T₂ curves slightly expands and changes from a single peak to bimodal ones with the decrease of p_c when p_c is not less than 3MPa; But when the confining pressure is unloaded (i.e., no p_c), which is similar to the conventional spontaneous imbibition experiment, T₂ curve dramatically expands and behaves more obvious bimodal characteristic, indicating that a new pore-fracture system has been formed, which is observed in our SEM experiment. It is found that the imbibition characteristics in our samples are controlled by the differential pressure and the effective pressure, but the change of the pore-fracture system in imbibition experiments loaded confining pressure is less obvious than the ones in the previous results. So, the effect of pressures on imbibition in shale should be considered more in future study and application.