Experimental NMR Analysis of Oil and Water Imbibition during Fracturing in Longmaxi Shale, SE Sichuan Basin

The decrease in fracturing fluid flowback rate in the process of hydraulic fracturing and the improvement of initial gas production rate after well shut-in for a period of time were investigated using weight and nuclear magnetic resonance (NMR) T-2 spectra of dried shale cores and shale cores in the "as received" state after spontaneous oil and water imbibition. Experimental results indicated that the rate and weight during spontaneous water imbibition were much higher than those of spontaneous oil imbibition, contrary to capillary-driven imbibition models. Therefore, spontaneous water imbibition was related to both capillary pressure and additional hydration stress generated by clay swelling. NMR T-2 spectra of different imbibition fluids (oil and water) demonstrated that signals between 0.01 ms and 1 ms were total responses of pore fluids and irreducible water, and excess imbibed water was mainly present as irreducible water in shale. Excess imbibed water flowed into the matrix and reacted with minerals to generate many micro-fractures which may connect isolated organic pores to mitigate water blocking.