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

Abstract

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) T2 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 T2 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.

A Critical Review of Osmosis-Associated Imbibition in Unconventional Formations

“…In shale, the sample with higher clay content could imbibe more volume and at a faster rate than the sample with lower clay content. This was later confirmed in other experimental measurements, such as NMR [87], and the excessive imbibed water beyond capillary-driven water remains as irreducible water in the clay of shale. In addition, the imbibition of fluid with additives was also different in various clay content shale samples.…”

NMR Investigation of Brine Imbibition Dynamics in Pores of Tight Sandstones under Different Boundary Conditions

“…The relationship between the imbibition volume and time is given by Handy's equation (eq 10), which assumes a piston-like nonwetting phase displacement 10,15,30 i k j j j j j j y…”

“…Similar zones have also been observed elsewhere. 10,15 However, some variations in the imbibition profiles were observed because of the different boundary conditions. It is noteworthy that the direction of the core sample does not have any influence on the imbibition profile because the sample is fully immersed in water.…”

“…The effect of boundary conditions on spontaneous imbibition in oil saturated sandstones was investigated by Lyu et al 14 They observed the highest imbibition recovery rate in samples subjected to allface-open (AFO) boundary conditions. Other studies on boundary conditions give more consideration to capillary imbibition than diffusion imbibition 10,15 and only a few studies have mentioned the influence of other forces such as diffusion, osmosis, or surface hydration. 8,16 A low flowback rate suggests that the overall imbibition process may also be greatly influenced by diffusion imbibition, which is slower because capillary imbibition mainly occurs at the early stage of the imbibition process while diffusion takes over as capillary forces decrease at the later stage of the imbibition process.…”

“…The effect of boundary conditions on spontaneous imbibition in oil saturated sandstones was investigated by Lyu et al They observed the highest imbibition recovery rate in samples subjected to all-face-open (AFO) boundary conditions. Other studies on boundary conditions give more consideration to capillary imbibition than diffusion imbibition , and only a few studies have mentioned the influence of other forces such as diffusion, osmosis, or surface hydration. , …”