Effects of Rock Texture on Surfactant-Assisted Spontaneous Imbibition: An Experimental and Stochastic Study in Tight Liquid-Rich Reservoirs

Abstract

The addition of surfactants to completion fluids in ULRs is widely believed to boost productivity and prolong well life. Previous laboratory studies have revealed that this approach is capillary-driven via spontaneous imbibition. This is a result of the interaction of surfactant solutions with the oil/water/rock interface. Nevertheless, published works regarding the effect of pore-size distribution (PSD) and the textural characteristics of the rock on the mechanism of surfactant-assisted spontaneous imbibition (SASI) are scarce at most. The purpose of this research is to provide a complete workflow for assessing the effectiveness of SASI and unveil a target pore size range for SASI EOR through a combination of experimental results, computed tomography (CT), scanning electron microscope (SEM), and nuclear magnetic resonance (NMR). In addition, interfacial tension (IFT), contact angle (CA), zeta potential, surfactant adsorption isotherm, and spontaneous imbibition experiments were all performed as part of the initial data gathering process. Ten SASI experiments were conducted at reservoir temperature using different surfactants on quartz- and carbonate-rich side-wall core samples obtained from the Wolfcamp formation in the Midland Basin. CT-scan technology was used to visualize the process of oil expulsion from the core plugs and assess fluid movement throughout the imbibition process. SEM was used to match the NMR PSD and obtain the surface relaxivity conversion factor for each core sample. This conversion factor was used to find the equivalent spherical radius (ESR) at each relaxation time. The target pore size range and fluid distribution for SASI EOR in the Wolfcamp formation was determined from the NMR results. The primary SASI EOR production mechanism is highly influenced by wettability alteration and IFT reduction. The SASI experiments showed optimistic oil recovery results in both quartz- and carbonate-rich core samples, with up to 36% and 40% of the original oil in place (OOIP), respectively. Measurements of the adsorption isotherm showed a positive correlation between the wettability alteration performance and amount of surfactant adsorbed onto the rock surface. The NMR results revealed that PSD plays a significant role in SASI EOR; the majority of the imbibed fluid was observed in smaller pores larger than those of clay-bound water (CBW) but smaller than those of high permeability streaks or bedding planes. Consideration of PSD has a significant impact on successful surfactant selection and proper EOR process design. CT scanning was also used to validate the NMR results, which revealed a direct relationship between CT imaging and NMR outcomes. The novelty of this research comes from the insight it offers into the essential role of PSD in SASI EOR through the use of CT, SEM, and NMR technologies. In addition, a new workflow for surfactant selection is proposed that unveils the real potential of SASI in low and ultra-low permeability clastic and carbonate reservoirs.