Abstract
Low-permeability sandy conglomerate reservoirs are inevitably vulnerable to interaction with foreign fluids, which seriously affects oil recovery. Hydraulic fracturing, which is normally achieved by squeezing high-viscosity fracturing fluids into the reservoir, is the primary method to exploit low-permeability reservoirs. Although fracturing fluids can rupture rocks and produce fractures, they also cause formation damage. Therefore, it is critical and essential to explore the formation damage caused by fracturing fluids to protect the formation and enhance oil recovery. In this paper, two types of conventional fracturing fluids (i.e. slickwater and guanidine gum gelout) were used to study the mechanisms of pore-scale formation damage, which was assisted by the three-dimensional mineral distribution reconstruction based on in-situ micro-computed tomography. During the formation damage caused by slick water, gaps at pore edges were found to be corroded and closed. In contrast, during the guanidine gum gelout-caused formation damage, the formation of flocculent deposits led to the accumulation of particles in the pores. Porosity decreases are observed in all cores flooded with fracturing fluids and distilled water. Gray-scale values of minerals in the micro-computed tomography images were identified based on the mineral distribution map determined by scanning electron microscopy and energy-dispersive X-ray spectroscopy. These gray-scale values were compared with mineral contents obtained by X-ray diffraction. Eventually, a three-dimensional digital core was established based on mineral features, and clay minerals were demonstrated to be critical in the formation damage by blocking pores and thus separating large pores into small pores.
NSTL