Experimental study of CO2 huff-n-puff in a tight conglomerate reservoir using true triaxial stress cell core fracturing and displacement system: A case study

To reveal the enhanced oil recovery (EOR) potential of CO2 huff-n-puff in fractured tight conglomerate reservoirs, hydraulic fracturing combined with CO2 huff-n-puff experiments were conducted using a true triaxial stress cell core fracturing and displacement system. To better understand the potential mechanisms behind CO2 huff-n-puff, the change in rock properties and formation water properties caused by the interactions among supercritical CO2/formation water/oil/rock were investigated. Experimental results demonstrate that CO2 huff-n-puff is viable to boost oil recovery in tight conglomerate reservoirs with recovery factor of 23.2% after hydraulic fracturing and depletion process. Rock surface morphology, mineralogical composition and formation water composition analysis indicate that supercritical CO2 leads the water-oil-rock system to more water-wet by increasing rock surface roughness. Geochemical simulation results combining the surface complexation and ion-exchange shows that supercritical CO2 weakens the crude oil adsorption on rock surface, which also contributes to the oil recovery increment. These results reveal the EOR feasibility of CO2 huff-n-puff in fractured tight conglomerate reservoirs, and provide insights to characterize geochemical features of carbonated brine/oil/rock.