Studying the impact of pore sizes on gas flow and distribution in volatile carbonate reservoirs using a new triple-porosity model

Carbonate reservoirs exhibit varying pore sizes that significantly impact gas distribution and flow dynamics. Current models fail to adequately address the flow mechanisms within the diverse matrix pores of carbonate reservoirs. To address this gap, we propose a triple-porosity model that incorporates small pores, large pores, and fractures, alongside a capillary pressure equation that accounts for pore radius and saturation. Additionally, a new transient shape factor was derived. Utilizing the triple-porosity model, we investigated the processes of gas separation and dissolution from oil, gas distribution, and the effects of gas on water flow. Our findings reveal that gas is primarily dissolved during the initial water injection stage or at low gas saturation levels. Once the pressure reaches the gas initiation threshold, gas transport becomes the dominant mechanism. Both dissolution and transport can reduce gas saturation by a factor of 1:10. In small pores, capillary pressure induces a gas locking phenomenon, resulting in higher free gas presence compared to large pores. Moreover, the presence of the gas phase accelerates the spread of injected water. Implementing depletion followed by water injection decreases oil recovery. This study elucidates the processes of gas separation, distribution, flow, and dissolution, providing theoretical guidance for managing complex flow dynamics in volatile carbonate reservoirs.