Search

Shale oil production is vital for meeting the rising global energy demand, while primary recovery rates are poor due to the ultralow permeability. CO2 huff-n-puff can boost yields by enabling key enhanced oil recovery mechanisms. This review examines the recent research on mechanisms and formation factors influencing CO2 huff-n-puff performance in shale liquid reservoirs. During the soaking period, oil swelling, viscosity reduction and CO2-oil miscibility occur through molecular diffusion into shale nanopores. The main recovery mechanism during the puff period is depressurization with oil desorption and elastic energy release. The interplay between matrix permeability and fracture network directly determines the CO2 huff-n-puff performance. Nanopore confinement, wettability alterations, and heterogeneity also significantly impact the huff-n-puff processes, with controversial effects under certain conditions. This work provides an integrated discussion on the mechanistic insights and formation considerations essential for the advancement of CO2 huff-n-puff application in shale reservoirs. By synthesizing the recent research findings, we aim to spotlight the key challenges and opportunities in considering reservoirs for this process, thereby contributing to the advancement of CO2 huff-n-puff applications for enhanced oil recovery. [ABSTRACT FROM AUTHOR]

Despite the significant progress made in tight gas exploration and development in recent years, the understanding of the dynamic mechanisms of tight gas accumulation is still limited, and numerical simulation methods are lacking. In fact, the gap between theory and field application has become an obstacle to the development of tight gas exploration and development. This work sheds light on the dynamic mechanisms of hydrocarbon accumulation in tight formations from the aspect of capillary self-sealing theory by embedding calculation of pressure- and temperature-dependent capillary force in a pore network model. The microscale dynamic mechanisms are scaled up to the reservoir level by geological simulation, and the quantitative evaluation of reserves based on real geological sections is realized. From the results, several considerations are made to assist with resource assessment and sweet spot prediction. Firstly, the self-sealing effect of capillary in the micro-nano pore-throat system is at the core of tight sandstone gas accumulation theory; the hydrocarbon-generated expansion force is the driving force, and capillary force comprises the resistance. Furthermore, microscopic capillary force studies can be embedded into a pore network model and scaled up to a geological model using relative permeability curve and capillary force curve. Field application can be achieved by geological numerical simulations at the reservoir scale. Finally, high temperature and high pressure can reduce capillary pressure, which increases gas saturation and reserves. [ABSTRACT FROM AUTHOR]