Shale Oil Reservoir Production Characteristics in Microscopic Pores Developed by Water/CO₂ Huff-n-Puff

In order to clarify the effect of deuterium water huff-n-puff (D2O HnP), carbon dioxide huff-n-puff (CO2 HnP) and CO2 HnP after D2O HnP (CAD HnP) on the production characteristics of crude oil in shale micro-nanopores, the microscopic space and pore connectivity of shale reservoir were characterized by scanning electron microscopy, high-pressure mercury injection, and shale spontaneous imbibition experiment. The NMR and MRI experiments of D2O HnP/CO2 HnP were carried out to study the effects of different huff-n-puff media enhancing oil recovery in shale pores from the microscopic scale. It was determined that the pore diameter distribution range of the target shale reservoir was wide (2 nm ∼ 10 μm), the slope of spontaneous imbibition curve was small, and the pore connectivity was general. The recovery degrees of D2O HnP, CO2 HnP, and CAD HnP were 20.16, 36.81, and 39.41%, respectively. The oil recovery degree of CAD HnP in large pores (r > 20 nm) and small pores (r ≤ 20 nm) reached 31.86 and 7.55%, respectively. With the increase of huff-n-puff rounds, the recovery degree of a single round gradually decreased. Combined with MRI, the utilization of shale oil developed by CAD HnP was clarified. In the initial stage of matrix shale, water and CO2 mainly used crude oil in the surrounding area of the core. With the increase in the number of CO2 HnP rounds, the crude oil inside the core was gradually utilized through CO2 diffusion. Prolonging the huff-n-puff rounds and soaking time appropriately could improve the CAD HnP recovery. The CAD HnP method of injecting water for 1 round and CO2 for 2 rounds was a better way to development. The preferred soaking time was 24 h for laboratory experiment. The research results could provide some reference and guidance for the field test of water, CO2, and CAD HnP.