High-Temperature Imbibition for Enhanced Recovery from Diatomite

Abstract

Abstract It is difficult to produce oil from diatomite due to unique rock properties such as low permeability, high porosity, fine laminations, and fractures. Steam injection is a promising enhanced oil recovery method for diatomite, but the generation and injection of steam may lead to large energy cost and, potentially, well failures. Injection of hot water instead of steam provides a means to decouple pressure and temperature while still achieving the benefits of thermal recovery. This study investigates and compares oil recoveries from fractured diatomite cores (1.5 ″, 3.5 ″, and 5 ″diameter) using hot water and steam. Both hot water and steam injection at 200 °C were applied under spontaneous and forced imbibition conditions. Cores from two different reservoirs (A and B) were used. The B reservoir oil and brine formation were adopted for all cases. Synthetic steam boiler feed was used to create hot water and steam. An X-ray CT scanner was used to visualize in-situ phases saturations and characterize the porosity/oil saturation distribution. Wettability alteration toward water-wet conditions was observed clearly at elevated temperature (200 °C) for all cores. The Amott index, Iw, for hot water and steam injection around 200 °C with reservoir A conditions were 0.41 (A#1) and 0.36 (A#2), respectively. The final oil recovery from A cores at 45 °C and 200 °C were 62- 64 % and 78- 82 % OOIP, respectively. The A reservoir whole core with 5 ″ diameter at 45 °C showed greater oil recovery from spontaneous imbibition than the 1.5 ″ diameter core. The Iw and final oil recovery from core A with both steam and hot water injection at 200 °C was nearly identical. With B core, the Iw increased from 0.28 (45 °C, water) to 0.34 (200 °C, steam). The Iw of B core was equal to 0.33 after both hot water injection and steam injection. The final oil recoveries of both hot water and steam at 200 °C showed 76 % and 80 % OOIP, respectively. Both in situ visualization and effluent analysis shows that rock dissolution and fines migrations contribute the evolution of fracture and pore networks within the core. Therefore, results suggest that hot water provides the benefits of thermal recovery, in some regards, while decoupling pressure and temperature.