Pore-Scale Investigation of Micron-Size Polyacrylamide Elastic Microspheres (MPEMs) Transport and Retention in Saturated Porous Media

Knowledge of micrometer-size polyacrylamide elastic microsphere (MPEM) transport and retention mechanisms in porous media is essential for the application of MPEMs as a smart sweep improvement and profile modification agent in improving oil recovery. A transparent micromodel packed with translucent quartz sand was constructed and used to investigate the pore-scale transport, surface deposition-release, and plugging deposition–remigration mechanisms of MPEMs in porous media. The results indicate that the combination of colloidal and hydrodynamic forces controls the deposition and release of MPEMs on pore-surfaces; the reduction of fluid salinity and the increase of Darcy velocity are beneficial to the MPEM release from pore-surfaces; the hydrodynamic forces also influence the remigration of MPEMs in pore-throats. MPEMs can plug pore-throats through the mechanisms of capture-plugging, superposition-plugging, and bridge-plugging, which produces resistance to water flow; the interception with MPEM particulate filters occurring in the interior of porous media can enhance the plugging effect of MPEMs; while the interception with MPEM particulate filters occurring at the surface of low-permeability layer can prevent the low-permeability layer from being damaged by MPEMs. MPEMs can remigrate in pore-throats depending on their elasticity through four steps of capture-plugging, elastic deformation, steady migration, and deformation recovery

Experimental Investigation on the Pyrolysis and Conversion Characteristics of Organic-Rich Shale by Supercritical Water

Organic-rich shale oil reservoirs with low-medium maturity have attracted increasing attention because of their enormous oil and gas potential. In this work, a series of experiments on pyrolysis of the particle and core samples were carried out in a self-made supercritical water pyrolysis apparatus to evaluate the feasibility and benefits of supercritical water in promoting the transformation efficiency and oil yield of the low-medium maturity organic-rich shale. Core samples had a mass loss of 8.4% under supercritical water pyrolysis, and many microcracks were generated, which increased the pyrolysis efficiency substantially. The oil yield of shale pyrolysis could reach 72.40% under supercritical water conditions at 23 MPa and 400 °C, which was 53.02% higher than that under anhydrous conditions. In supercritical water conditions, oxygen-containing compounds are less abundant than in anhydrous conditions, suggesting that supercritical water can inhibit their formation. Also, supercritical water conditions produced higher yields for light fraction, medium fraction, and heavy fraction shale oil than those under anhydrous conditions. These results indicate that supercritical water pyrolysis is feasible and has excellent advantages for low-medium maturity organic-rich shale

Effect of Ionic Strength on the Transport and Retention of Polyacrylamide Microspheres in Reservoir Water Shutoff Treatment

Knowledge of the effects of ionic strength (IS) on the transport and retention of polyacrylamide microspheres in porous media is essential for their application in reservoir water shutoff treatment, especially under high-IS conditions. In this work, retention and release experiments were conducted in a transparent sand-packed micromodel at various IS values from 0.001 to 0.20 M to investigate the pore-scale transport–retention–release processes of polyacrylamide microspheres, as well as their retention mechanisms and spatial distributions in porous media. DLVO interaction profiles, chamber dissections, and mass-balance calculations were used to quantitatively analyze the effects of IS on the transport and retention of polyacrylamide microspheres during reservoir water shutoff treatment. The results indicated that the retention of polyacrylamide microspheres increased with IS because of the diminution/elimination of the energy barrier at high IS levels. Straining at pore throats was the largest contributor to the retention of polyacrylamide microspheres for all IS conditions. IS reduction was beneficial to the release of polyacrylamide microspheres that were loosely retained on sand grain surfaces by secondary energy minima. Nevertheless, most of the released polyacrylamide microspheres were once again retained by straining, which would further enhance the water shutoff performance. Irreversible retention by primary energy minima ranged between 0.37% and 4.65% with increasing IS, and the elimination of the energy barrier under high-IS conditions could enhance this process