Foam flow in low-permeability Berea Sandstone cores: a laboratory investigation

Foam flow in porous media has been a subject of extensive investigation for the last 37 years because of its application in enhanced oil recovery (EOR) and stimulation operations. In acidizing, foam is used to facilitate the diversion of acid into the low-permeability strata when multiple layers of contrasting permeabilities are present. Very little has been done to investigate the flow behavior of foam in low-permeability rocks (1–10 md), mainly because of equipment limitations caused by the very high pressure gradients encountered when foam is injected. This paper discusses the results of several single-core, constant-quality, steady-state foam flow experiments, using a 9-md fired Berea Sandstone core and a 3500 psi (24 MPa) rated coreflooding unit. Factors, such as surfactant type, foam quality, liquid and gas velocities, were varied to investigate their effect on foam mobility, mobility reduction factors, and pressure gradients. To simulate the post-foam acid stage used in acidizing, brine was injected following foam injection in each experiment and the residual permeability and foam stability were observed. The foam qualities used varied from 55% to 90%, and the injection rates varied from 5 to 25 ft/day (1.5–7.5 m/day). Significant reductions in mobilities were observed for all the cases during steady-state foam injection. A new parameter called the persistency index is proposed to quantify foam stability during the post-foam acid stage, which may prove to be critical to predicting the foam diversion behavior. Also, more consistent results were obtained when dimensionless parameters such as mobility reduction factors rather than mobilities were compared. Finally, low-permeability results were compared with those for the higher permeability to identify uniqueness of foam generation in low-permeability formations.