Abstract

Foam flow in porous media has been a subject of extensive investigation for the last 30 years because of its application in enhanced oil recovery (BOR) 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 (11" -long, 1-1/2 diameter) and a 3500-psi-rated coreflooding unit. Factors, such as surfactant type, foam quality, liquid and gas velocities are 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 is injected after the foam in each experiment and the residual permeability and foams stability were observed. The foam qualities varied from 35% to 90%, and the injection rates varies from 5 to 25 ft/day. Significant reductions in mobilities are 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.

Introduction

Acidizing is one of the most commonly-used stimulation techniques to treat the damaged and the low-permeability formations in order to improve hydrocarbon recovery. However, when multiple layers of different permeabilities are present, acid flows more into the high-permeability layers than the low-permeability layer. Foam is considered to be the ideal fluid of choice because it is clean and it can be easily removed after treatment. Theoretical and experimental works on foam flow through porous media indicate that foam performance is influenced by factors such as absolute permeability, permeability contrast, foam quality, flow rate, surfactant type, surfactant concentration, and others. Identifying the effects of each of these factors is critical to a successful foam diversion treatment.

Effect of Permeability

Bernard and Holm first identified the effectiveness of foam in reducing gas mobility with increase in the absolute permeability and its effectiveness to control channeling due to permeability variation. Raza reported that foam can improve sweep efficiency of fluid injection processes because of its flow impeding characteristics and its inherent capability of providing greater flow impedance in high permeability porous media than in low permeability media. He also reported that in heterogeneous reservoirs, the generation of foam in the high permeability streaks will not only impede the entry of fluids in these streaks but will also divert the flow into the tighter streaks. Hirasaki postulated that permeability can affect mobility through its effects on gas mobility and on the foam texture. This observation was analogous to what was observed in smooth capillaries.

Khatib et al. speculated that the gas mobility could be an increasing or decreasing function of the permeability as different mechanisms dominated the foam stability and thus the foam texture.

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