Abstract

Foam is used during matrix stimulation to promote the diversion of acid from the (undamaged) high permeability streaks to the (damaged) low permeability ones. Often foam is first placed in the formation, near the wellbore, and then acid is injected. Diversion is obtained from the strong reduction of liquid and gas mobility, associated with the formation of foam films.

In this paper, foam flow in porous media has been investigated under conditions that are relevant for acid diversion induced by foam. First, a simple model for foam placement and liquid injection following foam has been developed. Then core flood experiments were conducted using Bentheim (quasi-homogeneous) and Berea (heterogeneous) sandstone cores, and nitrogen (N2) foam stabilized with sodium dodecyl sulfonate (SDS). The pressure drops over several core sections were mornitored as a function of time. In addition, the fluid partitioning was determined in real time using the X-ray computed tomography (CT) during the core floods.

The experiments using quasi-homogeneous cores show that flow in a core containing surfactant solution (placement) exhibits a front-like pattern. An unexpected secondary liquid desaturation, which becomes dominant after foam breakthrough, has also been observed. The injected liquid following foam develops into a stable finger. In the heterogeneous porous medium a fingering pattern, controlled by the heterogeneity features of the core, develops during foam flow in the presence of surfactant. Liquid desaturation appears to be relatively stronger, but the final liquid saturation remains higher than for the homogeneous cores. Also the injected liquid displays a complicated fingering pattern. Trapped gas finally is estimated to be 70 and 60% for the homogeneous and heterogeneous cases, respectively. These observations have been successfully interpreted using the proposed model.

Introduction

Foam induced acid diversion consists in placing foam in the near-well region to ensure that acid used for matrix stimulation is diverted from the (undamaged) high permeability streaks to the (damaged) low permeability ones1,4. The physical principle underlying the diversion process is the strong reduction of liquid and gas mobility, associated with the formation of foam films. This phenomenon has also been exploited widely in gas control5,6, as well as in enhanced oil recovery using foamed CO2, air or steam, to reduce gravity override and improve hydrocarbon sweep efficiency7,8. The primary focus of this study is foam diversion, but its outcome is expected be relevant for the other foam applications.

Although foam diversion has been used extensively in field operations, it has not reached a complete maturity. The reason for this is mainly that most studies of foam in porous media have not emphasized sufficiently the transient near-wellbore foam flow regimes, inherent in the diversion process. The insight gained into the process of acid diversion induced by foam is mainly due to Rossen and co-workers9–11, who conducted systematic studies using two-stage experiments, which is consisted of foam placement followed by liquid injection. In the first stage (foam placement), these authors observed that the pressure gradient increases as a function of time to a Plateau value. Furthermore, they found that the pressure gradient behaves in a rather similar manner in different core sections, except, in a number of cases, in the first section (near the inlet) where the pressure gradient remains relatively low. The low inlet pressure gradient has been observed repeatedly but was ascribed to a vague foam retexturing. Recently, Tanzil et al.12 and Gauglitz et al.11 interpreted this effect by the existence of minimum pressure gradient for foam generation, which is not attained in the first core segment near the inlet.

In the second stage (liquid injection following foam) Rossen et al.9,10 found that the pressure gradient declines in two steps. Soon after the liquid injection the pressure gradient first falls rapidly but levels off to a value that remains relatively high. This is followed by a rather gradual decrease that leads to a quasi-steady state of liquid flow in the presence of significant trapped foam.

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