Abstract

Foam has been used successfully in the field as a diverting agent for matrix acidizing when layers of contrasting permeabilities are present. However, the mechanisms governing the diversion performance of foam are still not properly understood and only a limited number of foam diversion papers can be found in the literature. This paper discusses the results from several dualcore experiments using 11- and 22-inch long and 1.5-inch diameter fired Berea sandstone cores (permeability range: 10 to 1000 md) in which parameters such as foam quality, total injection rate, foam slug size, etc. are varied systematically for different permeability contrasts. Each experiment consists of a gas-free surfactant injection phase, followed by a foam injection phase, which is then followed by a brine injection phase simulating the post-foam acid injection stage. The pressure data along each core at four locations and the weight of the liquid effluents for each core are recorded continuously. An attempt is made to quantify the diversion performance by the introduction of various diversion factors. The effect of each of the variables such as foam quality, injection rate, slug size etc. on the diversion factors are then compared for different permeability contrasts. Results show strong dependence of the diversion performance on permeability contrast, foam quality and total flow rate. This study provides important guidelines for designing acid jobs for different permeability contrasts and ranges when foam is used as a diverting agent.

Introduction

Quite often hydrocarbon reservoirs present themselves as layers of contrasting permeabilities caused either by natural phenomena or by mud invasion damage. Poor acid jobs cause more damage to the low-permeability zones than to stimulate them. Since the low-permeability zones in the reservoir cannot be stimulated like the high-permeability zones, a more efficient technique is needed to divert the stimulation fluid into the low-permeability zones. A diverting agent is a materiel used with a stimulation treatment to distribute the treatment more uniformly throughout the zones in the well. It is intended to plug the high-conductivity areas temporarily so that the more stimulating fluid is placed in the lower permeability zones. In the recent years foam has been used extensively as a diverting agent because of various reasons: its ability to block or minimize flow into high-permeability regions, its ability to perform diversion at various temperatures, its ability to flow through gravel-packed completions, and its ability to be removed easily after the treatment unlike some solid diverters, etc.

In 1969, Smith et al. conducted both laboratory and field tests to study the use of foam as a non-damaging diverting agent and found that foam (generated in-situ) improved the acid treatment by allowing acid to penetrate zones that would otherwise be left unstimulated. Burman and Hall analyzed the effects of staged foam injection and foam injection during the first third of the treatment and used production data to evaluate foam diversion performance. They found that foam diverted matrix HF acid treatments were successful in treating for the inorganic damage removal and that staged 65% quality foam treatments produced encouraging results. Burman and Hall also conducted some dual core laboratory tests using 10 to 158 md. In one of the tests, the 158-md core was initially taking 2 times as much fluid as the 97-md core. But after injecting 74% quality foam and switching back to unfoamed fluid, they observed that the volume going through the 158-md core decreased by 20%. P. 585^

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