Abstract

Seventeen wells were drilled in an aquifer to supply water for injection in an oil field in central Saudi Arabia. Although these wells produced high volumes of clear water on initial completion, during field start-up most wells produced poor quality water, and only at much reduced rates. A stimulation program to restore the productivity of water supply wells was initiated. The program included injection of 15 wt% HCl along with a friction reducer and a corrosion inhibitor. Samples of all injected and produced fluids were collected to analyze the solid particles present in the samples, and measure the concentration of key ions in the acid returns. The results of these analyses have been used to determine the mechanism by which damage to the wells occurred.

In this paper we present conclusive evidence, drawn from analyses of the returned acid cleaning fluids, which indicates that biomass and microbial corrosion products, produced by active sulfate-reducing bacteria (SRB) populations during shut-in periods, were responsible for much of the damage experienced by these wells. The investigation has highlighted an absolute requirement for an effective downhole SRB control program to prevent subsequent damage to water supply wells which have been restored by the cleaning program.

Based on the results of this investigation, an optimized cleaning procedure which will restore water productivity, and at the same time will minimize casing corrosion, has been developed. This program includes acid additives to minimize precipitation of calcium sulfate, elemental sulfur and ferric hydroxide, which can damage the formation and reduce the productivity of water supply wells.

Introduction

Water from an aquifer has been used to support production of sweet, superlight crude from an oil field in central Saudi Arabia. Seventeen wells were drilled to draw water from this aquifer. Initial productivity tests indicated that each well could produce, via a submersible pump, in excess of 20,000 bpd. However, later tests indicated that the majority of these wells could produce water at only a fraction of this rate. This result indicated that these wells had experienced some type of damage which would reduce flow from these wells. This paper discusses treating one of the damaged wells. This well will be referred to in the rest of this paper as Well A.

The aquifer is a sandstone reservoir which contains 1.4 to 2.1 wt% clay minerals as indicated by bulk XRD analysis (Table 1a). XRD analysis of the clay fraction indicated that kaolinite (migratable clay) and montmorillonite (swelling clay) are two dominant clay minerals in the formation (Table 1b). The aquifer water has a total dissolved solids of 17,000 to 21,000 ppm. Table 2 shows a geochemical analysis of a water sample collected from Well A. It is worth noting that this water contains a high concentration of sulfate ion (4487 ppm). This high sulfate ion concentration will have a great impact on the damage observed in water supply wells, as well as on the efficiency of the stimulation treatment, as will be explained later.

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