Simulation of Injection Water Supply Wells in Central Arabia

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.

Optimization of Hydrogen Sulfide Scavengers Used During Well Stimulation

“…To remove near wellbore damage and restore well productivity, the wells were stimulated with hydrochloric acids. 2 Analysis of acid returns of the first wells indicated that they contained high concentrations of sulfide ions. Solids present in the acid returns contained elemental sulfur.…”

“…2 The concentration of sulfide ion was measured in the acid returns. Figs 14 and 15 show the concentration of sulfide ion in the acid return.…”

“…16 Fe 3+ + 8 H 2 S → S 8 + 16 Fe 2+ +16 H + (2) Elemental sulfur is insoluble in acids, difficult to remove, and can cause formation damage. Also, ferrous ion reacts with sulfide ion to form iron sulfide which is insoluble at pH > 1.9 (Equation 3) 6,12…”

Iron Sulfide Scale: Formation, Removal and Prevention

“…4 Accumulation of iron sulfide and biomass around downhole screens and perforations can cause loss of productivity of water supply wells. 5 Build-up of iron sulfide scale in the tubing can create problems during wireline work and can reduce well deliverability. [6][7] The presence of fine particles of iron sulfide in the produced crude oil can cause many operational problems in oil-gas separation plants (GOSPs).…”

“…An example of the biotic mechanism is the hydrogen sulfide produced in injectors that utilize waters with high sulfate contents, e.g., seawater 3 or shallow water aquifers. 5 In these injectors, hydrogen sulfide is produced due to the activities of sulfate reducing bacteria (SRB). For abiotic mechanisms, hydrogen sulfide is produced due to thermochemical sulfur reduction, thermal hydrolysis of organic sulfur compounds, or hydrolysis of metal sulfides.…”