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.
A method is described for the assay of [35S]sulfate reduction in which filter paper wicks are used to trap [35S]sulfide. The simplicity of the technique enables large numbers of samples to be conveniently processed. Enhanced sensitivity is achieved since all acid-volatile [35S]sulfides produced during the incubation period are counted. Recovery of radioactivity from added Na235S is excellent (mean, 100.1%; standard deviation, 1.81; n = 9) and is unaffected by sulfide concentrations of up to 400 ,ug per sample. Field trial results with anoxic sediment samples are presented.
To control sulfate-reducing bacteria (SRB) populations, and minimize operational problems noted in the water supply wells, a biocide/corrosion inhibitor squeeze program was applied. Because of the weak nature of the formation and potential sand production, an extensive core flood study was conducted to determine the effect of a commercial biocide (formaldehyde, nitrogen and phosphorous compounds) and a biocide-enhanced corrosion inhibitor on the permeability of reservoir core plugs. Core Flood experiments were conducted using reservoir core plugs at reservoir temperature (50 °C) and pressure (2000 psi). The results obtained indicated that high concentrations of the biocide or the biocide enhanced corrosion inhibitor caused up to 90% reduction in the initial permeability of reservoir cores. Several damaging mechanisms were identified: dissolution of the cementing material by the biocide (acidic), swelling of montmorillonite present in the formation and precipitation of calcium sulfate. Based on lab results, a detailed biocide squeeze treatment was designed to treat water supply wells. The treatment was able to control SRB populations while maintaining the integrity of the formation. This paper examines the design of the treatment and the results obtained from extensive lab testing and field trials. Introduction Seventeen wells were drilled in a shallow aquifer to supply water for injection in an oil field in central Saudi Arabia. The sandstone aquifer contains 1.4 to 2.1 wt% clay minerals (Table 1). XRD analysis of the clay-size (i.e., less than 2 microns) fraction indicates that kaolinite (migratable clay) and montmorillonite (swelling clay) are the two dominant clay minerals present in the formation (Table 2). The aquifer water has a total dissolved solids (TDS) content of 16000 - 24800 mg/L, depending on location within the field.1 The water contains significant amount of sulfate ion that varies from 3000 to 6000 mg/L. Table 3 gives the chemical analysis of the water produced from three wells in this aquifer. Oxygen content of the water is below detection (less than 5 ppb). In addition, iron content is less than 5 mg/L. Tests performed on the water produced from these wells indicated that they are contaminated with SRB, with population densities as high as 104 per mL detected. The presence of SRB has caused corrosion products (iron sulfide) and biomass to accumulate around the screens, which resulted in reduced well productivity and deterioration in produced water quality.2–4 Well completion. Completion of the wells producing water from this aquifer includes a perforated 7" casing, placed uncemented in a 8.5" hole. Because the aquifer sandstone is poorly consolidated, stainless steel wire-wrap screens were used for sand control.2 The screen-wrapped perforated casing is generally distributed in sections across the producing zone of the aquifer. The screens have a slot size=0.012", contributing to their tendency to trap fine particulate debris. The total depth of the wells is in the range of 4000 to 4200 ft. Aquifer pressure is low (1500 psi), necessitating the use of submersible pumps to produce water from this aquifer. Damage to water supply wells. Flow tests conducted just prior to field commissioning indicated a majority of the water supply wells could produce water at only a fraction of the required design rate. Water which was produced contained significant quantities of suspended solids, which were identified as a variable mixture of sand, clays, corrosion products and biomass. The results suggested the wells had been damaged by an accumulation of particulate solids. Previous studies2–4 detailed the impact of sulfate-reducing bacteria (SRB) on formation damage in these wells. Well completion. Completion of the wells producing water from this aquifer includes a perforated 7" casing, placed uncemented in a 8.5" hole. Because the aquifer sandstone is poorly consolidated, stainless steel wire-wrap screens were used for sand control.2 The screen-wrapped perforated casing is generally distributed in sections across the producing zone of the aquifer. The screens have a slot size=0.012", contributing to their tendency to trap fine particulate debris.The total depth of the wells is in the range of 4000 to 4200 ft. Aquifer pressure is low (1500 psi), necessitating the use of submersible pumps to produce water from this aquifer. Damage to water supply wells. Flow tests conducted just prior to field commissioning indicated a majority of the water supply wells could produce water at only a fraction of the required design rate. Water which was produced contained significant quantities of suspended solids, which were identified as a variable mixture of sand, clays, corrosion products and biomass. The results suggested the wells had been damaged by an accumulation of particulate solids. Previous studies2–4 detailed the impact of sulfate-reducing bacteria (SRB) on formation damage in these wells.
This paper was prepared for presentation at the 1999 SPE International Symposium on Oilfield Chemistry held in Houston, Texas, 16-19 February 1999.
Introduction Deep subsurface terrestrial and marine environments are effectively shielded from the chemical and microbiological influences of surface soils and groundwaters. Physical isolation of the deep subsurface environment has the effect of separating it from normal biogeochemical nutrient recycling. With time, biologically-mediated changes in the deep subsurface environment become insignificant, relative to geothermally- or geochemically-mediated processes. Under such conditions, geologic changes continue until the isolation of the subsurface environment is broken. Abstract Water-based drilling mud formulations routinely include organic polymers as viscosifying and fluid loss control agents. These organic polymers, which are generally of either plant or microbiological origin (hence, "biopolymers"), can be degraded and utilized as nutrients for growth by naturally- occurring oilfield bacteria. This may occur despite the inclusion of biocide in some polymer preparations. Microbial growth in the mud can result in contamination of the well and near-wellbore zone. Fouling, corrosion and reservoir souring may then occur during subsequent operations. If growth in the mud is extensive, significant biopolymer degradation can be expected, with consequent loss of the mud's rheological properties. In these circumstances, additional polymer must be added to the contaminated drilling mud in an attempt to maintain viscosity and prevent excessive fluid loss. As additional biopolymer is added, and biodegradation continues, the cost of the drilling mud increases. This paper addresses the requirement for control of microbiological activity in water-based drilling muds representative of those used in Saudi Arabia. A laboratory investigation of the relative efficacy of alkaline pH (lime) and biocide (glutaraldehyde) treatments in limiting microbiological activity in drilling muds is presented, and minimum treatment levels for absolute control of general aerobic bacteria (GAB) and sulfate-reducing bacteria (SRB) are provided. Application of these findings can be expected to minimize drilling mud biodegradation, and costly post-drilling operational problems such as production tubing and pipeline corrosion. Water injection and oil production occur across the poorly defined physical boundaries separating surface and subsurface environments. They are, by nature, processes which initiate and promote exchange between surface and subsurface environments. Through this process, biogeochemical activity, long curtailed by the physical separation of the two environments can be re-initiated. The result can be renewed or enhanced microbiological activities in both environments. Drilling Activities. Communication between surface and subsurface oilfield environments is initiated through the drilling process. This process requires circulation of fluids (drilling mud) from the surface to the drill bit to carry cuttings out of the borehole. In so doing, chemicals and microbes from the surface are circulated into the deep subsurface, energy-rich oil-bearing strata, and hydrocarbon-laden cuttings are brought into the oxygen-rich, moderate temperature surface environment. Through this process, microbiological activities can be initiated in both surface and subsurface environments which would have been unlikely had exchange between the two environments not occurred. Downhole microbiological activities can cause the following problems:Microbiological corrosion of well tubulars and screens.Biomass plugging in injection wells and in the formation.H2S production deep in the formation, leading to microbial reservoir souring. P. 329^
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