Abstract

The unique and reactive chemistry of acrolein enables a 3-fold approach to oilfield problems stemming from sulfate reducing bacteria (SRB):

  1. it is an effective biocide,

  2. it scavenges H2S, and

  3. it dissolves iron sulfide.

In this study, the utility of acrolein for remediation of problems with SRB and biogenic sulfides in a gas plant water disposal system at an facility near Mobile, Alabama is presented. The disposal system, designed to handle produced water from offshore, process water from gas plant operations, and surface water drainage within the plant, was losing capacity due to iron sulfide fouling of the surface equipment and plugging of the disposal wells.

Bacterial monitoring indicated that SRB originating from the drain water had become established in the tanks and flowlines. These SRB were generating 8–10 ppm dissolved H2S that was ultimately responsible for the iron sulfide formation. The iron sulfide resulted in poor water quality (Millipore® rates = 200–300 ml/5 min), tank fouling, filter plugging, injection line scaling, and loss of injectivity due to near well bore damage in the disposal wells.

The treatment design consisted of 1) initial remediation of injection wells by acrolein squeeze stimulations using a coiled tubing unit, 2) removal of iron sulfide pads from the tankage and iron sulfide build up in the disposal lines, and mitigation of SRB populations via batch treatments with acrolein, and 3) preventive maintenance using a continuous and batch treatment program with acrolein.

Immediately following squeeze treatment of the disposal wells, a 30% increase in capacity was observed. The topside treatment program has resulted in a 400 to 500% improvement in Millipore® rates, decreased levels of SRB, greater than 90% decrease in soluble H2S and a 65% decrease in filter replacements. Overall benefits of the program include acceptable injection rates and pressures and reduced operating costs through decreased filter changes, remedial expenses to maintain the disposal wells, and water trucking expenses.

Introduction

Biogenic Sulfides. The impact of biogenic sulfides on oilfield operations has been well documented and includes a host of associated problems ranging from ineffective oil and water separation, fouling and corrosion of topside equipment to loss of injectivity and production, and even reservoir souring. The two common forms of biogenic sulfides are hydrogen sulfide (H[2]S) and iron sulfides (Fe[x]S[y]) which results from the reaction of soluble iron ion with H[2]S. These sulfides are biologically generated by sulfate reducing bacteria (SRB) which reduce dissolved sulfates to sulfide ions. Although various solutions have been designed to reduce or remediate the damage from biogenic sulfides, the ultimate control must also address the root of the problem which is microbial, i.e., the establishment of SRB in the system.

Conventional Treatments. Remediation of system damage caused by SRB activity is typically attempted by acid treatment, chelation, sulfide scavenging, and/or biocides. Each of these approaches has merits as well as limitations. Although acids are effective at dissolving iron sulfide deposits, they are extremely corrosive and in some cases allow redeposition of the iron sulfide downsteam in the system. Chelation of soluble iron is another approach which can inhibit iron sulfide formation, but the reaction is reversible and impacted by the presence of H[2]S. Sulfide scavengers target the sulfide ion and have the benefit of both removing H[2]S and inhibiting Fe[x]S[y] formation. However, with traditional triazine-based scavengers this reaction can be inefficient and often results in solids formation which can be detrimental to downstream filters and injection wells. Biocide treatments are able to target the SRB populations and mitigate the source of biogenic sulfides. But not all biocides are effective at penetrating oil wet solids and biofilms to attack persistent SRB populations established in vessels, lines, and the well bore. Furthermore, most biocides have little capacity to directly remove both H[2]S and iron sulfide scale already existent in the system (one exception is THPS which has iron-chelating properties but still does not react with sulfides). Ultimately, none of the conventional chemicals described have the capacity to provide a comprehensive treatment for SRB, H[2]S, and iron sulfide.

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