ABSTRACT

Deposition of inorganic mineral scale on downhole completion equipment contributes to significant downtime and loss of production within the oil and gas industry. High temperature/high pressure (HT/HP) fields have reported build-up of lead sulfide (PbS) scale as a consequence of reservoir souring; and the resultant reaction between dissociated sulfide anions from hydrogen sulfide (H2S) and heavy metal cations. In this work, laboratory apparatus enabled simulation of scale precipitation under turbulent emulsion-forming multiphase conditions, with behavior of PbS particles at the oil/water interface and subsequent adhesion onto anti-fouling surfaces measured at a range of polymer concentrations. Introduction of polymer sulfide inhibitor (PSI) product to the formation brine at concentrations of 500mg/L reduced overall PbS deposition whilst addition of 5000mg/L further reduced scale crystallisation but resulted in complete emulsification of the light oil phase. The tendency of soluble polymers to act as surfactants led to increased stabilisation of the formed oil in water (o/w) emulsion with heightened PSI concentration. Optical microscope, gravimetric and rheological measurements explained depositional behaviour; whereby enhanced o/w emulsion viscosity and stability due to amphiphilic polymer adsorption onto both PbS scale and oil droplet interfaces resulted in uniform deposition upon all surfaces.

INTRODUCTION

Evolution of H2S gas within oil reservoirs can occur through both microbiological and geochemical means; a consequence of the activity of Sulfate Reducing Bacteria (SRB) and chemical reactions resulting from seawater injection, respectively 1. The dissociation of H2S to its constituent anions in water can be seen in equations 1 and 2, with Pb2+ cations within produced water reacting readily with sulfide based species to form PbS (galena), as seen in equation 3.

equation

Flow assurance complications in oil and gas production associated with deposition of sulfide scales are becoming increasingly frequent in high temperature/high pressure (HT/HP) fields. This paper builds on previous work 2, 3, where adhesion of PbS nanoparticles onto surfaces was found to be the overwhelmingly dominant deposition mechanism; with fouling behavior contingent on formation of an oil in water (o/w) Pickering emulsion and wettability of the foreign surface in a multiphase system. As such, oil wetted hydrophobic fluoropolymer surfaces were found to significantly limit the adhesion of PbS scale 2. Whilst the scaling mitigation potential of both anti-fouling surfaces and inhibitors has been investigated extensively on an individual basis 4-7, this work is the first on their combined efficacy in sulfide forming multiphase processes. As operators become increasingly intent on applying anti-fouling coatings onto downhole equipment to prevent the deposition and build-up of scales, understanding the synergy (or lack thereof) between chemical and surface mitigation techniques is critical.

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