ABSTRACT

We present a systematic laboratory investigation on the impacts of enhanced oil recovery (EOR) chemicals, i.e., surfactant and polymer, on calcium carbonate scale formation and inhibition. Commonly used inhibitor products, which are based on tri-phosphonate, penta-phosphonate and polyacrylate, are evaluated in the absence and presence of the EOR chemicals. The inhibition efficiency is determined using a modified static bottle procedure and the dynamic tube blocking method. In addition, scale precipitates are characterized with scanning electron microscope and X-ray diffraction techniques. Test results show that the EOR chemicals can have a significant influence on inhibitor performance and scale morphology and polymorphs. These results suggest that new treatment program should be developed beforehand to ensure effective scale control in the chemical EOR project.

INTRODUCTION

Enhanced oil recovery (EOR) by chemical flooding has been demonstrated to increase oil recovery factor in lab studies, pilot testing, and field depolyments.1-5 The process involves the injection of three chemical types; polymer, surfactant, and alkaline. Each chemical has its unique function: Polymers increase the injection water viscosity and decreases the mobility ratio of solution to oil thereby improving the sweeping efficiency; Surfactants lower interfacial tension and improves the porous rock wettability, allowing water to flow faster displacing more oil; Alkali is used to decrease the adsorption of surfactant and polymer on rock surface and activate indigenous surfactants present in acidic or heavy crude oils). Combined use of these additives can yield significant synergistic oil recovery and sweep efficiency effects.

These chemicals can also create severe production chemistry issues, which complicate the oil extraction and treatment operations. One such problem is the formation and prevention of inorganic scales within production wells and topside facilities.6,7 Preventing calcium carbonate (CaCO3) scale can be unmanageable in the ASP flooding process.5,8 Karazincir et al. examined the feasibility of combining chelating agents to prevent carbonate scales.9 They found that in waters with 1,000 ppm divalent cations, high concentration of complexing agent (11,000 ppm) was effective up to 10.5 pH 5, and the required chelant concentration was reduced 50% when adding 200 ppm to 500 ppm of phosphonate or polyvinyl sulfonate-based scale inhibitor. Lo et al. studied scale control in a formation water containing low Ca2+ concentration (< 100 mg/L) and found that a penta-phosphonate inhibitor provided effective scale control.8 When the previously injected seawater was also co-produced, CaCO3 scale control was achieved only over a limited formation water-seawater mixing range by the penta-phosphonate inhibitor.10 Using alternative inhibitor chemistries and synergistic effects of different additives, they showed much improve scale control in produced waters with high seawater content.11

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