Deepwater Subsea Valve Flow Assurance—Compatibility Between Hydraulic Control Line Fluids and Completion Brines

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ProposalHistorically, systematic selection of completion brines for conventional completions has been based on several key critieria: hydrostatic density requirements, true crystallization temperature (TCT), pressure crystallization point (PCT), formation compatibility, and compatibility with reservoir fluid systems. However, as the number of deepwater oil and gas completions has increased, new concerns have arisen about the interaction between subsea hydraulic control-line fluids and completion brine systems. In offshore deepwater applications where subsea valves are not readily accessible, ensuring compatibility of hydraulic control-line fluids and completion brine systems is an essential part of the flow assurance process.Compatibility issues with completion brines become a concern during the initial seating of surface control units such as a subsea wellhead. All the control lines and orifices are filled with hydraulic fluid prior to running in the well to prevent collapse from subsea pressures. After the units are set in position, excess completion fluid has a tendency to displace the control-line fluid or intermix with it. Intermixing of completion brine and hydraulic fluid is critical because many of these fluids circulate through a extremely small orifice (usually less than 3/16 of an inch). Any precipitation of completion brine salts or separation of hydraulic fluid components may result in plugging and loss of hydraulic control.This paper will discuss and show results of a laboratory study to investigate compatibility between the most widely used water-based hydraulic fluid systems for subsea control applications and a wide range of completion brine systems. Furthermore, since most hydraulic fluids form insoluble solids when mixed with calcium-based brine systems, recommendation of calcium-based completion brines in deepwater applications has been limited. Therefore, a search for a subsea hydraulic fluid system compatible with calcium-based completion brines was conducted. The paper also outlines the results of a study with a subsea hydraulic fluid found to be compatible with calcium-based completion brine systems and discusses the completion advantages of this system.