As deepwater oil and gas exploration in the Gulf of Mexico and international waters have become key activities, new concerns have arisen about the interaction between subsea hydraulic control line fluids and completion brine systems.

In deepwater applications where many subsea components are not readily accessible, compatibility of the hydraulic control-line fluids and completion brine is an essential for flow assurance. Historically, systematic selection of completion brines for conventional completions has been based on several key criteria; hydrostatic density requirements, true crystallization temperature (TCT), pressure crystallization point (PCT), formation compatibility and reservoir fluid systems compatibility.

In the late 1970s and early 1980s, mineral and hydrocarbon-based hydraulic fluids were developed for controlling subsea production and control systems. But with evolving requirements of high-pressure/high-temperature (HP/HT) production wells and increasing environmental concerns during the early 1990s, new subsea control fluids were developed to meet application parameters. These HP/HT fluids were specifically water-formulated and designed to perform under HP/HT conditions as well as in colder temperatures and at higher seabed pressures. Water-based hydraulic fluids have two major advantages over mineral-based fluid systems - lower viscosity and allowable venting to the sea.

Surface-controlled reservoir analysis and management systems (SCRAMS) are used to actuate subsea control valves on equipment ranging from one or two functions on a production oil and gas line to complex manifolds with more than 300 valves. Most of these subsea control valves depend on hydraulic control-line fluids to operate piston-actuated valves and there are several areas where completion brines and hydraulic control line fluid may come in contact. The two-fluid systems can come in contact in any equipment that is either being set, controlled, or cycled by hydraulics. This can occur in pack-off tubing hangers, subsea wellheads, control valves, sleeves, tubing hangers and certain types of packers.

Concern

Compatibility with completion brines becomes a concern during initial seating of surface control units such as a subsea wellhead. All 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, excess completion fluid has a tendency to displace the control-line fluid or intermix with it. Intermixing can also occur during the cycling. During hydraulic equipment cycling, a slight surge can push completion brine into an open valve, causing intermixing of hydraulic fluids and brine. Intermixing is critical because many fluid systems circulate through extremely small orifices - usually less than 3/16 of an inch. Any precipitation of completion brine salts or separation of hydraulic fluid components in these orifices may result in plugging, and loss of surface hydraulic control.

Hydraulic fluid compositions vary by manufacturer. However, the basic composition of the water-based fluid systems is designed to have the lowest possible viscosity while retaining the characteristics necessary for a functional hydraulic fluid. Hydraulic fluids usually include a colored dye additive, lubricity package for anti-wear characteristics, corrosion inhibitor for metal protection, and 10% or greater glycol, to prevent crystallization in colder subsea temperatures, and to enhance thermal stability and help control micro-biological agents.

Compatibility

Many major operators are evaluating completion fluid compatibility testing as a new requirement for subsea control fluids used in SCRAMS completions. This results from reports of several new hydraulic fluid systems being recalled from the market after exposure to calcium-bromide based completion fluids resulting in the formation of a solid, sticky ball on contact.

Based on these reports, laboratory testing was initiated to investigate compatibility between the most widely incorporated water-based hydraulic fluid systems used in subsea control applications and a wide range of completion brines. Studies were conducted with two different manufacturers' water-based hydraulic fluid systems and varying salt compositions and concentrations of completion brines.

Testing

Testing for this study involved mixing samples of each hydraulic fluid and the designated completion brines in varying ratios and recording fluid characteristics after incubation.

The study was conducted with the two different manufacturers' hydraulic fluid systems, designated hydraulic fluid systems A and B, at different ratios with a wide range of completion brine compositions after one-hour incubation at 38?F (3?C).

The studies were conducted with the two hydraulic fluid systems and different completion brine compositions at room temperature 73?F (23?C) for 72 hours.

Figure 1 shows hydraulic fluids systems A and B as well as 14.2 ppg CaBr2 (Calcium Bromide) completion brine before mixing.

Figure 2 is a photograph of the resulting fluid mixtures when hydraulic fluid A was mixed with the 14.2 ppg CaBr2 completion brine in varying ratios. Figure 3 is shows the mixtures when hydraulic fluid B was mixed with the 14.2 ppg CaBr2 completion brine.

Range

Compatibility tests between the two hydraulic fluids and completion brines displayed a wide range of test results. Tests between hydraulic fluid A and calcium chloride-based completion brines did not result in solid precipitation or breakout of a layer of blue material. However, compatibility tests between hydraulic fluid A and calcium chloride/calcium bromide-based brines, calcium bromide-based and calcium chloride/ calcium bromide/zinc bromide-based completion brines resulted in breakout of a blue layer of material.
Figure 2 shows the breakout of this blue layer when hydraulic fluid A was mixed with the 14.2 ppg calcium bromide completion brine in varying ratios. Compatibility tests with hydraulic fluid A and sodium bromide-based and sodium formate-based completion brines also had break-out of the blue layer of material to the top of the mixture which did not disperse upon mixing.

Compatibility tests conducted with hydraulic fluid B show formation of a white solid precipitate with the calcium chloride, calcium chloride/calcium bromide, calcium bromide, and zinc bromide/calcium bromide/calcium chloride-based completion brine systems in all of the mixing ratios. Figure 3 shows the solid precipitation that occurred when hydraulic fluid B was mixed with the 14.2-ppg calcium bromide completion brine. However, hydraulic fluid B did not form any white precipitate or have any component breakout when mixed with the sodium bromide or sodium formate-based completion brines.

These results show the varying compatibilities that hydraulic fluid systems may have with different completion brine systems. Precipitation of brine salts and separation of control fluid components occurred almost immediately on contact. Furthermore, with some of the control-line fluids, contact with the completion brine resulted in separation of the control-line dye, lubricity package and corrosion inhibitor, suggesting a possible loss of control-line performance. These results bring attention to potential problems, which may exist between the selected hydraulic fluid and completion brines.

As for conventional completions, selection of completion brines for deepwater has also been based on: hydrostatic density, TCT, and (PCT, formation compatibility, and compatibility with reservoir fluids. However, incompatibility problems between hydraulic control line fluids and completion brines draws attention to another area of concern that should be addressed when systematically selecting completion brines for improving deepwater completions.

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