An operator in the deep-water block planned to drill additional wells targeted into the gas cap of depleted reservoirs via sidetrack operations. Historically in this field, as part of the completion systems design, the use of non-equalizing safety valves were selected for the applications largely due to concerns associated with integrity of the well completions over the life of the well.
The surface controlled subsurface safety valves (SCSSV) were set at depths in the wellbore below the hydrate stability region, to mitigate concerns of hydrates around the safety valve, as well as slam closure specifications required for high velocity flow rates of hydrocarbon fluids (oil/gas) through the safety valve. Since the SCSSVs were non-equalizing, in order to open the valve, hydraulic pressure typically generated by a liquid column, is applied to equalize across the valve’s closure mechanism prior to opening with the control line fluid.
During completions and interventions operations, the liquid column to equalize the safety valve at the subsea safety valve depths of 1000 meters from the rig rotary table and use of 9 5/8" 53.5 ppf landing strings, could be upwards of 300 bbl of liquid. This presents a problem when the valve is equalized in a gas well environment because the liquid is expected to drop into the wellbore once the safety valve is equalized and fully opened. In the case of depleted gas reservoirs, there is an increased risk against the gas reservoir naturally flowing and cleaning up this liquid volume from the well, and is further compounded with high flow line pressures, resulting in an inability to produce the well. After careful consideration, the above risks were identified, and a possible solution of using a self-equalizing SCSSV to mitigate the need to use a liquid column to equalize the system and enhance the natural ability of the well to flow was advised.
Selection of Downhole Safety Valves in Deepwater applications is most significant aspect of well completion design; especially in depleted reservoirs. An operator should judiciously select from Self-Equalizing and Non-Equalizing Safety Valves available in the market.