A More Accurate Approach for the Design of Subsea Chemical Storage Systems Regarding Volume Requirements of Valve Leakage Tests

Abstract

Subsea production systems require different chemicals to be injected into the wells, Xmas trees, manifolds, or flowlines to prevent or mitigate flow assurance issues, such as hydrate formation. Moving the chemical injection system subsea is one of the initiatives of the oil and gas industry to reduce field development costs and debottleneck topside facilities. Different studies present the potential benefits of adopting this technology, and the technological readiness level of the different concepts. One premise for successfully adopting a subsea chemical storage and injection system is to have sufficient storage capacity and an efficient re-filling strategy. To achieve that, it is essential to have an accurate estimate of the required volume of injected chemicals during the service life of a well. One of the procedures which demands most of the injected volume of chemicals is the equalizing of pressures across a valve after a leak test has been performed. One single tree can easily have more than a dozen valves needed to be tested periodically. The modelling of fluid behavior when equalizing the pressure across the valve to estimate the required volume of injected chemicals may be simplified, for instance, by employing an ideal gas law approach. However, a more sophisticated approach employing equations of state capable of determining properties based on fluid composition might be needed to properly simulate the behavior of reservoir fluids with changing composition and under different pressure and temperature conditions. For the gas-producer wells modelled in the study cases of this paper, the results obtained indicate a clear difference in estimated chemical injection volumes when using the proposed equations of state approach instead of the simplified ideal gas approach. This equation of state method may be used to design more accurately the volume capacity of a subsea chemical storage and injection system, in particularly as the subsea production system’s demands evolve with time.