Subsurface Safety Valves Prevent Self-Flow in Steam-Injection Wells in Heavy Oil Field
Chris Carpenter_
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 223125, “Case Study: Performance Analysis of Subsurface Safety Valve in Steam-Injection Wells for Thermally Assisted GOGD Heavy Oil Field,” by Noufal Nofali, SPE, Muhallab Al Riyami, SPE, and Mohsin Amri, Petroleum Development Oman, et al. The paper has not been peer reviewed.
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Field Q is a heavy and sour oil field characterized by a naturally fractured, dome-shaped carbonate reservoir with a compact structure. The recovery mechanism implemented is thermally assisted gas/oil gravity drainage (TA-GOGD). The steam injectors are shallow, deviated, openhole wells drilled at the top crest of the reservoir, penetrating a gas cap containing 3% hydrogen sulfide, which allows for the self-flow of sour gas when steam injection ceases. The complete paper describes the application of subsurface safety injection valves (SSSIV) in an extremely high-temperature environment.
Introduction
Field Q is a thermal enhanced oil recovery project producing heavy, viscous oil (16° API and 220 cp) from a shallow, naturally fractured carbonate reservoir at a depth of 210 m true vertical depth. It is a dual-porosity reservoir with extensive fractures, a matrix porosity of approximately 30%, and a matrix permeability varying between 5 and 20 md.
Field Q has been developed using the TA-GOGD recovery mechanism because of its highly fractured carbonate formation, high oil viscosity, shallow depth, gas cap, and thick oil column. In this process, steam is injected at the top of the reservoir while oil is produced from the bottom, with fractures playing a key role in distributing steam and collecting oil. The TA-GOGD mechanism works by creating a hydrostatic imbalance through gas-filled fractures. This effect is further enhanced by steam condensing on the fracture surfaces, which leads to thermal expansion of the oil and reduced viscosity. A steamflood or drive would not be feasible in this field because it would fail to create the required pressure difference between the fracture network and the matrix.
The steam-injection wells are shallow, deviated wells located at the top crest of the reservoir. The gas cap pressure is approximately 32 bar, and the closed-in tubinghead pressure for these wells ranges between 28 and 32 bar. Depending on the completion size and reservoir injectivity, steam-injection rates in these wells range between 600 to 1,400 tons/day. When steam injection is stopped, these wells behave as gas wells with the capability for surface flow, meaning that it is generally safer to maintain continuous steam injection. Given the reservoir’s extensive fractures, continuous dynamic filling is required during any workover activity as a well-control measure.
Requirement of SSSIV and Steam-Injection-Well Design
The presence of a secondary sour gas cap necessitated a comprehensive evaluation of using a surface-controlled subsurface safety valve (SC-SSSV), given the risk of a blowout if well-control barriers fail. In line with company policy, all free-flowing sour oil and gas wells must be equipped with tubing-retrievable SC-SSSVs to mitigate this risk.
Various pressure-release scenarios were analyzed for dispersion modeling in steam-injection wells. Both well and dispersion modeling clearly underscore the need for SC-SSSV installation. Since the commissioning of the Field-Q project, however, no SC-SSSV capable of withstanding the high temperatures in these steam-injection wells has been commercially available. Consequently, with appropriate risk-mitigation measures in place, it was concluded that SC-SSSV installation was not mandatory.