Abstract
In this work, a slimhole steam injector well using new generation insulated tubing as the production casing string is proposed. Its mechanical integrity is evaluated using High Temperature Post Yield (HTPY) design approach. The thermal impact of this application and the implications of cementing the insulated tubing on its integrity as a casing are examined. It is shown that using insulated tubing as a casing is feasible and viable.
A well consisting of insulated tubing (3.5″ × 5″) as the production casing and a 7″ surface casing is considered. The insulated tubing is cemented to surface. Steam is injected through the insulated tubing to minimize heat loss to formation during steam injection and avoid aquifer temperature increase. Different steam injection conditions are considered. Thermal analysis of the process is conducted using a multiphase flow thermal simulator capable of handling steam. Loads on both the surface casing and the insulated tubing as production casing are then calculated considering the response of the inner and outer wall of the tubular. A High Temperature, Post Yield design approach as described by Suryanarayana and Krishnamurthy [SPE 178473] is used to examine integrity for appropriate loads. In particular, the low cycle fatigue life of the tubular body and connections is calculated and used as basis for design.
Thermal analysis of the steam injection process shows that the steam is delivered to the formation at the same steam quality as the injected surface steam quality. The surface casing satisfies all applicable loads using Working Stress Design (WSD) approach. The insulated tubing satisfies WSD criteria for all drilling loads. The thermally dominated loads, e.g. steam injection, satisfy the HTPY design criteria. Sensitivity to the top of cement below the surface is also analyzed to investigate the effect of buckling under high temperature. The insulated tubing undergoes elastic buckling in the un-cemented section, resulting in acceptable stresses. The number of cycles to failure calculated here are adequate for a well life of 20 years with 3 shut-ins per year.
Insulated tubing is often used as a tubing string in thermal wells for steam injection. Its use as a casing string has not been considered before. The thermal response when insulated tubing is used as casing is different. It is shown here that the insulated tubing can be used as a casing for in thermal wells with specific care exercised in its design. In particular, the thermal performance should be warranted for typically 20 years at high temperature. This means the insulation material should be selected such that it is unaffected by vacuum deterioration over time.
