Design Criteria for Completion of Steam Injection Wells

Abstract

Abstract Casing failure in production wells stimulated with steam is a serious economic limitation in the application of steam injection to accelerate oil production from viscous reservoirs. This paper discusses the causes of casing failures and develops design criteria for completion and stimulation of new and existing wells. Casing failure occurs initially when temperature-generated compressive stresses exceed the yield strength of the casing. This forces the casing material or joint to permanently deform during the steam injection period. Subsequent cooling while the well is shut in or producing relieves the compressive stress. However, the deformation induced during steam injection creates a tensile stress as the casing temperature returns to its normal level. Often this tensile force buildup results ill joint failure by fracture or pullout. The mechanisms of casing failures are discussed in detail to provide a basis for selecting safe casing temperature changes for any well completion. Design criteria derived from these mechanisms are proposed as guidelines for the safe stimulation of old wells and the completion of new wells specifically for high temperature steam injection. Field experience is presented to confirm the utility of these guidelines for the protection of casing in productions wells stimulated with steam. Introduction Steam stimulation of production wells can subject the casing to high thermal stresses. Casing failures after first-cycle steaming have been numerous, particularly in areas where the injection temperatures are greater than 500F. At least 30 casing failures have been verified in California steam stimulation projects. Failures have also occurred in the Mid-Continent area from Texas to Wyoming. Generally, failures are found in the joint, although a few instances of collapse have been reported. Discussion Joint failure is a symptom rather than the cause of mechanical problems in steam injection wells. As the caring is heated it will normally tend to elongate in direct proportion to the change in temperature. Since some length of casing in most wells is not free to elongate, the tendency to elongate is replaced by compressive stress buildup in the casing. What happens to the casing can be understood if the behavior of a typical casing during a steam injection-production cycle is considered. The pipe body and the joint are assumed indistinguishable and both will be termed "the casing". Fig. 1 is a stress-temperature change diagram for casing heated in a steam injection well assuming that the initial stress is zero. Path A-B is the elastic portion of the compressive stress T curve. For plain carbon steels the compressive stress generated by thermal expansion forces is about 200 T psi. JPT P. 15ˆ