Steam Injection Casing Design

Abstract

Steam injection is a successful operation to increase heavy-oil production, but the casing failure rates in steam injection wells are very high. Severe thermal-loading to the casing is resulted from steam injection operations and is attributed to the high casing failure rates. The casing failures can be casing parted due to fatigue and/or tensile load at cooling period, and can also be casing ID restriction due to collapse and/or severe buckling. This paper will briefly review the common casing design practice, and provide mathematical modeling of casing stresses in steam injection wells, and discuss ways of reducing casing failures, including the use of high strength casing (such as P-110 grade) to reduce casing hot-yield in most steam injection wells. The field data at Cymric 1Y field, Bakersfield, California are also presented to support the use of high strength casing (P-110 grade) in steam injection wells, based on the reduction of casing failures in the past two years. Introduction It has been common practice in the oil industry for many years to use low-strength casings (K55, N80, and L80) in steam injection wells, despite a higher casing casing failure rate thaninconventional wells. Previous studies have shown that when low strength casing is well-cemented, the support of the cement may prevent casing failures (buckling and collapse),[1,2] even when the casing is hot-yeilded under extreme axial thermal compressive load during stream injection period. Of course, the support of the cement cannot prevent the low strength casing failure under tension load during soak or production. As shown in Fig. 1, the common steam injection practice recognizes that the casing axial compressive stress increases with the starts of steam injection and the casing becomes hot-yielded at curtain temperature.After the casing is hot-yielded, the casing axial compressive stress remains the same (Dietrich-Whillhite model) or reduces slightly (Material work hardening model), as the temperature continues increasing. But with the support of cement, the casing does not fail due to hot-yielding during steam injection. Instead, the casing fails in tension during soak or production period, when casing temperature drops and axial tension stress develops and reaches the casing material yield strength. However, a well-cemented casing condition is not always achievable or maintainable in cyclic steam injection wells. Poor cement jobs can result in no or poor cement outside sections of casing; cement fall-back at the end of cement pumping can leave the top casing section uncemented; perforating may cause damage to cement near the perforations; cyclic steam injection may fracture the cement. Those may explain why the casing failure rate has been very high in steam injection wells. The casing can fail under hot-yield period in axial compression due to insufficient cement support, or under the cold-yielded period in axial tension no matter whether cement is good or not. The casing failures related to hot-yield include casing ID restriction due to collapse and/or severe buckling. The following table shows the casing temperatures that could cause casing hot-yield in compression for different grades of casing. The initial temperature is assumed to be 15 oC (60 oF) and the initial casing axial stress is assumed to be zero. Casing material yield strength reduction at high temperature is included. It is seen that low strength casing material K/J 55 and N/L 80 will all become hot-yielded, when casing is heated-up to a temperature of 400 F. deg. which is lower than steam injection temperatures in most steam injection projects. Table 1.Casing Grade Hot-Yield Temperature

Elastoplastic Analysis of Casing Stress in Thermal Recovery

“…Under normal circumstances, the annulus between the tubing and casing production will inject nitrogen or other low pressure gas, which plays good insulation properties in the process of steam injection in high temperature steam. While the depth of insulated tubing is above the position of oil layer casing perforating interval, as a result, the casing under the packer that near the reservoir will be fully exposed to the high temperature steam, which was said "the dangerous section" in literature [6].Make following assumptions to simplify the calculation:…”

Finite Element Analysis of Casing Material Behavior in Steam Injection Well Operations

“…These casing materials have a high casing failure rate in such environment of high temperature and pressure. It has been shown that when these casing materials when well-cemented, the support of the cement tends to prevent collapse or buckling of casing during steam injection period at which high thermal compressive loads are exerted [3,7,9]. The mechanical properties of different casing grades are mentioned in the Table 1.…”

“…[5] Properties such as yield point of the material vary with temperature [6]. Although, several researchers have extensively performed experiments on this subject along with field surveys; usage of current tools and software such as FEM/Ansys makes research not only faster but also diverse in terms of environment definition [3,7,8,9]. Given that different materials respond differently to stresses, this paper ventures out into determining the nature of response, using Finite Element Analysis, for two frequently used casing grade materials, J-55 and L-80 steel.…”

Thermal Buckling of Casing in a Slanted Thermal Production Well

“…The casing, cement sheath, and formation are integrated after cementing, and the axial length is greater than the casing diameter; therefore, the above case can be regarded as a strain issue. The extension value of the casing can be calculated by the following equation (Wu et al, 2005):…”