Cyclic Steam Well Design: A New Approach to Solve an Old Problem of Cement Sheath Failure in Cyclic Steam Wells

Abstract

Proposal Steam injection is often used to increase oil production in heavy-oil fields. While the concept of thinning heavy oil by raising the temperature though steam injection is a fairly simple one, the associated challenges of designing and constructing a well for this environment can be complicated. One specific challenge is to construct a well where the cement sheath(s) will not fail and allow interzonal communication. The temperature fluctuations and the associated mechanical stresses transmitted to the cement sheath can cause the cement to crack. This cement failure can lead to steam injection migrating to zones other than the injection target zone, loss of production, communication of corrosive fluids in the annulus, casing movement, and other associated problems. Another factor that can affect the durability of the cement is the mechanical properties of the formations in which these wells are drilled. Excessively soft or low Young's Modulus formations exacerbate the problem by providing minimal confining stress on the cement sheath. In addition, wells drilled in a steam-flood field can often be plagued by lost- circulation problems, requiring use of low-density cements to address problems in cement placement. This paper provides details on how to help construct a well under these difficult conditions with minimal or no cement-sheath failure in critical production zones. Finite elemental analysis (FEA) shows that conventional and even specialized cement blends alone may not stand up to stresses encountered under these conditions. A unique technique is provided and verified with FEA to show how a well might be constructed without cement-sheath failure or at least minimal failure in critical zones. In addition, FEA analysis is provided to show how to help (1) optimize casing sizes and weights and (2) minimize costs of construction. Introduction Drilling and completion of cyclic steam and steam injection wells can entail many challenges. Lost circulation is often a problem during drilling and cementing, and the extreme temperatures involved require a robust well design. A significant concern in the well-construction process is placing a durable cement sheath around the various casing strings, especially the last string, which may cover the target steam-injection zone(s) and/or production zone(s). Although the placement of a competent cement in wells that often have problems with partial or total lost circulation while drilling is a significant issue and could justify a paper on its own merits, it is not the subject of this work. The focus of this paper is how to construct a well where the cement sheath will provide zonal isolation around critical depths to prevent loss of steam through interzonal communication and other problems associated with a failed cement sheath. Papers have been written about certain cement systems that address the issue of failed cement sheath due to thermally induced stresses.[1–5] Also, recently the cementing service industry has published a number of papers describing modeling programs that can help predict whether or not a given cement system will or will not fail under specific well conditions.[6] The body of this paper provides results of how what was believed to be the best cement system available was used in a model for a steam injection well constructed in the Central California Diatomite formation. Modeling results indicated that conventional construction methods with a superior cement system will result in cement sheath failure. This failure is predicted even when hole size, casing size, and casing weight are varied. Failure is also predicted when mechanical properties of the cement are adjusted to values not typically available and/or not yet achievable (tripling the tensile strength for instance). The main reason for this persistent failure is explained; i.e., not just the high thermal stress loadings. The new construction method is described in detail and FEA analysis results are provided to show how well design, including casing depth and weight, can be optimized to achieve a potentially more durable cement system under extreme thermal stress conditions. Results also show how one may be able to actually reduce well construction costs through optimization of the casing program.