Liners in the unconsolidated sand reservoirs common to SAGD applications are subjected to a unique combination of loads that arise due to re-established formation stresses combined with large temperature changes. Temperature changes that occur following frictional constraint of the liner induce axial strain that can yield the liner, which reduces its capacity to resist ovalisation under non-uniform re-established formation stresses. These types of liner deformations are undesirable because they can restrict wellbore access, compromise other liner functions (e.g., sand or inflow control) or, in the worst case, drive a complete collapse failure.
The magnitude, non-uniformity, and evolution of re-established formation stresses present in SAGD wells remains highly uncertain. Consequently, many of the liner designs deployed in early wells relied on approximations of the re-established formation stresses, for example, from experimental testing and analytical approximations provided by van den Hoek et. al. (2000b).
Industry experience with liners deployed in thermal applications has been mixed. Some projects have indicated very low rates of liner failures. Conversely, other projects in the same formations, or in formations with similar characteristics, have experienced higher failure rates with measured liner ovalisations that suggest re-established formation stress (REFS) distributions can be more severe than previously assumed.
This paper describes how REFS impacts long-term integrity of thermal liners in unconsolidated sand reservoirs and how uncertainty in those stresses can be the difference between success and failure. The sensitivity of thermal liner structural performance of several common liner systems to re-established formation stresses is examined. The results align with field experience and highlight the importance of developing a strong understanding of the downhole loading environment to ensure liner designs promote long-term integrity and sustained production. The paper recommends several activities that will enable industry to more rigorously determine re-established formation stress and proposes refinements to the design and selection basis for liners deployed in thermal wells to manage the risk of deformations that result from thermal loading.