ABSTRACT: Loading history during drilling, completion and operations of the well can affect wellbore integrity. Debonding along well cement interfaces leads to the creation of micro-annuli and potential leakage pathways. We investigate the risk of debonding by modeling the integrity of an example well throughout its life-time using a finite element analysis tool. Well cement properties under downhole conditions are uncertain and therefore treated in a probabilistic manner. Simulation results for an example well show that a flexible annular cement reduces the risk of debonding and creation of micro-annuli along well cement interfaces. A quantitative measure of risk reduction is provided by probability density functions. Overall, probability functions of model output parameters from simulations with real well data can be used as input to a well integrity risk assessment framework that can incorporate diverse data and uncertain information.
Structural integrity of wells is ensured through multiple well barriers that provide structural strength and effective zonal isolation. Wells are usually completed with a steel casing and cement placed in the annulus between the casing and the formation. The integrity and the sealing capacity of the annular cement are therefore essential for ensuring well integrity and must be maintained throughout the life of a well.
Annular cement quality is evaluated in the completion phase, shortly after placement of cement, by cement evaluation logs. During the subsequent operational phase (production, injection), the casing and cement will likely be exposed to large fluctuations in pressure and temperature exerted by the fluids circulating through the well, leading to possible damage and mechanical failure of the cement sheath. Time-lapse well-logging to monitor the quality of annular cement properties is rarely performed and well cement properties under downhole conditions are therefore uncertain.
The failure modes of annular cement are diverse and complex. Failure modes that could create a continuous leakage pathway along the well are of primary interest for assessment of well leakage risks. The most significant leakage mechanism is likely related to the flow of fluids along micro-annuli formed by debonding of the well cement interfaces (Bachu and Bennion, 2009; Carey et al., 2010). A micro-annulus represents a circumferential fracture formed by tensile failure or axial shear along the casing-cement or cement-formation interface (Fig. 1 and Fig. 2). Micro-annuli formed at a particular depth can propagate upward by accumulation of fluids under pressure behind the casing (Dusseault, 2000). Flow paths for caprock leakage along microannuli enable migration of hydrocarbons or injected fluids from a producing reservoir or storage formation to shallow fresh-water aquifers and the ground surface. Besides well leakage through caprocks, hydrocarbon wells can also provide potential pathways for shallow gas migration. For example, shallow gas leakage along hydrocarbon wells is likely of regional significance in the Central North Sea where accumulations of biogenic methane are frequently found in the shallow subsurface above deep hydrocarbon reservoirs (Vielstadte et al., 2017).