Abstract
Abstract Well integrity during hydrocarbon production from an oil or gas well drilled through naturally occurring gas hydrate bearing sediments (HBS) may be problematic due to hydrate dissociation during well production. Assessment of well casing integrity is complicated because of the many variables involved such as:character, distribution and volumetric concentration of gas hydrate habitat;physical characteristics, in-situ stress-strain history along with thermal and engineering properties of the sediments overlying, within and vertically adjacent to hydrate bearing zones;specific design and installation details of the production well(s) including well tubular depths, sizes and the undisturbed formation temperatures;thermal characteristics of the various well components including tubing, casing, packer fluids, cement, anddetails of produced hydrocarbons including production flowrates over time, reservoir pressures over time, duration of production, produced fluid composition over time and well temperature profile during production. In this paper, assessing well integrity using numerical modeling of wellbore stability in a natural deepwater offshore environment in the face of in-situ gas hydrate dissociation due to heating is performed and described in detail herein. The simulation includes thermodynamic stability analyses of the hydrates, assessment of fracture development and soil movement prediction considering seepage and soil consolidation effects followed by soil-well interaction modeling to determine potential stresses in the well components. Fracture models of crack dimensions are premised on either (1) development of a pattern of laterally expanding horizontal fractures evenly spaced vertically throughout the hydrate-bearing zone and centered on the wells, or (2) development of vertical fractures which will intersect the seafloor to allow gasses from hydrate dissociation to vent into water column. For this study, hydrate accumulation (character, saturation and distribution) for field development area is interpreted using seismic inversion and rock physics transform along with available geophysics seismic data and offset well log data. Sensitivity analyses with parametric variations of sediments and wells indicate that well design (in terms of insulation) and an increase in soil capacity for fluid flux will have a distinct impact on reduction of soil motions and casing stresses. With computational efficiency, the numerical simulation can provide bounding estimate of gas hydrate dissociation effects around producing wells. The model is very useful for a sound understanding of key parameters that determine HBS response to hydrocarbon production and the well integrity.
Published Version
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