This paper outlines a solution approach for evaluating the stability of casing and faults due to reservoir compaction. Firstly, a geomechanics model is presented for the evaluation of casing failure due to reservoir compaction. Secondly, a threedimensional finite element analysis is coupled with the developed geomechanics compaction model for the detailed casing failure analysis. Deformations and stresses are determined on a cylindrical surface surrounding the length of the newly drilled or completed wellbore in the regions of interest. This cylindrical surface is sufficiently remote from the wellbore so that the wellbore has no or little influence on the stresses and displacements due to the reservoir compaction on this surface. The calculated displacements on the cylindrical surface are then used as boundary conditions for a focused near-wellbore stress and strain analysis using finite element technology. This hybrid analysis affords evaluating the near wellbore details that are often glossed over with a fastly compacted solution not requiring multimillion FEA cells. Yet, it preserves the fine details around the wellbore and allows for incorporating fault loading and macro influences of geologic structures and reservoir extent. It preserves the material balance and does not alter the pressure volume relationship in the reservoir void space. Interface elements can account for the slippage between the casing and the cement and between the formation rock and the cement. Field cases are presented for both the geomechanics model and hybrid finite element model.
1. INTRODUCTION
Depletion can generate large vertical deformation in the vicinity of reservoirs, especially in the Gulf of Mexico, where reservoirs are typically deep, multilayered, over-pressured and weakly cemented sands and silt sequences. Rock geomechanics plays a major role in both the recovery mechanisms and the integrity of the reserve delivery via well survivability. Hence it becomes necessary for the operators in these fields to carry out geomechanics assurance of reserve delivery by assessing the risk of well and casing failure, evaluating fault seal integrity during production, and by analyzing the impact of reservoir compaction on the integrity and recovery of the reservoir's resources. The results of these evaluations would assist the operators in devising appropriate strategies for more optimal recovery of the hydrocarbon from deepwater reservoirs. The studies must be aimed at a careful evaluation of the reservoir's pore volume compressibility, the impact of pressure depletion on reservoir recovery, production rates, fault movements, and well casing integrity [1-7].
Issues related to the impact of compaction on pressure maintenance, total recovery, and the survivability of well casing and completions have been well documented in various reservoirs worldwide such as in the North America fields in California, Western Canada and in the Gulf of Mexico. Many fields have experienced well failures (as illustrated in Figure 1) and loss of productivity following pressure depletion and reservoir compaction.
This risk adds to the economic and technical challenges of developing deepwater reservoirs. While the industry has access to large coupled finite elements (FE) software that can model rock and casing deformations caused by pressure changes, widespread use of these packages is severely hampered by several vital factors, such as, lack of long-term reservoir performance forecasts and strategy studies needed by the reservoir engineers, unnecessarily large number (several million) of model cells required for the FE analysis, and lack of sufficiently detailed description of the rock properties for these cells.
