The paper discusses the impact of reservoir production on wellbore integrity and survivability when the compacting reservoir behaves as a deformable permeable body. The discussion is carried out using results of a numerical geomechanics model developed for estimating reservoir compaction, the subsequent surface/mudline subsidence; and the displacements, stresses and strains along the wellbore trajectories. The model has the unique aspect of efficiently coupling rock geomechanics with the actual reservoir simulation results. The input data include such relevant reservoir properties as pore volume compressibility, net-to-gross, porosity; corner point coordinates of each block from the reservoir simulator (the simulator blocks are identical to the elements used for the geomechanics model), wellbore trajectories and rock properties of the overburden. The deformation, stress and strains in the rock and along the well casings are evaluated at distinct time intervals. The initial, intermediate and final reservoir pressure profiles calculated by the reservoir simulator are central to the calculations. The model also has explicit analytical relationships that can be used to represent distinct variations in overburden lithologies.
In the model the reservoir is divided into many elements, where each element is approximated as a strain nucleus. The model estimates rock deformation in a semi-infinite body using a basic strain nucleus solution. The displacement field within the overburden, induced by reservoir compaction, is generated by integration of the Green's functions derived in the model using the basic strain nucleus solution.
For any specific well trajectory, the model approximates the deformations and stresses in and above/below the reservoir that are generated on the well casing due to reservoir pressure depletion. The calculated deformations include rock movement within the overburden and displacement along the well trajectories, possible fault movements or bedding plane slippage as well as mudline/surface subsidence.
In this paper the physics and mechanics of the modeling approach are briefly described, followed by some examples. Numerous reservoirs have been evaluated using this model in order to analyze reservoir compaction and related mudline/surface subsidence, and to assess the casing integrity of reservoir wells. The evaluated cases have included onshore, offshore and deepwater offshore scenarios. There have been discreet, "isolated," single reservoirs, as well as multi-reservoir situations and offshore deep-water reservoirs. The results from these analyses have assisted in devising improved strategies for reservoir exploitation and management.
The exploration and exploitation of deepwater fields require fewer, higher rate and more expensive wells than is customary. Most reservoir are multi-layered, over-pressured and weakly cemented sands and silt sequences. The combination of large pressure depletion and high rock compressibility leads to large deformation in the reservoir horizons. 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 operator in these fields to carry out geomechanics assurance of reserve delivery by assessing the risk of well and casing failure, fault or seal integrity during production and to analyze the impact of reservoir compaction on the integrity and recovery of the reservoir's resources. The results of these evaluations would assist the operator 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 [1–8].