Ujung Pangkah Field which located at offshore East Java Indonesia, is known for its challenging nature from geological, reservoir and drilling perspectives. Drilling experiences in this area shows severe wellbore instability in overburden shale and in fractured carbonate reservoir. Hydrocarbon production directly exacerbate drilling problems and production issues that were not expected came earlier than predicted, for example early water breakthrough. At least two or three operators facing similar severe wellbore instability problems in the area. Due to the complexity of subsurface systems and coupled interactions between depletion and stresses, the present-day stress state in Ujung Pangkah Field which have undergone production will be different from the pre-production stress state. Therefore, a comprehensive analysis will require numerical modelling involving coupling of 3D geomechanical model with fluid flow during production operations from dynamic model. Present-day stress state is subsequently used for wellbore stability analysis of planned development wells in Ujung Pangkah Field. Investigation of the behavior of natural fractured reservoir during depletion and its impact to reservoir management is also attempted. Two-way coupling of geomechanic and dynamic models were conducted whereby porosity and permeability update due to production were simulated based on uniaxial pore volume compressibility tests. Hence, porosity and permeability of fractures are not considered static anymore but dynamic due to stresses changes and production. The result of coupled simulation is able to reduce wellbore instabilities significantly in the planned well. The stable mud weight windows for planned wells are extracted from the model. The stable mud weight window in the reservoir interval is narrow to no stable drilling window in all the planned wells due to depletion. In general, the preferred direction to drill, requiring lowest mud weights, is in the direction of minimum horizontal stress which in this case is Northwest-Southeast (NW-SE). However, it was found that azimuthal dependency of mud weight is insignificant due to low horizontal stress anisotropy. Reservoir compaction and sea-bed subsidence were also calculated using the outputs from the model. The result is useful for completion and platform integrity.
A method of constructing high productivity and high wellbore dynamic stability wells has been developed for EOR field development application. It integrates a heuristic reservoir engineering modelling method for determining optimum drainage points with high dynamic flow and a 3D coupled reservoir geomechanical modelling method for identifying low sanding propensity regions within the entire reservoir. The reservoir geomechanical method couples dynamic reservoir modelling with geomechanical modelling. It can account for in-situ stress changes associated with reservoir pressure change, and predict any potential geomechanical-related physical events for the remaining life of the field. Correspondingly, the generated drainage map and 3D sand production critical drawdown cubes can then be combined to identify global optimum well placement locations within the reservoir, layer by layer. Drainage points selected by this heuristic reservoir modelling method can be correlated with their respective EUR (Estimated Ultimate Recovery) values, while the 3D critical drawdown cubes can identify reservoir regions with low sanding propensity. This combined approach can therefore lead to the development of multi-layer commingle wells having various angles of reservoir penetration, for optimizing well productivity and EUR value without the requirement of sand control. Illustrated by a case study in brownfield reservoirs, a non-linear well trajectory which maximizes reservoir fluid contact in a prolific sand layer can be designed without any sand control completion for sanding mitigation through optimization and management of production plan.
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