For a deviated well planned in a highly compressible deep water reservoir in the Gulf of Mexico, a typical perforation strategy of 140° phasing with 16 shots per foot (spf) is unlikely to meet flux requirements at the anticipated production rate. Perforating the production casing twice to double the flow area is one option to avoid exceeding the maximum flux limit. But will the high-density perforations impact the casing strength enough to cause casing failure as the reservoir compacts, given the reservoir operating limits? This cannot be assessed either by using existing analytical or numerical models, or from available empirical or experimental data.
To evaluate the feasibility of using a double perforation approach, a 3D finite element (FEM) model has been developed to determine the effect of the additional holes on casing failure. The model considers variations in both casing thickness and perforation pattern under expected wellbore and reservoir conditions. Both perforated and unperforated casings were analyzed to compare casing deformation characteristics and to determine the effect of the perforation on critical depletion pressures.
The FEM model shows that casing deformation is not uniform, resulting from nonuniform strain loading across reservoir boundaries and nonuniform distribution of casing strength as a result of the perforation. For the unperforated casing, the maximum strain is associated with bending near the reservoir boundaries. In contrast, the maximum distortion of the perforated casing is concentrated around the perforation holes at the ends of the perforated intervals. Allowing for a critical strain of 6% in the casing, the depletion limit would have to be reduced from 3500 psi to 2600 psi if the second set of perforations could be applied uniformly relative to the first set. A nonuniform distribution of the second set could further reduce the casing strength and lower the critical depletion pressure to 2200 psi. Finally, the study also shows that increasing the casing thickness by 0.0625 inches would have almost no effect on casing deformation or the attendant reservoir operating limit.
The study quantifies how much casing strain to expect in the scenario described – and that is critical for evaluating the risk of casing failure in expensive deep water wells. The results suggest that models like the one described here in fact offer a feasible way to assess casing deformation in highly compressible reservoirs where high density perforation may lead to casing failure.