4D Geomechanical Analysis to Support Field Strategy on Production and Well Positioning in an Ultradeep-Water Reservoir

Abstract

Abstract Fault reactivation due to reservoir depletion in normal faulting areas is well-documented in the technical literature. Ultra-deep water (1550m) combined with low overburden gradient impose additional operational and environmental constraints in fault-bounded reservoir developments. Under this scenario, an accurate assessment of fault reactivation potential is required. This paper presents the results of a risk assessment analysis for a deepwater offshore field located in the post-salt section of Santos Basin, Brazil. The initial 1D geomechanical model of existing wells indicated a stress state close to the frictional limit. A 4D geomechanical (Finite Element) analysis was performed to gain understanding on the pre-production stress state at the reservoir scale and its variation with depletion, via a one-way coupling to the reservoir flow (simulation) model. Stresses were mapped to seven fault planes, over which three index-ratios were estimated (Sh_ratio, τ_ratio, OT_ratio), which were later transformed to a traffic-light type representation of the risk of reactivation. Results show an overall stress-path trend towards a more stable condition inside the reservoir (away from the frictional limit) with increasing depletion. Some isolated points, however, show favorable conditions for faulting that should be avoided in future drilling campaigns. Regarding reservoir delimiting faults, a stable but close to the frictional limit condition was observed in all planes for the pre-production state of stresses. A tendency to more stability is expected with depletion, similar to the central reservoir’s response. Some fault planes show local patches with potential for reactivation following different time sequences. This analysis highlights the importance of considering the potential risk of a hydraulic communication between the reactivated patches and shallower zones where a low burial condition could facilitate the opening of the fault. The main uncertainties in the analysis are related to fault strength parameters and the accurate estimation of differential pressures across fault planes, both having a decisive impact on the risk assessment. An accurate estimation of fault reactivation potential is of great relevance in well planning, construction of safe mud windows and in particular the definition of safe limits for injection/depletion pressures. The later have the potential to impact the production forecast and economy of the project.