Reducing Fault Reactivation Risk on Deepwater Drilling

Abstract

Abstract Drilling in deepwater presents risks associated to fault reactivation, what includes severe drilling fluid losses, connection of different reservoirs and, in the worst case, connection of the reservoir to the sea floor, causing oil and gas seeps. Recent accidents along the Brazilian coast showed that such phenomena are real and associated risks should be evaluated before any well planning. This work presents a method to identify and quantify risks of fault reactivation while drilling. An upper limit on the safe drilling window is proposed to account for these risks. The method also includes tridimensional stress analysis, where maps, slices and cubes with reactivations pressures over the faults are determined, what can be used to reduce risks by changing well positioning, casing points and mud weight plan. The proposed method is based on two main pillars. The full 3D geomechanical characterization of the area, which is done through the 3D-Mechanical Earth Model, a consistent way to determine the stresses around faults; and a proper criterion to determine the potential risk of fault reactivation during drilling. For this, a criterion of slip tendency analysis based on frictional constrains is adopted. The method can be applied under two methodologies. The first one represents the general case, where enhanced fault and in situ stresses characterizations are required. Through this approach, each fault plane is analyzed and more precise values of fault reactivation pressure are obtained. The second one is a simplification of the first one, and considers that all media is potentially faulted, where each potential fault is critically orientated with respect to the far field stress. This approach is recommended for exploratory or semi-exploratory scenarios, where faults are not very well mapped and main stresses directions are unknown. A case study for a planning phase of a drilling campaign on a deepwater field located in Santos Basin, Brazil is presented. Fault reactivation risks were identified and quantified through the proposed method. Safe mud weight windows were determined accounting for fault reactivation pressures, which was valuable during well planning. Recommendations for casing points and mud weight plans could be done including fault reactivation risks. Examples of risk mitigation by avoiding problematic zones and by re-positioning planned wells were presented. Results showed that the proposed method is an important tool to identify, quantify and reduce fault reactivation risks while planning any drilling campaign.