3D Geomechanics Study for Basement Characterization and Sweet Spots Identification in Western Offshore, India

摘要

With known basement hydrocarbon accumulation, Mumbai High field in Western Offshore, India is a priority area for extending the concept of geo-mechanical fracture characterization in metamorphic basement reservoirs. Basement in Mumbai High is hydrocarbon bearing in areas proximal to major fault zones and intersections of major regional tectonic cross trends. Basement reservoirs have always been a challenge considering the lateral variation in rock properties with varying stress profile. The field under study has few wells producing hydrocarbon from varying depth intervals within Basement. Considering lower ROP, higher drilling cost and varying stress azimuth, a study has been conducted covering 3D Geomechanical numerical simulation and discrete fracture network stability analysis to identify sweetspots for new well locations targeting basement reservoirs while history matching field observations in offset wells. Basement reservoirs are often characterized by fracture sets which are conduit at present stress regime in far field condition. Some fracture sets are aligned within 20deg-30deg to maximum horizontal stress azimuth with few to be 90deg away from the maximum horizontal stress azimuth. To capture variation of fracture stability at field level, an analysis has been conducted at each offset well location, where geophysical logs, drilling parameters and geological information are integrated to construct a Mechanical Earth Model (Plumb et.al, 2000). Rock mechanical properties are calibrated with available core test results. Horizontal stress profile has been estimated based on poro-elastic horizontal strain method and advanced far field shear radial profiles. Validation of ID Mechanical Earth Models (MEMs) is conducted through predicted failure analysis and critical stressed fracture identification against drilling, production logging and testing results. A 3D MEM is constructed incorporating structural model, seismic velocity and ID MEM based rock mechanical properties. The 3D principal stress field from the geomechanical simulation has been redistributed over fracture planes of Discrete Fracture Network (DFN) model to identify critically stressed fractures using a slip criterion. It involves estimation of parameters like slip tolerance and critically pore pressure change to understand the proneness of fracture/fault planes to shear dilation (Barton et.al 1995). Stress regime varies from normal fault to strike slip fault depending on geological settings with maximum horizontal stress orientation varying between NNE-SSW to NE-SW. Analysis resulted in preparation of pseudo 3D sweet- spot maps governed by slip tolerance parameter indicating openness of fractures. It suggests that, most of the high angled fractures located in the proximity of intersection of faults are critically stressed subject to orientation reference to maximum horizontal stress direction and calculated slip tolerance. There is no specific azimuth of such fractures. Critical stressed fractures are present at varying depth: 50m to 250m from Basement top in the field as seen in offset wells where production logging and temperature data show good match of Hydrocarbon flow with predicted high slip tolerance fracture set locations. New locations with proposed well deviations and azimuths have been identified targeting the most probable prolific producers. The discussed multidisciplinary approach of fracture analysis has also been successfully used to predict and validate the contributing intervals in basement reservoirs of Mumbai High.