Field Scale Geo-Mechanical Analysis To Identify Fracture Sweet Spots Within Deccan Trap, Western Onshore, India

Abstract

Hydrocarbon exploration continues to venture into new avenues. This paper elaborates the 3D geomechanical study carried out to identify sweet spots in Deccan Trap Basalts in depth ranging from 500m-1100m in Cambay basin field of India. The main challenge is wide variation in the rock mechanical properties and stress profiles along various azimuths resulting from different tectonic incidents over the geological ages. Several drilling complications and held ups during electro logging in highly deviated wells are also reported. The normal fault tectonic framework has the imprint of two sets of faults viz., NNW-SSE and ENE-WSW. Deccan Trap acts as reservoirs due to the presence of connected open fracture network and to assess the potential reserves a comprehensive 3D Critically stressed fracture analysis has been performed using 3D numerical simulation-based rock properties, in-situ stress and seismic data. Open hole geophysical logs like sonic dipole and borehole images have been used to estimate rock mechanical properties and stress profiles in 18 key wells. Available core data of Basalt in the area have been used for dynamic to static rock properties estimation along with available published literature data. Critically stressed fracture analysis using 1D MEM outputs and dips dataset has been performed at well scale to history match production logging and testing results of 23 wells located in different fault blocks. 3D stress model has been built using plasticity model while taking into account faults and fracture sets. Utilizing 3D Geomechanical properties and Discrete fracture network model, critically stressed fracture sets have been identified across the field with slip tolerance and effective drawdown pressures. The study suggests that structurally high locations are good producers if seals are present above Trap. Sub-horizontal fractures have a higher closing tendency with decline in pressure in layers with SHmax>SHmin>Sv inside stiff Trap layer. There is variation of slip tolerance in the range of 0.2-1.4 in fracture sets which indicates slip tendency to be varying both vertically and laterally. Faults with ENE-WSW strike seem to be fluid migratory conduits and their intersection with NNW-SSE discontinuities are the areas where fracture sets have a higher slip tendency. Most of the producing layers are within 25m-55m of Trap with water being encountered at deeper depth intervals. These are mostly weathered fractured layers within the trap. The stress map suggests rotation of the maximum horizontal stress azimuth from NW to E which also affects fracture intensity in the field. Few fracture sets have tendency to be slip prone even with depletion up to 300psi-800psi while others will require stimulation or acid clean up job. Eight exploration wells drilled based on the study have shown good flow rate on initial well testing in the area providing validation to the study.