Geomechanical Aspect of Successful Deep Tight Gas Sandstone Exploration Involving Drilling to Completion
Authors: Zacharia Joseph , Kumar Rajeev Ranjan , Shrivastava Chandramani , Ghosh Arnab , Talreja Rahul , Shrivastava Disha , Subbiah Surej Kumar , Vinod A. K , Bhagat Jayant , Biswal S. K 
Published: April 2020

Abstract
Exploration wells in India are being drilled to target tight sandstone reservoirs of Mesozoic age with potential hydrocarbon accumulation in the Gulf of Kutch (GK) & Gulf of Saurashtra (GS). Typical permeability ranges from 0.1 md to 2.5 md with effective porosity in range of 8% to 13% in good sandstone layers. These boreholes are seen to have substantial risks in drilling and completion, as evident by the severe wellbore instability events in the offset wells. Presence of wide breakouts and stress rotations along the wellbore axis with variation in dips along fractures are often observed, indicating complicated wellbore failure modes. Considering the increasing exploration activity and field development plans for these basins located in western offshore India, maintenance of hole integrity till open hole logging and optimizing perforation depths for hydro-fracturing jobs are the main challenge in upcoming wells. An advanced acoustic geomechanics workflow for the estimation of formation horizontal stresses from the inversion of four velocity radial profiles was implemented in shallower formations of offset wells with the help of advanced borehole 3D acoustic profiling measurements. Based on the estimation of stress parameters, stable mud weight windows and rock mechanical properties were estimated for a planned well. Reported over-gauged hole condition, caving, and stuck tool incidents are found at the depth intervals where the estimated breakout mud limit exceeds the mud weight used during drilling of offset wells. Pore pressure value is in range of 13.7 ppg to 14.1 ppg EMW with sudden change in regime during transition from Trap formation to Mesozoic formation. Stable mud weights to avoid breakouts are in range of 14.2 ppg to 14.7 ppg EMW for vertical well. The local normal fault regime is identified with few interbedded stress barriers. Simulated ECD value ranges 15.6 ppg to 16.1 ppg for an 8.5-in section with 600 gpm. The rock mechanical properties constrained by inverted stresses and wellbore failure were used to optimize bit and BHA design in the planned well. Real-time monitoring has been performed in the planned well using logging while drilling data, borehole images, and cutting analysis. Intermediate advanced acoustic profiling logs and drilling parameters were used in the planned well to directly estimate stress gradients and proactively keep mud weight within the stable mud weight window. By monitoring magnitude of breakouts present in LWD images and caving type, further remedial actions were taken. Few high angle features with gas peaks were encountered as seen in offset wells. Hole cleaning was performed regularly through pumping high viscous pills and cutting load was monitored at each connection through ECD and ESD variations. Near balance equilibrium was maintained to minimize hoop stress related skin around the sandstone layers. The modifications resulted in significant reduction of nonproductive time in the planned well as compared to the offset wells by 40%. Successful casing and cementing of 7-in liner to planned depth with good cement bond was also achieved, unlike offset wells where wellbore instabilities were severe. Upon designing an appropriate stimulation strategy in a HPHT and high-stress environment, closure pressure and breakdown pressure ranges were 12,300 psi to 13,500 psi and 13,000 psi to 14,800 psi, respectively, in sandstone with 8% to 13% effective porosity. To optimize expensive hydraulic fracturing stimulation treatments, the open hole advanced 3D acoustic logs were analyzed to identify and test gas bearing intervals. Intervals with suspected critically stressed natural fractures as indicated by 3D shear moduli changes were identified. The optimized intervals flowed gas to surface at good rates with minimal stimulation, proving commercial viability of the field.