Advanced Acoustic Measurements Aided Rock Mechanics Calculation for Successful Hydrofracturing Design and Wellbore Stability Analysis in Unconventional Reservoirs of Western Onshore, India
Authors: Paul Debashree , Shenmar Rajnish , Kumar Rajeev Ranjan , Zacharia Joseph , Kumar Ajit , Agarwal Pranav , Subbiah Surej Kumar , Bandyopadhyay Atanu , Talreja Rahul , Nandi Anindya , Kumar Vibhu 
Published: November 2020

There are number of small oil fields located in western onshore India, undergoing a fast track development campaign, wherein wells have witnessed NPT due to various drilling challenges such as tight hole, stuck pipe etc. Bad borehole condition has affected the data quality being recorded for formation evaluation and poor cement quality. Most of these drilling problems are reported in shallow shale formations like Tarapur, Post-Kand and Babaguru. Target reservoirs like Kalol and Cambay shale are relatively silty with porosity in range of 8%-15% which requires hydro-fracturing jobs to enhance production. This paper summarizes these challenges and provides current learning based on results of a geomechanical workflow implemented in the field.

Far field and near wellbore advanced acoustic measurements have been recorded in recent wells to estimate direct horizontal stress profile and map regional variation of tectonics across two fields. 1D Mechanical Earth Models (MEMs) have been constructed for different wells in the area to develop a geomechanical understanding of the reservoir as well as in the overburden and underlying layers. Anisotropic stress profile provides better understanding on rock fabric-based tectonics effects to check feasibility of optimized hydro-fracturing operations. History match of predicted failures using 1D MEM with caliper and drilling events suggest that shales and coals are relatively weaker than sands in both fields with 10%-15% variation of stress gradient magnitude laterally. Triaxial test results provided UCS calibration for better wellbore stability analysis (WBS) and stable mud weight window prediction in new wells.

In the first field targeting Kalol formation, presence of weak coal layers near target sandstone layers make perforation and hydro-fracturing design more challenging. Stress barriers are located around 30m-40m away from target zones with frac height expected in the range of 50m-80m depending on frac job volume and geological variations. There is a stress contrast of 200psi-500psi between target Kalol layer with potential shale barrier layers using anisotropic rock properties and stress profile. Closure pressure ranges 3700psi-4200psi in target zones with good petrophysical properties. Perforations were carried out away from coal layers towards bottom of reservoir with thickness of 6m-11m. In another field with Cambay formation as target, the well was perforated using integrated petrophysical and geomechanical findings in different well profiles. Frac half-length obtained after net pressure match is around 40m-50m as confirmed with temperature log which suggests that the frac job was effective. Height barriers match very well with micro seismic events conducted afterwards.

Production rate has improved by 150%-200% in two fields with new integrated strategy of reservoir quality and completion quality in deviated wells. Implementation of stable mud weight window concept helped to improve hole condition significantly in new wells and increase in effective rate of penetration (ROP) by 15%-20% with no major drilling events.