Integration of Acoustics and Geomechanical Modelling for Subsurface Characterization in Tectonically Active Sedimentary Basins: A Case Study from Northeast India

Drilling wells in the remote northeastern part of India has always been a tremendous challenge owing to the subsurface complexity. This paper highlights the case of an exploratory well drilled in this region primarily targeting the main hydrocarbon bearing formations. The lithology characterized by mainly shale, siltstone and claystone sequences, are known to project high variance in terms of acoustic anisotropy. Additionally some mixed lithological sequences are also noted at particular depths and have been identified at posing potential problems during drilling operations. Several issues became apparent during the course of drilling the well, the main factor being consistently poor borehole condition. An added factor potentially exacerbating the progressively worsening borehole conditions was attributed to the significant tectonic activity in the area. To address and identify these issues and to pave the way for future operations in this region, a Deep Shear Wave Imaging analysis was commissioned to identify near and far wellbore geological features, in addition to addressing the geomechanical response of these formations. In this regard, acoustic based stress profiling and acoustic anisotropy analysis was carried out to estimate borehole stability for the drilled well section and provide insights for future drilling plans. Significant losses were observed while drilling the well, in addition to secondary problems including tight spots and hold ups and consequently the well had to be back reamed multiple times. Of particular note were the losses observed while transitioning between the main formations of interest. The former consisting relatively lower density claystone/siltstone formations and the latter, somewhat shalier interlayered with sandstones, displaying a generally higher density trend. This transition zone proved to be tricky while drilling, as a high density sandstone patch was encountered further impeding the drilling ROP. Overall, both formations were characterized by significantly low rock strength moduli with the exception of the sandstones projecting characteristically higher strengths. In light of these events, analysis of integrated geological, geomechanical and advanced borehole acoustic data analyses were used to identify the nature of the anisotropy, in terms of either stress induced, or caused by the presence of fractures in the vicinity of the borehole. The extensive analysis further identified sub-seismic features impeding drillability in these lithologies. Further, the holistic approach helped characterize the pressure regimes in different formations and in parallel, based on corroboration from available data, constrained stress magnitudes, indicating a transitional faulting regime. Variances in stress settings corresponded to the depths just above the transition zone, where significant variations were observed in shear wave azimuthal trends thereby indicating the presence of potential fracture clusters, some of which were revealed to be intersecting the borehole thereby causing stress. The analysis shed light on these near well fractures- prone to shear slip, causing mud losses during drilling while drilling with high mud weights. Finally, the encompassing multiple results, an operational mud weight window was devised for the planned casing setting depths. Given the presence of numerous fractures, the upper bound of the operational mud window was constrained further to account for the presence of these fractures. In summary, an integrated approach involving a detailed DSWI study in addition to traditional geomechanics has brought about new perspectives in assessing borehole instability. By actively identifying the sub surface features, (sub seismic faults and fractures) decisions can be taken on mud weight and optimizing drilling parameters dynamically for future field development.