Stress state in parts of North-East India: borehole collapse modelling with sensitivity analysis

Structural heterogeneities and tectonic forces in North-East India give rise to changeable in-situ stresses with varying orientations in this part of India. Wells located in the seismic gap in Upper Assam, Naga Thrust and Chittagong-Mizoram-Tripura fold belt of Mizoram are considered for studying the stress state and borehole collapse models for the area. The absence of stress studies in the Mizoram area acts as a stimulator to take up stress studies. Poroelastic modeling shows an average ratio of maximum horizontal to vertical stress to be 0.79 for normal faulted, 1.18 for thrust faulted and 1.12 in strike-slip faulted regimes. The SH direction varies from 193°N in Upper Assam to 213°N in Mizoram areas. The image log in a well of the Mizoram area shows the rotation of SH direction (≈85°) from 500m to 3707m due to structural heterogeneity. The thrust and strike-slip regimes under the study area pose the major threat for safe borehole drilling in this complex terrain. To mitigate this issue, Mohr-Coulomb (MC) and Mogi-Coulomb (MG) rock failure criteria are discussed to predict minimum mud weight for borehole drilling. MG predicted mud weight (MW) ensures borehole stability in wells in normal faulted sediments while MC predicted MW prevents shear failure in wells in thrust and strike-slip regimes. A disc plot is used to model a stable wellbore drilling path with minimum MW is modeled using a disc plot. A vertical well is stable in a normal faulted regime whereas horizontal drilling is preferable in the fold-thrust belt. Sensitivity analysis of geomechanical input parameters on MW using Monte Carlo Simulation shows that SH has the maximum effect on MW regardless of the faulting regimes.