Complex Settings Need Complex Equations – Comparison Between Numerical and Semi-Quantitative Geomechanical Solutions

Simplified semi-quantitative equations are used in 1D geomechanics workflows to predict the rock’s behavior during drilling and production. While such methods allow for getting a time- efficient solution, it does lose out on accuracy. In addition, by simplifying equations, we limit our ability to predict behavior of the borehole wall only i.e. near wellbore solutions. We lose the ability to predict full field behavior in response to drilling and production activities. For example, when constructing a field-wide drilling plan or a field development plan for a complex subsurface setting, a simplified approach may not be accurate enough and on the contrary, can be quite misleading. A 3D numerical solution on the other hand, honours subsurface features of a field and simulates for their effect on the stresses. It generates solutions which are more akin to reality. In this paper, this difference between a simplified semi-quantitative well-centric approach (1D) and a full field numerical solution (3D) has been presented and discussed. The subsurface setting considered in the study is quite complex – an amalgamation of high dipping beds with pinch outs and low angled faults in a thrust regime. Wellbore stability and fault stability models have been constructed using both a well-centric approach and a full field-wide 3D numerical solution. It is clearly observed that field-based approach provided us more accurate estimation of overburden stresses, variation of pore pressure across the field, impending changes in stress magnitudes and its rotation due to pinch-outs and formation dips. For example, due to variation in topography, the estimated well-centric overburden at the toe of deviated well at reservoir level is lower by 0.21gm/cc (~1.75ppg~0.9psi/ft) as compared to the 3D model. It is also observed that within the field itself stress regime changes from normal to strike slip laterally across the reservoir. In comparison to 1D model, considerable differences in stable mud weight window of upto 1.5ppg is observed in wells located close to faults. This is primarily due to effect of fault on stresses (both magnitude and azimuth). Stress states of 4 faults were assessed and all 4 faults are estimated to be critically stressed with elevated risk of damaging three wells cutting through. However, a simple 1D assessment of stress state of faults at wells cutting through them, shows them to be stable. By comparing the differences between 1D and 3D solutions, importance of 3D numerical modelling over 1D models is highlighted.