New Iterative Resistivity Modelling Workflow Reduces Uncertainty in the Assessment of Water Saturation in Deeply-Invaded Reservoirs

Abstract

A new iterative modelling workflow has been designed to reduce uncertainty of water saturation (Sw) calculations in the tight Barik sandstone in the Sultanate of Oman. Results from this case study indicate that Sw can be overestimated by up-to twenty saturation units if the as-acquired deep resistivity is used in volumetric calculations. Overbalanced drilling causes deep invasion of water-based mud (WBM) filtrate into porous and permeable rocks, leading to radial displacement of in-situ saturating fluids away from the wellbore. In low-porosity reservoirs drilled with WBM the inability of the filtration process to quickly build impermeable mudcake translates into long radial transition zones. Under certain reservoir and drilling conditions, deep resistivity logs cannot reliably measure true formation resistivity and are therefore unable to provide an accurate assessment of hydrocarbon saturation. The effect of mud-filtrate invasion on resistivity logs has been extensively documented; processing techniques utilize resistivity inversion and tool-specific forward modeling to provide uninvaded formation resistivity logs which are much better suited for in-place resource volume assessment. However, sensitivity analysis shows that the accuracy of invasion-corrected logs dramatically decreases as the depth of invasion increases whereby the inversion process needs to be further constrained. The new workflow is designed to reduce the non-uniqueness of true formation resistivity models, so that they honor multiple and independent petrophysical data. The inversion routine utilizes a Bayesian algorithm coupled with Markov-Chain Monte Carlo (MCMC) sampling. Inversion results are iteratively modified based upon two rock property models: one derived from rock-core data (helium expansion porosity and Dean- Stark saturations), and the other using an equivalent log interpretation of thick reservoir intervals from oil-based mud (OBM) wells. Simulated borehole-resistivity are compared to field logs after each validation loop against rock property models. The new inversion-based workflow is extensively tested in the unconventional tight Barik formation across water-free hydrocarbon and perched water intervals and inversion-derived Sw models are independently validated by capillary pressure-derived saturation-height models and fluid inflow rate from production logs.