Assessment of In-Situ Hydrocarbon Saturation in the Presence of Deep Invasion and Highly Saline Connate Water

This paper describes a field study undertaken to quantify the effects of mud-filtrate invasion on resistivity induction logs. The objective. is to assess in-situ gas saturation in a low-porosity carbonate formation. A large discrepancy between the salinity of connate water and drilling mud is responsible for the presence of a substantial low-resistivity annulus in the near-wellbore region. This annulus suppresses the sensitivity of electromagnetic induction currents to detecting gas saturation in the virgin zone. A quantitative explanation for the presence of the low-resistivity annulus is presented based on the physics of mud-filtrate invasion. The process of mud-filtrate invasion is modeled with a two-dimensional chemical flood simulator that includes the effect of salt mixing between mud filtrate and connate water. Radial resistivity profiles are obtained from the simulated spatial distributions of water saturation and salt concentration using Archie's law. These profiles confirm the presence of the low-resistivity annulus in the transition region between the flushed and virgin zones. Numerical simulation of induction logs validates the agreement between the mud-filtrate invasion model and the available wireline induction logs. An extensive sensitivity analysis is performed to quantify the effect of several petrophysical parameters on the spatial distributions of water saturation and salt concentration. Results from this study show that the pre-annulus and annulus segments of the radial resistivity profile remain insensitive to initial water saturation, thereby impeding the estimation of in-situ gas saturation from resistivity induction logs alone. Modeling of the process of mud-filtrate invasion is the only possible way to estimate in-situ hydrocarbon saturation from induction logs. It is also found that laterolog measurements are only marginally affected by the presence of a low-resistivity annulus. The sensitivity analysis described in this paper provides a rigorous quantitative method to assess the effects of different types of muds on the invaded zone prior to drilling.