NMR logs have traditionally been used to differentiate producible fluids from capillary bound water. When this assumption is valid, NMR is useful for predicting water cut in hydrocarbon bearing formations.
When wells are drilled with oil-base mud (OBM) however, there is evidence that the OBM-filtrate displaces some of the capillary bound water, possibly due to the presence of surfactants in the OBM (Chen, 2004). The presence of OBM-filtrate in the capillary bound pore space will affect the NMR log response, and differentiating capillary bound fluid from producible fluid becomes difficult.
We have been studying core samples from four hydrocarbon bearing formations from a well drilled with oil-base mud. To quantify the effects of the OBM-filtrate on the NMR response we have performed lab NMR measurements on the core samples saturated with brine and OBM filtrate, brine and the base oil used in the OBM, and brine and a non-wetting lab oil, Fluorinert (F77).
The results will be compared to NMR log data and recommendations will be made to optimize the log analysis for estimation of water-cut, irreducible water saturation and permeability.