Middle Bakken wettability evaluation using NMR T2 forward modeling and mineralogy

Abstract

Determination of the wettability of rocks is one of the crucial elements in reservoir characterization. It affects the recovery efficiency, relative permeability, and volumetric reserves calculations. Assuming wrong wetting conditions may result in inaccurate reserves estimation and could be detrimental to improved and enhanced hydrocarbon recovery processes. There are several conventional methods based on fluid displacement, such as USBM and Amott-Harvey, to determine wettability indices. However, these methods are time-consuming and expensive in tight unconventional reservoir core plugs. The approach developed by Looyestijn and Hofman (2005) was applied to determine a wettability index of preserved Middle Bakken core plugs using Nuclear Magnetic Resonance (NMR) transverse relaxation time (T2) data. In addition, a new simple approach was developed to predict likelihood wettability indices using the mineralogical content from X-ray Diffraction (XRD) data. The overall goal of computing wettability index (WI) is to identify ways in which we can significantly reduce the amount of time and cost of developing a representative wettability index in preserved Middle Bakken plugs, without changing the preserved state of the core. The main assumption in our approach is that cleaned, adjacent core properties are representative of the preserved core properties, such as helium connected porosity, absolute air permeability, Dean Stark saturation, mineralogy, etc. NMR T2 measurements in the adjacent, cleaned plugs were measured to develop the pore size distribution that is representative of the preserved plugs. To determine NMR wettability indices of the preserved Middle Bakken plugs, T2 relaxation measurements of preserved, and cleaned adjacent brine saturated plugs, bulk produced brine and oil under ambient conditions are used in the Looyestijn and Hofman (2005) NMR forward model. From NMR forward modeling, it is concluded that the preserved Middle Bakken plugs appear to be intermediate to fairly oil-wet. NMR wettability indices vary between -0.04 and -0.54. Also, to predict the wettability indices of the Middle Bakken plugs, a very simple approach based on previously published contact angle measurements on polished carbonate and silicate mineral surfaces was developed. Our results from this analysis conclude that the given plugs are likely oil-wet. Predicted likelihood wettability indices vary between -0.08 and -0.64. Likelihood wettability indices were matched with NMR wettability indices. The significance of this research study is to reduce time and cost in determining the wetted state.