Wettability Alteration during Aging: the Application of Nmr to Monitor Fluid Redistribution

The changes occurring during aging of reservoir core samples in crude oil, which aims to restore the reservoir wettability, are complex. The dynamic mechanisms involved are still not entirely clear. Understanding the dynamics of wettability restoration in the laboratory will improve our ability to reproduce reservoir conditions before performing core tests and help to ensure that laboratory data are representative of the reservoir. This paper investigates the changes of fluid distribution occurring at the pore-scale during wettability restoration using a combination of NMR T2 relaxation data and electrical impedance measurements. Sandstone core plugs were aged in crude oil at various conditions. One sample was aged at atmospheric pressure and a temperature of 35 to 50° C, for more than 100 days while two other plugs were first aged at ambient conditions for 80 days and then at 85° C and 1.38 MPa for 50 more days. NMR relaxation and electrical measurements were performed on all the plugs at regular time intervals during aging. In addition, the Amott-Harvey wettability index was measured on clean and aged cores. The T2 distributions as a function of aging time clearly show changes in oil/water configuration at the pore scale. We infer that this can only be due to wettability changes resulting from interactions between the crude oil and the rock surface. Both NMR data and electrical measurements show that wettability alteration can occur before the 40 days traditionally used in the majority of studies, even at room temperature and pressure. These experiments demonstrate the ability of NMR to identify wettability changes and, under certain conditions, to infer the fluid distributions at pore level. INTRODUCTION During special core analysis experiments it is important to establish a wettability condition at the outset that is as close as possible to that originally found in the reservoir.