Reproducing Core Wettability in Laboratory Core Restorations and the Influence of Solvent Cleaning on Carbonate Wetting

Abstract

With the current reservoir engineering technology, it is not possible to measure wettability at downhole conditions. Therefore, laboratory work is necessary to correctly assess this parameter. Countless efforts have been made by reservoir engineers to obtain decent estimates of reservoir wettability. However, in many cases this objective remains elusive. This means that the current special core analyses (SCAL) protocols have not overcome this hurdle, and new approaches must be tested. In this work, the effects on wettability of two different sets of organic solvents are studied in carbonates, and a new cleaning and restoration protocol is tested to reproduce wettability in carbonate core samples. Two water-wet chalk core plugs were restored with the same initial formation water saturation (Swi=10%), and then were saturated and aged in crude oil to create an initial wetting. Spontaneous imbibition (SI) experiments confirmed the reproducibility of the restoration process used. After spontaneous imbibition, the two cores were cleaned with different methods, the first core plug was subjected to a mild cleaning process (kerosene-heptane) and the second one was cleaned with a harsh method (toluene-methanol). It was found that the mildly cleaned chalk core was slightly water-wet, and the harshly cleaned core appeared to have changed to a more water-wet state. Therefore, the solvent pair, kerosene-heptane, preserved more polar components at the carbonate surface than the toluene-methanol pair, the latter, was more effective in solvating and removing polar components from the rock surface, showing increased capillary forces in the SI test with heptane. The last stage of the study aimed to reproduce the first induced wettability, this was carried out in two cores after a mild cleaning process. It was possible to closely reproduce the initial wetting of these cores. This was accomplished by controlling the injected amount of mild cleaning solvents and crude oil during the second restoration process. These results represent a successful first phase of research towards wettability reproduction and improved reservoir wettability evaluation. Furthermore, it represents a solid and modern alternative to the traditional SCAL approach.