Wettability Quantification - Prediction of Wettability for Australian Formations
Authors: Goda Hussam Mohammed , Behrenbruch Peter 
Published: March 2015

As a rock-fluid interaction property, wettability is well recognized to influence the flow in multi-phase systems such as hydrocarbon reservoirs. In the laboratory, wettabilty measurements are made according to certain standard procedures and the results are expressed as indices for comparative purposes. The two most commonly used wettability indices are the USBM index, related to areas under capillary pressure curves, and the Amott-Harvey wettability index related to imbibition characteristics. If such measurements are not available, relative permeability curve characteristics may be used to quantify wettability.

As is the case with most special core measurements, wettability tests are expensive and time consuming, with the consequence that the number of plugs subjected to wettability testing is usually limited, often resulting in a poor definition of reservoir wettability characteristics. One objective of the study presented is to introduce a mathematical expression, which may be used to gauge relative wettability, as an alternative to the above-mentioned indices. The model has been validated using data from Australian hydrocarbon basins. A genetic algorithm approach was utilised to tuning parameters in the wettability model presented. The model compares favourably with laboratory measurements and may be used to predict USBM indices if experimental values are not available. As such, the formulation presented may also be used in wettability classification.

One of the relative permeability characteristics used to gauge wettability is the ratio of relative permeability end points. A second objective in the presented research is to predict this ratio, useful for the prediction of relative permeability characteristics. In considering possible analytical forms, the final derived formulae are extensions of the Carman-Kozeny equation.

Introduction

Reservoir wettability characterisation requires the knowledge of three types of data: rock related pore structure properties, fluid properties and rock-fluid interaction properties. The knowledge of the mineralogy is also helpful in characterising wettability. On the other hand, failure to properly characterise wettability may result in incorrect validation of special rock properties, such as relative permeability, leading to wrong recovery factors for a particular reservoir situation.

Wettability may be viewed in different ways. Fundamentally, from thermodynamics, a system may be considered in terms of an equilibrium state where wetting of a surface by a liquid can be correlated to the variation in the Gibbs free energy (Berg, 1993). The balance between cohesive forces within the liquid (tend to pull up fluid drops) and adhesive forces between liquid and solid (allow fluid to spread) are a measure of wettability (Embid, 1997). Quoting from the literature: "Wettability is defined as the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids." (Ahmed, 2001). Honarpour, et al. (1986) stated the following, "a term used to describe the relative attraction of one fluid for a solid in the presence of other immiscible fluids".