Methodology for obtaining contact angles in rock sample images using image processing and polynomial fitting techniques

P M Sakugawa,H Pedrini,G Camargo,M A Jaculli,C E A G Barreto,R M G Santos

doi:10.1007/s13202-020-00851-2

Abstract

Estimating the contact angle in very complex rock pores presents some challenges to accurately identify the fluid–rock contact surface. This work presents a methodology to estimate the contact angle formed by the brine–rock and the brine–oil interfaces on processing high-resolution images provided by micro-CT scan. We focus the discussion on the limitations of the most popular computational techniques used to determine the contact angle and discuss how to select a practical way to evaluate it. The method consists of four steps: (1) processing the images to determine each fluid present in the image, (2) selection of the pixels that will be part of the contact interface of fluids and the contact point, (3) fitting polynomial equations for each interface and selection of the equation that gives the lowest error, (4) estimation of the contact angle based on the more appropriate polynomial equation. The contact angle is calculated based on the slope of the interfaces’ tangents at the contact point. Several types of approaches were tested to determine the contact interface and the contact point. In order to evaluate the applicability of our method, we use an analytically generated image and rock sample images. Potential errors between the angle obtained from the analytically generated image and the angle calculated from the method show the impact of the right selection of pixels during the image processing step. High sensitivity is also observed for the tangent values in the presence or absence of pixels from the rock sample analysis.

Highlights

The depletion of reservoirs evokes the necessity of applying enhanced oil recovery (EOR) methods, in order to prolong the field life cycle and increase the recovery factor as much as possible
The abscissa corresponds to the contact angle value from the analytically generated image; the ordinate is the error percentage between the analytical values and the computed ones; and each color of the marker corresponds to the employed range of the polynomial fitting order
This work focused on discussing the difficulties of image processing techniques used to determine contact angles

Summary

Introduction

The depletion of reservoirs evokes the necessity of applying enhanced oil recovery (EOR) methods, in order to prolong the field life cycle and increase the recovery factor as much as possible. Wettability, by definition, is the tendency of a fluid to adhere to a solid surface in the presence of another immiscible fluid (Zisman 1964) and can be quantified, at the pore scale, by the contact angle formed at the interface of the two fluids in balance with a solid surface. Calculations based on measured contact angle are fundamental in determining rock wettability characteristics, such as if a rock is oil-wet or water-wet. The estimation of the contact angle helps to model multiphase flow in order to enhance oil and gas recovery (Pope and Braviere 1991) and geological CO2 storage (Chalbaud et al 2009; DePaolo and Cole 2013). One way to estimate the contact angle is through the use of high-resolution images of rock samples

Objectives

Methods

Results

Conclusion