Laboratory analysis of the predictive value of image-based computations for resistivity index

Abstract

This thesis presents a numerical and experimental investigation of resistivity index (RI) at low water saturations. A novel analysis of micro-CT images of small rock samples is combined with laboratory measurements to examine the predictive value of imagebased computations of RI. Experimentally measured porosity, formation factor, capillary pressure (Pc) and RI are compared with image-based computations using the same rock sample for both imaging and laboratory measurements. The fluid systems used are 2wt% NaCl brine and air. The porous plate and centrifuge methods are used to measure Pc and RI at ambient conditions. Two carbonate samples from the Middle East and two outcrop sandstone samples (Fontainebleau and Bentheimer) are investigated. Three of the samples are homogeneous and one of the carbonate samples is heterogeneous. The effects of injection boundaries on image based computations of single and multiphase flow properties of the rock samples are investigated numerically. The RI of Fontainebleau and Bentheimer sandstone is studied at water saturations as low as 10%. The laboratory measurements show that it is possible to conduct reasonable tests on the small rock samples which are used for image analysis. For homogeneous rocks these measurements compare well with measurements made using conventional size cores and the measurements are in reasonable agreement with image-based computations. Simulations are in good agreement with experimental measurements of capillary drainage RI using the porous plate method at water saturations as low as 10%. However, this agreement is a result of careful treatment of discretisation effects. Fontainebleau sandstone exhibits a percolating network of grain contacts, while the high-porosity Bentheimer sandstone does not. It is shown that this difference in the topological connection of conductive films at low water saturations is responsible for the non- Archie behaviour of Fontainebleau sandstone. It is shown that grain contact conductivity needs to be attributed to the grain contacts in Fontainebleau sandstone in order to reconcile experiments and computations. Conductive films organised as pendular rings around grain contacts are shown to be insufficient to explain this result.