Modeling Oil Recovery for Mixed Macro- and Micro-Pore Carbonate Grainstones

Ye Xu,Hubert E King,Qiuzi Li

doi:10.1038/s41598-017-09507-4

Abstract

In general, modeling oil-recovery is a challenging problem involving detailed fluid flow calculations with required structural details that challenge current experimental resolution. Recent laboratory experiments on mixed micro- and macro-pore suggest that there is a systematic relationship between remaining oil saturation (ROS) and the fraction of micro-pores. Working with experimental measurements of the pores obtained from X-ray tomography and mercury intrusion capillary pressure porosimetry, we define a digital rock model exemplifying the key structural elements of these carbonate grainstones. We then test two fluid-flow models: invasion percolation model and effective medium model. Although invasion percolation identifies the important impact of macro-pore percolation on permeability, it does not describe the dependence of ROS on micro-pore percentage. We thus modified the effective medium model by introducing a single-parameter descriptor, reff. Oil from pores r ≥ reff is fully removed, while for the remaining pores with r < reff, their contribution is scaled by (r/reff)2. Applying this straightforward physics to pore size distributions for the mixed-pore grainstones reproduces the experimental ROS dependence.

Highlights

Heterogeneity of pore size in carbonates is well known
Samples came from the Fullmer et al.[8] case study on rock cores from a large Cretaceous offshore oil reservoir
Carbonate studies are challenged for these rocks, as there is no recognized manner to restore reservoir state wettability[17, 18] a sampling strategy that included benign coring fluid and sample preservation coating was pursued to insure that all measurements were performed on native state core material

Summary

Introduction

Heterogeneity of pore size in carbonates is well known. An important class of such rocks exhibit abundant micro-pores in the same rock where macro-pores are present[2,3,4,5,6,7]. Focusing on Type I microporous carbonates, Fullmer et al.[8] studied the oil recovery characteristics when macro-pores are present. See the study by Fabricius[13] of burial diagenesis for chalk sediments As this proceeds the intra-particle pores become more interconnected and homogeneous in size, dominated by inter-crystalline space between micro-crystals. Mineralogical and textural characteristics of the original grains can influence microporosity formation, the grainstones under study exhibit porosities for grains that produce about 20% microporosity, typical of Type I These diagenetically driven variations will alter the pore geometry in a systematic manner. Consideration of a simple grainstone model allows us to create an ensemble of possible geometric variations Working within those bounds, we can examine how oil recovery factor is affected by the geometry of the pore system. Our aim is to identify the key structural parameters and to determine the sensitivity of ROS to variations in this geometry

Objectives

Results

Conclusion