Fluid Flow in a Touching-Vug Cretaceous Carbonate Outcrop: Measurements and Models from Millimeters to Kilometers

James W. Jennings¹, Liying Zhang², Narayan Nair³, Steven L. Bryant³, and F. Jerry Lucia^1
1 The University of Texas at Austin, Austin, TX
² BP America Inc
³ Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX

Relationships between matrix porosity, rock fabric, and permeability are well established in carbonates. However, some reservoirs exhibit flow rates that are higher than can be explained by the properties of matrix pore space alone. In some cases the additional flow capacity exists in touching-vug porosity: vugs that form a connected pore network independent of the matrix. These fabrics are difficult to study because the pore networks often occur on scales that are too large to quantify using thin sections or core samples.

The purpose of this study is to quantify the flow behavior of a vuggy Cretaceous caprinid buildup exposed near San Antonio, Texas. The caprinid mounds are composed of two facies, an in situ facies and a flanking debris facies. The in situ facies is composed of caprinids in growth position, but the vugs within the rudist shells are not touching. However, the flanking debris deposits contain many centimeter-scale touching vugs both within and between the rudist shells. The debris facies was studied with three experimental techniques: 0.5-mm resolution X-ray CT imaging of a 30-cm-scale sample, laboratory flow experiments, and well testing on the outcrop. From these data we conclude that the permeability is in the range of 1-100 darcies, but most of the flow occurs in a small fraction of the pore space and passes through some pore throats no larger than a few millimeters in diameter. The study culminated in a kilometer-scale flow model of the rudist mound complex.