The Impact of Heterogeneity on Stressed Steady State Permeability Measurements in the Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

Abstract

Matrix permeability is identified as one of the key parameters for EUR prediction in unconventional plays. Stressed steady state permeability measurements can be regarded as a viable method for quantification of matrix permeability in mud rocks. The aim of this study is to highlight the impact of the inherent heterogeneity at the sample scale on the measured stressed steady state permeability.

Steady state permeability analyses were conducted on a number of horizontal and vertical core plugs from the Vaca Muerta Formation in the Neuquén Basin, Argentina. Specially designed pump systems, seals and sleeves reduced leak rates significantly to allow permeability determination down to around 1nD. Constant flow conditions were achieved with constant pressure difference across sample, and equal inflow and outflow volumes, and sufficient time in between pressure steps. In addition to the physical measurements, we characterized the core plugs using thin section and high resolution CT-Scans. The entire core plug has been scanned at a resolution of around 11 microns/voxel, before and after stressed steady state permeability measurements. The CT-Scans and the petrographical analysis allowed us to link directly the observed physical measurement with the pore texture and heterogeneity of the rock.

The analysis of high resolution plug-scale 3D micro-textural volumes, pre- and post-test, revealed presence of abundant horizontal hairline bed partings in the Vaca Muerta, increasing its heterogeneity. Understanding heterogeneity is of paramount importance as variations in bedding interfaces, horizontal hairline bed partings and natural fractures can impact measured properties of the rock. The study showed that vertical natural fractures, even though mineralized, significantly enhanced vertical permeability. The origin and nature of horizontal hairline bed partings are rather ambiguous. The lack of mineralization could point to their origin being mostly induced and related to the acquisition of core and test plugs. However, observations of horizontal bed partings (partially or completely mineralized) in the Vaca Muerta outcrops and cores indicate that at least some bed partings are natural. These bed partings drastically impact the permeability measurements under stressed conditions but are prone to partial closure during pressurization. Nevertheless, initial compaction and closure of hairline partings dominate stress dependence and strong permeability hysteresis effects.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016