Stress-Dependent Permeability: Characterization and Modeling

Abstract

During the production lifecycle of a reservoir, absolute permeability at any given location may change in response to an increase in the net effective stress and a concomitant decrease in the value of in-situ permeability. This paper focuses stress dependent permeability in unconsolidated, high porosity sand reservoirs (offshore turbidites) and consolidated reservoirs (tight gas sands). Specifically we address:fundamental controls on stress dependent permeability, as identified through analysis of core samples,rock-based log modeling of stress dependent permeability in cored and non-cored wells, andimplications for production based on data from reservoir simulation. This work reveals a fundamental difference between the stress dependent permeability behavior of unconsolidated and consolidated sand reservoirs. In unconsolidated sand reservoirs, the greatest permeability reduction with stress occurs in the sands with the highest values of porosity and permeability. In cemented sandstone reservoirs, the opposite is the case: most of the reduction in permeability occurs in sandstones with the lowest values of porosity and permeability. This difference in behavior between unconsolidated and consolidated reservoir sands is controlled by pore geometry. We present a practical, rapid and cost efficient methodology to improve evaluation and enhance the productivity and management of stress-dependent reservoirs. The method is based fundamentally on the identification of "Rock Types" (intervals of rock with unique pore geometry). Thin section evaluation, together with integrated nuclear magnetic resonance and SEM-based image analysis of core material is used to quantitatively identify various Rock Types. Rock Type data is integrated with measurements of permeability at various levels of stress. Results demonstrate that, within a particular field, some Rock Types lose <10% of original permeability while others lose >90% of original permeability as a function of increasing stress. Rock Types are then identified using routine suites of wireline logs, allowing for field-wide determination of the net footage and distribution of each Rock Type in all wells and the foot-by-foot calculation of permeability at any value of net effective stress. Based on geological input, the reservoirs are divided into flow units (hydrodynamically continuous layers) and grid blocks for simulation. Several cases are presented of a conceptual, single well model of an overpressured, tight gas sandstone reservoir that include stress dependent permeability. Results of simulation analyses for varying conditions of reservoir stress demonstrate the importance of stress dependent permeability in more accurate forecasting of reserves and predicting optimum well bore producing conditions.

Numerical models for ground deformation and gravity changes during volcanic unrest: simulating the hydrothermal system dynamics of a restless caldera

“…For example, since an increase in the effective stress may cause a permeability and porosity reduction (Davies et al, 1999;Rutqvist et al, 2002), a drop in these hydrological parameters is expected where higher deformation are observed, namely at the centre of the domain and close to the fault zones. This may reduce the deformation and gravity change profiles over time.…”

Research on Stress Sensitivity of Fractured Carbonate Reservoirs Based on CT Technology

“…The stress sensitivity of fractured gas reservoirs, in terms of their flow properties, has become a focus of much attention of research [11,12]. Permeability is well known to negatively correlate with the effective stress [13], and the stress-dependent permeability becomes the norm of petrophyiscal characterization for, but not limited to, fractured reservoirs [14,15].…”

A new model for calculating permeability of natural fractures in dual-porosity reservoir

“…Then, some researchers (Jones, 1975;Jones et al, 1980;Walsh, 1981;Randolph et al, 1984;Jelmert et al, 1998) established a mathematical relationship between core permeability and effective stress, and found that the formula is applicable to naturally fractured carbonated reservoir and low permeability sandstone reservoir. Davies et al (2001) compared the stress sensitivity characteristics of different permeability cores, and found that for unconsolidated cores with high permeability, the larger the porosity and permeability, the stronger the stress sensitivity. However, for some finely cemented cores with low permeability, the smaller the permeability, the stronger the stress sensitivity.…”