Tight Reservoir Rock Integrity - Experimentally Measured Pre-Failure Permeability Response to Stress Changes

Armitage, Peter J.¹; Faulkner, Daniel R.¹; Worden, Richard H.¹
(1)Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom.

Detailed experimental studies of the development of permeability in storage domain rocks during deformation are essential to understand productivity from tight reservoir rocks and gas shales. The drilling of boreholes, and increasing of fluid pressure for hydrofracturing creates changes in stress, which will lead to the creation of a microfracture network. It has been demonstrated from experiments and modelling that accumulation of microfractures under differential stress before rock failure occurs systematically and leads to enhanced porosity, permeability and fracture surface area. Permeability was measured experimentally under various stress regimes, at stages in the stress path up to failure of the sample. Stress regimes were selected to recreate sub-hydrofracturing increases in pore fluid pressure and stress concentrations around a borehole. This was achieved using a high-pressure triaxial deformation apparatus, which can recreate changing reservoir pressure and fluid conditions. Samples tested included reservoir, caprock, and fluid flow barrier rocks from a natural tight gas storage system, and current CO2 storage reservoir, from the Krechba Field, Algeria. In the example of stress concentrations around a borehole, stress change was found to cause pre-failure permeability increase in all samples of up to two orders of magnitude.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.