Geomechanical Instabilities In Diagenetically Altered Unconventional Reservoirs Enhance Fluid Pressure And Production.

Abstract

Unconventional energy and mineral resources define the most abundant resources available on our planet and are attractive targets for novel exploration, stimulation and production techniques. Yet, often they are located in a deeper and hotter impermeable environment where they are inaccessible to conventional techniques. The main challenge is that they are typically trapped in a low porosity/permeability environment and are difficult to produce. An extreme end-member is the shale gas reservoir in the Cooper Basin (Australia) that is located at 3500–4000 m depth and ambient temperature conditions around 200°C. Shales of lacustrine origin (with high clay content) are diagenetically altered. Diagenesis involves fluid release mineral reactions of the general type Asolid <-> Bsolid +Cfluid and switches on suddenly in the diagenetic window between 100–200°C. Diagenetic reactions can involve concentrations of smectite, aqueous silica compound, illite, potassium ions, aqueous silica, quartz, feldspar, kerogen, water and gas. In classical petroleum engineering such interlayer water/gas release reactions are considered to cause cementation and significantly reduce porosity and permeability. Yet in contradiction to the expected permeability reduction gas is successfully being produced. We propose that the success of shale gas extraction from these deep reservoirs in the diagenetic window is based on geomechanical instabilities from volumetric compaction controlled by diagenetic reactions. These instabilities appear as ductile compaction and dilation bands, which act as fluid channels. In this contribution we present analytical solutions where ductile dilation and compaction lead under gravity and tectonic loads to a pattern of intersecting dilational and compactive instabilities which both appear as fluid channels. We conclude by showing experimental evidence of periodic instabilities predicted in this study in a controlled laboratory experiment (kindly provided by Papamichos pers.comm.). This experiment reveals instabilities that emerge in a low permeability environment, where we expect the fluid diffusivity to be at least an order of magnitude lower than the mechanical loading. The experiment has been done independently of our work and a more detailed comparison with the theory will be the subject of forthcoming studies.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015