Abstract
AbstractThe concept of effective stress is one of the basic tenets of rock mechanics where the stress acting on a rock can be viewed as the total stress minus the pore water pressure. In many materials, including clay-rich rocks, this relationship has been seen to be imperfect and a coefficient (χ) is added to account for the mechanical properties of the clay matrix. Recent experimental results during the flow testing (both gas and water) of several rocks (Callovo-Oxfordian claystone, Opalinus Clay, Boom Clay) and geomaterials (bentonite, kaolinite) has given evidence for stable high pressure differentials. The design of the experiments allows multiple measurements of pore pressure, which commonly shows a complex distribution for several different experimental geometries. The observed stable high pressure differentials and heterogeneous pore pressure distribution makes the describing of stress states in terms of effective stress complex. Highly localized pore pressures can be sustained by argillaceous materials and concepts of evenly distributed pore pressures throughout the sample (i.e. conventional effective stress) do not fit many clay-rich rocks if the complexities observed on the micro-scale are not incorporated, especially when considering the case of gas flow.
Highlights
The introduction of pore-fluid under pressure has a profound effect on the physical properties of porous solids (Hubbert and Rubey, 1961; Terzaghi, 1943)
In this paper we have shown that pore pressure distribution during different laboratory and full-scale experiments was far from simple during both water and gas injection testing
The pore pressure distribution observed in shear experiments and in the full-scale Large scale gas injection test (Lasgit) test suggest that pore pressures are not evenly distributed and can be localised into almost isolated areas, in the case of Lasgit this may stem from disequilibrium
Summary
The introduction of pore-fluid under pressure has a profound effect on the physical properties of porous solids (Hubbert and Rubey, 1961; Terzaghi, 1943). Up until approximately Day 237, the pressure within the guard rings appeared to be independent of the injection gas pressure This showed that within the test sample very little pore pressure change had occurred and gas had not started to enter the sample. The Angled Shear Rig (ASR, Figure 3a) was used to investigate water and gas flow within a kaolinite gouge sandwiched between two steel platens angled to the stressfield (see Table 1 for test material parameters and experimental boundary conditions). The apparatus was designed with the following specification to test two prepared samples of 60 mm × 60 mm × 21 mm; normal load of up to 12 MPa, injection pore pressure of 0.5 – 12 MPa, shear as slow as 1 mm per 3 month period, and vertical displacement measured to a precision greater than 60 nm. These observations show that gas flow was localised within the Lasgit deposition hole and that pore pressure was not evenly distributed throughout the deposition hole
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