Left: Location of the Dakeng well (Hole-A, elevation at 247 m); generalized geological map and interpreted structural profile across the Dakeng well. The lithology between 800 and 1300 m below the ground surface of Hole-A, determined from the core and the Gamma-ray log, is summarized, and locations of selected rock samples are marked. In the gamma ray-derived lithology, green, brown, and yellow represent shale, siltstone, and sandstone, respectively. Right: Stress dependent (a) permeability and (b) porosity of the sandstone and silty-shale, for sandstone (red dashed lines) and silty-shale (solid black lines).

The permeability of shale was more sensitive to changes in effective confining pressure than the sandstone, particularly at low effective confining pressures. Meanwhile, the stress sensitivity of porosity of different rock types (sandstone versus shale) was almost identical. Based on the experimental results, it can be inferred that the evolution of compaction and permeability of the tested sandstone is related to the relative movement of the grains; the particle crushing mechanism is not involved. It is also postulated that a microcrack network in the tested shales contributes to the stress sensitivity of permeability under low effective confining pressure. The laboratory work indicates that a power law is superior to an exponential relationship for describing the stress dependency of permeability and porosity of the tested sedimentary rocks. In permeability–porosity relationships, the porosity sensitivity exponent determined for the sandstones is close to the theoretical values calculated for porous mediums (3–5). In contrast, the porosity sensitivity exponent of the silty-shale is much higher than that of the sandstone.

Exponential law for permeability and porosity