Page 68 - Geology and Geochemistry of Oil and Gas

44 TEMPERATURE AND PRESSURE IN THE SUBSURFACE

Formation of AHFP lowers the effective pressure and results in the emergence of
undercompacted rock zones. AHFP is non-uniformly and discretely distributed in
rocks. That is why mapping of AHFP using uniform interpolation (and even more
so, extrapolation) is not recommended.
It may be stated that

(3.4)
V AHFP ¼ V elast V relax
where V AHFP ¼ the rate of formation (or destruction) of AHFP, V elast ¼ the rate of
formation of elastic stresses and V relax ¼ the rate of stress relaxation.
This equation clearly shows all conditions for the formation or destruction of
AHFP. It can only exist while the conditions for the formation of elastic stresses are
present, and the rate of the formation (accumulation) of elastic energy is greater than
the relaxation rate. A horizontal stress of 10 MPa at a depth of 1000 m will dissipate
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after 1 million years only due to diffusion (at a very low diffusion rate of 10 ) (V.A.
Sokolov and V.F. Lipetskiy, personal communication).
If stress in the rock matrix exceeds the plastic limit, the rock begins to fracture. As
mentioned before, despite an insignificant decompaction (the fracture porosity is
low), the permeability will drastically increase. In her compaction experiments using
hydrostatic compaction apparatus, N.N. Pavlova (personal communication) found
only 10–12% decompaction and increase in permeability.
Stress relaxation in rock matrix occurs through fracturing whereas reservoir
pressure dissipation is due to the emigration of fluids through the fractures (and
diffusion, but more slowly). The role of the fluids per se in the formation of fractures
also should not be forgotten. Of particular significance here is water, which is a
medium with the lowest compressibility. As a result, a very slight increase in the
amount of water within the same pore volume results in a significant increase in
pressure. The low compressibility of water makes it play a very important role in the
formation of hydraulic fractures.
It was long and erroneously believed that a high pressure is necessary for
hydraulic fracturing (overburden pressure plus additional pressure to overcome the
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rock strength ). This would be correct if the rocks had rigid, non-deformable matrix.
The information derived from the practical applications of hydraulic fracturing (tens
of thousands) indicates that hydraulic fracturing may be achieved at just 0.5–0.75 of
the overburden pressure. The reason is that the rocks compress under the effective
pressure. Rocks compact as a result of (1) decrease in the volume of water-filled
pores, (2) temperature increase, and (3) relative increase in the volume of water due
to its slow escape (low permeability). As a result, reservoir pressure significantly
increases, causing hydraulic fracturing. The orientation of fractures is determined by
zones of weakness in rocks (most often, bedding planes) and by the spatial position
of a zone with the minimum (hydrostatic) reservoir pressure. Most commonly,
fracturing occurs in the overlying or underlying reservoir rocks. The emerging
difference in the reservoir pressure between the shales and reservoir rocks may be, as
Buryakovsky et al. (1990, p. 187) pointed out, an additional reservoir energy source
3 14 2
Young’s modulus for sedimentary rocks is in the range of 3–9 10 g/cm .