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Pore pressure profiles in fractured and compliant rocks 1
Author(s) -
Yilmaz Özdoǧan,
NolenHoeksema Richard C.,
Nur Amos
Publication year - 1994
Publication title -
geophysical prospecting
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.735
H-Index - 79
eISSN - 1365-2478
pISSN - 0016-8025
DOI - 10.1111/j.1365-2478.1994.tb00236.x
Subject(s) - permeability (electromagnetism) , geology , fluid dynamics , pore water pressure , pressure gradient , mechanics , fluid pressure , geotechnical engineering , mineralogy , chemistry , physics , biochemistry , oceanography , membrane
Fluid permeability in fractured rocks is sensitive to pore‐pressure changes. This dependence can have large effects on the flow of fluids through rocks. We define the permeability compliance γ= 1/ k ( k /δ p p ) pc , which is the sensitivity of the permeability k to the pore pressure p p at a constant confining pressure p c , and solve the specific problems of constant pressure at the boundary of a half‐space, a cylindrical cavity and a spherical cavity. The results show that when the magnitude of permeability compliance is large relative to other compliances, diffusion is masked by a piston‐like pressure profile. We expect this phenomenon to occur in highly fractured and compliant rock systems where γ may be large. The pressure profile moves rapidly when fluids are pumped into the rock and very slowly when fluids are pumped out. Consequently, fluid pressure, its history and distribution around injection and production wells may be significantly different from pressures predicted by the linear diffusion equation. The propagation speed of the pressure profile, marked by the point where δ p p /δ x is a maximum, decreases with time approximately as and the amplitude of the profile also dissipates with time (or distance). The effect of permeability compliance can be important for fluid injection into and withdrawal from reservoirs. For example, excessive drawdown could cause near‐wellbore flow suffocation. Also, estimates of the storage capacity of reservoirs may be greatly modified when γ is large. The large near‐wellbore pressure gradients caused during withdrawal by large γ can cause sanding and wellbore collapse due to excessive production rates.