The Role of Fluids in Crustal Processes (1990) / Chapter Skim
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7. Time-Dependent Hydraulics of the Earth's Crust
7. Time-Dependent Hydraulics of the Earth's Crust
Pages 113-127
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From page 113... ...
To reconcile this conflict, the hypothesis is explored that crustal porosity, permeability, and hence pore pressure are in general time dependent due to the gradual closure of crustal pore space via healing, sealing, and inelastic deformation. It is found that when the hydraulic conductivity of the system is large, so that the ratio of porosity reduction rate ~ to permeability k, ¢/k, is small, the initially porous water-saturated crustal rock mass will gradually lose its porosity and fluid, until it becomes essentially dry.
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From page 114... ...
water expulsion events associated with natural hydraulic fracturing may explain such observations as the episodic nature of some types of ore deposits and veins, the recurrence time between large earthquakes, the emplacement of foreland thrust systems, and the seismic and mechanical nature of crustal detachment zones. INTRODUCTION One of the simplest yet profound questions that can be asked about the state of the Earth's crust is the depth to which free water extends.
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From page 115... ...
However, the presence of free water to much greater depths in the 8 vp it, ' - - -~0 :-f Vo 1~ -- ~-: ~ 1 -- ~-: \ Lit Hydr~aP.c 1-: t ' Lithostatic HyrJros~ic FIGURE 7.3 Schematic illustration of the development of a low compressional and shear wave velocity zone due to anomalously high pore pressure, based on extensive laboratory measurements (Nur and Simmons, 1969) and some field observations.
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From page 116... ...
On the basis of commonly accepted criteria for brittle failure, this requires that Pf ex AMOS NUR AND JOSEPH WALDER ceed the minimum principal confining stress at the time of fracture formation.
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From page 117... ...
examined the hypothesized relationship between seismically reflective zones in the crust and elevated Pf by combining simple models of Pf development with synthetic seismograms. Jones concluded that, although elevated pore pressure can significantly affect the existence and amplitudes of reflected waves, such effects persist for geologically significant periods of time "only for a permeability lower than that generally observed in laboratory measurements on crustal rocks." Clearly, a model is needed that can reconcile the two conflicting lines of evidence regarding crustal hydrology: evidence for ubiquitous high pore pressure and hence low permeability on the one hand, and evidence for relatively fast flow and fast pore pressure dissipation on the other.
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From page 118... ...
for 2 weeks. Their results indicate that porosity reduction rates in rocks subjected to tectonically induced deviatoric stresses can be fast, suggesting that porosity reduction rates in situ may also be geologically fast.
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From page 119... ...
, the rock will gradually lose its porosity and hence the pore fluid. If fluid escape is slow relative to pore pressure buildup, Pp will reach lithostatic pressure and cause natural hydraulic fractures to occur.
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From page 120... ...
I . , O 0.2 0.4 0.6 0.8 DIMENSIONLESS DEPTH 1.0 FIGURE 7.10 Fluid pressure as a function of depth for a 10-kmthick section undergoing uniform porosity reduction for several values of porosity reduction rate ~ (in s-l)
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From page 121... ...
From this simple analysis we can conclude that the total porosity reduction associated with one cycle of pore pressure buildup and natural hydraulic fracturing is only a small fraction of the porosity of typical crustal rocks. Consequently, maybe the porosity reduction process could involve many such cycles.
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From page 122... ...
A different estimate of the number of P cycles n can be p made by considering the amount of porosity reduction needed to raise Pp to lithostatic pressure per Pa cycle. Assuming that the mass of fluid in the decreasing pore space is conserved during the Pp buildup phase of each cycle and is being reduced only during the expulsion phase, we can write AVf Mop (7.7)
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From page 123... ...
Estimates of the time required for P to reach lithostatic p and the amount of porosity reduction, especially in tectonically active areas, suggest that this kind of episodic hydrological and mechanical behavior of the crust is quite probable. The number of cycles expected at a given site depends on strain rate, permeability, depth, and other rock parameters, but simple analysis suggests that tens to hundreds of cycles may be expected.
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From page 124... ...
(a) Pore pressure in the region of porosity reduction reached the minimum compressive stress and caused natural fractures to occur in the sealed rock surrounding this region.
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From page 125... ...
tore as long as enough water remains in the pore space. The possibility that crustal seismic reflectors may be high pore pressure zones and hence mechanically weak is especially intriguing in view of the growing evidence that these reflectors may represent subhor~zontal crustal detachment zones.
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From page 126... ...
Nur (1984~. The nature of seismic reflections from deep crustal fault zones, Journal of Geophysical Research 89(B5)
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From page 127... ...
Nur (1984~. Porosity reduction and crustal pore pressure development, Journal of Geophysical Research 89(B 13)
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