Fracture pressure gradient

Subsurface Rock Fracture Pressure (Fracture Pressure Gradient). The subsurface rock fracture pressure can be approximated by utilizing the known pore pressure at the same depth. The relationship between rock fracture pressure p (psi) and pore pressure p (psi) is [34]... 

The rock fracture pressure gradient at depth can be approximated by using Equation 2-174 and the variable Poisson s ratios versus depth data (Figure 2-58) and the variable total overburden stress gradients versus depth data (Figure 2-59). 

In Figure 2-57 the pore pressure gradient has been given as a function of depth for a typical Gulf Coast well. Determine the approximate fracture pressure gradient for a depth of 10,000 ft. From Figure 2-57, the pore pressure gradient at 10,000 ft is... 

This value of 0.90 psi/ft falls on the dashed line of Figure 2-57. The entire dashed line (fracture pressure gradient) in Figure 2-57 has been determined by using Equation 2-174. 

In general. Equation 2-174 can be used to approximate fracture pressure gradients. To obtain an adequate approximation for fracture pressure gradients, the pore pressure gradient must be determined from well log data. ALso, the overburden stress gradient and Poisson s ratio versus depth must be known for the region. 

There is a field operation method by which the fracture pressure gradient can be experimentally verified. Such tests are known as leak-off tests. The leak-off test will be discussed in Chapter 4. 

Compute the fracturation pressure gradient and fracturation pressure at 8,460 ft assuming a Poisson ratio of 0.4. 

Suppose that in some area the expected formation pressure gradient is 0.65 psi/ft and formation fracture pressure gradient is 0.85 psi/ft. A gas-bearing... 

Fig. 14 relates the pore-pressure data from all the wells to the minimum fracture pressure gradient of... 

Fig. 14. Relationships between pore-pressures, the hydrostatic gradient, the fracture pressure gradient (approximation to the minimal horizontal stress, Sf,) and the lithostatic pressure gradient (approximation to the vertical stress, S ). Pore-pressures from sea floor to base Pliocene equals hydrostatic. The yellow, dark blue and red pore-pressure trend-lines represent the pore-pressure versus depth gradients for the Paleocene-Eocene, Mid-late Cretaceous and Upper Jurassic-lowermost Cretaceous, respectively. The portion of the red trend-line below approximately 2550 m MSL equals the maximum reservoir pore-pressure trend-line of Fig. 13 and reflects the counter-pressure of the topseal controlling the pore-pressure distribution of hydraulic compartments II, III and (probably) IV.

Fig. 11. Plot of fracture pressure gradients versus depth. Fracture pressure gradients approach the lithostatic gradient with increasing depth and the two trends more-or-less coincide below 5000 m.

It is obvious that in the case of pressure gradients determined by the maximum controlled by buffer reactions in the pile of rocks, and by the minimum in fractures (in the case of open circulation Pf(n,in) - (hydr) of the column of fluid), mechanical movement of the fluid was in one direction — from rock to fracture. For movement in the opposite direction—from fracture to rock—it was necessary to create a corresponding pressure gradient. Such phenomena, in addition to diffusion along the concentration gradient, presumably have occurred in hydrothermal metamorphism with typical reactions of hydration and carbonation. However, for normal progressive metamorphism it is hard to imagine a mechanical model in which a fluid with a strictly constant value of / h,o " h,o introduced from... 

Fig. 4. Whilst the Smerbukk and Smerbukk Ser fields (Fig. la) today have pressures on the oil gradient just above the hydrostatic gradient, pressures in structures in Halten Vest are close to the estimated fracture pressures (from leak-off tests). Most wells in the overpressured region are dry, unless traps are sufficiently deep to avoid the fracture gradient, e.g. Kristin (Figs la 5), and this region coincides roughly with the area where the Spekk Formation is in the very last part of the oil window, or already over-mature.

However, some overpressured traps, such as Kristin (Figs 1 5), with a substantial column of condensate exist in Halten Vest and preservation of petroleum in such high pressure isolated fault-bounded reservoirs is fundamentally no different from a hydrostatic system, i.e. the occurrence of the accumulation simply reflects that the fracture pressure at the given depth is higher than the actual reservoir pressure (cf. general principles discussed in Holm 1996). Thus, Kristin is presently sufficiently deep to avoid the fracture gradient and has also a very thick caprock (cf Figs 4 7). No obviously visible gas chimney exists above Kristin, whilst such is indicated over the Lavrans trap. 

Gas Emission from adjacent seam creates higher methane concentrations in the fracture zone. However, the pressure gradient is almost zero, for the floating effect gas will gradually accumulate in the upper. When a cycle of pressure takes place, a lot of gas will be pressed into the face, causing gas exceeding the limit. 

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