Overburden gradient

Figure 4-325. Overburden gradient and Poisson ratio (a) variation of the overburden gradient with depth (b) variation of the Poisson ratio with depth. (Courtesy Editions Technip.)...

Gqb overburden gradient in psi/ft Gp = pore pressure gradient in psi/ft K = coefficient related to Poisson ratio... 

Compute the pore pressure at 15,000 ft using Eaton s equation, a normal gradient of 0.453 psi/ft, and an overburden gradient of 1 psi/ft assuming the well vertical. 

Subsurface Fluid Pressure (Pore Pressure Gradient). The total overburden pressure is derived from the weight of the materials and fluids that lie above any particular depth level in the earth. Of interest to the petroleum industry are the sedimentary rocks derived from deposits in water, particularly, in seawater. Such sedimentary rocks contain rock particle grains and saline water within the pore spaces. Total theoretical maximum overburden pressure, P (Ib/ft-), is... 

The average specific gravity of minerals in the earth s crust is taken to be 2.7. The average specific gravity of saltwater is taken to be 1.07. If the average sedimentary rock porosity is assumed to be 10%, then the total theoretical maximum overburden pressure gradient (lb/fp)/ft becomes... 

Equation 2 169 can be expressed in normal gradient terms of psi/ft. Equation 2-169, which is the theoretical maximum overburden pressure gradient, becomes... 

The foregoing theoretical overburden pressure gradient assumes that the sedimentary deposits together with the saline water are a mixture of materials and fluid. Such a mixture could be considered as a fluid with a new specific weight of... 

The fluid pressure in the rock at the bottom of a well is commonly defined as pore pressure (also called formation pressure, or reservoir pressure). Depending on the maturity of the sedimentary basin, the pore pressure will reflect geologic column overburden that may include a portion of the rock particle weight (i.e., immature basins), or a simple hydrostatic column of fluid (i.e., mature basins). The pore pressure and therefore its gradient can be obtained from well log data as wells are drilled. These pore pressure data are fundamental for the solution of engineering problems in drilling, well completions, production, and reservoir engineering. 

Figure 2-59 gives typical total overburden stress gradients versus depths for several regions in North America [36]. 

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). 

From Figure 2-59, the total overburden stress gradient is (i.e.. Gulf Coast curve)... 

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. 

Figure 2-59. Total overburden stress gradient vs. depth (from Engineering of Modem DiWng, Energy Publication Division of Harcourt Brace Jovanovich, New York, 1982, p. 82).

Figure 4-325a shows a plot of overburden pressure gradient versus depth for typical soft (1) and hard (2) provinces. Figure 4-325b gives similar data for Poisson s ratio. 

The relevance of the remarks on sulfur content is that, for reasons explained above, it is usually a valid index of the salinity of the environments of deposition. It was remarked earlier that the Eastern and Interior provinces have experienced different temperature/pressure/time histories. It should be added that coals of the Rocky Mountain, Pacific and Alaskan provinces most probably experienced yet further sets of conditions of metamorphism a locally increased geothermal gradient that produced relatively high temperatures at relatively low depths of burial and hence at relatively low pressures of overburden. 

Dipole development around a conductor immersed in an electrolyte with uneven oxidative properties is scale independent. Even tiny conductive or semi-conductive mineral grains in overburden will develop dipoles if there are redox differences across them. Almost all solids have some semi-conductive properties and therefore the nature of the conductive minerals is of less importance than the existence of a redox gradient. Large redox gradients in overburden contain polarisable minerals. 

It is, of course, very difficult to separate the effects of geothermal gradient from overburden pressure. In a given region the two are intercorrelated. 

The increased solubility of quartz in basic organic solvent systems appears to be caused by aqueous potassium hydroxide reaction at temperatures above the boiling point of the aqueous system alone. The organic solvent fraction serves as a substrate which permits attainment of elevated temperatures. Increasing the pressure at which basic aqueous reactions are performed would serve as an alternative method which would eliminate the need for addition of organic solvents. This prospect is especially attractive for in situ removal of silicates from oil shale since geothermal gradient and overburden may provide the elevated temperature and pressure necessary for efficient silicate removal. 

In addition to the parameters we found applicable to surface bursts, we must now add those that affect the lifting and containment effects of the overburden. These parameters include depth of burial and pressure gradient of the soil with depth (which is assumed linear), giving us... 

Fig. 13. Predicted effective stress versus depth using the seismic velocity and sonic data of Fig. 12. The curve labeled hydrostatic is obtained using a fluid pressure gradient of 0.465 psi/ft. The overburden pressures needed to generate the effective stress plots of this figure are obtained by integrating the appropriate density curve.

A brief comment should be made concerning the use of the Nernst-Planck equations for ion transport across the liquid film (e.g., Copeland and Marchello [1969], Kataoka et al. [1987]). This is a nonlinear, three-ion problem because of the presence of at least one coion at comparable concentration. The Nernst film model relies on the assumption of a linear concentration gradient in the liquid film. The film has no physical reality, and the calculation of nonlinear concentration profiles in it overburdens the model and offers little improvement over the much simpler linear driving force approximation. For higher accuracy, more refined and complex hydrodynamic models would have to be used (Van Brocklin and David, 1975). 

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