Overburden pressure

Assuming a normal pressure regime, at a given depth below ground level, a certain pressure must exist which just balances the overburden pressure (OBP) due to the... 

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... 

Figure 2-56 shows a plot of the theoretical maximum overburden pressure and the theoretical minimum pressure as a function of depth. Also plotted are various bottomhole fluid pressures from actual wells drilled in the Gulf Coast region [33]. These experimentally obtained pressures are the measurements of the pressures in the fluids that result from a combination of rock overburden and the fluid hydraulic column to the surface. These data show the bottomhole fluid pressure extremes. The abnormally high pressures can be explained by the fact that the sedimentary basins in the Gulf Coast region are immature basins and are... 

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 compaction effects are the most common. According to Terzaghi s work in soil mechanics, the overburden pressure mentioned earlier, is equal to [119]... 

Strength It should be compact, hard and strong to withstand abrasion as well as overburden pressure in the furnaces. 

The core is put into a Hassler sleeve inside a core holder where overburden pressure is applied outside of the sleeve by using water. Gas is not used, since it has the potential to diffuse through the rubber sleeve. The overburdened core holder is connected to the Isco pump, from which 1% acidified brine is pumped into the core. The needle valve that restricts flow from the output end of the core is opened slightly to allow the escape of air and air-saturated brine. The needle valve which restricts flow from the input end of the core is opened fully. Complete saturation is attained after several pore volumes of brine have been pumped through the core under pressure, and this fully saturated core is used throughout the experiment. 

Since the overburden pressure cannot have caused the increase of rank with increasing depth (as will be shown later), only increasing rock temperature can be the reason for the vertical rank variations. 

The examples show that pressure usually does not promote chemical coalificadon—but undoubtedly it changes the coal physically. As is well known, clays are very sensitive to the action of overburden pressure, under which they lose moisture and porosity. Peats and brown coals have much the same characteristics as clays. This can be readily demonstrated experimentally by compressing brown coals at room temperature. The process does not involve any chemical changes but only a compression and consolidation, attended by an appreciable shrinkage in volume and emission of moisture. Figure 10 shows the action of a pressure of 270 kg./sq. cm. corresponding to an overburden of 1300 meters, upon a xylite from a soft brown coal. The... 

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

Dr. Teichmuller Very often it is impossible to separate the influence of overburden pressure and the influence of rock temperature on rank of coal in a subsidized formation. 

In the formation model of the above hydrate concentrations, Torres et al. (2004) indicate that the pressure of hydrate crystallization can exceed the overburden pressure. This can occur at shallow subseafloor depth, to cause massive hydrate deposits such as those shown in Figure 7.28, if hydrates do not promptly cement the grains. This finding is similar to that observed by Sassen and coworkers in the Gulf of Mexico (Personal Communication, November 10, 2005). Torres and coworkers also suggest that methane dissolved in water alone is insufficient to cause the noted hydrate concentrations—there must be additional free gas present. 

The theoretical development by Groenevelt and Bolt [2] provides the following equation for the bundle of shrinkage curves for which the lines are for different load (overburden) pressures, P... 

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