【文章內容簡介】 overburden pressure gradient, just as the change with depth of the fluid pressure alone is expressed as a formation pressure gradient. These gradients will vary, depending on the height of the fluid column, the salinity (density) of the water, and the mineral makeup and porosity of the sediments. The overburden pressure gradient is normally about psi/ft ( kPa/m), and the reservoir pressure gradient varies from area to area: in the . Gulf Coast it is generally about psi/ft ( kPa/m)。 it is somewhat less in freshwater areas.Figure Gulf Coast reservoir pressures that deviate from those expected for a normal fluid pressure gradient. From Geology of Petroleum, 2d ed. W. H. Freeman and Company. Copyright 1967This reservoir pressure gradient can also be expressed in terms of the weight of the fluid (in oilfield terminology, pounds per gallon or ppg), by a simple conversion factor. A pressure gradient of psi/ft ( kPa/m) is exerted by an ppg (107 kg/m3) fluid, or conversely, an ppg (107 kg/m3) fluid in a wellbore will exactly offset a reservoir pressure resulting from a psi/ft ( kPa/m) gradient. This point is important in drilling, and will be discussed further in section , Drilling Fluids.Unfortunately, certain geological or geochemical processes can affect the pressure gradient, causing it to deviate from its normal trend, resulting in abnormal pressures that are higher than expected (and sometimes lower). Figure shows the deviation from the normal pressure trend measured in a group of . Gulf Coast oil fields. This type of deviation can be due to a number of processes rapid sedimentation piezometric surface contrasts chemical diagenesis fluid density contrasts structural movement chargingFigure Abnormally pressured sandstone as a result of rapid sedimentationRapid Sedimentation: Normally deposited silts, sands, and muds will be pacted as additional sediment is dumped on top of them. The clays will lose most of their fluid volume and will be pressed into shale. As the sediments are buried deeper, the sands pact slightly, but the shales are squeezed and lose more and more of their interstitial water. If the rate of sediment deposition is so great that the water within and below the shale cannot escape and therefore must help carry the load developed during subsequent paction, the reservoir pressure will be abnormally high. This is shown schematically in figure .Piezometric Surface Contrasts: The piezometric surface is the point to which the fluid in a reservoir will rise under a pressure head resulting from a difference in elevation (Figure ). The difference between the distance from the reservoir to ground level and to this “surface” will determine the degree to which the measured pressure gradient is greater or less than normal. An example of this phenomenon is the artesian well, where an above normal pressure results in a water well that flows on its own to the surface.Chemical Diagenesis: Chemical transformation of clays results in an expulsion of water bound within the clay particle layers, with increasing temperature. If this water cannot escape from the pressing shales, or is forced into interbedded sandstones, abnormal pressure can develop.Figure Piezometric surface contrast with ground level causes deviations from normal pressure gradient, Courtesy EXLOG.Fluid Density Contrasts: When a large column of oil or gas is trapped in a reservoir, the lower density of this can cause a deviation from expected pressure。This is because the reservoir fluids transmit the greatest pressure applied to them, much like hydraulic fluid in an automobile brake line. Figure shows how the normal pore pressure in the watersaturated portion of the reservoir is transmitted to the shallow end of the formation. The pressure at the top of the reservoir equals the pore pressure at the deep end, minus the fluid head exerted by the lighter (less dense) hydrocarbons.Structural Movement: If a normally pressured sand is lifted rather rapidly relative to geologic time, the overburden pressure may decrease more rapidly than the pore pressure can dissipate. This can occur as a result of piercement salt domes, plumes of lowdensity salt deposited by ambient seas, which force their way upward through the sediment, displacing the deposited layers.Charging: Abnormal pressures can be encountered in sands which, although originally normally pressured, have been placed in fluid contact with an abnormally pressured zone via a conduit (leaking fault, fracture, aquifer, borehole, or binations of these). Such a sand is said to be “charged” with a pressure greater than normal for its depth. This type of overpressure can be generated relatively quickly in the case of an underground blowout, as described in section , Well Control.There are several other mechanisms that contribute to the overpressuring of reservoirs. Clay can act as a semipermeable membrane, allowing osmosis to inhibit the flow of water from pacting shales as a result of an increase in ion concentration. The thermal expansion of water and thermal cracking of hydrocarbons both act to increase the volume of saturating fluids, and can help cause abnormal pressures in a confined reservoir rock. Figure Abnormal pressure as a result of fluid density contrastsThese contributions are thought to be minor, however.Another variable parameter that contributes to the hostility of subsurface environments is temperature. The temperature gradient, or geothermal gradient, is generally constant for a given borehole, although it may vary from area to area. Typically, the geothermal gradient is expressed in degrees per unit depth (℉/100 ft or ℃/100 m). The average is about 2 ℉/100 ft, or ℃/100 m, although ℉/I00ft and less than ℉/100 ft gradients are found (Levorsen 1967). A change in geothermal gradient can sometimes be an indicator pressured shales, foreshadowing the potential for an overpressured reservoir. In determining a bottomhole