During a period of erosion and sedimentation, grains of sediment are continuously building up on top of each other, generally in a water filled environment. As the thickness of the layer of sediment increases, the grains of the sediment are packed closer together, and some of the water is expelled from the pore spaces. However, if the pore throats through the sediment are interconnecting all the way to surface the pressure of the fluid at any depth in the sediment will be same as that which would be found in a simple colom of fluid. The pressure in the fluid in the pores of the sediment will only be dependent on the density of the fluid in the pore space and the depth of the pressure measurement (equal to the height of the colom of liquid). it will be independent of the pore size or pore throat geometry. The pressure of the fluid in the pore space (the pore pressure) can be measured and plotted against depth as shown in Figure 1. This type of diagram is known as a P-Z diagram.
The pressure in the formations to be drilled is often expressed in terms of a pressure gradient. This gradient is derived from a line passing through a particular formation pore pressure and a datum point at surface and is known as the pore pressure gradient. The reasons for this will become apparent subsequently. The datum which is generally used during drilling operations is the drillfloor elevation but a more general datum level, used almost universally, is Mean Sea Level, MSL. When the pore throats through the sediment are interconnecting, the pressure of the fluid at any depth in the sediment will be same as that which would be found in a simple colom of fluid and therefore the pore pressure gradient is a straight line as shown in Figure 1. The gradient of the line is a representation of the density of the fluid. Hence the density of the fluid in the pore space is often expressed in units of psi/ft.
This is a very convenient unit of representation since the pore pressure for any given formation can easily be deduced from the pore pressure gradient if the vertical depth of the formation is known. Representing the pore pressures in the formations in terms of pore pressure gradients is also convenient when computing the density of the drilling fluid that will be required to drill through the formations in question. If the density of the drilling fluid in the wellbore is also expressed in units of psi/ft then the pressure at all points in the wellbore can be compared with the pore pressures to ensure that the pressure in the wellbore exceeds the pore pressure. The differential between the mud pressure and the pore pressure at any given depth is known as the overbalance pressure at that depth (Figure 2). If the mud pressure is less than the pore pressure then the differential is known as the underbalance pressure. It will be seen below that the fracture pressure gradient of the formations is also expressed in units of psi/ft.
Most of the fluids found in the pore space of sedimentary formations contain a proportion of salt and are known as brines. The dissolved salt content may vary from 0 to over 200,000 ppm. Correspondingly, the pore pressure gradient ranges from 0.433 psi/ft (pure water) to about 0.50 psi/ft. In most geographical areas the pore pressure gradient is approximately 0.465 psi/ft (assumes 80,000 ppm salt content) and this pressure gradient has been defined as the normal pressure gradient.Any formation pressure above or below the points defined by this gradient are called abnormal pressures (Figure 3). The mechanisms by which these abnormal pressures can be generated will be discussed below. When the pore fluids are normally pressured the formation pore pressure is also said to be hydrostatic.
Institute of Petroleum Engineering, Heriot-Watt University