Pore pressure terminology

Abstract

Fluid pressures in the pore spaces of rocks are critical to several aspects of petroleum exploration and production. However, a general understanding of some basic concepts has been obscured by a lack of consistency in terminology. It is the intent of this paper to clarify the meaning of certain terms so that the many disciplines involved with, and affected by, pore pressures can communicate effectively and clearly. A most confusing aspect of pressure terminology arises from mixing the terms for pressure and pressure gradients. The word “gradient” is often dropped when referring to pressure increases with depth. Even when the gradient distinction is made, it can confuse because it can either mean pressure changes referenced from the surface or pressure changes measured over short depth ranges. It is important to understand pressures in absolute terms before beginning to work with gradients. Please note that the terms “pressure” and “stress” are used interchangeably in the following. They are not strictly the same, but can be so considered for this discussion. (Stress is a tensor while fluid pressure is isotropic.) Figure 1, a very stylized diagram of pressure versus depth for a fictional well, illustrates several concepts. The “hydrostatic” line gives the pressure due to a column of water. The slope would be .433 psi/ft for pure water, but is usually .45–.465 for formation waters. An important concept is that, for a simple porous rock with pore spaces continuously connected to the surface (i.e. an open system), the pressure of the fluid in the pore space is just the pressure exerted by the weight of the overlying fluids. This “normal or hydrostatic pressure” is simply the pressure due to a column of water. Figure 1. Pressure plotted against depth in a fictional well. Overpressure is the amount of pore pressure in excess of …