Pore pressure is an important parameter, in exploration and production of hydrocarbon resources. Lack of accurate knowledge about pore pressure, before drilling phase, leads to serious dangers in drilling process. Accurate knowledge about distribution of pore pressure in a field, leads to reduce risks in drilling, improve well planning and mud weight calculations.
During burial, normally pressured formations are able to maintain hydraulic communication with the surface. Pore pressure or formation pressure is defined as the pressure acting on the fluids in the pore space of a formation. So, this pore fluid pressure equals the hydrostatic pressure of a column of formation water extending to the surface and is also commonly termed as normal pressure.
Hydrostatic pressure is controlled by the density of the fluid saturating the formation. As the pore water becomes saline, or other dissolved solids are added, the hydrostatic pressure gradient will increase.
Also, sonic velocity, density and resistivity of a normal pressured formation will generally increase with depth of burial and the way such rock properties vary with burial under normal pore pressure conditions is termed its normal compaction trend. Pore pressure gradient is defined as the ratio of the formation
pressure to the depth and is usually displayed in units of psi/ft or equivalent mud weight units in pounds per gallon (ppg). Overburden pressure at any depth is the pressure that results from the combined weight of the rock matrix and the fluids in the pore space overlying the formation of interest. Overburden pressure increases with depth and is also called the vertical stress. Effective pressure is defined as the pressure acting on the solid rock framework. Terzaghi defined it as the difference between the overburden pressure and the pore pressure. Effective pressure thus controls the compaction that takes place in porous granular media including sedimentary rocks and this has been confirmed by laboratory studies.
The only method for predrill predicting pore pressure is based on the use of 3D seismic data. In this study, seismic velocity obtained from processing methods, will be calibrated with regard to sonic velocities measured at wells. Then using the relation between effective pressure and velocity the effective pressure is calculated. In order to distribute the velocity and pressure quantities in the whole field, we use geostatistical estimation methods (krigging or co-krigging). For using co-krigging we need to have more than one variable. We use acoustic impedance as the second variable. For this purpose firstly the 3D seismic volume was inverted to obtain an acoustic impedance. Usage of multi-variable estimation will consider lithologic and geologic variations of the layers and we have a better estimation in comparsion with one-variable estimate. The 3D pore pressure cube was constructed using these calibrated velocities. The validation of the results illustrates a successful pore pressure prediction in this carbonate field.
We also include some of definitions here for convenience.