Compressibility method for pore pressure prediction

Pore pressure is a key parameter in controlling the well in terms of reservoir fluid pressure. An accurate estimation of pore pressure yields to better mud weight proposition and pressure balance in the bore hole. Current well known methods of pore pressure prediction are mainly based on the differences between the recorded amount and normal trend in sonic wave velocity, formation resistivity factor (FRF), or d-exponent (a function of drilling parameters) in overpressured zone. The majority of the techniques are based on the compaction of specific formation type which need localization or calibration. They occasionally fail to proper response in carbonate reservoirs. In this research, a new method for calculating the pore pressure has been obtained using the compressibility attribute of reservoir rock. In the case of overpressure generation by undercompaction (which is the case in most of the reservoirs), pore pressure is depended on the changes in pore space which is a function of rock and pore compressibility. In a simple way, pore space decreases while the formation undergoes compaction and this imposes pressure on the fluid which fills the pores. Generally, rock compressibility has minor changes over a specific formation, but even this small amount must be considered. Thus, the statistical tools should be used to distribute the compressibility over the formation. Therefore, based on the bulk and pore compressibility achieved from the special core analysis (SCAL) or well logs in one well, the pore pressure in the other locations of a formation could be predicted.Vahid Atashbari, Mark Robert Tingay, Mohammad Hossein Zareia