| Popis: |
The pore pressure prediction is the most important process in the design of drilling wells. This paper depicts a new methodology to analyze pore pressure based on both, the normal compaction theory of sediments and the way that normal behavior diverges when it is interrupted. Much has been written on the topic; however, even today a high percentage of non-productive time (NPT) in drilling activities is related to pore pressure and wellbore instability problems. Here, a new methodology is proposed to improve the accuracy of calculated pore pressure from well logs and seismic data. Moreover, this new methodology allows, under specific conditions, to determine pore pressure in carbonates and other reservoir rocks. The compaction process defines the normal trend of porosity indicators with depth, the fluid retention depth and those rock bodies diverging from a normal compaction trend. The divergence detection procedure includes the identification of both, transitional changes of the porosity indicators (shale) and those that are parallel to normal compaction trend (reservoir rock); they allow to build a divergent area. When the divergent area is defined, the pore pressure calculation can be done using a pore pressure model based on normal compaction theory and well logs or interval velocity data from seismic. Misleading prediction of geopressures for a particular area are linked to: misunderstandings of pore pressure origins there, the limited scope of pore pressure models based on well logs and to miscalculations of the key parameters of pore pressure models. This work discuss the impact of these key parameters in the pore pressure prognosis. Analysis of actual cases showing the impact of miscalculation of overburden stress on pore pressure estimations, normal compaction trend definition and pore pressure calculations are presented using divergent area along with the Eaton model. The conclusions support the following statements: well log density data cannot be used to calculate the overburden pressure and under some conditions, the divergence methodology can be used to calculate pore pressures in carbonates. Furthermore, the divergent area method eliminate the use of shale points in pore pressure prognosis. |
