Abstract
The accurate prediction of pore pressures and fracture gradients has become almost essential to the drilling of deep wells with higher than normal pore pressures. Drilling costs and problems can be reduced substantially by the early recognition of abnormally high pore pressures.
Two new methods have been developed from well logs that can be applied universally to the determination of pore pressures and fracture gradient. The first method uses the principle of compaction concept where the porosity varies exponentially with vertical stress, and the second method uses the power-law relationship. Acoustic logs obtained from five wells in Ouargla and Garet El Bouib fields are used to validate the two methods.
The computed pressures matched the measured pressures obtained from repeated formation tests (RFT). The deviation between the first method and RFT varies from 3% to 6%, while, the deviation for the second method varies from 1% to 3%. The acoustic log predicts fluid pressure within an accuracy of 0.05 psi/ft. The standard deviations for the first method and the second method are 0.04 psi/ft and 0.02 psi/ft respectively. The analysis also shows that there are two overpressured zones in the well studied, which accurately matched the actual sequence of events while drilling the wells. The developed methodologies are illustrated and validated with several examples from the Hassi Messaoud field in Algeria.
