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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.
Introduction and Literature Review An important parameter for well planning is the knowledge about the formation pore pressure. The detection of abnormally high formation pressure, or overpressured zones, provides valuable information for exploration and exploitation purposes. This is essentially true or common in all Algerian oil fields. Overpressured sediments are generally caused by a sequence of events wherein water becomes trapped by faults or non-permeable barriers in sediments at depths. In a normally pressured formation the water was forced out by normal increases in overburden pressure. Abnormal pressure is also caused by the release of water into the pore system during clay diagenesis (smectite/illite transformation) and other mechanisms. High formation pressures are due to major changes in the behavior of subsurface rock. In overpressured shales, which contain pressured water, density is lower and porosity is higher than normal.
Formation pressure can be the major factor affecting the success of drilling operations. If pressure is not properly evaluated, it can lead to drilling problems such as lost circulation, blowouts, stuck pipe, hole instability, and excessive costs. Unfortunately, formation pressures can be very difficult to quantify precisely where unusual, or abnormal, pressures exist. The complete well planning process, with few exceptions, is predicted on knowledge of formation pressures. The pressure is the foundation for many segments of the well plan. If proper attention is not given to formation pressure predictions, the other technical portions of the well plan may be inadequate.
Many authors have outlined procedures for estimating formation pressure using data obtained from electrical and acoustical surveys. Also some author, namely Hubbert and Willis, Matthews and Kelly, and Eaton have outlined procedures for estimating fracture pressure. Knowledge of these two parameters (formation pressure and fracture pressure) is extremely important in planning and drilling future wells.
The purpose of this paper is to present new methods to estimate formation pore pressure and show how they may be used to enhance the interpretation process.
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