Summary

Pore pressure prediction based on under compaction model is one kind of longitudinal thinking. The essential of this thinking is that the vertical pressure plays the dominant role in pore pressure, whereas other factors, especially the tectonic compress stress, are ignored. When predicting abnormal pore pressure in northeastern of Sichuan, vertical and horizontal stresses must be taken into account simultaneously. The Poisson Ratio, defined as the ratio of vertical deformation to the horizontal deformation, is appropriate for prediction in northeastern of Sichuan. Possion Ratio is derived from the ratio of compressional velocity (Vp) to shear velocity (Vs). Since Vp is not sensitive to the pressure of gas reservoirs, the Generalized Poisson Ratio is defined by introducing a mathematical amplifier into the formula of calculating Poisson Ratio from velocities, following the principle of pseudo synthetic seismogram, which can then amplify the difference in ‘physical property’ between gas reservoirs and surrounding rocks. Pore pressure prediction is realized based on the Generalized Poisson Ratio. Predicted results from seismic data show that the uncalibrated pressure and the test pressure are similar in tendency, and the calibrated pressure and the test pressure are similar in pressure structure.

Introduction

Abnormal pore pressure is common in northeast of Sichuan while drilling, as a result of compressive tectonic stress. Pore pressure prediction before drilling is in exploration. important content. Velocity is first taken by Pennebaker to predict pore pressure (Pennebaker, 1968; Dutta, 2002a). Eaton proposed a compressional-velocity-powered function to predict pore pressure based on a hypothetical compaction condition (Eaton, 1972; Chilingar et al, 2002). Bellotti and Giacca proposed the formula of calculating stress of rock frame (Bellotti and Giacca, 1978; Meihou Y, 1996). Fillippone proposed the Fillippone Model for pore pressure prediction, after a combined consideration of velocity variation of rock frame and pore fluid with depth (Fillippone, 1982; Meihou Y, 1996). Dutta simultaneously studied the effect of geo-temperature, clay-porosity ratio and diagenesis on effective pressure (Dutta, 2002b). Pore pressure prediction model mentioned above are developed for clastic formation, and are not appropriate for northeastern Sichuan where the dominant control factor is compressive tectonic stress. Additional Stone predicted pore pressure and porosity from VSP data (Stone, 1982). Martinez was the first to predict porosity and pore pressure from seismic data (Martinez, 1985).

Carbonates have low velocity sensitivity to the change of pressure, and can sustain severe secondary pressure conditions with very small velocity change (Huffman, 2002). Hard rock with gas in its pore would have a soft rock character in its compressional velocity curve, whereas shear velocity is not sensitive to pore fluids, pressure prediction methods based on compressional velocity anomaly are not suitable for Carbonates. Poisson Ratio, defined as the ratio of the latitudinal shortens to the longitudinal growth,represents the mechanical behavior of rocks under uniaxial stress. Gas is the main factor of hyper-pressure in reservoirs laminated by fractures. Considering the difference in gas-sensitivity between compressional velocity and shear velocity, we adapt the concept of constructing pseudo-sonic curve of synthetic seismograms, to obtain the generalized Poisson Ratio.

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