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Technical Paper: Improved Pore-Pressure Prediction and Mechanical Earth Model Estimation Through Binary Decomposition of Seismic Inversion Data in Subresolution Clastic Sequences

Society: SPE
Paper Number: 90925
Presentation Date: 2004
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Abstract

Formation pore pressures that exceed the hydrostatic gradient are common in all basins worldwide. The most common cause is undercompaction, which occurs when the rate of shale sedimentation is high, thus preventing water to escape from the pore space in the underlying sediments. This leads, in turn, to a redistribution of the overburden stress, i.e., part of the overburden stress is maintained by the pore fluids and the effective stress on the rock fabric decreases by an equal amount. Porosity is preserved and assumed to be in balance with the effective stress. Other mechanisms that can generate increased pore pressure include: dewatering of clays, generation of hydrocarbons, cracking of oil to gas under increasing temperature, and volumetric increase of the fluids when temperature is increasing.

Uncertainty in the pore-pressure profile along the planned drilling path means, at the very least, increased cost to cover contingency plans, but can also include very expensive mitigation work, especially in deep- and ultra-deepwater environments. Worst-case scenarios can include loss of the well, loss of lives, and severe environmental impact. Current state-of-the-art methods for pore-pressure prediction include classification of lithology and pore fluids based on prestack inverted data yielding compressional and shear velocities as well as density. This allows the rock physics equations to be customized for improved accuracy in the predicted pore pressure.

Clastic basins consist predominantly of interbedded shales and sands. The vertical resolution of seismic data depends on the wavelength of the propagating seismic energy, but typical resolution in Tertiary basins is about 30 ft, which means the extracted elastic parameters often represent a net to gross (N/G) rather than the properties of individual sands and shales. The volumetric content of each lithology can be evaluated from the elastic parameters extracted from the seismic data. However, when there are hydrocarbons present, especially gas, the estimate of N/G and the elastic properties of the two lithologies are more difficult to evaluate.

In this paper, we take the inversion process one step further by decomposing the seismic elastic data into the individual properties of the sands and shales. In addition to improving the accuracy of the pore-pressure prediction and the mechanical properties of the two lithologies, we also get an estimate of N/G, hydrocarbon properties, and saturation in the sands. The ability to estimate elastic properties for sands and shales individually is also very important to get correct estimates of the mechanical rock properties.