School of GeoSciences

School of GeoSciences

A new methodology for predicting rock mechanical properties from porosity

The rock mechanical behavior of reservoir rocks is important in the design and implementation of drilling and production program. Traditionally rock mechanical properties are obtained from direct measurement on core samples or from mechanical calculations on acoustic wireline log measurements.I developed a new method of predicting rock mechanical properties directly from porosity. Porosity and rock mechanical property measurements were obtained from extensive rock mechanical testing on North Sea sandstone samples from an extensive range of oil and gas fields. The rults can be seen below

[Porosity Youngs Modulus relationship]
[porosity Poisson's ratio relationship]
[Porosity UCS relationship]
[porosity cohesion relationship]
[porosity internal friction correlation]
[Porosity traixial stress factor relationship]

For any orientation of well, the greater part of the rock mechanical properties predictor is applicable and should produce accurate estimates of elastic and strength property data. Sound conventional analysis techniques on the wireline data prior to analysis is essential to provide the most representative base porosity log. The correlations of the rock mechanical properties with porosity are significant. The general all sediment relationship is an excellent starting point, but if the depositional environment is known it is advisable to apply the depositional specific correlations backed up with actual experimental data from the well or field being analysed. When the rock mechanical property logs calculated from the wireline porosity are compared to the experimental results a good degree of correlation is observed. Therefore it can be concluded that the rock mechanical property routine is an accurate, representative and cost effective method of obtaining a rock mechanical profile of the reservoir.

In vertical and moderately inclined wells, the coherent rock mechanical units provide an immediate profile of the overall mechanical nature of the reservoir and allows comparison and correlation both with other beds studied, throughout the wellbore section and from well to well across the reservoir area. Within horizontal beds, the inaccuracy of the rock mechanical units is more pronounced, and is a function of the distance travelled within a certain bed rather than the actual thickness of the bed studied. The applications of this routine are many and the sanding potential predictor described summarises the versatility of this routine to provide valuable data for a wide range of petroleum reservoir disciplines.

The benefits of the routine are that obtaining a representative rock strength profile of the borehole is relatively cheap, quick and requires very little input data. The porosity derived calculations also allow a more detailed quantification of the mechanical properties of the reservoir as only the modulus of elasticity and Poisson’s Ratio can be obtained from acoustic calculations.