ABSTRACT:

Understanding the mechanical properties of petroleum reservoirs is important for fracturing, wellbore stability, sand control, and subsidence prediction. Static, triaxial measurements of these properties, however, can be significantly different from dynamic, acoustic log derived measurements. The static measurements are more representative of the reservoirs properties but dynamic measurements afford a much greater coverage of the reservoir. This study examines the differences between static and dynamic mechanical properties for the Rotliegendes sandstones in quadrant 48 of the southern North Sea. Cores from 4 wells spanning a large range of porosity (4% to 22%) and mechanical properties were tested to develop a correlation between static and dynamic properties for this area and to understand the micromechanical basis for the static/dynamic differences.

We find that dynamic Young's moduli are nearly twice the static values in the high porosity, low modulus samples but they are only 10% higher in the low porosity, high modulus samples. There are strong correlations between the dynamic/static modulus ratio and static modulus and between modulus and porosity. Petrographic analysis shows that porosity is controlled mainly by clay and quartz cementafion. Quartz cement appears much more effective in suturingrains together than clay cement based on differences in static/dynamic moduli between samples with clay versus quartz cement. Poisson's ratio also shows a strong dependence on porosity/cementafion. The strong dependence between cementafion and dynamic/static differences support recent theoretical (Sharma and Tutuncu, 1994; Guyer and McCall, 1994) models for the static-dynamic discrepancy. We find a linear relationship between the magnitude of the dynamic/static modulus ratio and the degree of non-linearity/hysteresis in the stress-strain curves.

INTRODUCTION

The mechanical properties of reservoir rock (Young's modulus, Poisson's ratio, failure strength, Biot alpha) are critical inputs for designing and modeling hydraulic fractures, for determining wellbore stability, and for predicting potential sand control and reservoir subsidence problems. These properties are usually measured in triaxial laboratory tests on core samples (static tests) but the increasing availability of compressional and shear wave velocities from well logs, cross-hole seismic and VSP allows the estimation of these properties (dynamic tests) on a much broader scale than before.

The problem remains that the dynamic tests sense only the elastic portion of the rocks response and therefore yield higher moduli and different Poisson's ratios than the static tests. The magnitude and rate of stress changes and rock deformations in the reservoir make static tests more representative of the reservoir's properties than dynamic tests. Therefore, dynamic data need to be corrected to equivalent static values before being used in fracture, stability or subsidence modeling.

A study was undertaken to develop correlations to correct dynamic data for the Rotliegendes sandstone in block 48 of the southern North Sea. Fifty eight core samples from 4 wells were tested. The samples spanned a large range of porosity (4% to 22%), mechanical properties, and degree of cementation. We find that not only do static and dynamic properties correlate well in this area but that variability in static and dynamic differences allows us to understand the fundamental mechanisms which may be causing the differences. The results are complementary to new theoretical models being put forth to explain static/dynamic mechanical properties.

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