Abstract

The rate dependence of dry, oil- or water-saturated high-porosity outcrop chalk is investigated based on whether the fluid effect could be excluded from a governing material parameter, the b-factor. The b-factor is used in geotechnical engineering to establish the difference in evolution of load between stress-strain curves when applying different loading rates. The material investigated is outcrop chalk from Stevns, Southern part of Denmark, with a porosity of 43 to 44% and subjected to varying loading rates. The Biot critical frequency is a function of the fluid properties viscosity and density, and the material properties porosity and permeability. The critical frequency is interpreted as a measure of the solid-fluid friction and hence relevant to use for subtracting the fluid effect to arrive at a pure material parameter. The results show a correlation between the b-factor and the Biot critical frequency.

1. INTRODUCTION

For hydrocarbon reservoir behavior upscaling from laboratory results to field behavior is important. One of the central parameters which are challenging to apply in the laboratory is the low strain or stress rate involved. These can range from as much as 1 %/hour for the laboratory tests to 10–4%/hour for a hydrocarbon reservoir under depletion [1]. Thus upscaling parameters measured in the laboratory is important. The time dependence is vital as it affects both the strength and the deformation such that a slow loading results in a weaker material response and a faster loading rate yields a higher strength and a stiffer response. Dealing with the time effect is exemplified by Rhett [2] for the Ekofisk Field, and by de Waal and Smits [3] for Goose Creek, Wilmington, Inglewood, Nigata, Boliva coast, and Groningen fields.

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