ABSTRACT:

Injection of water into chalk hydrocarbon reservoirs has lead to mechanical yield and failure. Laboratory experiments on chalk samples correspondingly show that the mechanical properties of porous chalk depend on pore fluid and temperature. Water has a significant softening effect on elastic properties of chalk as calculated from wave data, and the softening increases with increasing critical frequency as defined by Biot. The critical frequency is the highest frequency where wave propagation is controlled by solid-fluid friction. The Biot critical frequency is thus a measure of this friction and we propose that the fluid effect on mechanical properties of highly porous chalk may be the result of liquid-solid friction. Applying a different strain or stress rate is influencing the rock strength and needs to be included. The resulting function is shown to relate to the material dependent and rate independent bfactor used when describing the time dependent mechanical properties of soft rock or soils. As a consequence it is then possible to further characterize the material constant from the porosity and permeability of the rock as well as from pore fluid density and viscosity which is highly influenced by temperature.

1. INTRODUCTION

Water weakening in chalk has been extensively studied. Many explanations have been proposed including capillary effects with local menisci at the grain contacts strengthening the chalk [1, 2], pressure solution weakening the chalk [3], attractive and repulsive forces at the grain contacts [4], and chemical dissolution followed by precipitation which is affected by the pore fluid composition [5]. These theories apply to a static system of solid and fluid leaving a subject which seems not to have been covered: the aspect of the moving fluid relative to the solid. The most important aspect when considering this subject is the friction arising at the solid-fluid interface. For weakly cemented porous media undergoing time dependent deformation, not only the strength of the rock frame but also the draining of pore fluid plays a role. This drainage is affected by the solid-fluid friction and we propose that an entity describing this friction could be used as an indicator for the mechanical properties. In this paper we review the available literature on rock mechanical tests performed on highly porous chalk fully saturated with one fluid phase. The purpose is to look for underlying physical properties which might explain the water weakening.

1.1. Background

Biot's critical frequency defines the transition from where the fluid is dominated by viscous forces at low frequencies to where the fluid is dominated by inertial forces at higher frequencies [6, 7]. The drag that the solid motion makes on the fluid and whether this is in the viscous or inertial regime are basically matters of friction between the liquid and the solid. For the present application it is the relative fluid-solid movement that is in focus as there both exist a strain of the solid and a fluid expulsion during the rock mechanical test of the specimen.

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