ABSTRACT

ABSTRACT: A model is presented for predicting the swelling behavior of the Wellington shale placed in contact with water under elevated temperatures and pressures. The model is based on thermodynamic theory and laboratory derived experimental data.

The theory suggests that fluid movement into or out of a shale is driven by an imbalance in the partial molar free energy of the shale and the contacting fluid. Such energies are dependent on the temperature, pressure, and the presence of charged elements. By describing the free energy of each system in terms of a "Total Potential Pressures" it is possible to model transient pressures and strains using the diffusivity equation.

This theory was evaluated using equipment and experimental techniques which allow the monitoring of shale swelling at elevated temperatures and pressures, as functions of time and distance from a wetted shale surface.

Independent measurements of ionic content within the shale after testing showed that the Wellington does not act as an effective membranes and does allow the passage of ions.

Increasing the compaction stress acting on the shale reduced the rate of swelling, and increasing the hydraulic pressure of the fluid on the shale's wetted surface increased the rate of swelling. Increases of temperature increased the swelling rates.

This work represents a new method for predicting shale swelling and internal fluid pressures as functions of time and distance, under a wide variety of environmental conditions.

INTRODUCTION

The hydration of shales and subsequent alterations have long been recognized (Chenevert, 1970). Many theories have been presented as to the driving mechanisms, which include capillary pressure, osmosis pressures, hydraulic pore pressure imbalance, and the polar attraction of water by the charged clay surfaces within the shale, to name a few. None of these theories have presented a technique which satisfactorily models this water/shale flow and swelling behavior. The authors of this paper have taken a basic approach which hopefully includes the major driving mechanisms responsible for such flow.

When water moves into a saturated shale body held under constant compaction stress, the total volume of the body increases, therefore, swelling strains develop at the boundaries. The mechanism which drives the water transport is believed to be the -- differential between the partial molar free energy of the water in the shale (Gws) and the partial molar free energy of the water (?,?v)in contact with the surface of the shale. This basic concept of water movement caused by a difference in the partial molar free energy of 869 two systems was presented by Gibbs in 1875, and is accepted by most thermodynamicists.

The application of such thermodynamic theory to shale type formations penetrated by a drill bit and contacted by wellbore drilling fluids, is the subject of this paper. It is proposed herein that the partial molar free energy of the water in either system can be determined, and is controlled by three factors: temperature, pressure, and the concentration of all charged particles (both dissolved and solid) within each system. The challenge is to accurately describe the partial molar free energies in both systems, and thereby predict water flow. Techniques for achieving such determinations use thermodynamic principles, and laboratory derived relationships as shown below.

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