ABSTRACT:

The shale's ability for water adsorption or desorption (dehydration) is controlled by the salinity of the drill-fluid. A theory, based on the presence of a semi- permeable membrane, is developed for calculating the moisture induced strain in shales. An example, using the experimental data, is presented to illustrate the method.

INTRODUCTION

In drilling operations, the stability of wellbore is maintained to a large degree by the hydraulic pressure supplied by the drilling mud. While drilling through shales, the drill- fluid (or mud) is often treated with chemicals to control its salinity. It is well established (Chenevert, 1969a,b, 1970), that the.shale's ability for water adsorption or desorption (dehydration) is controlled by the salinity of the fluid. A theory, based on the relative chemical energies between shale and drill-fluid and on the magnitude of confining pressure, for calculating the moisture induced strain in shales is presented in this paper.

A semi-permeable membrane is assumed to present between the mud and the shale. The movement of ions in the drill-fluid and shale system is controlled by the semi- permeable membrane at the interface. The membrane inhibits the movement of ions from one component to the other. It is experimentally established that if there is a semi- permeable membrane between the drill-fluid and shale, the difference of ion concentration between the drill-fluid and rock produces an osmotic pressure difference which provides the driving force for water movement. For an example, if the fluid were treated with NaC1, the concentration of ions is high in the fluid. The high concentration of ions in the drill-fluid causes a lower water activity in the fluid (the water activity equals to unity in deionized water) and an osmotic pressure difference between the drill- fluid and shale. The difference in osmotic pressure causes the flow of water from shale to drill-fluid. As the water leaves the shale and enters the drill-fluid, the water activity in the drill-fluid increases while the water activity in the shale decreases. When the water activity in the drill-fluid equal to that in the shale, the osmotic pressure difference is zero and the movement of water in the drill-fluid and shale system stops. If there is no semi- permeable membrane between the drill-fluid and shale, the ions may migrate freely between the drill-fluid and the shale; and the system would eventually reach a condition of chemical equilibrium. The mechanism of water movement between the drill-fluid and the shale becomes complicated. More research is needed for a clear understanding of the phenomenon. It was shown by Chenevert (1970) that certain oil muds do provide a suitable membrane which inhibits the ion movement. Several authors have also suggested that the shale itself may act as a semi-permeable membrane.

In their study of the transport mechanisms of water and salt (NaC1) through clays, Mokady and Low (1968) demonstrated that the transport of water, NaC1, and its component ions is related to the activity and to the respective partial molar free energy gradients in the clay. They further demonstrated that the movement of water, NaC1, and its component ions were toward the lower value of the respective relative partial molar free energies.

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