The motion of single species of ions and water through clay shale is a coupled mass transfer phenomenon. In addition, the existence of charged particles and the effect of the double layer in clay shale lead to processes which occur at both micro and macro scales. The processes at both of these scales influence each other. The flows of water and ions in macro pores are usually due to the hydraulic gradient and/or concentration gradients, whereas flows of the fluid and ionic species in micro pores are dominated by electrical and osmotic effects. The purpose of this study is to propose a mathematical coupled macro-micro model, considering all the aforementioned effects in clay shales to explain the behavior of these rocks, subjected to different environmental changes and physical loads. Sources of each flow and the effect of simultaneous flows at the macro and micro scales have been investigated and considered in the model.


The ability of compacted clays to swell due to the adsorption of water is well known in geotechnical engineering. Clay shales, particularly those predominantly composed of smectite have high swelling potential. The source of this phenomenon is the existence of the electrical double layer (E.D.L.) around the platelets which form clay particles. Often the swelling of clay shale is not desirable and it causes problems. For example, in the petroleum industry the swelling of clay shale produces problems during the drilling process. One way to avoid this problem is to use oil-based mud or add salt to the drilling fluid thereby reducing the chemical potential of water within the well-bore (Sherwood 1994). Generally, in geotechnical engineering literature there are some methods to control the undesired swelling of clay shale (Sherwood 1994, Cooper & Roy 1994).

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