Due to the existence of hydration, underground complex accidents often occur in shale formation. Different from hydration swelling shale, the researches related to mechanical characteristics in carbonaceous shale that is symbiotic with coal-bed strata are less. The shale is rich in carbonized organic matter, often includes fossils like graptolite, and develops plentiful bedding planes. In order to study the influence of hydration on mechanical characteristics of carbonaceous shale, we have carried out some experiments, including mineral components, specific surface area, wettability, cationic exchange capacity, scanning electron microscope(SEM), triaxial compression experiment and shear strength experiment. The results show:
Carbonaceous shale is mainly composed of quartz and clay mineral and the main clay minerals are illite and illite-smectite mixed layer.
Carbonaceous shale is water wetness and oil wetness. As specific surface area, pore volume and the average diameter of pore are small, the capillary pressure of shale will be very high.
Carbonaceous shale has a mass of micro-cracks, micro-pores and bedding planes by scanning electron microscopy (SEM). The drilling fluid is easy to enter into the interior of the rock under the action of the pressure of the fluid column, the chemical potential difference and the capillary pressure, which result into the reduction of the strength parameters in the shale. Therefore, in the drilling process, the hydration function should not be ignored, which requires the strong sealing ability of the drilling fluid and the contact with the carbonaceous shale should be avoided as much as possible.
Malek Elgmati, Robert G, Younane A, Gareth R Chalmers investigated mineral composition and the content of clay minerals of typical hard brittle shale formation in Fort Worth basin, Appalachian basin, Anadarko basin and East Texas basin of North America by X ray diffraction method. Statistics results showed that the clay minerals content distribution in hard brittle shale formation was between 15% ~ 50%, the main clay minerals content is illite and clay minerals content of North America stratum is generally excluding illite-smectite mixed-layer and the content of illite in North America accounts for more than 75% of the total amount of clay minerals. Martin E Chenevert measured hydration swelling pressure when he was studying shale hydration, and the expansion pressure are expressed as the function of waters activity in the inside of shale, found water absorbing capacity increased with time in the process of shale hydration, the rock strength decreased with the increase of water absorbing capacity, and the one dimensional water absorption equation is given. Skipper, Chang FRC, Karaborni N T S, Emiel J and Smith based on computer molecular simulation concluded that the hydration process began when balance ion migrated on the surface of clay minerals, produced complete hydration process until the balance ion penetrated to intermediate sandwich of clay minerals and charge location had a great influence on the hydration process. H.C.H.Darley proposed that water would not disperse the hard brittle shale, but would enter the inner rock through tiny cracks in the hard brittle shale, causing the splitting and peeling of the rock. Yew C. H simulated the flow forms of water molecules within the shale hydration process by using thermal diffusion model, the influence of the water quantity of wall rock mechanics properties was concluded, and the wall stress distribution and stress was sensitive to shale water quantity. F.K.Mody found that the shale hydration changed the pore pressure and the effective stress state of the formation, and established a model of well wall stability based on chemical potential. Microstructure of hard brittle shale as a extremely important rock physical properties has been widely attention. Since Teraghi puts forward the concept of microstructure, the scholars have put forward various models such as the non-flocculated structure, flocculation structure, spatial dispersion structure to describe the microstructure of clay minerals.
