Deformation of Chalk Under Confining Pressure and Pore Pressure

Compression tests with and without pore pressure have been run on Danian and Austin chalks. The rocks yielded under increasing hydrostatic stress by pore collapse. The same effect was produced by holding a constant hydrostatic stress and reducing the pore pressure. This pore collapse reduced the permeability. The ultimate strength of the chalks increased with increasing confining pressure. The yield strength increased initially, but at higher confining pressures it decreased until it yielded under hydrostatic stress. Relatively high pore-pressure gradients developed when the chalks. were compressed. In these situations, the mechanical behavior tended to be a function of the average effective stresses. Introduction Hydrocarbons have been found in chalks in the North Sea, the Middle East, the Gulf Coast and midcontinent regions of the U.S., and the Scotian Shelf of Canada1; however, problems have been encountered in developing these reservoirs efficiently because of the unusual mechanical behavior of chalk. Chalks have three characteristics that interact to differentiate their behavior from most reservoir rocks. High Porosity. Porosities may be as high as 80070.1,2 Effects of burial and pore-water chemistry can reduce this porosity to less than 1%, but notable exceptions occur in areas of early oil placement and overpressuring where porosities in excess of 40% have been reported.2,3 Low Permeability Regardless of porosity, chalks have low permeabilities, usually around 1 to 10 md. Soft Matrix. Chalks are predominantly calcite, which has a hardness of 3 on Mohr's scale. These properties create problems in the following areas of reservoir development. Drilling. High porosity combined with a soft matrix material makes for a relatively weak and ductile rock. Efficient drilling involves chipping the rock and ductile behavior inhibits this process. Stimulation. The combination of high porosity and low permeability makes chalks prime candidates for stimulation by hydraulic fracturing or acid fracturing. The best production often is associated with natural fractures.2,3 Man-made fractures could open up new areas to production, but again ductile behavior inhibits the fracturing process. Production. In many cases permeabilities are low enough to trap pore fluids and cause abnormally high pore pressures.2 These high pore pressures help maintain the high porosities at depth by supporting some of the weight of the overburden. As the field is produced and the pore pressure lowered, some of the weight will shift to the soft matrix. The result may be pore collapse and reduction of an already low permeability. These problems indicate a need for basic information on the mechanical behavior of chalks. Determining methods of enhancing brittle behavior could lead to improved drilling and stimulation techniques. The ability to predict and prevent pore collapse could increase ultimate recovery. The approach taken in this study was experimental. Specimens of chalk were subjected to different combinations of stress and pore pressure in the laboratory, and the resulting deformations were measured.