Fracture Mechanics Evaluation of Parameters Associated with Horizontal Hydrofracturing

The horizontal portion of the borehole used to extract gas from shale deposits is made of an annulus of cement and a metal casing. This annulus has a series of diametrically opposed cracks made by a perforating gun. The annulus with the cracks is subjected to a fluid pressure or hydrofracturing. Little is known about the influence of the number, depth, and separation of the cracks on the fluid pressure efficiency in cracking the shale. The main objective of this study is to use fracture mechanics theory to evaluate the effect of the number, separation and depth of the gun-induced cracks on the hydrofracturing of the shale. The level of tensile stress induced by the hydrofracturing fluid is a function of the Mode I stress intensity factor K_I. This study indicated that K_I was the highest for the case of two single diametrically opposed vertical cracks. K_I was found to decrease in value as the number of diametrically opposed cracks increased in number and decreased in separation. Also, regardless of the number of the gun-induced cracks, the K_I value generally increased in value as the depth of these cracks increased in value.