1. Field of the Invention
The invention pertains to a method for enhancing the productivity of subterranean wells, namely oil and gas wells. The method generally involves passing a solvent down the well bore and through perforations in the well casing in order to dissolve a soluble portion of a subterranean formation and create a cavity. The size of the cavity is increased until the shear forces exerted on the cavity by the overlaying subterranean formations cause the cavity to collapse.
2. Description of the Prior Art
A number of methods are known in the art for enhancing the productivity of subterranean wells. One conventional method is commonly referred to as acid fracturing. Acid fracturing employs a non-reactive fracturing fluid, or “pad,” to fracture the producing subterranean formation. Typically, the pad is a viscous water-based fluid comprising a cross-linked polymer. Once the fracture is made, acid (typically about 28% HCl by volume) is introduced into the subterranean formation and etches the face of the fracture. This etching creates greater conductivity within the formation allowing for flow back to the well bore once the fracture closes back down. While this method has proven to enhance well productivity, it has a number of drawbacks. For example, acid fracturing requires injection of multiple fluids into the well in addition to using highly concentrated acid. Furthermore, the conventional acid fracturing process is unsuitable for use in softer rock formations such as chalk because the rock strength is typically inadequate to support the etched fracture face at the in-situ reservoir closure stresses and pore pressures. This problem is further compounded by “water weakening” of the rock. Water weakening occurs when certain formations, such as chalk, are exposed to water. Generally, chalk having a porosity of 36% will have only ⅙ the strength after being exposed to water. This water weakening occurs immediately and is irreversible. The combination of water weakening, closure stress and pore pressure effects cause the etched rock face to collapse immediately and lose any induced fracture conductivity.