Explosive Detonation Tested in Hydraulically Fractured Gas Wells

Abstract

In the tests described here, the well completed in open hole, which was extensively fractured vertically by hydraulic treatment, responded poorly to explosive fracture stimulation. Detonation through perforated casing, however, damaged only the joint of casing in the perforated interval, and improved flow capacity by 40 percent. Introduction Vast quantities of natural gas and liquid hydrocarbons exist in underground formations having low permeability. In many cases, the permeability is so low permeability. In many cases, the permeability is so low that economic production from a reservoir is prohibited. Even though such conventional treatments as hydraulic fracture stimulation, acidizing, and nitro shooting in the wellbore have improved permeability in many of these tight reservoirs, some formations have not responded. Research was begun in 1963 to develop an explosive technique that would give the increase in permeability needed to permit recovery from these tight permeability needed to permit recovery from these tight reservoirs. The emplacement and detonation of a liquid explosive in previously induced fracture zones is intended to increase permeability by creating additional fractures and enlarging existing fractures. The mechanism for formation fracturing by detonating an explosive in the wellbore is relatively simple. Rock is generally less resistant to tension than to compression. The high pressure developed by an explosion shatters the rock adjacent to the wellbore, and a shock wave is generated that moves outward through the formation. The pressure in this shock wave is initially positive when it arrives at a free surface and then falls rapidly to negative values. This implies a change from compression to tension with resulting rock fracturing. When detonating an explosive within an existing hydraulic fracture system, one encounters two distinctly different conditions. If a detonation occurs within a horizontal fracture, the high-pressure shock wave should shatter the adjacent faces of the fracture, with the shock wave traversing the medium and reflecting back from stratigraphic heterogeneities above and below the pay zone. The rock would be subjected to a compression wave followed by a tension wave and fracturing should occur. However, extension of the fracture radially in the horizontal plane is limited, because the ever-increasing area at the perimeter of the system would be available to dissipate the energy. It is suggested that if detonation occurs in a vertical fracture, the faces of the fracture again should be shattered, but the traversing shock wave usually has no immediate vertical reflection boundary and the energy is quickly dissipated. The stratigraphic boundaries above and below the interval may confine the expanding gases and contribute to extending the vertical fracture. In this environment, only a few explosively created fractures may be expected. Explosive fracture stimulation is not a new idea. U. S. patents related to this technique were granted to Zandmer, Brandon, Hanson and Hinson. Oilfield service companies have been actively engaged in similar research. However, much of that research has been performed to develop a slurry explosive that is pumped into a well and detonated opposite the formation to be fractured. In contrast, the Bureau of Mines has displaced a less viscous liquid explosive from the wellbore into existing formation fractures before detonation. JPT P. 403