A new model of propellant fracturing for well stimulation is presented. It involves the pressure build-up due to propellant burning, accompanied by the propellant gas generation, the wellbore pressurisation due to gas bubble expansion and the fracturing of rock, followed by the propagation of created fractures. Dynamic fracturing is included in the conventional hydraulic fracturing model as an instantaneous fracture creation and propagation after wellbore breakdown to the distance of the order of a few feet from the wellbore. Operational targets for pre-fracturing and productivity or injectivity stimulation are discussed.
Propellant fracturing is considered as a cheap alternative to the conventional hydraulic fracturing under those circumstances when either heavy pumping equipment is unavailable or well construction integrity during treatment can be compromised or cost of conventional treatment cannot be justified.
A new model of propellant fracturing is presented. The main focus is in on the qualitative analysis of propellant fracturing, using simplifications of rock properties, fracture propagation patterns and wellbore hydraulics.
In contrast to the conventional hydraulic fracturing simulations, the initial size of fractures is assumed to be equal to zero and the breakdown pressure known somehow together with the geometry of propagating fracture (in particular, the KGD model has been used). The dynamic phase of fracture propagation cannot be modeled within the conventional approach developed for hydraulic fracturing and it is represented by the fracture "jump" or instantaneous propagation to some distance from the wellbore.
Possible applications of propellant fracturing include 1) the pre-fracturing before conventional hydraulic fracturing to reduce the pressure of fracture initiation and the risk of undesirable fracture propagation (halo effect, fracture tortuosity effect, small stress barriers) and 2) the injectivity enhancement, which also may be useful for reinjection during well testing. The application of propellant fracturing to productivity stimulation seems to be limited to the formations under in situ stress contrast unless the efficient technique of fracture closure prevention (proppant placement) is implemented.
The field and lab experiments, which have been carried out in late 70's - early 80's [1-17], proved that wellbore pressurisation by means of propellant burning can be used effectively for the creation of multiple fractures in rocks around boreholes. This technique, which was using a full wellbore charge of propellant, was called high-energy gas fracturing. It was tailored to pressurize the hole and create multiple fractures, radiating from the wellbore, without crushing surrounding rock. The main targets at that time were considered to be natural gas reservoirs in low permeability formations and also tight shale formations, which could be used for the burial of dangerous wastes. In the first case, the fractures should increase near wellbore hydraulic conductivity, facilitating the gas flow to the wellbore. In the second application, the required injectivity should be established.
The first tests were carried out at the Nevada Test Site in dry wellbores in tuffs. The mine back in a testing tunnel complex allowed investigators to examine fracturing pattern, which has been studied versus the propellant amount and type, the in-situ stresses and the borehole diameter.