Fracturing effect of underwater shock waves generated by plasma-ignited energetic materials explosion

Summary form only given. Underwater shock waves have many applications, such as petroleum exploitation, sterilization, lithotripsy and so on. Based on high pulsed power technology, electrical explosion can be utilized to generate shock waves with fast front and short duration efficiently. In order to enhance the shock wave, we have proposed a new technique by taking advantage of energetic materials. In this paper, we investigated the fracturing effect of underwater shock waves generated by the explosion of an energetic materials load that ignited by pulsed discharge plasma. The energetic materials load in specifics contained three parts: 1) a metal wire as core; 2) energetic materials as mantle; 3) a thin walled propene polymer pipe as package. The experimental apparatus could achieve a controllable electrical energy conversion with a period of ~13.5 μs, a charging voltage of 25 kV and stored energy of ~1875 joules. The energetic materials mainly contained a certain proportion of aluminum powders (Al), ammonium nitrate (NH₄NO₃), and ammonium perchlorate (NH₄ClO₄) with an average charge density of ~1.38 g/cm³. The sample was a cuboid shale (762 mm in length, 762 mm in width, and 914 mm in height) from Changning, Sichuan Province. A wellbore (550 mm in depth, 80 mm in diameter) was excavated on the surface of the sample with an artificial well bottom and the discharge could occur at the geometrical center of the shale sample. During the fracturing process, a large-scale triaxial stress pressurizing equipment was used for simulating the practical crustal stress. The most evident experimental results showed that a large number of cell-shaped, multiple cracks were formed after the shock wave fracturing, and contributed to a great reduction of fracture pressure in the fracturing test. The fracture pressure Pf, the propagation pressure Ppro and the instantaneous shut-in pressure Ps were 25.03 MPa, 19.26 MPa, and 16.90 MPa respectively, which suggested that this technique might be an effective method of well stimulation in low permeability reservoir.