Explosively Created Permeability From Single Charges

Abstract

Abstract A Theoretical expression showing the radial dependence of permeability in geologic media as a function of the distance from the point of detonation bas been derived. This relationship shows that created permeability decreases as a function of radius (1/r5 around a spherical blast and 1/r4 around a cylindrical shot). Excellent correlation was found when this prediction was compared with permeability measurements made around the site of permeability measurements made around the site of the Hardhat Nuclear Event fired in granodiorite and a chemical explosive detonated in coal. Introduction There is considerable interest in predicting the distribution of permeability arising from the detonation of explosives emplaced in low-permeability formations. These predictions would be useful for projects such as massive explosive stimulation of projects such as massive explosive stimulation of tight gas reservoirs, in-situ retorting of oil shale, coal gasification, stimulation of geothermal reservoirs, and fracturing oil shale with explosives. Explosives are useful because standard stimulation techniques such as hydraulic fracturing and acidizing are at times ineffective. Explosives may also be used to initiate fractures before a hydraulic stimulation. For in-situ processes an array of explosive wells offers the possibility of creating a distribution of many fractures not achievable by other techniques. The size distribution of fractures resulting from the use of explosives in block and sublevel caving operations is a critical factor in determining the pressure drop through the rubblized zone. For example, large proportions of fine particles are undesirable because of their effect in particles are undesirable because of their effect in reducing the permeability of the rubblized zone. Hence, a knowledge of the size distribution of fragments created by the detonation of emplaced explosives is extremely important. It is the purpose of this article to present a theory that can predict the permeability and fracture distribution resulting from an underground explosion. THEORY To develop the necessary theory we begin with an expression for permeability (developed in the Appendix). Permeability is related to porosity and specific surface through the relation 3k,.......................(1)(1 −)2S2 where is porosity and S is specific surface area. To relate this formula to the complex phenomena involved in an explosive detonation, it is useful to divide explosive effects into two stages. The first stage is dominated by the large-amplitude stress wave. The second stage is characterized by the expansion of the cavity due to the high pressure gases from the detonation. The effects of the first stage on the media are of a dynamic nature, while those of the second stage extend over a much longer time interval and can be regarded as a quasistatic process. To obtain a description of permeability, we must relate these processes to permeability, we must relate these processes to Eq. 1. According to Kutter and Fairhurst, the principal role of the stress wave is to initiate fractures. The fracture density, n, is related to the porosity and specific surface by (see Eq. A-9) Sn = (1−)..........................(2)2w Griffith postulated a failure criteria for real materials. From tensile tests, Griffith learned that the average stress at rupture was small compared with the theoretical strength of the solid. From this he concluded that energy in the test piece was not uniformly distributed. At points where the cracks originate there must exist high concentrations of strain energy. We assume that these concentration points are macroscopic flaws in the material. points are macroscopic flaws in the material. SPEJ P. 495