Patent GB-1482023-A (GB)
Title: Explosive fracturing of deep rock
IPC: E21B43/263
Filed: 1974-07-25
Abstract: 1482023 Rock working &c E I DU PONT DE NEMOURS & CO 25 July 1974 [26 July 1973] 32879/74 Heading E1F In a technique for producing a network of fractures in an underground formation, chemical explosive charges are detonated in sequence to form overlapping fracture zones, the detonation of each charge after the first being delayed until liquid is present in the adjacent fractures, the liquid serving both as a lubricant to enhance fragmentation and as a means for preventing existing fractures from being rammed shut. The technique can be used to stimulate production via oil and gas wells but is particularly useful in preparing an ore body for a solution mining operation. The charges may be placed in sections of shotholes that have been locally enlarged, e.g. using springing charges or expansible bits, the charge in any given shothole, usually being detonated before drilling an adjacent shothole whose charge will produce a fracture zone overlapping that produced by detonation of the charge in the given shothole. The liquid used to fill the fractures around each charge may be water which can be naturallyoccurring or be pumped into the formation, or a lixiviant if leaching of ore is performed in the course of the detonation sequence. The successive shotholes preferably remain intact above the fracture zones, and injection holes are drilled which remain intact throughout the fracture zones, each detonation preferably being followed by a flushing operation in which fines are flushed out of the latest fracture zone by injecting liquid and gas, or gas alone, via a neighbouring injection hole and recovering liquid and gas and entrained fines via the upper intact portion of the shothole in which the latest detonation occurred, this circulation optionally being supplemented by injection of fluid (e.g. air) via neighbouring shotholes. Adjacent shotholes may be drilled by sidetracking from a common upper hole. In some cases charges in a group of adjacent holes may be detonated simultaneously to form the next in a series of overlapping fracture zones. In the case of vertical shotholes, the local liquid level at the time of detonation should be at least as high as the top of the charge, and when horizontal shotholes or tunnels are used the liquid level should extend above the charge by not less than the anticipated radius of fracture. Relatively remote charges, i.e. those not producing mutually overlapping fractures may be detonated simultaneously, the delay to allow flooding only applying to detonations of charges adjacent to previously-formed fracture zones. The suggested solution mining technique involves using the same circulation path as was used in the flushing steps, i.e. lixiviant is displaced by drive gas into the lower regions of the formation of interest via the injection wells and pregnant liquor is recovered via the intact portions of the shotholes, this circulation path ensuring that fines are not displaced from the more severely fractured regions to the less severely fractured regions. Suitable lixiviants include sulfuric acid/water or sulfuric acid/nitric acid/water for ores with low acid consumption, and NH 4 OH/water for ores with high acid consumption, and suitable drive gases include O 2 , air and NO x , the injection being either continuous or intermittent and at pressures exceeding local lithostatic pressure. In an example, shotholes are centred on the vertices of concentric equilateral triangles which in turn are centred on injection wells, and an aluminized water gel explosive is used containing 18.9% ammonium nitrate, 10.5% sodium nitrate, 29.6% methylamine nitrate, 30% aluminium and 11% water.