Experimenting with Deflagration for Stimulating Geothermal Wells

Abstract

Deflagration was introduced in Iceland in 2010 for stimulating geothermal wells. At that time the method and fuel material had proofed successful in stimulating geothermal wells in sedimentary formations such as at Soda Lake USA. The method was appealing as equipment for hydro-fracturing at high pressures were not available in the country. However, use of packers and pressure stimulation at relatively low pressures, well below the assumed formation fracturing pressure, have proofed successful in many instances in Icelandic geothermal fields. Icelandic geothermal fields are located in basaltic volcanic type formations with varying degree of alteration dependent on the reservoirs temperature. The rock matrix has relatively low permeability so the transmissivity of a given geothermal well is controlled by fracture permeability, which is concentrated to handful feeding points intersected by the well. In some instances the permeability of these observed feeds is not enough to make the well a producer or an injector. The potential of these feeds may be apparent so successful stimulation could shift the well to be economically useable. In Icelandic basaltic formations the stimulation options are generally limited to methods that can change the aperture/width of fractures that show indications to be active feed points. Acidification is normally out as few fractures have calcite scaling in them. So some form of pressurization methods or thermal sock methods are left as stimulation options. Deflagration with high energy gas flow fits into that category. The paper describes deflagration stimulation trials in four wells at the Reykjanes high enthalpy geothermal field in Iceland. The trials took place in three newly drilled wells in the period 2010 to 2013 and in an older production well during workover. During the trials 1-6 charges were burned at different locations in the well undergoing the stimulation. Torpedoes (carrying tools) loaded with charges of two different energy capacities have been tried. The deflagration was tried both inside liners and in open hole completions. Described is how the procedure has evolved between trials for selecting potential feed points for stimulation and for positioning the tool in the wells. Data obtained during stimulation trials is discussed and results given mainly from short injection tests that evaluate the short time injectivity change caused by the deflagration.