--> Experimental Study on Hydraulic Fracturing in Unconsolidated Sands for Its Application to Methane Hydrate Development

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Experimental Study on Hydraulic Fracturing in Unconsolidated Sands for Its Application to Methane Hydrate Development

Abstract

Depressurization is considered a promising stimulation method of gas production from reservoirs of methane hydrate (MH). In this method, the wellbore pressure is decreased to 3 or 4 MPa for MH to be thermodynamically unstable dissociating into gas and water. It means that when the depressurization is applied to MH in marine sediments offshore, the wellbore pressure must be reduced extremely. For a typical example that the reservoir is located at 300 m below sea floor at 1000 m water depth, the wellbore pressure at the reservoir depth should be reduced by 10 MPa from hydrostatic level of 13 MPa. Such a big drawdown will cause a severe flow of water and gas from the reservoir to the wellbore, and in consideration of soft reservoir formation, i.e. unconsolidated sands, the fluid flow will force fine solids in the reservoir to move and accumulate around the wellbore. Furthermore, the drawdown will lead to formation compaction. Those events will result in undesirable severe reduction of permeability in the near-well region. This situation can be improved by hydraulic fracturing. The created fracture will bypass the permeability reduction zone and rebuild the hydraulic communication between the wellbore and the reservoir, and it will contribute also to suppress sand production. However, the mechanism of hydraulic fracturing in sands has not been well understood yet. Thus we experimentally examined and clarified interesting phenomenon accompanying fracturing. To this end, a method to simulate hydraulic fracturing of unconsolidated strata in laboratory experiment was constructed. In the experiments using a specimen made from a mixture of silica sand and kaolin and a fracturing fluid of machine oil, a linearly extending fracture was formed similarly to the case of hard rocks when the proportion of kaolin was large. However, when the proportion of kaolin is small, fracture was not formed at all. This phenomenon can be interpreted if the fracture is formed by compressive deformation / failure, which is completely different from conventional theory of hydraulic fracturing. That is, since tensile force is not transmitted in unconsolidated sands, gaps are created by pushing the part where the fluid pressure acts and the grains in the vicinity thereof. Also, in the mixture of silica sand and kaolin, as the proportion of kaolin decreases, the interlocking between the grains improves and compressive deformation / failure hardly occurs, so it is considered difficult to form gaps / fracture in hydraulic fracturing.