Fluid Pocket Generation in Response to Heterogeneous Reactivity of a Rock Fracture Under Hydrothermal Conditions. Issue 20 (30th October 2017)

• Record Type: Journal Article
• Authors: Okamoto, A.; Tanaka, H.; Watanabe, N.; Saishu, H.; Tsuchiya, N.
• Is Part Of: Geophysical research letters. Volume 44:Issue 20(2017)
• Page Start: 10, 306
• Page End: 10, 315
• Publication Date: 2017-10-30
• Subjects: hydrothermal experiment -- mineral dissolution -- fracture permeability -- aperture structure -- X‐ray CT -- gouge; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1002/2017GL075476
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Fractures are the location of various water‐rock interactions within the Earth's crust; however, the impact of the chemical heterogeneity of fractures on hydraulic properties is poorly understood. We conducted flow‐through experiments on the dissolution of granite with a tensile fracture at 350°C and fluid pressure of 20 MPa with confining pressure of 40 MPa. The aperture structures were evaluated by X‐ray computed tomography before and after the experiments. Under the experimental conditions, quartz grains dissolve rapidly to produce grain‐scale pockets on the fracture surface, whereas altered feldspar grains act as asperities to sustain the open cavities. The fracture contained gouge with large surface area. The feedback between fluid flow and the rapid dissolution of gouge material produced large fluid pockets, whereas permeability did not always increase significantly. Such intense hydrological‐chemical interactions could strongly influence the porosity‐permeability relationship of fractured reservoirs in the crust. Plain Language Summary: Fluid flow within the Earth's crusts plays important roles of various geological phenomena, including seismicity, volcanism, developments of geothermal systems, and hydrothermal ore deposits. Fractures act as dominant fluid pathways in the crusts and as favorable sites of fluid‐rock interactions; however, our knowledge on the interaction between chemical and hydraulic processes at high temperatures is very limited. In this study, we developed a novel experimental apparatus, which enabled to hydrothermal flow‐through experiments at high‐temperature conditions (350˚C, 20 MPa), relevant to the deep in the geothermal areas. We investigated dissolution of fractured granite and revealed the detailed aperture structures of the fracture by X‐ray computed tomography. We found that a large amount of dissolution occurs and the "heterogeneous reactivity" of rock fractures is a key of the evolution of porosities and permeability. Quartz is dissolved rapidly, whereas "undissolved" feldspars act as asperities, which sustain open cavities. In addition, the feedback of rapid dissolution of gauges and fluid flow create a large fluid pockets and preferential fluid pathways along the fracture. Such intense hydrological‐chemical‐mechanical interactions could be critical in the high‐fluid‐flux regime in fault zones, linked to phenomena such as swarm seismicity and fluid injection in geothermal fields. Key Points: Flow‐through experiments were conducted on the dissolution of fractured granite under hydrothermal conditions and confining pressure Quartz was dissolved rapidly to create open cavities, whereas altered feldspars acted as asperities, which sustained open cavities Preferential dissolution of quartz and gouge may result in the formation of large pockets of stagnant fluids in the crust