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摘要
When rising cross-formational warm water encounters the cool water in a phreatic carbonate aquifer, their mixing changes the flux, temperature, and concentration of solutes in the mixed groundwater, and this may be locally important for the dissolution of carbonate rock. The primary objective of this paper is to provide quantitative insight into the early karstification of a hypothetic geothermal limestone basin with inflows from a surface and a bottom boundary, a central river for discharge, and two intersected fault damage zones in the left half aquifer, by mixing of cool meteoric water with warm cross-formational water under different boundary conditions, using a hydro-thermal-chemical coupled model. For the meteoric water which is highly under saturated with respect to calcite, the dissolution capacity of mixed groundwater with respect to calcite increases with its mixing fraction. According to the set up described, with partial pressure of carbon dioxide and pH remaining constant, high susceptibility to early karstification by thermal mixing corrosion during 0.1 Ma is found at the intersection of the faults, and along the water table in discharge regions. By increasing the input flux of cross-formational warm water from 0.001 m/d to 0.002 m/d, early karstification is accelerated by nearly 30,000 years due to the increase of flow velocity, whereas increasing the temperature of the supplied warm water from 50 degrees C to 90 degrees C decreases the average groundwater dissolution capacity by similar to 0.2 mol/m(3) and prolongs early karstification by around 10,000 years. For meteoric water saturated with calcite, the dissolution capacity increases with the mixing ratio of rising warm water, which restricts the dissolution along the water table. As a result, the calculated time for early karstification is about 0.1 Ma longer, and karst formation occurs as a vertical proto-conduit extending from the river bottom to depth. @@@ In all cases, the flow and dissolution in the left-half basin with fault damage zones is faster and more heterogeneous than the homogeneous aquifer, thereby increasing differences of conduit diameter in the aquifer with faults among different cases. The increase of the supply and temperature of warm water under the boundary condition of highly undersaturated meteoric water results in the largest discrepancies in groundwater flux and temperature, respectively. The loss of aggressiveness in meteoric water changes the Ca2+ concentration and the diameter of the enlarged conduits most significantly. This work provides some quantitative guidelines for understanding the behaviour of thermal mixing corrosion in geothermal carbonate reservoirs, inferring the variation of the hydraulic-thermal-chemical features when boundary conditions change.
- 出版日期2019-1
- 单位广东工业大学
