In the hydraulic stimulation, massive fluid is injected into subsurface rock through drilled wells. Then a number of micro-seismic events are commonly observed. By analyzing the data of microseismic event associated with hydraulic stimulation, we can estimate the orientation, i.e. dip and strike, of the fracture which slides to induce micro-seismic event. From the estimated orientation taking into account the in-situ stresses and the Mohr-coulomb criterion to describe the critical condition of fracture sliding, we can estimate the pore pressure at the location of sliding fracture and at the time when the sliding occurs, in other words, when micro-seismic event occurs. The estimated values of pore pressure are sorted in a certain manner for each equally-divided spatial region, i.e. block, to give spatial distribution of pore pressure and its variation with time during hydraulic stimulation. For an example, we applied this method to the micro-seismic data observed during the hydraulic stimulation performed in September 1993 at Soultz field in France. We obtained successfully the estimated pore pressure distribution.
A new simulation model for co-axial ground heat exchangers was developed to simulate the temperature behaviors of heat exchangers in heterogeneous formations. In the simulation model, the formation is vertically discretized into sub-layers and the cylindrical source function is applied to each layer. A nonlinear regression method, the polytope method, is applied to the simulation model for automatically estimating the vertical distribution of thermal conductivities along the heat exchangers. The reliability of the new procure is validated with the results of thermal response test conducted using fiber optical temperature sensors. The calculated temperature performances and vertical temperature profiles agreed well with the thermal response test results. The estimated distribution of thermal conductivity also correlated well with the geological and groundwater information around the ground heat exchanger.
Spatial continuity of geothermal temperatures has been analyzed by variograms using temperature data of 35 wells drilled in Yanaidzu-Nishiyama geothermal field, northeastern Japan. The trend that temperature increases with increasing depth is approximated by T(z)=T0+b·In(z-z0), where T(z) is temperature at elevation z, and b, z0, T0 are constants. In order to obtain temperature deviations from the trend, the function is applied to each well in the individually regressed case (IRC), or to all wells in the totally regressed case (TRC). The analysis gives the following conclusions : 1) 2-D variograms of measured temperatures, and both temperature deviations show patterns caused by the data point distribution ; 2) a vertical variogram of measured temperatures diverges, and do not show a sill ; 3) temperature deviations in IRC seem to be completely free from the trend, and their 3-D variogram is almost isotropic, though ranges varies between 200 and 70 m (the longest and shortest directions are vertical and horizontally W-E, respectively) ; 4) variograms of temperature deviations in TRC show the intermediate character between measured temperatures and temperature deviations in IRC, and hence it is not clear that temperature deviations in TRC are free from the general trend ; 5) a horizontal 2-D variogram in TRC was almost the same as that of measured temperatures (γ=0.9999), and ranges toward N40∼50°W and N10∼40°E directions are extremely long ; 6) N40∼50°W which is one of the directions having long ranges coincides with the strike of faults playing a pathway of hydrothermal fluids ; 7) wells are hoped to be distributed randomly, and be inclined toward many directions.
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