Hydraulics stimulation

Monitoring electric potential during hydraulic stimulation... 

Marquis, G., Darnet, M., Saii.hac, P., Singh, A. K. Gerard, A. 2002. Surface electric variations induced by deep hydraulic stimulation an example from the Soultz HDR site. Geophysical Research Letters, 29, 7.1-7.4. 

A Comprehensive understanding of the fracture distribution and hydro-geomechanical processes occurring during hydraulic stimulation is essential in both conventional and Hot Dry Rock (HDR)/ Hot Wet Rock (HWR) geothermal development. Some of this information can be obtained from well logs such as flow, temperature, pressure, BHTV, FMI, etc. However, they only provide restricted information near the well. 

The clustering analysis is another mean to find and evaluate structures in seismic clouds. Phillips et al, (1997) and Phillips (2000) proposed a repicking method, which produces the relative time difference of phases within a group of similar microseismic events, followed by master event relative source locations. For this analysis, waveforms are lined up after low pass filtering, with P- and S-wave arrival times are manually repicked from a similar portion of the waveform. The technique can be used to identify wave arrival times and improve the accuracy of relative source location determinations. Phillips (2000) applied this analysis to microseismic clouds created by hydraulic stimulation in the Soultz HDR reservoir and delineated two distinct intersecting planar structures. 

HDR/HWR geothermal development. These techniques have been applied to the microseismic data set obtained during the hydraulic stimulation at Soultz HDR field, France, in 1993. Detailed seismic structure of a seismic line, which was growing linearly at the early stage of the stimulation, has been analysed. It is estimated that this structure shows enhanced permeable paths and consist of sub parallel microstructures, which are oriented to the directions most easy to slip by the stimulation. 

In this study, simulated hydraulic stimulation tests were undertaken under supercritical water environments in order to examine the feasibility of creating water pathway in deep-seated supercritical rock masses. The present experimental results... 

Figure I. Schematic illustration of a high temperature-pressure autoclave system used for simulated laboratory hydraulic stimulation...

Examples of the borehole pressure records obtained from the simulated hydraulic stimulation tests are shown in Fig. 2. Figure 2(a) shows the variation of the borehole pressure with respect to time at a room temperature (25 C), under the confining pressure of lOOMPa and the injection rate of 5.0 mmVsec. Figure 3 (a) shows the fracture profile observed on the cross section perpendicular to the specimen axis. The specimen has been impregnated with a red dye penetrant for ten... 

Figure 2. Representative curves of borehole pressure versus time for simulated hydraulic stimulation tests conducted at (a) a room temperature (25°C) and (b) 600°C. The confining pressure is 100 MPa, and the injection rate is 5 mm3/sec.

The occurrence condition of shear fracture is examined on the basis of the Coulomb criterion. The averaged shear stress across the fracture plane in the simulated hydraulic stimulation tests is plotted in Fig. 5, as a function of the effective normal stress across the fracture plane. The steady-stale pore pressure distribution given from Equation (1) is averaged over the fracture plane and is used to compute the effective normal stress. Triaxial compression tests have been performed on the granite using the same apparatus shown in Fig. 

I (Takahashi, 2000). The peak shear stresses obtained from the triaxial compression tests are also plotted in Fig. A2. In the plot of Fig. 5, there is general agreement between the two types of the experimental results. Thus, it is thought that the occurrence of the shear fracture in the simulated hydraulic stimulation tests can be approximately predicted by the Coulomb criterion. Based on the comparison, the critical condition for the shear fracture due to hydraulic stimulation was estimated using the experimental results of the triaxial compression tests, as given in Fig. A2, and the averaged value of pore pressure. The detailed discussion of the triaxial compression tests can be found elsewhere (Takahashi, 2000.). 

Even though the experimental results are still limited, it appears that the predicted occurrence conditions provide the general trend compatible with the experimental results. Thus, it may be possible to examine the fracture process to be expected for hydraulic stimulation in supercritical rock masses on the basis of the fracture mechanism map given in Fig. 4. The ratio of the minimum horizontal tectonic stress to the vertical tectonic stress usually falls in the range of 0.5-1.0 for brittle rock masses (Brace Kohlstedt, 1980.). The actual difference of the tectonic stresses may be significantly smaller in supercritical rock masses due to the high temperature creep deformation (Fournier, 1999 Muraoka et al., 20()0). If we limit... 

Figure 5. Comparison of the shear strength data obtained from simulated hydraulic stimulation tests and triaxial compression tests. Those specimens which were pre-heated up to 500°C are designated (h).

Simulated hydraulic stimulation tests were performed using thick-walled cylindrical specimens of lidate Granite at temperatures up to 600°C and confining pressures up to 100 MPa. The objective of the laboratory experiment was to examine the feasibility of creating an artificial... 

Takahashi T. 2000. Fracture and fluid flow characteristics and hydraulic stimulation behavior in granite under supercritical water environments. Doctoral dissertation, Tohoku University. 

Figure AI. Temperature dependence of tensile fracture strength determined by simulated hydraulic stimulation tests...

The experiments conducted so far at the site in range of depths between 3000 and 3800 m show that hydraulic stimulation techniques can considerably increase the permeability in a rock volume that can extend up to several hundred meters around the well, thereby serving to establish connections between the wells. The major impact of the hydraulic stimulation could be explained by hydromechanical mechanisms (ruptures in fracture planes). However in the near well at the scale of some meters, a decrease of the impedancy is outlined via hydraulic vibration tests and is presently not well understood. As at this scale, the hydromechanical effect cannot be dissociated from the thermal effect due to the injection of a cold water in a hot media, the thermo-hydro-mechanical responses of the stimulated fractures must be investigated. 

In this paper, we suggest a method for estimation of pore-pressure during hydraulic stimulation of geothermal reservoir by using the induced microseismic multiplets. The results of the application to the microseismic events at Soultz field are demonstrated showing the feasibility of the method. 

We estimate distribution of critical pore-pressure of fractures for shear slip during hydraulic stimulation. 

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