DAMAST - Dams and Seismicity

Stress and Pressure Compilation

WP A.1, A.3, B.1, B.3, B.5

Stress and Pore Pressure Monitoring


Figure 1: If the effective stresses are reduced by an increase
in pore pressure (ΔPp ) the Mohr Circle can reach the
failure envelope and faults can be reactivated. Here a 2D
display of stresses is used.

To identify the dominant processes of induced seismicity around high dams, we use a simple mechanical model based on a Mohr-Coulomb approach, which combines the failure theory of Coulomb with Mohr´s hypothesis and Mohr´s description of the state of stress using the so-called Mohr circles.

 

Activities & Preliminary Results:

  1. Referencing of map information (different geodetic datum) to enable merging of information
  2. Identification of Fault structures from geological maps and in the field (May 2019, October 2019)
  3. Based on the Map studies and results from the field trip the drilling locations for the 2 deep wells have been identified
  4. Stress field investigations: To understand the modifications of the stress field due to the building and operation of the Enguri Dam and the Jvari reservoir, we compiled information about the stress field of the region before the impoundment of the reservoir.

 


Figure 2: Stress profile at the Khudoni dam
area including data from Rummel & Baumgärtner
1985 for DFG Project RU225-4 and assumptions
for the vertical stress gradient and the pore
pressure gradient.

One source for the regional stress magnitude information are results from a hydraulic fracturing measurement campaign in the vicinity of the planned (but never completed) Khudoni dam in 1985 (Rummel & Baumgärtner, 1985). With hydraulic fracturing the minimum stress component can be determined. The following figures shows the depth profiles for effective stresses in two shallow boreholes according to the results of Rummel & Baumgärtner and under the assumption of a vertical stress gradient of 26 MPa/km and a pore pressure gradient of 9.81 MPa/km.

Effective Stress profile with depth for the wells in the Khudoni region based on Rummel & Baumgärtner 1985 and under the assumption of a vertical stress gradient of 26 MPa/km and a hydrostatic pore pressure gradient.

Furthermore, in 1991 hydraulic fracturing stress measurements have been performed in the Tyrnauz deep borehole. The Tyrnauz borehole is a km deep well northeast of the Enguri area at a distance of ca. 60 km. thus, the stress field and was designed as a geothermal well as test well for municipal heating. It is located in granitic rock. There is not much to be found in non-russian literature besides a report by Kruger (1992) summarizing two Russian reports. The stress information from the Tyrnauz well results from hydraulic fracturing within the depth interval of 3721 to 3830 m. Thereby, the well was deviating by 25° from vertical and the hole azimuth is N 25°E. The flow rate for the first test interval was 0.4 m³/min.

In the Tyrnauz well on 24-25 February 1991 a hydraulic fracturing experiment has been performed. These tests had to stop soon due to failure of borehole infrastructure. The results of the first experiments show, that in a depth of ca. 3.7km the minimum horizontal stress is at 81 to 82 MPa. The vertical stress is 96 MPa and the hydrostatic pore pressure at 35.2 MPa.

 

Stress orientations: can be derived from focal mechanisms in the area as compiled in Tibaldi et al. (2019) and the World Stress Map (www.world-stress-map.org). These data indicate that the maximum horizontal stress SH is oriented N155-N160° E to N-S.

 


The in-situ stress data from hydraulic fracturing experiments are at a distance of ca. 25 km (Khudoni-site) and ca. 100 km (Tyrnauz-site) available.