Given that EGS geothermal development is accomplished by drilling boreholes that are several kilometers deep, the ability to estimate stresses is limited in these depths, as they are difficult to access. Thus, it is desirable to combine the data from various stress measurement methods and follow a set of steps to construct a reliable rock stress model.
- Best Estimate Stress Model (BESM)
The geological and morphological characteristics of an area of interest has to be considered before starting any in-situ stress measurements. To establish a BESM, the World Stress Map (WSM, www.world-stress-map.org) database should be consulted, offering a collection of existing stress measurement data.
- Stress Measurement Methods (SMM)
The most suitable measurement method for the field condition should be selected based on the BESM.
The vertical principal stress can usually be obtained by integrating the density of the logging data. The measurement of the horizontal stress in a vertical borehole is commonly performed by hydraulic fracturing (HF) or hydraulic tests on pre-existing fractures (HTPF). Observation of borehole breakouts (BO) and drilling induced tensile fractures (DIFS) by logging borehole images enable the estimation of the direction and magnitude of horizontal stresses. Core-based methods help to estimate stress, and these methods include borehole relief (BR) using Borre probe/CSIRO hollow inclusion cell and flat jack (FJ), anelastic strain recovery (ASR), differential strain analysis (DSA), core disking (CD), and Kaiser effect (KE) by acoustic emission. In addition, stress inversion from the focal mechanism of earthquakes (natural seismicity (NS) and induced seismicity (IS)) can provide information about the orientation and relative magnitude of the in-situ stress.
- Integrated Stress Determination (ISD) and Final Rock Stress Model (FRSM)
ISD integrates the stress measurement data obtained from BESM and SMM using various algorithms, including the least squares method. In addition to the direct measurement of stress, numerical modeling is also recommended as a complementary tool to predict and validate the in-situ stress for the establishment of the FRSM.
The reliability of the final rock stress model increases gradually as the results of each stress-estimation step are integrated. Fig. 2 shows that the most complete stress model result from performing all of the steps described in the figure.
Several case studies have presented integrated stress models by combining the results of various stress measurements (see table below).