What is in-situ stress?
In-situ stress is a set of far-field mechanical forces impacting underground structures. There are four types of in-situ stresses: gravitational, tectonic, residual, and terrestrial stresses. The gravitational and tectonic forces produced by the motion of crustal plates are the main sources of in-situ stresses, the others account only for in-situ stresses at shallow depths.
In-situ stresses within the earth’s crust are typically defined in terms of principal stresses and their orientations. The principal stress component caused by the force of gravity is usually a vertical stress; the other two are the minimum and maximum horizontal stresses. The in-situ stresses are classified as normal fault (NF), strike-slip (SS) and thrust fault (TF) stress regimes depending on whether the magnitude of the vertical stress is the largest, intermediate or the least (figure 1).
Rock stress is a fictitious quantity and cannot be determined accurately. The state of current stress is the result of a long series of past geological events, including physiochemical, thermal, and tectonic processes. Therefore, rock stress can be inferred by disturbing in-situ rock conditions, i.e., inducing strains, deformations, crack openings, or by observing the behavior of the rock.
Figure 1: three in-situ stress regimes (www.world-stress-map.org)
Why is the estimation of in-situ stress important for the exploration and production of geothermal energy?
Enhanced geothermal systems (EGS) based on hydraulic stimulation enhance the hydraulic connection of the fracture network in geothermal reservoirs. In consequence, the development of a reservoir depends on successful hydraulic stimulation. Breakdown pressure, direction in which hydraulic fractures propagate, injection pressure required to activate stimulation, reservoir’s response to hydraulic stimulation, evolution of permeability, and migration of induced seismicity are closely related to the in-situ stress. And also, the in-situ stress affects the stabilities and trajectories of deep boreholes: A high in-situ stress ratio relative to rock strength often results in instability of deep boreholes.
How can in-situ stress be estimated in a geothermal reservoir?
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).
Reviewers
Régis Hehn, és-Géothermie
Clément Baujard, és-Géothermie
References and further links
[1]
“ISRM suggested methods for rock stress estimation - part 5: establishing a model for the in situ stress at a given site”, in Rock Mechanics Rock Engineering by Stephansson and Zang, 2012
This paper suggests the best way forward for deriving the final rock stress model for a site or an area. A summary of this paper can be found in the answer to the third question of this report, i.e., "3. How could in-situ stress be estimated in a geothermal reservoir?" More detailed information on the procedures for establishing the final rock stress model can be obtained from this paper.
[2]
“Contribution of the exploration of deep crystalline fractured reservoir of Soultz to the knowledge of enhanced geothermal systems (EGS)”, in Comptes Rendus Geoscience by Genter et al., 2010
This paper presents an overview of the principal achievements at the Soult experimental geothermal site in France over the past two decades and highlights the scientific results, including the estimation of in-situ stress. It assists the overall understanding of EGS from exploration to reservoir development and hydraulic circulation.
[3]
Stress field of the Earth’s crust, by Zang and Stephansson, 2010
This book provides an overview of in-situ stress and also offers detailed information on methodologies and procedures for various stress measurements. Several stress measurement methods commonly used in stress estimation for geothermal reservoirs are mentioned in this report.
Geothermal Project (Country)
Rosemanowes (UK)
Fenton Hill (US)
Hijiori (Japan)
Soultz (France)
Cooper Basin (Australia)
Basel (Switzerland)
Pohang (South Korea)
Stress estimation method
HF, OC
HF, BO
ASR, AE, BO
HT, BO, DIFS
HT, BO, DIFS
HF, DSCA, BO, DIFS
HF, BO, DIFS, CD, KE
