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==Determination Methods of Stress==

==Determination Methods of Stress==

Determination methods of in-situ stresses can be classified into three categories as shown in [[:File:GeoWikiWriteOff2021-Tayyib-Figure8.png|Figure 8]]. The loading method involves disturbing the in-situ stress condition in the rock such as pumping high pressure fluid into the formation to create fractures. The relief method involves isolating the rock sample partially or completely from the surrounding rocks and observe the natural rock response to the in-situ stress. Other methods can be used to deduce the in-situ stress in the rock such as geological and geophysical method. Since stress cannot be measured directly, the methods rely on the measurements of any change in rock volume or shape (Strain). Figure 9 shows the integration of the different methods in one workflow to infer the in-situ stress state.

Determination methods of in-situ stresses can be classified into three categories as shown in [[:File:GeoWikiWriteOff2021-Tayyib-Figure8.png|Figure 8]]. The loading method involves disturbing the in-situ stress condition in the rock such as pumping high pressure fluid into the formation to create fractures. The relief method involves isolating the rock sample partially or completely from the surrounding rocks and observe the natural rock response to the in-situ stress. Other methods can be used to deduce the in-situ stress in the rock such as geological and geophysical method. Since stress cannot be measured directly, the methods rely on the measurements of any change in rock volume or shape (Strain). [[:File:GeoWikiWriteOff2021-Tayyib-Figure9.png|Figure 9]] shows the integration of the different methods in one workflow to infer the in-situ stress state.

 

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GeoWikiWriteOff2021-Tayyib-Figure8.png|{{Figure number|8}}Summary of the stress determination methods. (modified from Heidbach et al.<ref name=Heidbachetal>Heidbach, O., A. Barth, B. Müller, J. Reinecker, O. Stephansson, M. Tingay, and A. Zang, 2016, WSM quality ranking scheme, database description and analysis guidelines for stress indicator: World Stress Map Technical Report 16-01</ref> and Morawietz et al.<ref>Morawietz, S., O. Heidbach, K. Reiter, M. Ziegler, M. Rajabi, G. Zimmerman, B. Müller, and M. Tingay, 2020, An open-access stress magnitude database for Germany and adjacent regions: Geothermal Energy, vol. 8, article 25.</ref>).   

GeoWikiWriteOff2021-Tayyib-Figure8.png|{{Figure number|8}}Summary of the stress determination methods. (modified from Heidbach et al.<ref name=Heidbachetal>Heidbach, O., A. Barth, B. Müller, J. Reinecker, O. Stephansson, M. Tingay, and A. Zang, 2016, WSM quality ranking scheme, database description and analysis guidelines for stress indicator: World Stress Map Technical Report 16-01</ref> and Morawietz et al.<ref>Morawietz, S., O. Heidbach, K. Reiter, M. Ziegler, M. Rajabi, G. Zimmerman, B. Müller, and M. Tingay, 2020, An open-access stress magnitude database for Germany and adjacent regions: Geothermal Energy, vol. 8, article 25.</ref>).   

===In-situ Stress from Field Investigations===

===In-situ Stress from Field Investigations===

The following subsections showcase the different methods of in-situ stress determination:  

The following subsections showcase the different methods of in-situ stress determination:  

====Conventional Hydraulic Fracturing (Fracking)====

====Conventional Hydraulic Fracturing (Fracking)====

Hydraulic fracturing can be used to measure the maximum and minimum horizontal stresses at great depths below the surface. This method involves pumping fluid into an isolated target formation, until the fracture breakdown pressure is reached and the fracture is created. The fracture will propagate perpendicular to the minimum horizontal stress as shown in [[:File:GeoWikiWriteOff2021-Tayyib-Figure11.png|Figure 11]]. Then, the well is shut-in (pumping stops), causing the pressure to subside until it reaches the fracture closure pressure and the fractures will start to close. The pumping of fluid starts again until the fracture reopening pressure is reached and the previously closed fractures reopen. Multiple pumping cycles (minimum of three) are required to measure the reopening pressure. After the last pumping cycle, the shut-in pressure (Ps) is recorded and is considered equal to the minimum horizontal stress (σh). Then the maximum horizontal stress (σH) can be calculated as follows:

Hydraulic fracturing can be used to measure the maximum and minimum horizontal stresses at great depths below the surface. This method involves pumping fluid into an isolated target formation, until the fracture breakdown pressure is reached and the fracture is created. The fracture will propagate perpendicular to the minimum horizontal stress as shown in [[:File:GeoWikiWriteOff2021-Tayyib-Figure11.png|Figure 11]]. Then, the well is shut-in (pumping stops), causing the pressure to subside until it reaches the fracture closure pressure and the fractures will start to close. The pumping of fluid starts again until the fracture reopening pressure is reached and the previously closed fractures reopen. Multiple pumping cycles (minimum of three) are required to measure the reopening pressure. After the last pumping cycle, the shut-in pressure (Ps) is recorded and is considered equal to the minimum horizontal stress (σh). Then the maximum horizontal stress (σH) can be calculated as follows:

====Overcoring Method====

====Overcoring Method====

Overcoring method is used to determine in-situ stresses from a rock sample extracted from shallow depths, and released to expand freely. This method involves a sequence of steps illustrated in [[:File:GeoWikiWriteOff2021-Tayyib-Figure14.png|Figure 14]]. The process of cutting the hollow cylindrical rock, using the tool shown in figure 15, is called overcoring and the resulting change in shape is measured using a device called stressmeter. In general, the maximum expansion of the rock sample occurs in the direction of the maximum horizontal stress (σH).

Overcoring method is used to determine in-situ stresses from a rock sample extracted from shallow depths, and released to expand freely. This method involves a sequence of steps illustrated in [[:File:GeoWikiWriteOff2021-Tayyib-Figure14.png|Figure 14]]. The process of cutting the hollow cylindrical rock, using the tool shown in figure 15, is called overcoring and the resulting change in shape is measured using a device called stressmeter. In general, the maximum expansion of the rock sample occurs in the direction of the maximum horizontal stress (σH).

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File:GeoWikiWriteOff2021-Tayyib-Figure14.png|{{Figure number|14}}The sequence of overcoring method: (a) Drilling a large diameter hole. (b) Drilling a smaller pilot hole. (c) Placing the measuring device in the smaller hole. (d) Drilling the large diameter hole is resumed and the measuring device is overcored. (from Guo et al.<ref>Guo, Q., F. Ren, H.  Guo, L. Zhao, and Z. Yan, 2013, Strain sensors with temperature compensation employed for in situ stress monitor: TELKOMNIKA Indonesian Journal of Electrical Engineering, vol. 11, no. 11, p. 6296-6303.</ref>).

File:GeoWikiWriteOff2021-Tayyib-Figure14.png|{{Figure number|14}}The sequence of overcoring method: (a) Drilling a large diameter hole. (b) Drilling a smaller pilot hole. (c) Placing the measuring device in the smaller hole. (d) Drilling the large diameter hole is resumed and the measuring device is overcored. (from Guo et al.<ref>Guo, Q., F. Ren, H.  Guo, L. Zhao, and Z. Yan, 2013, Strain sensors with temperature compensation employed for in situ stress monitor: TELKOMNIKA Indonesian Journal of Electrical Engineering, vol. 11, no. 11, p. 6296-6303.</ref>).