Changes

==Determination Methods of Stress==

==Determination Methods of Stress==

Determination methods of in-situ stresses can be classified into three categories as shown in 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 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.

 

[[File:GeoWikiWriteOff2021-Tayyib-Figure8.png|thumbnail|Figure 8 Summary of the stress determination methods. (modified from Heidbach et al. [5.0], 2016, and Moawietz et al. [6], 2020)]]  

GeoWikiWriteOff2021-Tayyib-Figure8.png|{{Figure number|8}}Summary of the stress determination methods. (modified from Heidbach et al. [5.0], 2016, and Moawietz et al. [6], 2020)   

[[File:GeoWikiWriteOff2021-Tayyib-Figure9.png|thumbnail|Figure 9 Workflow of the integrated approach for in-situ stress determination. (from Hudson et al, 2003, as cited in Zhang, L. 2016) [3.3]]] 

GeoWikiWriteOff2021-Tayyib-Figure9.png|{{Figure number|9}}Workflow of the integrated approach for in-situ stress determination. (from Hudson et al, 2003, as cited in Zhang, L. 2016) [3.3]

===In-situ Stress from Historical Data===

===In-situ Stress from Historical Data===

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 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 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:

H = 3Ps– Po - Pr

:H = 3Ps– Po - Pr

Where Ps is the shut-in pressure.

::Where Ps is the shut-in pressure.

Po is the pore pressure, which is the pressure of fluid inside the rock.

::Po is the pore pressure, which is the pressure of fluid inside the rock.

Pr is the reopening pressure.

:::Pr is the reopening pressure.

[[File:GeoWikiWriteOff2021-Tayyib-Figure11.png|thumbnail|Figure 11 The direction of fracture propagation is perpendicular to the minimum horizontal stress. (from Hoeksema, 2015) [7]]]     

[[File:GeoWikiWriteOff2021-Tayyib-Figure11.png|thumbnail|Figure 11 The direction of fracture propagation is perpendicular to the minimum horizontal stress. (from Hoeksema, 2015) [7]]]     

The shape of the hole is identified using four-arm caliper tools or well imaging tools (see Figure 20). These tools are used during the drilling of the well for petroleum exploration and production. The four caliper arms push against the wall as they move along the wellbore, recording the shape of the hole, from which the orientation of the horizontal stresses can be inferred.

The shape of the hole is identified using four-arm caliper tools or well imaging tools (see Figure 20). These tools are used during the drilling of the well for petroleum exploration and production. The four caliper arms push against the wall as they move along the wellbore, recording the shape of the hole, from which the orientation of the horizontal stresses can be inferred.

[[File:GeoWikiWriteOff2021-Tayyib-Figure20.png|thumbnail|Figure 20 (a) Four-arm Caliper tool used to identify the shape of the well. (b) Well imaging tool used to detect breakouts. (from Heidbach et al., 2016) [5.1]]]

[[File:GeoWikiWriteOff2021-Tayyib-Figure20.png|thumbnail|Figure 20 (a) Four-arm Caliper tool used to identify the shape of the well. (b) Well imaging tool used to detect breakouts. (from Heidbach et al., 2016) [5.1]]]

==Applications of In-situ Stress==

==Applications of In-situ Stress==