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Line 70: − Conventional Hydraulic Fracturing (Fracking)+ Line 77:
Line 77: − + − + − Hydraulic Testing of Pre-existing Fracture (HTPF)+ − + − + + − Formation Integrity (FIT) Test or Leak-off Test (LOT)+ − FIT and LOT are used interchangeably by petroleum engineers because they follow similar procedures, however, the purpose of each is different. During FIT, the surface pressure increases until it reaches the required pressure to test the strength of the formation, without the intention of breaking the rock. The Leak-off test is used to determine the strength and the fracture pressure of the formation as well as the minimum horizontal stress. This procedure is similar to fracking by pumping fluids, however, instead of measuring the shut-in pressure, the leak-off pressure (Plo) is measured. The leak-off pressure is defined as the pressure at which the fluid will leak into the formation and it is equal to the minimum horizontal stress. − Extended Leak-off Test (XLOT) Line 96:
Line 95: − + − Figure 13 Recorded pressure during the extended leak-off test (XLOT) (from Kartevoll, 2009) [9]+ − − − Overcoring Method − + + + + + + − + − − − − Borehole Slotting − − Borehole slotting method involves cutting different 25-mm deep slots at the wall of a wellbore using a diamond saw. The cut slots form a half circle as shown in figure 16. The strain changes before, during, and after cutting the slots, are measured using a sensor and converted into stress readings. This method helps avoid the high cost of using other in-situ measurement methods. − − Figure 16 Slots are cut into the wellbore in different direction (from Kanduth, et al.1991) [12] − Lab Based Core Sample Testing
In-situ stress characterization and applications (view source)
Revision as of 19:28, 20 April 2022
, 19:28, 20 April 2022no edit summary
===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)====
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:
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]]]
Figure 11 The direction of fracture propagation is perpendicular to the minimum horizontal stress. (from Hoeksema, 2015) [7]
====Hydraulic Testing of Pre-existing Fracture (HTPF)====
Hydraulic Testing of Pre-existing Fracture is similar to the conventional fracking. However, no fractures are created by the HTPF method. It is used to reopen a pre-existing fracture in the formation (see figure 12). HTPF method requires the knowledge of the exact orientation and location of the pre-existing fractures before pumping fluid into the formation. HTPF can determine the normal stress acting perpendicular to the pre-existing fractures, which is equal to the shut-in pressure.
Hydraulic Testing of Pre-existing Fracture is similar to the conventional fracking. However, no fractures are created by the HTPF method. It is used to reopen a pre-existing fracture in the formation (see figure 12). HTPF method requires the knowledge of the exact orientation and location of the pre-existing fractures before pumping fluid into the formation. HTPF can determine the normal stress acting perpendicular to the pre-existing fractures, which is equal to the shut-in pressure.
[[File:GeoWikiWriteOff2021-Tayyib-Figure12.png|thumbnail|Figure 12 Hydraulic Testing of Pre-existing Fractures. (from Gaines, et al. 2012 as cited in Lin et al., 2018) [8]]]
Figure 12 Hydraulic Testing of Pre-existing Fractures. (from Gaines, et al. 2012 as cited in Lin et al., 2018) [8]
====Formation Integrity (FIT) Test or Leak-off Test (LOT)====
FIT and LOT are used interchangeably by petroleum engineers because they follow similar procedures, however, the purpose of each is different. During FIT, the surface pressure increases until it reaches the required pressure to test the strength of the formation, without the intention of breaking the rock. The Leak-off test is used to determine the strength and the fracture pressure of the formation as well as the minimum horizontal stress. This procedure is similar to fracking by pumping fluids, however, instead of measuring the shut-in pressure, the leak-off pressure (Plo) is measured. The leak-off pressure is defined as the pressure at which the fluid will leak into the formation and it is equal to the minimum horizontal stress.
====Extended Leak-off Test (XLOT)====
The extended leak-off test (XLOT) is an extension of LOT and it is similar to fracking which involves propagating of a fracture by pumping fluids. However, XLOT does not determine the stresses orientation. Figure 13 shows the recorded pressure during XLOT. The horizontal stresses values can be estimated as follow:
The extended leak-off test (XLOT) is an extension of LOT and it is similar to fracking which involves propagating of a fracture by pumping fluids. However, XLOT does not determine the stresses orientation. Figure 13 shows the recorded pressure during XLOT. The horizontal stresses values can be estimated as follow:
Where Plo is the leak-off pressure.
Where Plo is the leak-off pressure.
[[File:GeoWikiWriteOff2021-Tayyib-Figure13.png|thumbnail|Figure 13 Recorded pressure during the extended leak-off test (XLOT) (from Kartevoll, 2009) [9]]]
====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 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 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).
[[File:GeoWikiWriteOff2021-Tayyib-Figure14.png|thumbnail|Figure 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., 2013) [10]]]
[[File:GeoWikiWriteOff2021-Tayyib-Figure15.png|thumbnail|Figure 15 Overcoring Tool (from Hudson, 2003) [11]]]
====Borehole Slotting====
Borehole slotting method involves cutting different 25-mm deep slots at the wall of a wellbore using a diamond saw. The cut slots form a half circle as shown in figure 16. The strain changes before, during, and after cutting the slots, are measured using a sensor and converted into stress readings. This method helps avoid the high cost of using other in-situ measurement methods.
[[File:GeoWikiWriteOff2021-Tayyib-Figure16.png|thumbnail|Figure 16 Slots are cut into the wellbore in different direction (from Kanduth, et al.1991) [12]]]
====Lab Based Core Sample Testing====
Cores are cylindrical rock samples that are collected during or after well drilling. They are sent to the laboratory for further testing. The Anelastic Strain Recovery (ASR) is a method used to determine the in-situ stresses and their orientation from cores by measuring the strain over time.
Cores are cylindrical rock samples that are collected during or after well drilling. They are sent to the laboratory for further testing. The Anelastic Strain Recovery (ASR) is a method used to determine the in-situ stresses and their orientation from cores by measuring the strain over time.
The core is placed in a container filled with silicon and the strain is monitored (see Figure 17). The change in the core dimension is related to the microcracks creation in the rock when in-situ stress is relieved. The alignment of these micro fractures depends on the direction of the principal stresses. When the stress is relieved, the core tends to expand most in the maximum stress relief direction, and least in the minimum stress relief direction. The volume of the microcracks is proportional to the values of the in-situ stresses.
The core is placed in a container filled with silicon and the strain is monitored (see Figure 17). The change in the core dimension is related to the microcracks creation in the rock when in-situ stress is relieved. The alignment of these micro fractures depends on the direction of the principal stresses. When the stress is relieved, the core tends to expand most in the maximum stress relief direction, and least in the minimum stress relief direction. The volume of the microcracks is proportional to the values of the in-situ stresses.