Systems and methods for predicting shear failure of a rock formation
Systems and methods for determining shear failure of a rock formation are disclosed. The method includes receiving, by a processor, a plurality of parameters related to physical properties of the rock formation, applying the plurality of parameters to a predetermined failure criterion, and determining shear failure of the rock formation based on the failure criterion. In some embodiments the failure criterion is a modified Hoek-Brown failure criterion that takes into consideration an intermediate principal stress, and the difference between normal stresses and an average confining stress.
1. A method for determining shear failure of a rock formation, the method comprising:
drilling a bore into the rock formation to form a borehole;
positioning a tool within the borehole and acquiring data with the tool;
developing a plurality of parameters related to physical properties of the rock formation from the data;
applying the plurality of parameters to a predetermined failure criterion;
predicting shear failure of the rock formation based on the failure criterion; and
planning an exploration operation of a reservoir within the rock formation by fracture monitoring to determine the predicted shear failure of the rock formation;
wherein the failure criterion comprises:
1
2
[
(
σ
1
-
σ
2
)
2
+
(
σ
2
-
σ
3
)
2
+
(
σ
1
-
σ
3
)
2
]
=
σ
ci
(
m
b
σ
ci
(
ω
σ
2
+
(
1
-
ω
)
σ
3
)
+
s
)
a
where σ l is the maximum principal stress at failure; s and a depend on fracture geometry and surface conditions; m b depends on the fracture and intact rock matrix properties; ω (0≤ω≤1) is the weight term that quantifies the different degrees of the effects of the intermediate principal stress, σ 2 , and the minimal principal stress, σ 3 , on the shear failure of the rock formation; σ ci is the unconfined compressive strength; and the term of ωσ 2 +(1−ω)σ 3 on the right side of equation is the average confining stress during a poly-axial test.
2. The method according to claim 1 , wherein the failure criterion is a modified Hoek-Brown failure criterion.
3. The method according to claim 1 , wherein the failure criterion comprises an intermediate principal stress.
4. The method according to claim 1 , wherein the failure criterion comprises a difference between normal stresses and an average confining stress.
5. The method according to claim 1 , wherein the failure criterion is configured to determine that shear failure of the rock formation occurs when the term of the normal stress differences reaches a predetermined value that is dependent on the unconfined compressive strength (σ ci ), rock matrix and fracture properties (m b , s, a), and the average confining stress, ωσ 2 +(1−ω)σ 3 .
6. A system for determining shear failure of a rock formation, the system comprising:
a borehole obtained by drilling a bore into the rock formation;
a tool positioned within the borehole operable to acquire data;
a processing unit; and
a memory unit in communication with the processing unit, the memory unit comprising program instructions that when executed by the processing unit cause the operations of:
receiving a plurality of parameters related to physical properties of the rock formation, where the plurality of parameters were developed from the data;
applying the plurality of parameters to a predetermined failure criterion; and
performing fracture monitoring to predict shear failure of the rock formation based on the failure criterion,
wherein the failure criterion comprises:
1
2
[
(
σ
1
-
σ
2
)
2
+
(
σ
2
-
σ
3
)
2
+
(
σ
1
-
σ
3
)
2
]
=
σ
ci
(
m
b
σ
ci
(
ω
σ
2
+
(
1
-
ω
)
σ
3
)
+
s
)
a
where σ 1 is the maximum principal stress at failure; s and a depend on fracture geometry and surface conditions; m b depends on the fracture and intact rock matrix properties; ω(0≤ω≤1) is the weight term that quantifies the different degrees of the effects of the intermediate principal stress, σ 2 , and the minimal principal stress, σ 3 , on the shear failure of the rock formation; σ ci is the unconfined compressive strength; and the term of ωσ 2 +(1−ω)σ 3 on the right side of equation is the average confining stress during a poly-axial test.
7. The system according to claim 6 , wherein the failure criterion is a modified Hoek-Brown failure criterion.
8. The system according to claim 6 , wherein the failure criterion comprises an intermediate principal stress.
9. The system according to claim 6 , wherein the failure criterion comprises a difference between normal stresses and an average confining stress.
10. The system according to claim 6 , wherein the failure criterion is configured to determine that shear failure occurs when the term of the normal stress differences reaches a predetermined value that is dependent on the unconfined compressive strength (σ ci ) of the rock formation, rock matrix and fracture properties (m b , s, a), and the average confining stress, ωσ 2 +(1−ω)σ 3 .
11. A non-transitory computer readable medium comprising a computer program, that when executed by the processing unit of claim 7 , is configured to:
receive a plurality of parameters related to physical properties of the rock formation, where the plurality of parameters were developed from the data acquired by the tool positioned within the borehole;
apply the plurality of parameters to a predetermined failure criterion; and
perform fracture monitoring to predict shear failure of the rock formation based on the failure criterion,
wherein the failure criterion is as follows:
1
2
[
(
σ
1
-
σ
2
)
2
+
(
σ
2
-
σ
3
)
2
+
(
σ
1
-
σ
3
)
2
]
=
σ
ci
(
m
b
σ
ci
(
ω
σ
2
+
(
1
-
ω
)
σ
3
)
+
s
)
a
where σ 1 is the maximum principal stress at failure; s and a depend on fracture geometry and surface conditions; m b depends on the fracture and intact rock matrix properties; ω(0≤ω≤1) is the weight term that quantifies the different degrees of the effects of the intermediate principal stress, σ 2 , and the minimal principal stress, σ 3 , on the shear failure of the rock formation; σ ci is the unconfined compressive strength; and the term of ωσ 2 +(1−ω)σ 3 on the right side of equation is the average confining stress during a poly-axial test.
12. The medium according to claim 11 , wherein the failure criterion is a modified Hoek-Brown failure criterion.
13. The medium according to claim 11 , wherein the failure criterion comprises an intermediate principal stress.
14. The medium according to claim 11 , wherein the failure criterion comprises a difference between normal stresses and an average confining stress.
15. The medium according to claim 11 , wherein the failure criterion is configured to determine that shear failure occurs when the term of the normal stress differences reaches a predetermined value that is dependent on the unconfined compressive strength (σ ci ) of the rock formation, rock matrix and fracture properties (m b , s, a), and the average confining stress, ωσ 2 +(1−ω)σ 3 .