Rotary drill bits with back-up cutting elements to optimize bit life

1. A multi-layer downhole drilling tool designed for drilling a wellbore including a plurality of formations, comprising:

a bit body with a rotational axis extending therethrough;

a plurality of blades disposed on exterior portions of the bit body;

a plurality of primary cutting elements disposed on exterior portions of some of the plurality of blades; and

a plurality of back-up cutting elements disposed on exterior portions of different blades of the plurality of blades than the primary cutting elements, each back-up cutting element located at the same radial position with respect to the rotational axis as a corresponding primary cutting element, the back-up cutting elements under-exposed with respect to the primary cutting elements based on a critical depth of cut as a function of a radius of the bit body.

2. The drilling tool of claim 1 , wherein

an amount of under-exposure between the back-up cutting elements and primary cutting elements varies based on the location of the back-up cutting elements on the blades.

3. The drilling tool of claim 1 , wherein

an amount of under-exposure between the back-up cutting elements and primary cutting elements is based on a target critical depth of cut and varies such that the back-up cutting elements are configured to engage the wellbore at the target critical depth of cut.

4. The drilling tool of claim 1 , wherein an amount of under-exposure between the back-up cutting elements and primary cutting elements varies based on a first target critical depth of cut for a first radius of the bit body and a second target critical depth of cut for a second radius of the bit body.

5. The drilling tool of claim 1 , wherein the back-up cutting elements are configured to control a depth of cut of the primary cutting elements at a first drilling depth.

6. The drilling tool of claim 5 , wherein the back-up cutting elements are configured to engage the wellbore and act as major cutting elements at a second-drilling depth.

7. The drilling tool of claim 1 , wherein the back-up cutting elements are disposed on blades at an angular distance between approximately 10 degrees and approximately 350 degrees from the corresponding primary cutting elements.

8. A method of designing a multi-profile layer drill bit for drilling a wellbore including a plurality of formations, the method comprising:

obtaining drill bit run information for a pre-existing drill bit;

generating an actual depth of cut as a function of a drilling depth based on the drill bit run information;

estimating a wear of each of a plurality of primary cutting elements on some of a plurality of blades as a function of the drilling depth;

generating a designed depth of cut as a function of the drilling depth based on the estimated wear of each of the primary cutting elements;

estimating a first drilling depth where the primary cutting elements are worn such that at least one of a plurality of back-up cutting elements engage the wellbore based on the actual depth of cut and the designed depth of cut; and

configuring the back-up cutting elements on different blades of the plurality of blades than the primary cutting elements based on the first drilling depth, each back-up cutting element located at the same radial position with respect to a rotational axis of the drill bit as a corresponding primary cutting element, the back-up cutting elements under-exposed with respect to the primary cutting elements based on a critical depth of cut as a function of a radius of a bit body.

9. The method of claim 8 , wherein configuring the back-up cutting elements further comprises determining an amount of under-exposure between the back-up cutting elements and the primary cutting elements based on the first drilling depth.

10. The method of claim 8 , wherein configuring the back-up cutting elements further comprises determining an amount of under-exposure between the back-up cutting elements and the primary cutting elements based on the first drilling depth, the amount of under-exposure varying based on the location of the back-up cutting elements on the blades.

11. The method of claim 8 , wherein configuring the back-up cutting elements further comprises determining an amount of under-exposure between the back-up cutting elements and the primary cutting elements based on the first drilling depth and a target critical depth of cut, the amount of under-exposure varying such that the back-up cutting elements are configured to engage the wellbore at the target critical depth of cut.

12. The method of claim 8 , wherein configuring the back-up cutting elements further comprises determining an amount of under-exposure between the back-up cutting elements and the primary cutting elements based on the first drilling depth, a first target critical depth of cut for a first radius of the bit body and a second target critical depth of cut for a second radius of the bit body.

13. The method of claim 8 , wherein the back-up cutting elements are configured to control a depth of cut of the primary cutting elements at the first drilling depth.

14. The method of claim 13 , further comprising estimating a second drilling depth where the primary cutting elements are worn such that the back-up cutting elements engage the wellbore and act as major cutting elements based on the actual depth of cut and the designed depth of cut.

15. The method of claim 8 , further comprising

generating primary cutting zones for the primary cutting elements, the primary cutting zones corresponding to one or more primary drilling depths where the primary cutting elements act as major cutting elements;

generating back-up cutting zones for the back-up cutting elements, the back-up cutting zones corresponding to one or more back-up drilling depths where the back-up cutting elements engage the wellbore; and

reconfiguring the back-up cutting elements if the one or more back-up drilling depths corresponding to the back-up cutting zones are greater than the one or more primary drilling depths corresponding to the primary cutting zones.

16. The method of claim 15 , wherein reconfiguring the back-up cutting elements includes adjusting the under-exposure between the back-up cutting elements and the primary cutting elements.

17. A method of designing a multi-profile layer drill bit for drilling a wellbore including a plurality of formations, the method comprising:

obtaining an expected drilling depth;

generating an expected depth of cut as a function of a drilling depth based on the expected drilling depth;

estimating a wear of each of a plurality of primary cutting elements on some of a plurality of blades wear as a function of the drilling depth;

generating a designed depth of cut as a function of the drilling depth based on the estimated wear of each of the primary cutting elements;

estimating a first drilling depth where the primary cutting elements are worn such that at least one of a plurality of back-up cutting elements engage the wellbore based on the expected depth of cut and the designed depth of cut; and

configuring the back-up cutting elements on different blades of the plurality of blades than the primary cutting elements based on the first drilling depth, each back-up cutting element located at the same radial position with respect to a rotational axis of the drill bit as a corresponding primary cutting element, the back-up cutting elements under-exposed with respect to the primary cutting elements based on a critical depth of cut as a function of a radius of a bit body.

18. The method of claim 17 , wherein configuring the back-up cutting elements further comprises determining an amount of under-exposure between the back-up cutting elements and the primary cutting elements based on the first drilling depth.

19. The method of claim 17 , wherein configuring the back-up cutting elements further comprises determining an amount of under-exposure between the back-up cutting elements and the primary cutting elements based on the first drilling depth, the amount of under-exposure varying based on the location of the back-up cutting elements on the blades.

20. The method of claim 17 , wherein configuring the back-up cutting elements further comprises determining an amount of under-exposure between the back-up cutting elements and the primary cutting elements based on the first drilling depth and a target critical depth of cut, the amount of under-exposure varying such that the back-up cutting elements are configured to engage the wellbore at the target critical depth of cut.

21. The method of claim 17 , wherein configuring the back-up cutting elements further comprises determining an amount of under-exposure between the back-up cutting elements and the primary cutting elements based on the first drilling depth, a first target critical depth of cut for a first radius of the bit body and a second target critical depth of cut for a second radius of the bit body.

22. The method of claim 17 , wherein the back-up cutting elements are configured to control a depth of cut of the primary cutting elements at the first drilling depth.

23. The method of claim 22 , further comprising estimating a second drilling depth where the primary cutting elements are worn such that the back-up cutting elements engage the well bore and act as major cutting elements based on the actual depth of cut and the designed depth of cut.

24. The method of claim 17 , further comprising

generating primary cutting zones for the primary cutting elements, the primary cutting zones corresponding to one or more primary drilling depths where the primary cutting elements act as major cutting elements;

generating back-up cutting zones for the back-up cutting elements, the back-up cutting zones corresponding to one or more back-up drilling depths where the back-up cutting elements engage the wellbore; and

reconfiguring the back-up cutting elements if the one or more back-up drilling depths corresponding to the back-up cutting zones are greater than the one or more primary drilling depths corresponding to the primary cutting zones.

25. The method of claim 24 , wherein reconfiguring the back-up cutting elements includes adjusting the under-exposure between the back-up cutting elements and the primary cutting elements.