Cutter Forces and Wear Severity Determination for a Drill Bit for Drilling a Wellbore

Wellbores may be drilled into a subsurface formation using a drill bit having a number of cutters. It can be challenging to know a cutter wear severity of a given cutter during drilling of the wellbore.

The description that follows includes example systems, methods, techniques, and program flows that embody aspects of the disclosure. However, it is understood that this disclosure may be practiced without these specific details. In some instances, well-known instruction instances, protocols, structures, and techniques have not been shown in detail in order not to obfuscate the description.

In contrast to conventional approaches, example implementations may include estimation of cutter forces and cutter wear severity on cutters of a drill bit in real time while the wellbore is being drilled using the drill bit. Accordingly, example implementations may determine such severity at a cutter level. This is in contrast to conventional approaches that are limited to determining severity across the entire drill bit. Accordingly, example implementations may estimate a cutter wear severity for each cutter of a drill bit during drilling of a wellbore. For example, a cutter wear severity for each cutter of the drill bit may be estimated or determined along a drilling depth of the wellbore using drill bit response data.

For example, in some implementations, the following may be pre-calculated before drilling operation for an assumed drill bit wear level and a depth of cut for a given rock strength: 1) a ratio of primary cutter weight on bit (WOB) to drill bit WOB, 2) a ratio of primary cutter torque on bit (TOB) to drill bit TOB, 3) a scaled cutter axial force distribution, and 4) a scaled cutter torque distribution. At a given drilling depth, a bit response (that may include WOB, TOB, rate of penetration (ROP), and drill bit rotation speed (e.g., rotations per minute (RPM)), etc.) may be determined. For example, sensors in the drill bit may be used to determine these responses. Some implementations may calculate a cutter axial force, a drag force, a cutter power, a cutter energy, a cutter wear volume, a cutter wear depth, and a cutter wear severity based on this bit response (as further described below). The cutter wear severity may then be used to monitor the bit drilling performance. If the bit drilling performance is below a threshold, drilling may be stopped to bring the drill string to the surface of the wellbore to replace the current drill bit with a new drill bit. The drill string may then be lowered back down into the wellbore to resume drilling.

an elevation view (partially cross sectional) of an example well system, according to some implementations. In particular,is a schematic diagram of a well systemthat includes a drill stringhaving a drill bitdisposed in a wellborefor drilling the wellborein the subsurface formation. While depicted for a land-based well system, example embodiments can be used in subsea operations that employ floating or sea-based platforms and rigs. The drill bitis an example drill bit for which simulation of abrasive wear and damage as described herein can be performed.

The well systemmay further include a drilling platformthat supports a derrickhaving a traveling blockfor raising and lowering the drill string. The drill stringmay include, but is not limited to, drill pipe, drill collars, and down hole tools. The down hole toolsmay comprise any of a number of different types of tools including measurement while drilling (MWD) tools, logging while drilling (LWD) tools, mud motors, and others. A kellymay support the drill stringas it may be lowered through a rotary table. The drill bitmay include roller cone bits, polycrystalline diamond compact (PDC) bits, natural diamond bits, any hole openers, reamers, coring bits, and the like. As the drill bitrotates, it may crush or cut rock to create and extend a wellborethat penetrates various subterranean formations. The drill bitmay be rotated by various methods including rotation by a downhole mud motor and/or via rotation of the drill stringfrom the surfaceby the rotary table. Attributes of drilling the wellbore may be adjusted to increase, decrease, and/or maintain the rate of penetration (ROP) of the drill bitthrough the subsurface formation. Attributes may include weight-on-bit (WOB) and rotations-per-minute (RPM) of the drill string. In some embodiments, the drill bitmay become dull and lose efficiency, thus requiring more WOB and/or RPM to maintain a target ROP. A pumpmay circulate drilling fluid through a feed pipeto the kelly, downhole through interior of the drill string, through orifices in the drill bit, back to the surfacevia an annulus surrounding the drill string, and into a retention pit.

The well systemincludes a computerthat may be communicatively coupled to other parts of the well system. The computercan be local or remote to the drilling platform. A processor of the computermay perform simulations (as further described below). In some embodiments, the processor of the computermay control drilling operations of the well systemor subsequent drilling operations of other wellbores.

An example of the computeris now described.is a block diagram of an example computer, according to some implementations.depicts a computerthat includes a processor(possibly including multiple processors, multiple cores, multiple nodes, and/or implementing multi-threading, etc.). The computerincludes a memory. The memorymay be system memory or any one or more of the above already described possible realizations of machine-readable media. The computeralso includes a busand a network interface.

The computeralso includes a simulation processorand a controller. The simulation processorand the controllercan perform one or more of the operations described herein. For example, the simulation processorcan perform data processing and simulation operations as further described below. The controllermay perform various control operations to a wellbore operation based on the simulations. For example, the controllercan modify a drilling operation based on the simulations.

Any one of the previously described functionalities may be partially (or entirely) implemented in hardware and/or on the processor. For example, the functionality may be implemented with an application specific integrated circuit, in logic implemented in the processor, in a co-processor on a peripheral device or card, etc. Further, realizations may include fewer or additional components not illustrated in(e.g., video cards, audio cards, additional network interfaces, peripheral devices, etc.). The processorand the network interfaceare coupled to the bus. Although illustrated as being coupled to the bus, the memorymay be coupled to the processor.

Some implementations may include a nonlinear cutter wear model for at least one of step wear of a cutter and nonlinear velocity of a cutter during drilling. To illustrate,is a graphical representation of a cutter of a drill bit, according to some implementations. In, a cutterincludes a cutting elementand a body. In some implementations, the cutting elementmay be composed of diamond. The bodymay be composed of other less costly material (such as carbide).

As shown, a critical cutter energy (Ec) may be defined as the energy associated with a wear depth of the cuttersubstantially equaling the chamfer size. A critical wear depth (Wc) may be defined as the depth wherein the cutting elementis worn away to a point where the bodyis starting to be in contact with the formation being drilled.

To further illustrate,is a graph defining an example relationship between the cutter wear volume and energy of a cutter of a drill bit, according to some implementations.depicts a graphthat includes an x-axis that is the energy (E) 402 of a cutter and a y-axis that is the cutter wear volume (V)of the drill bit. The graphincludes a linethat starts at an Ec(when V is zero). A first partof the lineis based on (a constant), Ka (the rock abrasive factor), and Kb (the diamond cutter wear resistance factor). A second partof the lineis based on, Ka, Kb, and Kc (the carbide body element wear resistance factor).

In some implementations, the nonlinear cutter wear model for a cutter of a drill bit may be defined as follows. If E is less than Ec, V=0 (as shown in the graphof). However, if E is not less than Ec, a determination is made of whether the cutter wear depth (Wd) is less than Wc. If Wd is less than We, the cutter wear volume (V) may be defined by Equation (1):

However, if Wd is not less than We, V may be defined by Equation (2):

Also, as part of the nonlinear cutter wear model, a nonlinear velocity of the drill bit may be determined. A critical velocity (Vc) may be defined as the last cone cutter's velocity of the drill bit. Vc may be fixed in a simulation for a drill bit (even when drilling parameters are changed.

Cutter power (Pt) may be defined as follows. If the velocity (Vel) of the cutter is less than or equal to Vc, P may be defined by Equation (3):

Additionally, cutter energy (E) may be defined by Equation (5):

Additionally, a cutter wear severity may be defined based on a cutter wear depth. To illustrate,is a graphical representation of a cutter of a drill bit, according to some implementations.depicts a cutterhaving a cutter wear depth and at a back rake angle (BRa). A cutter wear depth may be a function of back rake angle, cutter diameter, chamfer size, cutter shape and cutter wear volume. A cutter wear severity (Sc) may be defined by Equation (6):

If Sc=4, cutter wear depth=0.5 diameter. Therefore, Sc=4 may be defined as the maximal cutter wear. If Sc>4, the cutter may be considered lost.

Example operations are now described. Example operations for determining a cutter wear severity based on input drill bit response are described.

Example operations for determining cutter wear severity are now described.is a flowchart of example operations for determining a cutter wear severity for cutters of a drill bit during drilling of a wellbore, according to some implementations. Operations of a flowchartofare described in reference to the well systemofand the computerof. Operations of the flowchartstart at block.

At block, an input drill bit response is determined. For example, the drill bit response may include at least one of WOB, TOB, ROP, rotation speed, etc. of the drill bit. With reference to, the processormay retrieve these measurements from one or more sensors positioned at the surface and/or downhole in the wellbore. The energy input to bit may include the primary cutters, backup cutters, Depth of Cut Controllers (DOCCs) and blades. However, the energy input to primary cutters may need to be determined. Accordingly, an estimation of how much at least one of WOB or TOB is applied to the primary cutters may be determined. To illustrate,is a graph of weight on bit (WOB) contributed from primary cutters, backup cutters, Depth of Cut Controllers (DOCCs) and blades at a bit wear level, according to some implementations. In particular,includes a graphhaving lines for the primary cutters, the backup cutters, and the DOCCs and blades.is a graph of a ratio of WOB from the primary cutters to WOB at the bit wear levelto, according to some implementations. In particular,includes a graph.

In some implementations, WOB ratio may be determined based on Equation (7):

In some implementations, TOB ratio may be determined based on Equation (8):

Both λw and λt may depend on depth of cut (DOC) and bit wear level. These ratios may be pre-calculated once bit is designed and saved for subsequent use. Returning to, operations of the flowchartcontinues at block.

At block, cutter forces (for at least one cutter of the drill bit) are determined based on the input drill bit response. With reference to, the processormay make this determination. In some implementations, the processormay retrieve pre-calculated scaled cutter axial and torque forces distributions for a given DOC and a bit wear level for the current bit used in drilling. In some implementations, the cutter forces being determined may be the axial force and the drag force for the at least one cutter.

The processormay then determine the axial forces coefficient (fa) based on the Equation (9):

The processormay then determine an axial force (Fa) for each cutter based on Equation (10):

is scaled cutter axial force of cutter i

The processormay also determine a torque coefficient (ηd) based on Equation (11):

The processormay then determine a torque (Td) for each cutter based on Equation (12):

is scaled cutter torque of cutter i

The processormay determine a drag force (Fd) for each cutter based on Equation (13):

To further illustrate,are graphs of an example of a scaled cutter axial force distribution and cutter torque distribution at different depths of cut (DOC) across the different cutters of a new drill bit, according to some implementations.are graphs of an example of a scaled cutter axial force distribution and cutter torque distribution at different depths of cut (DOC) across the different cutters of a used drill bit, according to some implementations.