Estimation of Rock Abrasiveness and Rock Strength Using Bit Response Data

Knowledge of rock mechanical properties, including unconfined compressive strength (UCS) and confined compressive strength (CCS), allows for accurate geomechanical evaluations. Two important rock properties include rock abrasiveness and rock strength. The estimation of rock abrasiveness and rock strength after drilling a well may be useful in understanding dysfunctions that may have occurred in drilling and useful in selecting a bit or drilling conditions for future well drilling. Usually, these rock properties may be measured by measurement while drilling (MWD) and/or logging while drilling (LWD) tools indirectly.

and the operations described herein are examples meant to aid in understanding example implementations and should not be used to limit the potential implementations or limit the scope of the claims. None of the implementations described herein may be performed exclusively in the human mind nor exclusively using pencil and paper. None of the implementations described herein may be performed without computerized components such as those described herein. Some implementations may perform additional operations, fewer operations, operations in parallel or in a different order, and some operations differently.

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.

As described herein, example implementations may use down hole bit response data including weight on bit (WOB), torque on bit (TOB), rate of penetration (ROP), and rotations per minute (RPM) of the bit to directly estimate rock abrasiveness and rock strength. Example implementations may include a method to estimate rock abrasiveness and a new method to estimate rock strength. The estimation of rock abrasiveness and rock strength may be useful to understand bit performance and, hence, to redesign a bit or to select another bit for future drilling.

Bit response data may be obtained using one or more in-bit sensors configured to provide direct, in-bit measurements of weight, torque, bending, vibration, rotational speed, etc. A final bit cutter dull severity may also be obtained by a cutter wear analysis tool. For example, a cutter wear analysis tool utilizing computer vision and one or more machine learning algorithms may be used to capture and precisely dull grade every cutter on the drill bit. The tool may be configured to determine the final bit cutter dull severity through rapid analysis of hundreds, or even thousands of individual cutter dull grades. However, other techniques may also be possible.

In some implementations, the in-bit sensors within the drill bit may measure different drill bit response data. Examples of such drill bit response data measured may include Weight on Bit (WOB), Torque on Bit (TOB), Rate of Penetration (ROP), rotations per unit of time (e.g., Rotations Per Minute (RPM). Additionally, rock strength index and rock abrasiveness index along drilling depth may be available using logging while drilling (LWD) tools (such as gamma ray, sonic logging, etc.). Data regarding final cutter and bit dull of the drill bit after drilling may also be available. Using this different data as input, a bit-rock interaction simulation may be developed that simulates drilling along drilling depth of the offset well.

The bit-rock interaction tool may be used for simulation at different distances (such as foot by foot) along the drilling depth of the section of the offset well that was drilled. The performance of the base drill bit (also referenced as a base drill bit performance) may be evaluated using this simulation. This simulation may, for example, include estimating energy input to the drill bit and energy required by the drill bit.

Example implementations may include calculating an input energy to the bit along drilling depth from mechanical specific energy (MSE). A required energy to wear the cutters may be calculated from the cutter dull severity as determined via the cutter wear analysis tool above. An average rock abrasiveness may then be calculated by equating the total input energy to the total required energy.

In drilling, the mechanical specific energy (MSE) that is required for a sharp cutter to successfully drill through rock may be proportional to the unconfined compressive strength of the rock. This same relationship, modeled by Equation (1), may be used to estimate confined compressive strength (CCS) during drilling of a wellbore:

wherein MSE is the specific energy and n is the bit drilling efficiency.

MSE may be calculated from measured Weight on Bit (WOB), Torque on Bit (TOB), Rotations per Minute (RPM), Rate of Penetration (ROP), etc. (as shown in Equation (2)):

wherein MSE is the specific energy, WOB is the weight on bit, A is the cross-sectional area of the hole drilled by the bit, TOB is the torque on bit, RPM is the revolutions per minute of the drill bit, and ROP is the rate of penetration of the drill bit.

In Equation (1), η is related to bit drilling efficiency. However, this may lead to inaccuracies as η may change along the drilling depth of the wellbore. Furthermore, such a conventional approach for determining the CCS (using Equation (1) may be not accurate enough to be used in drilling optimization. This lack of accuracy may arise for multiple reasons. For example, traditional techniques may assume a constant drilling efficiency between 0.3-0.4, as stated above. This however is not true, as drill bit drilling efficiency (bit DE, also referred to as η) changes with depth when drilling a well and is not a constant. The bit DE may depend heavily on the design and wear of the drill bit. Therefore, the estimation of η should consider the design and wear of the drill bit. A second reason that Equation (1) may not be accurate enough to be used in drilling optimization is that the measured MSE during drilling includes two parts. The first part is related to pure drilling. This first part neglects cutter wear. The second part is related to wear of the cutter(s) along the bit face (i.e., frictional forces in drilling).

Therefore, a two-part solution may include calculating bit DE along the drilling depth of a well and calculating an MSE for pure drilling, MSE_c. Using MSE_c in Equation (1) may provide a more accurate estimation of rock strength along the drilling depth.

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 implementations may 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 may 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 implementations, 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 computermay be local or remote to the drilling platform. A processor of the computermay perform simulations (as further described below). In some implementations, 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 controllermay perform one or more of the operations described herein. For example, the simulation processormay 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 controllermay 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.

Example operations are now described with reference to a bit-rock interaction simulation software tool (also referred to as the “bit rock interaction tool” and “bit rock interaction model”) used in determining rock abrasiveness, bit drilling efficiency of a sharp bit with sharp cutters and worn bit with worn cutters, and estimating rock strength. The simulation processorofmay be coupled with the bit rock interaction tool.

In some implementations, a bit-rock interaction simulation software tool may be used to calculate bit drilling efficiency for any bit with cutter wear. The bit wear may be divided into several levels. For example, bit wear may be divided into 9 levels from 0 to 8. At each bit wear level, a cutter wear profile for each cutter may be calculated, including a wear contact area and wear depth. A value of bit drilling efficiency may be calculated at each depth of cut and at each wear level. This procedure may be referred to as the pre-calculation.

During drilling, the accumulated input energy to bit at a drilling depth may be calculated using MSE and accumulated removed volume at that depth. Therefore, input energy as a function of drilling depth is available. Cutter wear volume may be proportional to input energy to cutter. Therefore, bit wear level is also proportional to the input energy to bit. This relationship is used to determine, at a drilling depth, the bit wear level. Once bit wear level at a depth is determined, and depth of cut is known, DE is available from pre-calculations. Therefore, the bit drilling efficiency or coefficient n in Equation (1) may then be available at a drilling depth.

are flowcharts depicting example operations for calculating an estimated rock strength along drilling depth, according to some implementations. Operations of a flowchartofand a flowchartofare described in reference to the well systemofand the computerof. Also, operations of the flowchartofand the flowchartofcontinue between each other through transition point A. Operations of the flowcharts-start at block.

At block, a bit response file and measured bit dull file are retrieved. Data regarding the drilling parameters and drilling conditions from the drilling of the section of the offset well may be retrieved for input via the bit-rock interaction model. For example, with reference to, the processormay retrieve the data from any type of machine-readable media (local or remote). For example, with reference to, the data may be related to drilling of the wellbore(which may be an offset well). The data may include a bit response file (including WOB, TOB, RPM and ROP), a rock abrasive index of the subsurface formation, and a measured bit dull file. At least a portion of the data may be provided via simulation, via one or more in-bit sensors, etc. Flow progresses to block.

At block, the processormay retrieve WOB, TOB, RPM, ROP, and may calculate bit MSE. For example, the WOB, TOB, RPM, ROP, etc. may be retrieved alongside the bit response file and measured bit dull file of block. The MSE may be calculated with reference to Equationusing the retrieved WOB, TOB, RPM, and ROP of the drill bit of interest. Flow progresses to block.

At block, the processormay calculate an energy input into the drill bit along the drilling depth. For example, an energy input (or spent) after drilling through a layer with thickness ΔS (in ft) may be calculated using Equation 3:

wherein R is the hole radius in inches, MSE_layer is the mechanical specific energy of the layer of formation rock, and ΔS is the formation layer thickness.

The processormay calculate a total input energy to the drill bit via a summation of energy spent after drilling through the various layers of a simulated well, offset well, etc., as shown in Equation 4:

where Ebit is the total input energy to the bit at a drilling depth.

Therefore, input bit energy may be modeled as a function of drilling depth. The drilling depth refers to the measured depth (MD) of at least a portion of an example well (real-world or simulated). In some implementations, the processormay obtain a relationship between input energy as a function of drilling depth via one or more in-bit sensors after the drilling of an offset well. This relationship is depicted in.is a plot depicting the relationship between input bit energy and drilling depth, according to some implementations. A plotincludes an X-axis depicting drilling depth(in ft) and a spent (total) bit energymeasured in lb-in. As expected, the total input energy to the bit increases with drilling depth. Flow progresses to block.

At block, the processormay retrieve the pre-calculated following values at each bit wear level and at each depth of cut: 1) WOB, 2) TOB, 3) drilling efficiency 4) WOB to the primary cutters, 5) TOB to the primary cutters, 6) WOB friction, 7) TOB friction, 8) pure cutting WOB, 9) pure cutting TOB, and 10) scaled cutter force distributions. For example,include various drilling parameters calculated at different levels of bit wear as part of the pre-calculation step of block.

is a plot depicting depth of cut vs. weight on bit at various bit wear levels, according to some implementations. A plotincludes an X-axis of depth of cut(in inches per revolution of the bit) and a Y-axis of a weight on bit(WOB) in kilo-pound force (klbf).

is a plot depicting depth of cut vs. torque on bit at various bit wear levels, according to some implementations. A plotincludes an X-axis depicting a depth of cutin inches/revolution and a Y-axis depicting a torque on bitin kilo foot-pounds (kft-lb).

is a plot depicting depth of cut vs. drilling efficiency at various bit wear levels, according to some implementations. A plotincludes an X-axis depicting a depth of cutin inches/revolution and a Y-axis depicting a bit drilling efficiency(Bit DE) as a percentage of a theoretical perfect drilling efficiency. A sharp-bit drilling efficiencydepicts a drilling efficiency of a bit with a wear level of 0 as a function of cut depth, and a worn-bit drilling efficiencydepicts a drilling efficiency of a bit with a wear level of 8.

is a plot depicting weight on bit vs. torque on bit at various wear levels, according to some implementations. A plotincludes an X-axis depicting a weight on bit(measured in klbf) and a Y-axis depicting a torque on bitmeasured in kft-lb.

As determined via block, the energy input to the drill bit along the drill depth may be spread amongst various components of the cutting face of the drill bit. For example, the energy input to the bit of blockmay include the primary cutters, backup cutters, depth of cut controllers (DOCCs) and blades. However, the energy input to primary cutters may be of particular interest. The bit dull analysis tool may be configured to record only cutter dull severity measurements for primary cutters; therefore, drill bit optimization via the bit-rock interaction tool may be performed with reference to the primary cutters.

To calculate the energy input to the primary cutter(s), the processormay estimate how much of the WOB (and/or the TOB) is only applied to primary cutters. The processormay perform this calculation by first determining a ratio of the WOB and TOB experienced only by the primary cutters to the total WOB and TOB exerted on the drill bit, as shown in Equations 5-6:

where WOB_p is the weight on bit experienced by the primary cutters, TOB_p is the torque on bit experienced by the primary cutters, λw is the ratio of the weight on bit experienced by the primary cutters to the total WOB on the drill bit, and λw is the ratio of the torque on bit experienced by the primary cutters to the total TOB on the drill bit.

Both λw and λt may depend on the depth of cut (DOC) of the primary cutters and on bit wear level. These values may be pre-calculated and saved for use in later calculations. Examples of the above relationships may be depicted via.

is a plot depicting contributions to WOB between various components of a drill bit, according to some implementations. At an example bit wear level of 5, the plotincludes an X-axis of depth of cut(measured in inches/revolution) and a Y-axis of WOBin lbs. The total WOB may be decomposed into numerous trends. For example, a primary cutter trendmodels the WOB experienced by the primary cutters (WOB_p) with respect to depth of cut, a backup cutter trendmodels the WOB experienced by the backup cutters, and a DOCCs +Blades trendmodels the WOB experienced by the DOCCs and blades.

is a plot depicting the weight on bit experienced by the primary cutters across a plurality of bit wear levels, according to some implementations. A plotincludes an X-axis of depth of cut(measured in in/rev) and a Y-axis depicting a rationof WOB_p/WOB (also referred to as λw). In some implementations, the processormay calculate λw and λt across all bit wear levels as part of the pre-calculations.

The cutter dull severity (Sd) may be estimated from cutter wear depth (Wd) and cutter diameter (Dc) using Equation 7:

If, for example, Sd=4, this means that half of the PDC (Polycrystalline Diamond Compact) cutter is worn out.