Determining a Drilling Path in a Subterranean Formation

The present disclosure describes systems and methods for determining a drilling path in a subterranean formation and more particularly, determining a drilling path with a bottom hole assembly that includes a milling assembly that determines an angular drill path.

Whipstocks are wedges designed to anchor on an inside wall of a joint of casing and enable milling operations through the wall of the casing and into a subterranean formation. In the oil and gas drilling industry, this operation is typically referred to as a casing exit operation or a cased-hole sidetrack. A successful casing exit operation requires creating a sideways hole, departing at an angle away from the inclination of the original casing string. This angular direction (or build up) is typically in the range of 2-3 degrees relative to the original casing string. If the buildup fails, the sidetrack hole ends up parallel to the original casing string (i.e., right next to the casing string). Such a failure is called “casing tracking” and, when the drilling operations trips in hole again with the next drilling assembly to start drilling the rest of the well, the drill bit will start drilling against the metal casing string instead of drilling through the rock formation. This can result in damage to the casing string and to other components of the drilling assembly. The drilling operation is then forced to pull the drilling assembly out of hole, take remedial action, and use a new drilling assembly afterwards.

In an example implementation, a downhole tool includes a milling assembly configured to couple to a tubular string and run into a vertical wellbore formed from a terranean surface to one or more subterranean formations. The milling assembly includes a tubular mandrel; a first mill coupled to the tubular mandrel at a downhole end of the tubular mandrel; and a second mill coupled to the tubular mandrel uphole of the first mill. The downhole tool includes an inclination sensor mounted in or to the tubular mandrel between the first mill and the second mill. The inclination sensor is configured to determine an angle between a lateral wellbore formed by the milling assembly from a window in the vertical wellbore and the vertical wellbore.

In an aspect combinable with the example implementation, the inclination sensor is mounted within a pocket formed in a wall of the tubular mandrel.

Another aspect combinable with one, some, or all of the previous aspects includes a cover plate coupled to the tubular mandrel over the pocket to enclose the inclination sensor.

Another aspect combinable with one, some, or all of the previous aspects includes one or more fasteners configured to couple at least one of the inclination sensor or the cover plate to the tubular mandrel.

In another aspect combinable with one, some, or all of the previous aspects, the inclination sensor is configured to determine a first angle between the lateral wellbore formed by the milling assembly from the window in the vertical wellbore and the vertical wellbore; and a second angle between the lateral wellbore formed by the milling assembly from the window in the vertical wellbore and the vertical wellbore.

In another aspect combinable with one, some, or all of the previous aspects, the inclination sensor includes a printed circuit board that includes at least one accelerometer; and at least one memory module communicably coupled to the at least one accelerometer.

Another aspect combinable with one, some, or all of the previous aspects includes a third mill coupled to the tubular mandrel uphole of the second mill.

In another example implementation, a drilling system includes a whipstock configured couple to a casing positioned in a vertical wellbore formed from a terranean surface to one or more subterranean formations; and a bottom hole assembly including a milling assembly configured to couple to a tubular string and run into the vertical wellbore through the casing. The milling assembly includes a tubular mandrel; a plurality of mills, each mill coupled to the tubular mandrel; and an inclination sensor mounted in or to the tubular mandrel between two of the plurality of mills. The inclination sensor is configured to determine an angle between a lateral wellbore formed by the milling assembly from a window in the vertical wellbore and the vertical wellbore.

In an aspect combinable with the example implementation, the inclination sensor is mounted within a pocket formed in a wall of the tubular mandrel.

Another aspect combinable with one, some, or all of the previous aspects includes a cover plate coupled to the tubular mandrel over the pocket to enclose the inclination sensor.

Another aspect combinable with one, some, or all of the previous aspects includes one or more fasteners configured to couple at least one of the inclination sensor or the cover plate to the tubular mandrel.

In another aspect combinable with one, some, or all of the previous aspects, the inclination sensor is configured to determine a first angle between the lateral wellbore formed by the milling assembly from the window in the vertical wellbore and the vertical wellbore; and a second angle between the lateral wellbore formed by the milling assembly from the window in the vertical wellbore and the vertical wellbore.

In another aspect combinable with one, some, or all of the previous aspects, the inclination sensor includes a printed circuit board that includes at least one accelerometer; and at least one memory module communicably coupled to the at least one accelerometer.

In another aspect combinable with one, some, or all of the previous aspects, the plurality of mills include a first mill positioned at a distal end of the tubular mandrel and a second mill positioned uphole of the first mill.

In another aspect combinable with one, some, or all of the previous aspects, the inclination sensor is mounted in or to the tubular mandrel between the first and second mills.

In another example implementation, a drilling method includes running a downhole tool into a vertical wellbore formed from a terranean surface to one or more subterranean formations. The downhole tool includes a milling assembly that includes a tubular mandrel; a first mill coupled to the tubular mandrel at a downhole end of the tubular mandrel; a second mill coupled to the tubular mandrel uphole of the first mill; and an inclination sensor mounted in or to the tubular mandrel between the first mill and the second mill. The method includes urging the milling assembly through a window in a casing of the vertical wellbore with a whipstock positioned in the vertical wellbore; milling at least a portion of a lateral wellbore from the window; and during the milling, determining an angle between the lateral wellbore and the vertical wellbore with the inclination sensor.

In an aspect combinable with the example implementation, milling the portion of the lateral wellbore from the window includes milling the portion of the lateral wellbore with the first and second mills.

Another aspect combinable with one, some, or all of the previous aspects includes installing the inclination sensor within a pocket formed in a wall of the tubular mandrel.

Another aspect combinable with one, some, or all of the previous aspects includes securing the inclination sensor on or to the tubular mandrel with one or more tension bolts.

In another aspect combinable with one, some, or all of the previous aspects, determining the angle between the lateral wellbore and the vertical wellbore with the inclination sensor includes determining a first angle between the lateral wellbore and the vertical wellbore; and determining a second angle between the lateral wellbore and the vertical wellbore.

In another aspect combinable with one, some, or all of the previous aspects, determining the angle between the lateral wellbore and the vertical wellbore with the inclination sensor includes determining the angle with at least one accelerometer.

Another aspect combinable with one, some, or all of the previous aspects includes storing the determined angle in at least one memory module communicably coupled to the at least one accelerometer; and subsequent to running the milling assembly out of the vertical wellbore to the terranean surface, accessing the stored determined angle from the at least one memory module.

Implementations of systems and methods for determining a drilling path according to the present disclosure may also include one or more of the following features. For example, implementations according to the present disclosure can allow for placement of a near-mill sensor in milling assemblies to give a well operator an early indications on the success of casing exit operations, and an early warning on the failure of casing exit operations. As another example, implementations according to the present disclosure can therefore provide a milling process in which early remedial action can be taken when needed (if failures are detected) and unnecessary pre-cautionary runs are not performed when not needed (when milling success is confirmed). As another example, implementations according to the present disclosure can enable near-mill sensor placement safely and reliably in higher stress components, but also maintain the sensor as close to a mill downhole end for optimal readings.

The details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

The present disclosure describes example implementations of a milling assembly for a bottom hole assembly that includes one or more inclination sensors mounted between mills on the milling assembly, such as between a primary (for example, downhole most) mill and a secondary mill. During operation of the milling assembly, such as to form a window in a casing in a vertical wellbore and initiate a lateral wellbore, the inclination sensor(s) can measure angular displacement from the vertical wellbore to ensure that the sidetrack for the lateral wellbore is angled properly.

is a schematic diagram of a drilling operationthat can include a bottom hole assembly that includes a milling assembly according to the present disclosure. For example, drilling systemincludes a drilling rigconfigured to form a vertical wellboreand one or more lateral wellbores(from the vertical wellbore) from a terranean surface(which can also be a body of water). The wellboresandextend from the terranean surfaceinto one or more subterranean formations. During drilling operations, as shown in the circular view, a bottom hole assembly (BHA)that includes a drill bitis operated through a casing(for example, installed on the vertical wellbore) to form the laterals. During formation of the laterals, the drill bitdrills through the casingwith the help of a whipstockto form a window. The BHAforms the lateralsfrom the respective windowsat an angle offset from the vertical wellbore. Once a portion of a lateralhas been formed at an appropriate angle (for example,-degrees) away from the vertical wellbore, further drilling operations can commence to form the rest of the lateral.

is a schematic diagram of an example implementation of the BHAwith a milling sub-assemblyaccording to the present disclosure. As shown in this example, the BHAis run into the wellboreand through production casingto the whipstock. As shown in this example, the whipstockis positioned uphole of a seal/slip assemblythat is anchored to the casing. The BHA, in this example, is coupled to a downhole conveyance, such as a drill string comprised of tubular sections coupled together.

In this example implementation, the milling sub-assemblyincludes multiple mills mounted on or connected to a tubular mandrel(that can be a single tubular section or multiple tubular sections). For example, a first or primary millis positioned on the tubular mandrelat a downhole end of the milling sub-assembly. Separated from the primary millby a portionof the tubular mandrelis a second or secondary mill. Optionally, and separated from the secondary millby a portionof the tubular mandrelis a third or tertiary mill. In operation, the mills,, and (optionally), can be operated with the BHAto mill windowin the casingand, through angular contact with the whipstock, initiate a lateral wellborethat is angled from the vertical wellbore.

As shown in this example implementation, an inclination sensoris positioned in or on the portionof the tubular mandrel. In operation, during milling of the lateralfrom the wellbore(with the milling sub-assemblyangled by the whipstock), the inclination sensormeasures at least one angle between the lateral wellboreand the vertical wellbore. In some aspects, the inclination sensorcan be activated at the surface (for example, activated in a memory mode) before the start of the milling operation by the BHA. The inclination sensorprovides angle readings to the measured inclination above the opened windowand at the bottom of the drilled lateral. The measured angular inclination readings can be stored in the inclination sensorand, after a particular amount of milling time or length of milling performed by the BHA, the inclination sensor(along with the BHA) can be run out of the wellboreso that the stored readings can be obtained. If the measured inclination readings are correct, for example,-degrees, the continued milling or drilling operations are continued. If the measured inclination readings are small, such as near zero degrees inclination, this can indicate casing tracking and remedial operations are taken before running in the wellborewith the next drilling assembly.

is a schematic diagram of an example implementation of the inclination sensorused in the BHAwith the milling sub-assemblyaccording to the present disclosure. In this example implementation, the inclination sensorincludes a housingin which a printed circuit board (PCB)is enclosed. In this example implementation, the PCBincludes at least one accelerometerand a memory module. The accelerometer, in example aspects, measures the inclination angle between the lateral wellboreand vertical wellbore.

In some aspects, the measured angle includes two measured angles. For example, turning briefly to, this figure shows a schematic diagram of an angle between the sidetrack (lateral) and a primary wellbore (vertical wellbore). As shown in this example, as the BHA(including milling sub-assembly) is milling the lateral, the inclination sensorcan measure (for example, per time or milled distance) an angle(in the x direction), an angle(in the z direction), or both.

As the measurement readings (of one or two angular deviations of the lateralfrom the vertical wellbore) accumulate, the accelerometerprovides the readings to the memory module. The inclination angle readings are then stored in the memory module(for example, as analog or digital data) until the BHAis run out of the wellboreand to the surface. On the surface, the memory modulecan be interrogated or otherwise provide the stored inclination readings for analysis.

are schematic diagrams of an example implementation of portions of the BHAwith the milling sub-assemblythat includes the inclination sensoraccording to the present disclosure. As shown in this example (and as shown in), the inclination sensoris positioned in the tubular mandreladjacent the primary mill(and between the primary milland the secondary mill). In this example, the inclination sensoris inserted or otherwise positioned in a pocket(or slot) that is milled in the wallof the tubular mandrel. Once positioned in the pocket, a covercan (optionally) be secured over the inclination sensorto enclose the sensorin the pocket.

are schematic diagrams of an example drilling operation using the BHAwith the milling sub-assembly according to the present disclosure. For example, as shown in, the whipstockcan be installed in a seal/slip assembly, which is anchored in the wellbore(for example, to the casing). An orienting subcoupled to a downhole end of the whipstockallows the whipstockto be secured in the seal/slip assemblyand angled so that the face of the whipstock(on which the milling assemblyis run) is opposite a desired window location (for window). In, the whipstockis secured in the seal/slip assemblyand the milling assemblyis in contact with a setting studon the whipstock, which helps orient the milling assemblyto mill windowin the casingand start lateral wellbore. The millsandmill through the casingand angle from the vertical wellboreto start the lateral wellbore. During the milling, the inclination sensormeasures the angle (or multiple angles) between the vertical wellboreand the lateral wellbore. The measurements can be stored in the inclination sensorand then analyzed when the BHAis run out of the vertical wellbore(after initiating the lateral wellbore.

If the measurements of the angle between the vertical wellboreand lateral wellboreshow a proper angle (for example, about 2-3 degrees), then a drilling assemblycan be run into the vertical wellboreto finish the lateral wellboreas shown in. However, if the measurements of the angle between the vertical wellboreand lateral wellboreshow an improper angle (too close to the vertical wellboreor too far from the vertical wellbore), then the milling assemblycan be run back into the vertical wellborefor remedial action.

are graphs that illustrate test results for an inclination sensor used in a BHA with a milling sub-assembly according to the present disclosure. For example, a prototype inclination sensorwas built and tested to validate: sensor output at controlled inclinations, and sensor functionality at a required or desired temperature range.shows a graphof measured rotations taken by the inclination sensorover time. Graphincludes x-axisof time (in hours, minutes, and seconds) and a y-axisof rotation (in degrees).shows a graphof operation of the inclination sensorto measure angular displacement at a temperature range between about 20-130° C. over time. Graphincludes x-axisof time (in hours, minutes, and seconds) and a y-axisof temperature (in degrees C.). Curverepresents the temperature of the inclination sensorover time, while curverepresents angular measurements taken by the inclination sensorover time. Graphsandshows that the prototype inclination sensorprovided accurate measurements during the testing over time and at the desired temperature range.

Other testing on the prototype of the inclination sensorwas performed, including wireless communication testing, vibration testing, and shock testing. The wireless testing confirmed that a PCB on the inclination sensorcould communicate with a remote PCB so that stored angular measurements could be analyzed. The vibration testing and shock testing confirmed that a housing for the inclination sensorcould withstand the environment of the wellbore during milling operations. The vibration testing environment includes 30 g random profile at 2 Hz to 1 kHz, in all three directions, as well as 30 g with 2 kHz sine wave in all three directions. The shock testing environment included 500 g at 1 ms and 2 ms durations.

Field testing of the inclination sensorincluded operation of the sensoron a BHA to test angular measurements at depth as shown in graphof. Graphshows angular measurements taken by a measurement-while-drilling (MWD) tool and the inclination sensor. Graphincludes x-axisof wellbore depth (in fect) and y-axisof angular measurements between a vertical wellbore and a lateral (or angled wellbore) (in degrees). Curverepresents the measurements from an MWD survey, while curverepresents measurements from the inclination sensor prototype. As shown in graph, the window of the lateral wellbore is at aboutfeet, and a comparison of sensor readings and the MWD survey confirms the accurate measurement of the angle difference before and after the window by the prototype inclination sensor.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, example operations, methods, or processes described herein may include more steps or fewer steps than those described. Further, the steps in such example operations, methods, or processes may be performed in different successions than that described or illustrated in the figures. Accordingly, other implementations are within the scope of the following claims.