Sidetracking Wells is drilling a hole around unrecoverable junk, such as a stuck drill string ( Check also Pipe Sticking problem), it is necessary to place a cement plug above the junk at a required depth that will allow sufficient distance to kick off the cement plug and drill around, bypassing, the original hole and junk. High-density cement plugs are usually recommended for this application.
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In other words, oil/ Gas Wells may require sidetracking in order to re-drill the well to a new target, maybe to pass a fish or because of hole problems. To sidetrack any Oil / Gas well, you have to develop a lateral force to allow the drilling bit to cut on the side of the hole. This force will be provided by the drilling BHA geometry.
Problems That May Occur During Drilling Sidetracking Well.
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- The most common is a failure to deviate because the plug is too soft. This can be corrected by setting a longer plug and dressing it off correctly.
- Drilling around the plug and back into the original hole, especially. in soft formations, is a less common problem that may be corrected by setting a longer plug and sidetracking with a higher build angle.
- Hard formations may cause special sidetracking problems, especially with soft plugs and sometimes even with good, hard plugs. Some formations are actually harder than the cement plug, so the drilling bit ( check also: types of drilling bits) will preferentially drill the plug. This can be corrected by setting the hardest plug possible. It is possible to use a longer plug so that there is more distance for sidetracking. Drilling with reduced weight or possibly “time drilling” with an aggressive deviation assembly also is helpful.
- Sidetracking in holes containing oil mud reportedly causes problems, but it shouldn’t if the plug slurry is designed and positioned correctly using adequate spacers. Other remedies include setting a longer plug with extra slurry and using a higher sand content.
The main reason for failure to sidetrack successfully (with one plug) is drilling before the slurry hardens properly. Other reasons include using slurry volume (check also: casing cement slurry volume calculation) that are too small so that the plug is too short, contaminating the slurry during placement, and not deviating the hole aggressively during kickoff. The underlying reason may be a failure to design a good slurry. It is important to be patient. One can always consider using cement slurry accelerators, but cement retarders should be omitted if possible, or only the minimum amount should be used. Most failures require plugging back and sidetracking a second time, an additional and usually unnecessary expense.
It is common to locate the horizontal position of the kick off point based on measurements taken during drilling (check also Measurement While Drilling). The alternatives are to measure with a wellbore surveyor accept target limits within a cone of uncertainty. This usually is acceptable for sidetracking around a fish and for large targets with few limiting hard lines. One of the three measuring systems for measurement and orientation during sidetracking should be used.
Sidetracking In Open Hole
The sidetracking in the open hole is accomplished by first setting a sidetracking cement plug and drilling the extra cement to the kickoff point. The concentric and parallel versions of the steering tool measuring system are described here for measurements and orientation.
Sidetracking Well Procedures Using Concentric Steering Tool
For the concentric steering tool measuring system, it is necessary first to build a sidetracking drilling mud motor assembly, similar to a deviating motor assembly, with a steering tool measurement sub. Sidetracking procedures will be as following:
- The tool face correction is measured and recorded, which is the angular difference between the tool face and the indicating magnets.
- The assembly is lowered to the top of the plug by tripping pipe.
- The instrument measurement package is lowered inside the drill pipe (check also: API drill pipe specifications) with a shielded electrical conduit (cable) on the drum of a winch on a cable truck.
- The instrument package is seated in the measurement sub.
- A swiveling pressure pack-off is installed on top of the drill pipe and connected to the mud hose.
- The mud pump is started in order to circulate mud and the drilling bit is rotated with a motor.
- The direction of the tool face is observed on the data display monitor. It is normal to set required corrections in the surface readout equipment so that it reads the corrected tool face direction. This usually includes the difference between the tool face and the indicating magnets and the correction to true north.
- The drill string (check also: drill string components) is turned to point the bit in the required direction and locked to prevent it from rotating (usually by locking the swivel on the traveling block).
- Drilling of the sidetrack hole begins by lowering the drill string slowly and applying weight to the bit, increasing the weight slowly until the weight is within the specifications of the motor and bit.
Note: It is important to monitor the drift and direction of the hole and the tool face as drilling continues, orienting again as needed. This is accomplished by unlocking the swivel, turning the drill pipe to the correct direction, and locking the swivel to prevent the drill pipe from rotating. Drilling resumes.
- Precise measurements are recorded periodically by allowing the deviating tool to stop momentarily
- The next step is to add 1-3 joints of drill pipe to the drill string when the top of the drill pipe is near the rotary.
- The mud pump is stopped and the pack-off is disconnected.
- The instrument package is pulled out of the hole with the winch on the cable truck.
- The instrument package is lowered into ajoint of drill pipe in the mouse hole and the pack-off is connected to the top of the joint.
- The joint of drill pipe is lifted out of the mouse hole, and another joint is placed in the mouse hole and connected it to the bottom of the first joint.
- Another joint of drill pipe may be connected if there is sufficient mud hose length and space in the mast.
- These joints are lifted and connected to the top of the drill string.
- The instrument package is lowered inside the drill string with the cable, and seated in the measurement sub.
- The pack-off is sealed and the mud pump is started.
- The sidetracking assembly is oriented, the drill string locked, and sidetrack drilling resumes.
- If the drift angle is not correct, it may be adjusted with different bit weights and rotational speeds.
- If necessary, it is possible to trip and change the bottom hole assembly as described for deviating in the open hole.
- The instrument package may be replaced if it fails by pulling it out of the hole from inside the drill string with the cable on the cable truck and lowering another instrument package into the hole.
- If the cable parts for any reason, it may be recovered by fishing or pulling the drill string.
- Drilling continues, sidetracking the original hole until the new deviated hole is in the correct direction with an established upward curvature. The final step is to drill directionally or horizontally by one of the procedures of deflecting tools.
Sidetracking Well Procedures Using Parallel Measuring Tool System
Sidetracking with the parallel measuring tool system is similar except that the lower part of the cable holding the instrument package is inside the drill string, and the upper part is outside. The cable passes from inside the pipe to the outside through a side-door sub. The sub contains a seal assembly for sealing around the cable and allowing drilling fluid to be pumped through the drill string. Normally, the sub is positioned so that the cable is outside the drill pipe in a vertical section of cased hole. These limitations may be modified depending upon specific hole conditions.
Sidetracking Well Procedures Using Parallel Steering Tool Measuring System
For the parallel steering tool measuring system,
- The first step is to lower a sidetracking motor assembly with a steering tool measurement sub into the hole to the location for the installation of the side-door sub.
- The instrument package is lowered into the drill pipe and seated in the measurement sub.
- A side-door sub is connected in the drill string, the cable is passed through the sub, and it is sealed.
- The sidetrack assembly is lowered by tripping while simultaneously lowering the cable with the cable truck until the assembly is near the bottom of the hole.
- The kelly is connected, and the mud pump is started.
- Orienting and sidetracking are similar to the procedures for sidetracking with measurement instruments run in the parallel system.
- Standard drill pipe connections are made.
- The drill string and sidetracking assembly are pulled out of the hole and the instrument package is replaced if it fails.
- Then the assembly is lowered, oriented, and sidetrack drilling begins as described.
- If the conductor line parts either while drilling or tripping, the connected section is pulled out of the hole, sometimes while pulling the drill string and fishing when necessary.
- Drilling continues until the original hole is sidetracked with a new deviated hole drilled in the correct direction with an established upward curvature.
- Then drilling continues directionally or horizontally.
Sidetracking Well Procedures Using Time Drilling
Some sidetracking plugs are too soft to sidetrack by the method described but may be sidetracked by time drilling. The procedure also may apply while sidetracking in very hard formations in which the cement hardness is similar to or less than formation hardness.
- First a deviation assembly is run with the maximum reasonable angle-build section.
- The top of the dressed-off plug is touched (tagged) and the assembly is picked up until there is a small amount of bit weight on the plug, usually only noticeable on the sensitive needle or pointer of the weight indicator.
- The actual weight on the cement top should be almost negligible.
- The sidetracking assembly is oriented and directional drilling begins.
- After about 5 to 20 minutes, the drill string is lowered a few inches while continuing to rotate the bit and circulating.
- The procedure continues until about 5-10 ft are drilled.
- It is important not to use noticeable bit weight in the early part of this procedure.
- The drilling penetration rate is about 2-4 ft/hr depending upon the bit, plug hardness, and the formation.
- The next step is to begin increasing the bit weight very slowly.
- Normally, the drilling response will show if the bit is sidetracking correctly into the formation or following the old hole. If the procedure is successful sidetracking continues. Otherwise, it is necessary to try it again.
- If the hole is not successfully sidetracked on the second try, then the soft plug must be drilled out completely and another one set.
Sidetracking Cased Hole Wells
Cased holes are sidetracked by one of three methods, listed here in order of increasing risk:
- Sidetracking through a milled casing section.
- Whipstock through a milled casing section.
- Whipstock through a casing window.
Each has advantages and disadvantages. Measurements are recorded with one of the three measurement systems for orientation depending upon the type of sidetracking. The most applicable method is selected based upon depth, casing size, hole condition, the reason for sidetracking, and operator preference. Here we will discuss the operations through milled casing sections. In the article Whipstock Drilling, we shall cover the sidetracking using whipstock.
What Is The Difference Between Cased Hole And Open Hole Sidetracking
Sidetracking fundamentals in cased and open holes generally are similar. However, one major difference is the removal of a section of casing by milling or milling a hole through the side of the casing. Other differences are the methods of plugging back, sidetracking procedures, and some of the tools. The cased wellbore is surveyed with a gyro survey tool to locate the position of the kickoff point if necessary. The cone of uncertainty may be used if it is applicable.
Sidetracking in cased holes is often a higher risk operation than sidetracking in open holes. Smaller diameter casing (check also: casing design) requires smaller tools that have less strength than larger tools. Operations are more difficult in smaller holes, and they usually take longer because of the involved procedures and the necessity of removing a section of casing or milling a hole through it. The drill string may rub and wear against the milled hole through the casing and, in the worst case, become stuck (check also: Pipe sticking problem).
Sidetracking Through A Milled Casing Section Procedures
Sidetracking through a milled casing section is the most common sidetracking procedure and involves the least risk. It is used for both high and low angles of build, for long sections, and in most other cases. It is a common procedure for reentering an old vertical cased hole for drilling horizontally. Preferred casing size is 7 in. or larger since more operating problems occur while sidetracking inside smaller casing sizes. Larger casing sizes may be necessary if the deviated hole section requires more than one string of casing (check also: types of casing ). Anyone of the three measurement systems may be used.
The use of measurement-while-drilling (MWD) will be described here for purposes of illustration (see Fig. 3-7). It is common to plug the lower hole before milling the casing, depending upon formation conditions exposed in the lower hole compared to those in the section where the casing will be removed. A drillable cement retainer is common for plugging. The procedures will be as following:
- The first step is to connect the cement retainer to the bottom of the drill pipe and lower it in to the hole to the location selected for plugging. This frequently is the same depth as the bottom of the sidetrack plug.
- Then the retainer is set and mud is pumped through it into the formation, ensuring that the casing is open.
- The third step is to mix about 25 bbls of cement slurry and pump them into the drill pipe. Mud or water is pumped behind the slurry and displaced through the retainer into the casing below the retainer.
- A back pressure valve in the retainer seals and contains pressure below the retainer after pulling the drill pipe. The cement and retainer serve as a double plug. An alternative procedure is similar except that about half the cement is displaced below the retainer.
- The next step is to pick up the drill pipe out of the retainer and displace the remaining cement on top of the retainer. This ensures a seal with cement above and below the retainer.
- Then the drill pipe is pulled out of the hole.
- Milling casing starts at a point about 20 ft above the projected sidetrack depth. About 60-80 ft of the casing are milled and removed.
- A sidetracking cement plug is set as previously described.
- The bottom of the plug is placed at least 50-100 ft below the bottom of the milled casing section.
- The plug is extended through the milled section and into the upper casing.
- After it hardens, the excess cement is drilled or milled so that the top of the plug (kickoff point) is about 20 ft below the top of the milled section of casing.
- Sidetracking is accomplished in the same general manner as sidetracking in the open hole, allowing for the different type of measuring system, measurement-while-drilling (MWD). A measurement or instrument sub holds the MWD equipment. The MWD measurement sub is connected in the sidetracking motor assembly.
- The next step is to measure and record the tool face correction, the angular difference between tool face and the indicating magnets.
- The assembly is then lowered into the hole. A mud pulse sensor or other type of sensing instrument is installed at the surface, depending upon the MWD system, and the data display monitor also is installed.
- The Kelly is connected to the drill string and the mud pump is started in order to circulate and to rotate the bit.
- The direction of the tool face should be checked on the monitor. It is normal to set the corrections in the surface readout equipment for true north and the difference between the tool face and the indicating magnets so that it reads the corrected tool face. Orienting is done by turning the drill string to point the tool face in the correct direction.
- Then the rotary is locked to prevent rotating the drill string.
- The swivel is locked on the traveling block if the kelly is not used.
- The drill string is lowered slowly and sidetrack drilling begins. Precise measurements are taken periodically for verification by allowing the drill string to come to a full stop momentarily. The allowances for bit walk and reactive torque may be omitted, since MWD equipment gives the correct direction of the tool face. The direction and orientation are monitored again by turning the drill string as required.
- The drill pipe connections are made in the normal manner.
- The drill string is lifted out of the hole to replace the MWD equipment if it fails.
- It is possible to sidetrack a few cased holes in order to bypass an unrecoverable fish, and the lower part of the hole may be redrilled by blind sidetracking. This is used when it is not necessary to monitor and control the direction of the sidetracked hole.
- The inclination is still monitored, but sidetracking continues without directional control.
- Nonmagnetic collars are omitted, and the hole is drilled vertically using regular drift measuring instruments.
- A hole with junked casing is sidetracked similarly.
- Gyroscopic surveys may not be necessary after the new hole is 50-75 ft in a straight-line distance from the nearest section of casing in the original cased hole, depending upon casing size and hole drift. The magnetic influence of the casing is negligible at this distance, so the operator may change to a more economical measuring instrument, depending upon the type of sidetrack hole.
- Drilling is continued until the new sidetrack hole points in the correct direction and has an established upward curvature.
- Then continue drilling the directional of horizontal well with the same planned BHA before sidetrack.
Considerations While Sidetracking Wells
2 Main Issues to consider when select Kick-off Point.
Ok let’s understand how we select the kickoff point in simple steps
- Choose a soft or soft to medium formation so that you can increase the chances of a successful kick-off. However, if only medium to hard formations exist at the required sidetrack depth, you will need to be more patient.
- Control your ROP till it is confirmed that you got a successful kick-off.
Consider The Following When you Set A Kick-off plug For Drilling Wells Sidetracking
- SET a 300 – 500 ft NEAT CMT PLUG (Class G in the production hole section) at 17.0 ppg (batch mixer will be more reliable in this situation if possible).
- WOC WAIT ON CMT for 12 hours minimum.
- DRESS OFF 20 ft with low parameters, and WEIGHT TEST the plug
- Now you have 2 possibilities
- Plug is firm : DRESS OFF a further 20-30 ft comparing drillability with that of the original hole
- Plug appears good: WAIT a further 12 hours before attempting to kick-off
- Plug has no signs of compressive strength: Then WASH DOWN more CMT to be able to place second 300 ft plug at a suitable depth for sidetracking.
Take Care While Cutting Casing Windows.
When it is about initial cut, try to avoid casing collars and centralizers (minimum of 8 ft above a casing collar). Generally, the lengths of the window to facilitate an effective sidetrack are:
Choose Adaptable Drilling BHA To Perform Successful Hole Sidetrack
A suitable assembly shall be agreed upon between the Directional Driller in consultation with the Drilling Supervisor. The number of non-magnetic components can usually be reduced by applying a survey correction algorithm to survey data.
Sidetracking Through the Low Side of the Drilling Hole
- The low side hole sidetrack have higher risk of creating high doglegs as it allow you to take off rapidly. So that it is much better to come off low side to the right / left than purely low side.
- Once the new hole has been cut, lateral separation should be maintained to prevent collapse of the old hole into the new hole.
Maximizing Kick-off & Sidetracking Success
- If there are external factors which may reduce the chances of performing successful kick-off such as (Eg., hard formations, deep kick-offs, or restricted window lengths), you will have to consider the following drilling BHA to maximise kick-off probability: PDC bits, mud motor, 1.5 degree bent housing, MWD, UBHO sub, DC’s.
- Initially, the assembly shall be time drilled until approximately 30 ft with 100 % formation returns, then the BHA shall be pulled to reset the deflection.
- bent sub size will depend on the drilling hole size, formation hardness and required deviation.don’t forget to check the geometry of the assembly to ensure that there is a lateral loading with the motor length and bent sub for the hole size that will be sidetracked.
- Mud Motor / Bent Sub combination is the most economical choice for well sidetrack then you can change to a more optimum drilling BHA.
- If you consider to drill ahead after the kick-off with the same drilling BHA, it is much better to choose a motor bearing bit.
- You can run MWD tool directly above the bent sub and utilize survey correction algorithms.
- In-case of sidetracking through a casing window, orient the string using tool face high side readings or a UBHO with steering tool above the MWD.