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The Benefits of Top Drive Drilling

By John D. Warren, Varco International Inc.

The Top Drive Drilling System is recognized as one of the most significant advancements in drilling technology since the introduction of the rotary table.  When compared to conventional rotary drilling rigs, those equipped with Top Drive Systems consistently drill faster and safer, with far less instances of stuck drill pipe.  In addition, drilling with a Top Drive allows operators to reach areas and types of formations that would not be accessible with conventional rotary drilling.  Extended reach and horizontal drilling have brought about dramatic increases is production rates from specific fields, and these types of wells can only be drilled with Top Drives.  These benefits, along with improved well control and better hole conditioning, all contribute to the indisputable financial justification for the Top Drive.

Top Drives are drilling machines that hang below, and travel with, the traveling block and rotate the drill pipe from the top of the string, as opposed to using a rotary table and kelly drive.  The Top Drive connects to the top of the drill string, providing rotational torque to the string while simultaneously supplying drilling fluids and supporting the string weight (similar to the function of a standard drilling swivel); Top Drives are sometimes called “power swivels” for this reason.  The drilling torque produced by the Top Drive motor is reacted through a guide beam or guide rails and transmitted into the mast or substructure.  The differentiator of a Top Drive System from a power swivel (a swivel with a power drive) is in the Pipehandler connected directly underneath the Top Drive transmission.  This Pipehandler is remotely controlled by the driller and makes up and breaks out the Top Drive lower connection for applying the torque to the drill string without manual intervention.

As the Top Drive drills down and approaches the rig floor, rotation and circulation are stopped, slips are set on the pipe to support the string weight, and the Top Drive is disconnected from the string by the driller utilizing the Top Drive Pipehandler.  The driller then raises the blocks, and thus the Top Drive, to the fingerboard level, where another stand of pipe (approx. 30m) is delivered to a conventional set of drill pipe elevators hanging from the Top Drive.  This stand is raised and stabbed into the drill string and the Top Drive is then lowered until the drive stem engages the upper connection of the stand.  The Top Drive motor is engaged, spinning the drive stem, and both the upper and lower connections of the drill pipe stand are made up simultaneously.  A single backup tong is used at the drill floor to prevent rotation of the drill string as the connections are being made.  After the connections are properly made up, the slips can be released, circulation can resume, and the Top Drive can begin drilling the string down to the floor once again.  The entire process, from the time the slips are set on the string, a new stand is added, the connections are made up, and the slips are released allowing drilling to resume, typically takes less than 90 seconds to perform.

Refer to Figure 1 for a schematic representation of the drilling process when using a Top Drive.

drilling process

The process of adding a new stand of pipe to the string, and drilling down to the floor, involves fewer actions and requires less involvement from the drill crew, as compared to kelly drilling.  Drillers and rig crews usually become comfortable and proficient in drilling with Top Drives in a matter of hours.  Built-in features such as thread compensation, remote-controlled valves to stop the flow of drilling fluids, and mechanisms to tilt the elevators and links to the derrickman or floor crew add to the speed, convenience and safety associated with Top Drive drilling.

If needed, Top Drives can also be effective when drilling with single joints (10m) of pipe, although greater benefit is achieved by drilling with triples (stands of pipe).  With the drill pipe being supported and rotated from the top, an entire stand of drill pipe can be drilled down at one time.  This extends the time the bit is on bottom and produces a cleaner bore hole.  Compared to kelly drilling, where a connection is made after drilling down a single joint of pipe, Top Drive drilling results in faster drilling by eliminating every two out of three connections. 

Improved drilling speed is often referred to as the single largest benefit of Top Drive drilling, and is definitely an advantage over kelly drilling.  Fewer connections are made, and what connections are made are typically performed faster than those made while kelly drilling.  Numerous reports of 17-20% improvements in the time it takes to complete a well program have been documented (see Figure 2 and references).

drilling programs chart

Certain drilling programs have justified the use of Top Drives based on the speed benefit alone.  However, the Top Drive offers many other benefits, each of which act to increase the performance of the drilling rig and improve the return on investment for the well.  For instance, Top Drives reduce the instances of stuck pipe. 
Historically it has not been considered uncommon to get a drill string “stuck” in the hole from time to time, and the potential of stuck pipe increases with hole depth and the particular formation being drilled through.  Regardless of the depth or type of the formation, drilling with a Top Drive drastically reduces the instances of stuck pipe.  Drilling with 30m of pipe at a time allows more time for hole conditioning and circulating solids to the surface.  Also, because there are fewer connections to be made, the pumps are stopped less often.  This results in less circulation time required to achieve uniform distribution of annular cuttings load.  All of these factors help keep the bit and the string free to rotate and help prevent sticking.

Besides the stuck pipe that could previously occur while drilling, drill strings can also encounter tight spots when tripping in or out of the hole.  If a tight spot is encountered during a trip on a conventional rig, it becomes a major effort to pick up the kelly and begin circulating and rotating the pipe through the trouble zone.  However, when tripping on a rig equipped with a Top Drive, rotation and circulation can be achieved at any point within a matter of seconds.  The driller simply needs to set the slips, lower the Top Drive to engage the drive stem, make up the connection with the Top Drive Pipehandler, and begin circulation.  This feature provides the driller with the added benefit of being able to “back-ream” whenever necessary.  Entire sections of the well bore can be reamed through without significantly impacting trip times.  The result is a conditioned and clean borehole, ensuring a successful casing run.

The ability to quickly and easily connect to the drill string during trips provides benefits that extend beyond just preventing stuck pipe.  For instance, consider the situation when a kick is encountered during a trip.  On a kelly rig, the crew has little recourse and will find it very difficult to contain the fluids escaping the well without taking drastic measures.  In the case where a Top Drive encounters the same situation, the slips can be set and the Top Drive connected immediately, thus containing and controlling the well within seconds of a kick being detected.  These rapid responses to well kicks have increased the safety of the rig floor and have helped to protect drilling personnel from possible injury.

Other aspects of Top Drive drilling have led to increased awareness of safety all around the rig floor.  When kelly drilling, the rotary table and kelly bushing are spinning rapidly at the rig floor, while the crew is in close proximity.  Since Top drives eliminate the need for the kelly drive mechanism, and the rotary table is not used to rotate the pipe, the only thing rotating at the drill floor is smooth drill pipe.  Also, since the Top Drive eliminates 2 out of 3 drilling connections, the drill crew is less exposed to possible injury; less exposure to possible injury results in less injuries.

While improvements in drilling time and crew safety are well documented, and these features can benefit any drilling rig, certain aspects of Top Drive drilling have allowed drastic improvements in oil and gas recovery from reservoirs.  Enhanced recovery of hydrocarbons has been achieved through a combination of extended reach and horizontal drilling programs.  Extended reach, or highly deviated wells, increases the horizontal area of a reservoir that can be tapped from a given location.  Horizontal completions allow a major increase in the ultimate recovery from a given reservoir.  Both of these offers tremendous financial incentive from the operator’s perspective, and both of these situations can only be achieve by utilizing a Top Drive Drilling System.

In the case where geological, geographic, or economic factors limit the placement of drilling locations, it may be beneficial to deviate the wells drilled from a given location in order to access certain areas of a reservoir.  This is achieved by drilling at angles from 70 – 90 degrees from vertical for extended measured depth.  When drilling with a Top Drive, and taking into account other parameters such as drilling fluid composition, it is now considered commonplace to extend the horizontal reach to beyond 5km; specialized situations have extended reaches to beyond 10km (see references).  If a drilling location were limited to a maximum angle of 40 degrees (the highest angle easily achieved with a kelly rig), the total horizontal area of reservoir that could be tapped would be approx. 10.6km2.  If the maximum angle is increased to 80 degrees from vertical (only achievable with a Top Drive), the total area accessible goes up by a factor of 30:1, to nearly 320km2.  This particular benefit has provided cost savings in certain fields, primarily offshore, of over $2 billion, and has led to the standardization of Top Drive use on virtually all platforms and mobile drilling rigs alike.

Extent of Horizontal Reach

As is the case with extended reach drilling, horizontal drilling increases financial return, but does so in a very different manner.  Interestingly, land rigs can realize as much benefit from horizontal completions as offshore installations.  The basic premise is that, when compared to conventional completions, production data from horizontal completions indicate rate benefits of 300%, and a major increase in ultimate recovery (see references).  This is due to the fact that conventional wells pierce the reservoir vertically, thus production zones are limited to the thickness of that particular area of the reservoir.  However, if the well is completed horizontally, the length of the pay zone can be increased.  The concept of horizontal wells has been around since the 1940’s but only with the introduction of the Top Drive have they become technically and economically feasible.

Horizontal wells can improve production rates and ultimate recovery in a variety of ways.  As stated above, the horizontal section of the well bore allows longer completed intervals, allowing increased production rates.  In reservoirs overlying an aquifer or located under a gas cap, the increased standoff from the fluid contacts can improve the production rates without causing “coning”.  Additionally, the longer well bore length reduces the drawdown for a given production rate and further reduces coning tendencies (see references).  In a fractured formation, longer horizontal well bores are more likely to intersect fractures and will thus improve both production rate and ultimate recovery.  Horizontal drilling, and the use of a Top Drive, may allow development where conventional techniques and equipment would be uneconomical.

direction-profile

With the speed, safety, performance, and production improvements achievable by using Top Drive Drilling Systems, it is no wonder that most operators and contractors in the world now consider Top Drives as “standard equipment” on both their exploration and production wells.   At this moment, approx. 92% of all mobile offshore drilling rigs are equipped with Top Drives, and every new build offshore rig built in the last 9 years has been delivered with a Top Drive installed.  Although offshore rigs were the first to fully realize the benefits of drilling with Top Drives, land contractors and operators have now recognized the value and are installing Top Drives at a record pace.  In fact, due to the popularity of Top drive drilling on land, manufacturers are producing systems at record high levels, with over 72% of all systems sold in the past three years destined for land rig applications.  Top Drive Drilling Systems have been proven to drill faster, safer, and better, and represent “the industry standard” method of drilling for oil and gas.
 
References:

R.C. Wilson and D.N. Willis, Mobil Exploration. SPE paper # 15465 “Successful High Angle Drilling in the Statfjord Field”

J.P. Wilkerson and J.H. Smith, Standard Oil Production Co., and T.O. Stagg and D.A. Walters, Standard Alaska Production Co.  SPE paper # 15372 “Horizontal Drilling Techniques at Prudhoe Bay, Alaska”

D.W. Sherrard, B.W. Brice, and D.G. MacDonald, Standard Alaska Production Co.  SPE paper # 15376 “Application of Horizontal Wells at Prudhoe Bay”
 
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