Copyright 2007, Society of Petroleum Engineers This paper was prepared for presentation at the 2007 SPE Production and Operations Symposium held in Oklahoma City, Oklahoma, U.S.A., 31 March–3 April 2007. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, Texas 75083-3836 U.S.A., fax 01-972-952-9435.

Abstract In conventional vertical drilling, the drill string is tripped in, allowed to drill to the predetermined depth and tripped out so that a bit of a smaller diameter can be attached to continue drilling the hole. Extendable arm drill bit is essentially a rotary drill bit in which the roller cones can be extended so that the diameter of the bit corresponds to the required diameter of the hole to be drilled. This extension is done hydraulically by diverting some of the pressure of the circulating drilling mud to compress an assembly which would push down the roller cones, thus increasing the diameter. Now as drilling continues, the diameter of the bit can be adjusted by diverting a lesser amount of circulating pressure on the bit thereby “contracting” it to achieve a smaller diameter. It could be imagined like a tripod standing on its three legs. The idea behind this proposal is to save the time, power and the cost of tripping operations of the drill string to change the bit to a smaller diameter one, by reducing the frequency of tripping the string, if not completely eliminating the process. This would result in an increase in the overall rig efficiency, both in terms of time and mainly money. Thus, the time and money saved can be diverted, if required, towards problems arising due to unexpected hole problems and pipe stuck-ups.

This paper gives the description of the most suitable design of such a drill bit and the possible modifications of this concept to be applicable in future developments.

Introduction Through the history of oil well drilling, individual concentric lengths of pipe are used to line the borehole. This process results in stability of the hole and avoiding the hole from

caving in. But it also leads to structural redundancy of the casing that is used as a bore-hole liner and results in an increase in the cost of both the material and logistics involved. During the drilling operation, there is a need to isolate sections of the well depending upon a variety of geological factors which range from abnormal pore pressure to wellbore instability to hydrocarbon or water bearing zones. From the time rotary drilling was employed, wells have been drilled in a step-wise manner i.e. drilling a larger diameter hole at the surface and proceeding to the reservoir with a receding hole diameter. The bore-hole drilled is cased in between changing of the bit to drill a smaller diameter hole. A single diameter bore-hole cannot be drilled throughout its entire length due to certain parameters arising largely due to geothermal pressure gradients. But still, two techniques, namely the expandable tubulars and drilling with casing methods drift apart from the traditional drilling operations. In cases where there is an uncertainty about the bore-hole instability, simultaneous lining or casing of the hole alongside drilling removes some concern. But this method, which is employed in casing while drilling operations, reduces the annular space between the casing and the bore-hole as it involves drilling with concentric casing strings rather than the traditional drill pipes. Certain grades of steel have much greater ductility which allows adequate plastic deformation to be achieved without much harmful work-hardening, in such a way that it is feasible to increase the diameter of a pipe uniformly and without any uneven thinning of the cross section. The development of pipe connections which are capable to accommodate this increase in diameter has brought the concept of mono-bore wells in the limelight. One of the convenient methods for drilling a mono-bore well is employed by the casing while drilling method. The drilling BHA passes through the casing of restricted diameter to get attached at the bottom of the casing to drill till the pre-determined depth. Later the BHA is retrieved by wireline at the end of drilling. But since the BHA has to pass through the casing, the size of the pilot bit and the underreamers used suffers. Also a smaller size of motor is used which reduces power output and increases sliding drilling ratio due to its flexibility.

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Extendable Arm Drill Bit: A Novel Idea Yash Gupta, SPE, and Sudeepto N. Banerjee, SPE, Maharashtra Inst. of Technology

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Due to such reasons, there was a requirement to develop a technology which would assist in completion of the first phase of the mono-bore well dream. Though in its infancy, the extendable arm drill bit would help in diverting some attention towards development of a drill bit capable of “opening up” so as to provide a larger or smaller hole size as and when required. Further research and development would lead to complete success of the extendable arm drill bit and completion methods corresponding to it. Also to cover up the more conventional aspect, the extendable arm drill bit, once completely developed and tested in the field, may prove to be a perfect substitute for conventional drill bits, if not for all, then for at least a few intermediate sizes. By employing this bit, the frequency of the tripping operations could be cut down by a major factor. This would result in phenomenal savings which would be in the form of both time and money. Also this would resulting greater well control, as most problems arise while tripping operations for changing the bit and proceeding drilling. Design Basis The basic idea behind designing the extendable arm drill bit is to provide simplicity and effectivity in the operation of the BHA without any complications. Time and again different mechanisms were thought over for performing the desired function. These mechanisms varied from using energy governors to external hydraulic jars to screw jacks. In the end the idea came back to the fact that using hydraulic energy would prove to be the most efficient method. Using the hydraulic energy imparted by the circulating fluid seems to be the most feasible method. It is simple to divert the mud hydraulic energy to any BHA component and even more convenient to control the energy imparted by the mud. The diversion is possible by strategically placing valves wherever required and energy application can be varied by changing the density of the circulating mud. The mechanism for extending the arms is hydraulically actuated by pressure differential from the fluid flowing through the bit. An internal coil spring is used to return the arms to their initial closed position. This would take place every time the mud circulation is stopped.

Bit components

The basic arrangement of the extendable arm drill bit is almost similar to the conventional rotary bit with some additions. The extendable arm drill bit consists of the normal rotary PDC cutters which are attached to arms, which can be extended by the use of hydraulic energy imparted by the circulating mud. The arms are specially fabricated in this particular shape. The arms have slots grooved in their body which helps in holding the extended arm in place. These arms are made of the same material as the normal bit i.e. carbon molybdenum alloy with a variable amount of cobalt which helps in improving the steel hardness. The PDC cutters are to be installed with tungsten carbide inserts for application in areas of hard rock. The

technical configurations like the bit offset and the journal angle have to be dealt with care as we are using the same drill bit to drill through different types of formations and through different hole sizes. Figure 1 shows a schematic of the drill bit arm along with the slots that are grooved in them. As we are using hydraulic energy to operate the extendable arm drill bit, we need some kind of a plunger system to transform this hydraulic energy into mechanical energy. We use a dual plunger mechanism for this specific function. Dual because we intend to transform a small amount of hydraulic energy to a proportionately larger amount of mechanical energy. The plunger is filled with an incompressible fluid like lube oil which acts as the medium for the transfer of energy from the upper to the lower plunger. The lower plunger is specially crafted so that it fits in to a slot especially machined at the top of the arms. The arms are made to pivot at the top, with the help of the bottom plunger. Thus, it can be said that the basic movement of the arms is controlled by the dual plunger installed in the drill bit. A schematic of the dual plunger is shown in Figure 2.

Internal coil springs are used in the drill bit to hold the arms in place once they are extended in the desired position. The coil springs are released in the slots present in the arms and tend to hold the arms in that certain position. The coils are so calibrated that they would retract only when the force acting on them is zero. This is possible only when drilling and mud circulation are stopped. This means that the complete force of drilling the formation acts on the coil strings installed in the drill bit. The complete schematic setup of the drill bit is shown in Figure 3.

Modus operandi The main and probably the only source of energy for this method is the hydraulic energy stored in the circulating mud. The drill bit consists of the arms which pivot about their rearmost end. This pivoting action is retained in a housing which is attached to an outer hydraulic cylinder. This hydraulic cylinder does not carry any axial drilling forces. The hydraulic cylinder houses a dual plunger and bypass valves controlled from the surface. The drill bit is initially kept in closed position. Any kind of opening of the arms is done by the circulating mud. The bypass valves are located alongside the dual plunger and are initially in the open position. Once the drilling has to start, the drilling mud is pumped through. The bypass valves are closed and the complete force of the mud is diverted over the plunger. The amount of pressure being applied on the plunger can be controlled by either changing the circulation rate or by changing the viscosity of the mud. The bit is earlier tested in the laboratory to open up under specific pressure conditions, and accordingly calibrated. This information is available at the field during the drilling operation to assist the driller in applying the prescribed amount of pressure. The downward movement of the plunger under the influence of pressure differential draws the bit arms over a profiled surface, causing them to open outwards, to the expanded position. Now as the

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arms move outwards, the internal coil spring comes into action. The arms are retained in slots machined at the back of the bit, as a means of transmitting the drilling torque. The open position of the arms in the drill bit are shown in Figure 4. Retaining pins, mounted on the arms, act on grooves machined into the arm to actively retract the cutters as the bit closes. The entire drilling weight is directly applied to the head of the bit and onto the PDC inserts in the rollers. The arms are not directly loaded with any compressive forces applied to the pivoting end. The pivot only acts as a means of locating the rear end of the arms. The internal coil springs hold the arms in their appropriate position during drilling. These coil springs are pre-calibrated so that they tend to close when the pressure differential acting on them falls below a certain level. This great amount of pressure differential is only achieved when the mud circulation has been completely stopped. Pulling out of hole imparts a down thrust onto the arms, promoting a closing a closing action, in addition to that imparted by the return spring. This procedure facilitates the opening and closing of the drill bit.

Further developments The topic discussed in this paper is in a very crude form. The performance of this drill bit cannot be assumed without practical testing in the field. Further developments can be sought after when the bit has been tested in the actual environment. But at this theoretical level one can only assume the improvements that should be made. Through the medium of the paper so far, one can challenge the credibility of this drill bit. Some of these challenges are discussed in this section. The first thing that one would challenge is the ability of the internal coil springs to sustain the forces of drilling the formation. The coil springs are especially designed to hold the arms in their respective positions while drilling. This is possible due to the tightly wound return spring in the coil. The spring(s) retract in such a way that they attach into the grooves made in the slots machined in the arms. The grooves are machined in the slots at specific intervals which would correspond to the opening of the arms at a particular diameter. The coils are calibrated in the laboratory prior to any field application. This calibration allows the coils to retract automatically when the pressure acting in them falls below a certain reading. This is only possible when circulation has been completely seized and the required interval has been drilled. Another factor one would focus upon is the pressure differential required to open the arms to a certain extent. The drill bit is first tested in the laboratory and the results are noted to be directly used in the field. One of the main results is the calibration of the opening of the arms to a certain angle. This opening would be directly related to the pressure differential acting on the dual plunger. These calculations would be available at the drill site with the driller to assist him in choosing the appropriate circulating pressure to be applied.

As we are using a single drill bit to drill multiple size holes, one can challenge the overall bit life due to continuous interface with the formation rock. We are using the same drill bit for both hard and soft formations, thus we have to design the journal angle and the offset in such a way that it is efficient for use in most of the formations. Also we would have to consider the material used for manufacturing the different bit components. As stated earlier, carbon molybdenum can be used with variable proportions of cobalt to increase the hardness of the bit, but the actual composition has to be determined by considering the type of formation present at the site. The type of formation can be assumed by the wild cat wells or the earlier wells that have been drilled conventionally in the area. Also the tungsten carbide inserts used in the PDC rollers have to be designed for easy removal and replacement of the inserts in case of any damage and wear & tear, which is evident after constant use of the drill bit over a large interval. Overall the design should allow the drill bit to be fully field serviceable, with all mechanical components having an extended operating life. Only the inserts would be replaced to present a new cutting structure. The inserts could be supplied with an individual cutting structure to suit particular drilling parameters and also each set of rollers, and eventually the arms, would enable a unique expansion ratio to be achieved within the maximum design diameter. The rollers could also be redressed with a new cutting structure a number of times before eventual replacement. High strength corrosion resistant alloys have to be selected for all component parts coming into contact with drilling fluids, to improve reliability and extend service life. Conclusions The idea proposed in this paper is very much in its infancy and it requires much research and thought both theoretically and practically in the field. Due to lack of appropriate infrastructural facilities, the required results and correlations cannot be obtained. But this would be easily developed in the industry amidst the more experienced and concerned personnel, who would develop this idea into a field application. As the idea of the mono-bore well takes form, this idea would possibly prove to be beneficial in developing single diameter boreholes. After further finalising of the design and the durability, the extendable arm drill bit would surely be counted among technologies like casing while drilling and solid expandable tubulars, which are currently being used in drilling mono-bore wells. Along with the economic savings incurred due to the drill bit in the form of saving of tripping time, this bit could be combined with the casing while drilling process to result in even greater economic efficiency. Further developments of the extendable arm drill bit completely depend on the competency of the bit in the field, but one can surely count upon this idea as one of the possible method for achieving economic stability in the field.

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Figures:

Figure 1. Drill bit arm Figure 2. Dual Plunger Mechanism Figure 3. Complete setup of the Extendable Arm Drill Bit

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Figure 4. Initial and operating positions of the bit