Deepwater under-reaming

Transcription

1 Deepwater under-reaming Reaming with non conventional drilling assemblies has always been difficult and often problematic. In addition, high down hole and surface torques often accompany drilling operations, yielding violent, transverse vibration on bottom hole assembly (BHA) components. Such destructive energy alone is sufficient to fail down hole tool components prematurely. For this reason, underreaming well bores was usually discounted in the well construction activities, relegated to a clean-up, or wiper trip, prior to running casing. As the industry commenced to explore in greater water depths, with drill string trip time and associated costs to consider, drilling optimization and minimal trips became an essential priority. e.g. Eliminating one trip in deepwater, can save in excess of $100,000 for the operator. In simplicity, < BHA changes and trips results in reduced tripping time, > efficiency and enhanced cost savings. At present, deepwater drilling is one of the new frontiers for under-reaming and hole opening technology applications e.g. Typical top hole sections may require 5 days of rig time at a rig costs often in excess of $200,000/day. Under reaming to drill sections in one trip can thus provide significant benefits. Typical Under-reaming Practices The following practices should be considered when under-reaming. 1. With roller bearing cutters, controlled drilling or weight on bit cannot often be maintained due to excessive down hole or surface vibrations and rough drilling effects. Consider reducing and/or changing the rotary RPM by 20 or 30 RPM to avoid premature gauge wear on the cutter cones and arm shirt-tails that can lead to losing the cones in a very short period of time. 2. If the under-reamer arms will not close e.g. drilled debris blocking the arm pockets, pump at the highest rate in combination with higher RPM. It may be possible to bounce the drillstring off bottom and dislodge the foreign material from the arm pockets. 3. When very fast under-reaming is possible, ensure that there is sufficient RPM to prevent cutting a spiral wellbore. In this case > 100 RPM+ is recommended. 4. When opening the hole, the use of an expandable blade stabiliser behind the under-reamer may assist in maintaining verticality of the wellbore. 5. Placing a suitably stabilised bit, taper mill, assembly below the underreamer may not guarantee vertical hole. It can however prevent any orbiting tendency of the under-reamer and ensure original hole and/or pilot hole is closely followed. 6. On making the initial under-reamer cut, rotate for at least 15minutes at a reduced rate of RPM with full pumps prior to drilling ahead. This allows adequate time for a full gauge initial cut to be made. Note ; The harder the formation to be under-reamed, the longer the time required before drilling ahead should commence.

2 7. After having under-reamed for approximately 3m, stop rotating, pick up the string with the pumps on, and lower the string in an attempt to tag the ledge that should have been cut. - If the formation is firm enough and has not washed out, this will give an indication that the hole is being cut. - If the formation is very soft or badly washed out, this procedure will serve no purpose. Hole-open or under-ream? The essential difference between hole openers and under reamers are that hole openers enlarge an existing pilot hole from the surface of the wellbore and contain fixed cutters of a pre-determined diameter. Conversely, underreamers enlarge the hole below a restricted tubular bore allowing annular clearance to be able to run additional casing strings. The cutters are kept closed during the passthrough and are activated once the tool reaches the point that requires under-reaming. Turning the pumps off deactivates the under-reamer cutters with this technology. Today, under-reaming offers several product options, including: - bi-centre bits, - reaming wing tools, - expandable bits, - and under-reamers. Under reaming and drilling & under reaming simultaneously are regularly performed with PDC under reamers, with no loss of arms, even in horizontal holes. These under reamers have no locking devices. For a reaming tool to work in many situations with operator confidence, improved designs are however needed. E.g. illustrated example from Security DBS, a division of Halliburton Energy Services is a short fully-stabilized Near Bit Reamer (NBR). This tools can be used to drill oversized holes, more efficiently than with some bi-centre bits, reaming-while-drilling tools, or other types of reamers. Unlike most bicentre bits and reaming-while-drilling tools, the NBR has been used on both conventional mud motor and rotary steering directional assemblies. Successful runs have been recorded offshore Norway, e.g. a record run of 2,213 meters of continuous drilling/under-reaming, 20 successful runs offshore Norway and 17runs in the Gulf of Mexico to date.

3 Hole enlarging methodology The accompanying tables in this section, lists the strengths and limitations for under-reamers, hole openers, bi-centre bit and reaming wing tools application in deep water drilling operating environments. Essentially under-reaming in deepwater softer formations can lead to impressive gains in penetration rates. Also specifically for examples in the Gulf of Mexico, under-reaming has allowed both flexibility and contingency to accommodate design for multiple casing strings that may or may not be required to isolate shallow geo-hazards, e.g. shallow water flows. Problematic areas however can include cutters becoming mud packed, balled etc. due to poor hydraulics, cutter cleaning, inadequate nozzling size and design, reduced rate of penetration, unable to close cutters etc. making it extremely difficult to pull out of hole. Note: On some designs PDC under reamers don t show any balling behind the arms, because there is a nozzle dedicated to each arm wing to keep it clean. Historically, such factors swayed decision making in regards to under-reaming and as such under-reamers still have a way to go in terms of improving mechanical integrity robustness and total reliability. To improve the mechanical integrity of traditional arm type under-reamers, the development of bi-centre bits, reaming wing tools and polycrystalline diamond compact under reaming cutters are working towards eliminating the traditional problems associated with mud packing and arm breakage. E.g. Typical tool life being extended to 10 runs or more. A further procedural innovation in under reaming involved the removal of all cutters from worn roller cones, and replacing with new cutters. Such cutters being stub-welded in place once fatigue became an issue. Three-cone under-reamers are also capable of enlarging holes by as much as 50%, and are typically selected according to formation type so as to optimise performance. The tools also allowed full volume circulation at all times. Reamer classification It is convenient to classify the various reamers as either drilling or reaming types. The reaming type serves only to enlarge an existing hole, involving complete section re-drill. Conversely, the drilling type allows for the simultaneous reaming of a pilot hole as it is drilled. It can also be used to under-ream existing holes and its design allows mudflow to be diverted to the bit or can even be used with guidance system such as a bull nose for pilot-hole re-entry. The latter has been reported to give good hold or slight build tendencies, depending on formation and other factors. Thus, its performance resembles a packed-hole assembly.

4 Table 1: Underreaming operating characteristics Hole enlargement in deep open water drilling Underreamers Disadvantages Eccentric cutting action leads to hole bellying, increasing likeliness of hole instability, tool & equipment problems. Can be more costly than Bicenter bits and/or hole openers. Perception of arm breakage and failure, hence greater risk Requires stabilisation of bottom hole assembly. Risk of mud packing and inability to retract arms Fatigue and bending stresses on arms, body and connections Hydraulics i.e. efficient nozzling and cutter cleaning often a problem. If not afforded balling can occur. In traditional designs, pivot arms unsuited to deviated wells. In some tools difficult to optimise nozzle flow. Advantages Enlarge wellbore size below last set casing string Offer some flexibility of operation Concentric cutting action means a smoother hole Aids in reducing vibration, delivering better hole stability quality, and gauging prior to running casing Simultaneous drilling and reaming Arms retract when pumps off. Advantageous when pulling out and through casing. Most tools designed with roller cones, better suited to open water formations PDC cutters better suited to more consolidated formations Greater certainty of hole gauge. Can be a vital consideration for tripping, logging and casing running/cementing. Table 2: Hole opening characteristics Hole openers Disadvantages No flexibility of operations. Advantages Concentric cutting action means a smoother hole, decreased torque and lateral vibration. Robust strong and sturdy tool design. Hydraulics i.e. efficient cutter cleaning often a problem Requires stabilisation of bottom hole assembly. Hydraulics i.e. efficient nozzling and cutter cleaning often a problem Need to keep tools moving in deviated wells to prevent side-tracking the hole. Two trips often required to drill section. Most tools designed with roller cones, better suited to open water formations Aids in reducing vibration, delivering better hole stability, quality, and gauging prior to running casing PDC cutters better suited to more consolidated formations Certainty of hole gauge. Can be a vital consideration for tripping, logging and casing running/cementing. Often advantageous for hole cleaning, getting data acquisition from pilot hole.

5 Bicenter bits and reaming wing tools Bicenter bits In the 1990 s, bit manufacturers had developed bi-center bits that were capable of drilling enlarged hole sections beneath up to 13 3/8 casing strings. Increased demand and use of for these bits through the 90 s however highlighted certain technological limitations, e.g. poor directional, notably inclination, drilling performance essentially linked to the lack of a full-gauge stabilizer above the bi-centre bit, that resulted in bottom hole assembly (BHA) stabilization issues. It was concluded that without properly afforded stabilization, directional requirements depend almost entirely on the bent motor housing to steer. Consequently, to make changes in inclination, drillers spent too much time slide drilling that can be difficult at the best of times, let alone with the added complication of running a bit that is geometrically unstable. From this perspective, there was an understandable apprehension towards running bicentre bits. As bit/bha steering issues continued to hinder directional performance, bicenter bit manufacturers continued however to develop more preferred design and operating methods to improve bit & BHA directional behaviour. E.g. A realization that bi-centre bits could to an extent be steered by weight and that longer bit profiles helped to stabilize BHA performance. The success of bi-center bits however does not remove the need for hole openers in many applications, or eliminate the need to drill the interval twice. E.g. in deep open water environments, pilot holes are often required to gain valuable data acquisition, to identify geo hazards, in a hole size where problems can be more readily dealt with. Often data is required first to make more valued, risked based and cost efficiency savings later. From the drilling engineers' point of view, a single integral component will however often reduce while drilling risks, associated times and costs as compared to two or three component requirements. All these factors when duly considered however confirm a place of the bicenter bit in the industry and potential for use in deep open water applications. Note: Bi-center bits it is felt are often the excitation force to often induce vibrations that are detrimental to drillstring and BHA components, It is also felt that they do not drill out shoes as efficiently as predicted. Finally recent calliper logs run in the GOM illustrated that large sections of the wellbore had not been enlarge opened up at all.

6 Reaming wing tools. Finally, a new technology is the reaming wing tools, that feature polycrystalline diamond compact bit with a ream while drilling capabilities. These tools differ from bi-center bits in that it they are essentially a two-piece design, allowing for greater flexibility in pilot bit selection. Directional control would be conducted through a steerable motor. Again with all factors duly considered reaming wing tools may have some potential for use in deep open water applications, notably in shallow high angle wellbore field developments. Table 3: Bicenter characteristics Hole enlargement in deep open water drilling Bicenter bits Disadvantages Eccentric cutting action leads to hole bellying, that may increase likeliness of wellbore instability, tool and equipment vibrational problems. Advantages Enlarge wellbore size below last set casing string Requires stabilisation of bottom hole assembly. Diameter of enlarged hole varies between pilot hole and desired gauge. Increased lateral vibration and torque with larger OD tools, especially in highly drillable formations. Aids to reduce vibration, delivering better hole stability quality, and gauging prior to running casing Relatively cheaper than under-reamers Integral component with less chance of breakage, failure, lost in hole consequence. Difficult to keep wells vertical, lending to difficulties running casings. Risk of mud packing and inability to clean cutters Hydraulics i.e. efficient nozzling and PDC cutter cleaning often a problem. If not afforded balling can occur Simultaneous drilling and reaming PDC cutters generally best suited to more consolidated formations Potential casing damage while tripping, no flexibility of activation Tendency for BHA to drop angle in directional section. Unsuited for soft formations.

7 Table 4: Reaming wing tools characteristics. Reaming wing tools Disadvantages No flexibility of operations. PDC cutters only. Hydraulics i.e. efficient cutter cleaning often a problem Requires stabilisation of bottom hole assembly. Hydraulics i.e. efficient nozzling and cutter cleaning often a problem Need to keep tools moving in deviated wells to prevent side-tracking the hole. Two trips often required to drill section Potential for casing damage while running in/pulling out Advantages Concentric cutting action means a smoother hole, decreased torque and lateral vibration. Robust strong and sturdy tool design. Low lost in hole consequence. Aids in reducing vibration, delivering better hole stability, quality, and gauging prior to running casing PDC cutters best suited to more consolidated formations Certainty of hole gauge. Can be a vital consideration for tripping, logging and casing running/cementing. Often advantageous for hole cleaning, getting data acquisition from pilot hole. Suited to most formations Long duration of run in more consolidated formations Simultaneous reaming while drilling.

8 Near bit reamer The NBR provides hole enlarging while drilling, can be placed just above the bit, or higher in the BHA, and was designed for optimum performance in directional applications where traditional hole opening techniques have proven difficult. They short in length to be run below directional drilling assemblies, no additional pressure drop is created, no vibration induced, safe casing shoes drilling operation NBR s are often run in tandem (one cutting one stabilising). Recent experiences also show that sliding in directional work is dramatically reduced when using NBR s. Unlike bi-centre bits and reamingwhile-drilling tools, the NBR is designed symmetrically with three cutter assemblies to provide balanced operation for smooth torque and minimal vibration. The tool can be used with any bit type, fixed cutter, or roller cone, to precisely match the formation and application for optimum drilling performance. The tool is hydraulically actuated, eliminating problems commonly associated with mechanically actuated tools. There is no mechanical locking mechanism, thus preventing the arms from inadvertently sticking in the open position. The tool is also equipped with shear pins that prevent the cutting structure from opening while inside casing. i.e. highly favourable for drilling out float equipment. Once out from under casing, the shear pins retaining the arms can be sheared, allowing for opening the hole while drilling.

9 Applications, benefits Traditional methods of hole enlargement underneath casing include underreamers, bi-centre bits and reaming-while-drilling tools as previously discussed within this section. While these tools have proved successful in many applications, use in harsh environments and difficult directional applications have often proven to be less reliable and optimal than desired results. However even in the harshest of North Sea environments, the NBR has been used to successfully open up pilot holes and while drilling directionally. With the NBR being one of the first reaming-while-drilling tools to be used in conjunction with a rotary steering system to drill oversized holes. Also the short, fully stabilized configuration of the NBR provided low impact forces while steering directional assemblies to result in minimal bottom hole assembly destructive vibrational energy, efficient bit/bha operations, longer tools runs and increased rates of penetration. Finally the tool in conjunction with a rotary system can thus significantly improve the economics of hole opening operations and must be viewed as a contender for deep open water and deepwater directional drilling requirements.

10 The hole enlarging future Three main objectives to best meet deepwater hole enlarging show promise for the future. 1.) Increased production, 2.) Cost cutting, 3.) Technical innovation in well design Moreover, strong market drivers in the short, medium, and long term will tend to encourage conditions apt for growth. Factors worthy of mention are. 1.) Reducing the environmental footprint left by drilling and production activities may soon become an operators strategy or regulatory requirement. 2.) Reduction in hole sizes and volumes go hand in hand with the costs associated with expensive mud s, casing strings, and cement. e.g. such an approach institutionalised by the UK finder well concept, seeking to reduce the average costs of drilling and exploration that will be imposed by demanding regulatory requirements. 3.) Technical innovation in slender wells will provides a strong mechanism for cost cutting. Allied to slender wells are two novel technologies. 4.) Expandable casing applications in the short to medium term 5.) Mono bore drilling and completion in the long term. Such methodology will undoubtedly push the limits of current deepwater under-reaming technology, inevitably resulting in new "stepping-stone" technologies e.g. retractable bits and under-reamers, vital to the long-term vision of a viable mono-diameter wellbore concept. Where the current operational limitations standing between today's capabilities and the ultimate goal of a mono-diameter well, will likely be resolved through the development of solid tubular expansion (STE) techniques. Until then, the adaptation of wellhead design, drill-bit development, and so on, have no commercial justification until expandable technology fully matures, whereas the marriage of STE and under-reaming technologies have intermediate applications throughout the well development cycle.