Most slim wells are drilled and completed with a down-sized conventional approach using completely conventional drilling rigs, or modified workover hoists, as this is the cheapest solution. The conventional rigs may be one or two sizes smaller than if the same well were full sized, and some of them will have a "Retrofit" package of (rented) equipment installed. This applies to land, jackup, swamp, and floating rigs. In a few cases, the lower hook loads will enable wells to be drilled with a mobile (or platform) crane.

Small, specially constructed or modified rigs will be used in a few specialised land, Arctic and swamp drilling campaigns. These rigs, although physically smaller than conventional rigs, will be more expensive as the day rate will have to cover the investment cost of the new equipment. Older rigs are offered at rates which do not include repayment of capital. The drilling performance of these new rigs will not be very much better than conventional rigs drilling the same sized hole as drilling performance is related mainly to the subsurface technology, which largely "belongs" to the service companies and operators. Therefore, the use of these rigs will be limited to operations where factors such as the environmental impact of the well site, or logistics, or the high costs of personnel, dominate the operation and can be used to justify the premium on the day rates.

The hull size of floating and jackup rigs is dominated by environmental conditions. The well size is only a secondary influence. However, by offering slim wells the contractor might extend the capability of older smaller units, and reduce the logistical complications of working offshore. Thus a smaller, older, cheaper drill ship could have its water depth extended by going slim. Drilling slim wells in an area may reduce the tonnage of materials shipped by supply boat by 60-70%. For a semi-submersible the logistics could be reduced to those of a drill ship where it is possible for the rig to return to sheltered waters or port between wells for restocking. In a remote area this could avoid setting up a local supply base.

1. Introduction

A typical objective is to drill, evaluate and complete fit-for-purpose wells at the lowest overall cost while meeting locally applicable standards in areas such as:

·safe operation,

·well quality (produceability, impairment, information, etc.),

·environmental impact.

The precise contractor rig chosen for any slim well will be influenced by such factors as:

·Well objectives, and resulting alternative well designs.

·Local rig and service company availability, technical/managerial capability.

·Operator contracting strategy.

·Number of wells to be drilled, evaluated, and completed during campaign.

·Geological, technical, and economic risks perceived by operator and contractor managements.

·Local infrastructure and regulatory and business environment.

·Expected weather and other environmental conditions during the drilling campaign.

·Logistics and transportation.

·In-house knowledge, technical skills and other resources allocated to the project by the operator and available outside from consultants and contractors..

For continuous coring rigs, a general set of recommendations have been drawn up. These recommendations have been split into those which are essential for the well to be drilled safely and those which are desirable. It is highly likely that many rigs considered for slim wells will not meet some of the recommended specifications. It will then be up to management to decide whether the specific wells should be drilled on the basis of a risk assessment. Clearly the cost/time/effort of modifying a given rig will be a factor in these decisions.

There is no list of recommendations for rigs for destructive drilling because the contractors active in this far larger market are expected to have a background in hydrocarbon drilling, and to be more used to incorporating the flexibility to cope with well control incidents in their rig design. However, many of the recommendations made in the section on rigs for continuous coring may prove relevant.

2 Land rigs for continuous coring

2.1 Currently available rigs for continuous coring

As indicated in the management overview, it is unlikely that this technique, which is still in an immature state of development, will be much used by Group Opcos. Any application in the near future will be a "one off" with numerous non-technical factors influencing the choice of rig.

The following companies have continuous coring rigs which have been used to drill for hydrocarbons: Nabors, Longyear, Parker, Forasol, Kenting, Rockwell (Australia) and Slimdrill (Australia).

2.2 Requirements for rigs drilling with continuous coring geometry

Most contractors offering continuous coring services have extensive experience of the technique. Much of this experience has been gained in mineral exploration drilling and often in non-permeable rocks and at relatively shallow depths. Likewise many of the supervisory personnel have come from that environment. When evaluating rigs and personnel, this factor and the need for well control training should be considered.

The presumption is that the technique will be used for remote area exploration drilling, requiring high core recovery, in a low pressure geological environment and that the approach should be fit-for-purpose. It is also assumed that the less interesting parts of the geological sequence will be drilled destructively and only the objective will be cored. The specification is deliberately written not to exclude modified mining rigs.

2.3 Essential pressure control elements of the specification.

Appropriate BOP stack, wellhead, choke manifold, mud gas separator, flare line, etc. rated to maximum expected surface pressure. It is unlikely that any Group company will use this technique in areas requiring more than 5000 psi (34,500 kPa) MSWP pressure control equipment in the near future.

Kick detection system capable of providing kick and loss detection during: drilling, reaming, making connections, tripping, running casing, wireline logging in open hole, core recovery on wireline, and any other rig operations where a kick can occur.

The system should be:

·Either unmanned, and capable of providing automatic alarms to the driller at a volume of influx or loss not exceeding 50 metres of the smallest annulus, with a false alarm frequency of not more than one per day. In this case it must be adequately capable of recognising different rig operations automatically, and compensating for dynamic and compression effects from the starting and stopping of the mud stream.

·Or fully manned around the clock, with alarms which are also repeated at the drillfloor.

Capable of working with the expected muds in use. Provision for handling clay balls by bypassing the flow out meter.

Shut-in procedure and equipment capable of closing in the annular preventer within one minute of the point of minimum flow after the string has stopped rotating and the pumps have been shutoff. Precise procedure for avoiding shutting in on own pump pressure i.e. avoid ballooning. Hard shut in to be standard. Provision of hydraulic pressure to shut in well not to be dependent on a prime mover running, i.e. an accumulator system is required.

The procedures and equipment should be prepared in advance to cope with the following:

·Establishing a connection with the same working pressure as the BOP stack (or maximum expected shut-in pressure, MESIP) onto the drill rod after a kick and the well is shut in during any operation. Several mining rigs have kelly, hoses, and pumps with pressure ratings below 100 bar.

·Shutting in during tripping, wireline logging, running casing, reaming, and other well operations.

·Stripping in. Calculations as to the weight/length of drill rod which can be stripped in under its own weight against a given pressure.

·Detecting kicks while recovering core barrel with slick line, for shutting in well and for conducting a circulation, or volumetric kill as appropriate. A tension meter on the slick line to detect swabbing in the well is recommended. Sometimes, in prospective hydrocarbon intervals, recovering of the inner barrel is done through an oil saver with circulation down the drill rod and up the annulus to avoid swabbing. In other cases the DP is open ended in the rotary table.

·Installing lubricator to recover inner barrel when the well is under pressure. Consideration how to cope with a bird's nest in the oil saver or in the lubricator and/or across the lubricator lower valve/BOP.

·Pressure testing all equipment which can be exposed to well pressure and spare parts/consumables for equipment and test tools

·Killing well by circulation, volumetric kill, and bull heading. Consideration to be given to detecting and coping with drill rod leaks and partings during the kill. Also the possibility of an annulus packoff creating a pumpout force on the string should be considered.

Inclination only survey tool (Totco) for all drill rod sizes. Coil for magnetising the core barrel and bottom joint of each casing string as it is being picked up to be run. This is to be able to increase the chance of homing in on the blowing well if no directional surveys are run.

If appropriate, precautions for H2S gas and for H2S and CO2 corrosion.

2.4 Essential operational elements of the specification

·Compliance with appropriate well safety, personnel safety, and safe area standards. Adequate lighting and safe access and escape routes.

·Hook load of maximum string weight, including casing, in air plus margin.

·Drill rod and BHA for each hole size in good condition, free of internal rust, connections pressure tested to MESIP or planned circulating pressure, which ever is the greater. Consider drifting to OD of inner barrel.

·Procedure and equipment for handling pipe into and out of derrick and running same into hole with safe lifting and handling equipment.

·Equipment and procedures for back reaming the single, making a connection, and reaming down to bottom reasonably quickly.

·Ability to mix mud and cement, and pump mixture downhole. Ability to weigh up the active mud volume in a reasonable time.

·Ability to run casing and tubing with appropriate control on makeup torque.

·Ability to pump mud during a kill with an acceptable level of reliability and safety. A pump stroke counter and good pressure gauges on stand pipe and choke manifold are required.

2.5 Desirable operational features

Reliable main power system with minimum number of common node failure points.

If the system is hydraulic, special attention should be paid to oil cooling in the tropics (and to oil viscosity in Arctic regions), keeping the system uncontaminated during moves, ensuring that adequate filters are fitted, etc. The system should be designed for the minimum number of hydraulic connections to be broken during a move. Larger hydraulic systems (above 200-400 HP) used for continuous service have sometimes given less reliable service than electrical systems of a similar size, especially in high vibration environments and when connections are repeatedly broken during rig moves.

If the system is electric, area classification is usually the main problem.

Reliable mud pumps capable of delivering 180 ft/min annular velocity with 2 HHP/square inch for a full hole bit for all hole sizes below surface pipe. More is better.

Solids control equipment capable of treating the total flow below surface casing with:

·250 mesh shaker screen.

·Decanting centrifuge at 3000 g, if costs of dumping are high (Alfa Laval 518, or equivalent).

·Sand trap and desanders/desilters are effective in surface hole, but the economics of desanders/desilters, unless already on the rig may be marginal if a centrifuge is fitted.

Consider handling of clay balls.

High shear mixing system for polymers capable of shearing mud and cement mix water at high shear rates. Lower shear mixing system for other chemicals, salts, weighting materials, etc.

If formate or other high density brines are to be used, rain covers for tanks, or preferably closed tanks, with gas vents are recommended. Such brines are very hydrophilic and if shipped to the rig as salt crystals in sacks it will be rock solid on any contact with air.

Sufficient mud tankage to avoid dumping mud after the biggest cement job expected. Consider using rubber pillow tanks, or lined earth pits, as reserve tanks.

Fishing tools for all tubulars. Consider fishing jars.

Directional surveying equipment and motors for sidetracking may be limited.

Smooth feed off system for string while drilling. Sensitive hook load sensor below block in addition to one on dead line may be desirable on some rigs. Sensitive pressure gauge for use while drilling.

(Hydraulic) Power swivel with soft torque MkII. Screw-on rather than chuck type is strongly preferred. Continuous rotation at slow speed without over heating should be possible.

Ability to batch mix all critical cement jobs. Ability to pump and displace cement in a controlled manner, preferably using the kick detection system to monitor the cement displacement for losses in real time. Ability to rotate the casing slowly while cementing.

Capability to round trip from TD at 3 hours per 1000 feet, hole conditions permitting. This is one third the rate of a conventional triples rig. To minimise the chance of getting stuck while the pipe is stationary without circulation (e.g. while handling core on surface) consider being able to rotate the drill rod in rotary table.

Ability to round trip the inner core barrel at 3000 feet per hour with slick line.

Core handling system in derrick to avoid subjecting the core to bending and to speed up handling the core, similar to that fitted to the Nabors 180.

On-site core analysis unit (e.g. BHI Core Byte, or Amoco GEM).

Tubing tong with gauge and rpm and torque limiting system (flow rate/pressure regulator) for making up drill rods, tubing, casing, etc.

Production test (DST) string with appropriate connections (Gas/Oil, HP/LP).

3 Rigs for destructive drilling

3.1 Background

To date practically all destructively drilled slim wells have been drilled with totally conventional rigs (and in some cases workover hoists), operated by established oilfield drilling contractors. This may be the trend in the future. Hole sections of 4 1/8" and smaller have been successfully drilled from land rigs, jackups, drill ships and semi-submersibles. The choice of rigs which can destructively drill slim holes is therefore considerably greater, although cost savings were often foregone by using an oversized rig.

Some of these future wells will be for exploration, some for production, and some will be (horizontal) sidetracks from existing wells. A few may be drilled for reservoir pressure and temperature monitoring. Some wells will be shallow, some deep, with obvious implications for the required hook load, mast size, mud/cement volumes, pump size, etc. The number of wells in the campaign, relative cost of labour, logistics, and expected environmental conditions will also impact the rig design options. Many of the requirements identified in the section on continuous coring rigs will be relevant but will not be reiterated as the more established oilfield drilling contractors will presumably be more aware of the potential complications inherent in drilling for hydrocarbons.

3.2 The land drilling contractor's dilemma

If the drilling, evaluation and completing of slim wells is to be efficient then land drilling contractors will have to:

1.either bid oversized rigs, in which case the full environmental, infrastructure, and cost benefits of slim wells will not be realised,

2.or invest additional capital in building small rigs or upgrading workover rigs for an efficient drilling service.

Upgrading would include 5,000 psi (34,500 kPa) pumps and circulating systems, soft torque hydraulic power swivels/small top drives or other string rotating device, improved drawworks feed off, instrumentation and solids removal equipment, KDS, etc. Much of this upgrading may be part of the integrated service companies "Retrofit" package. Many established land drilling contractors do not have a strong technological (as opposed to practical/operational) base in subsurface drilling, evaluation, and completion technology and frequently lack the overview of the objectives and technological interfaces which is taken for granted within the operator's organisation.

In the short term, drilling contractors will try to keep their bigger rigs working as they have more flexibility to bid for subsequent jobs and require less maintenance. Provision of dedicated small rigs for slim wells, with the current financial outlook for drilling contractors, and in the absence of long-term contracts, requires a common long-term vision. The only purpose-built slim rigs to date have been the "Forasol Euroslim Rig", the Microdrill rigs, and two Nabors continuous coring rigs. Most were built with an assured work load for at least two years, and/or were (partly) financed by an oil company or government organisation.

3.3 Can the offshore contractors 'help'?

The conventional wisdom is that subsurface technology is developed on land before being taken offshore. However, the absolute potential cost gains offshore are greater than on land, and offshore drilling contractors often have a broader and stronger technological base. On occasion, this can result in a more technically sophisticated management team. Additionally, the absolute costs of logistical support, where many of the savings in slim holes arise, are much higher.

Many offshore mobile (jackup/semi-submersibles) rigs have substantial hull life remaining, but their drilling equipment may be nearing the end of its economic life. Astute adoption of the slim well concept may enable one or two of the more competent offshore contractors to reduce the costs of refitting a hull to meet current safety standards and to gain leadership in a market segment. Candidate segments include remote area exploration, HP/HT and ultra deep water exploration drilling. The gains versus new building or retrofitting a rig to drill with conventional hole sizes could be substantial. In the medium term, development of a slim subsea production concept may reduce the costs sufficiently to expand that market. The current uncertain outlook for the oil price means that few offshore prospects are financially attractive and imaginative use of slim wells may assist in making some projects viable. However, the small production conduits associated with slim wells may limit applicability.

Consistent with the above arguments, the drilling contractors who are the first to build small land rigs are those who operate both offshore and onshore such as Forasol and Nabors. Sedco-Forex is also studying rigs for slim wells as part of their "SIMPLER" project. Such contractors can rationalise the decision to go slim on land as an investment in experience for future offshore drilling units. More wells are being drilled under integrated contracts with a fixed price mobilisation of the rig and well consumables. Such contracts tend to be awarded to the more flexible, sophisticated contractors, who are often those who have an offshore presence. Under such contracts, drilling contractors can see more clearly the substantial savings in transport and logistics which slimmer wells can bring. In environmentally sensitive areas, the drilling contractor's field personnel are closely involved in the practical aspects of waste and spillage minimisation, (as well as sharing some of the potential liabilities) and so they can also see the benefits of down-sizing in this area.

3.4 Rigs from other industries

A further potential category of slim well drilling contractor are those that come from outside the established hydrocarbon drilling industry. Microdrill, who originated in the Swedish mining industry, is such an example. Such contractors have the advantage that they do not have an excess of oversized equipment which they are tempted to use for slim wells. They are also more used to an integrated, low cost approach to well construction. One of the growth areas in civil engineering is tunnelling and other underground work. A great number of small "rigs" of various kinds are working in this area and there is a significant amount of equipment, expertise and personnel which could be applicable to small exploration and production/sidetrack drilling.

3.5 The "Retrofit" concept

For down-sized conventional wells one approach is to use a standard rig or workover hoist, usually chosen largely on availability and minimum mobilisation and day rate cost, with a "Retrofit" package from a major service company or consortium of such companies. The equipment itemised below would be complemented by a small number of multi-skilled personnel who would do the work traditionally done by several service companies. Such a package would be capable of working on any rig, fixed or floating, and in the vast majority of cases represents the most efficient use of the service industries capital in delivering down-sized cheap wells to the oil company. Such packages will be much cheaper to mobilise than a special rig, and, once the concept has been demonstrated to be profitable to both service industry and oil company, would permit the fastest implementation of slim wells in appropriate applications.

The precise contents of the package will vary with well type, well objectives, rig type, length of campaign, etc., but in general would consist of the following major categories.

·Downhole drilling equipment (for smaller hole sizes only): Bits, BHA components, small drillstring tail, MWD and survey equipment, fishing tools, muds, LCM, etc.

·Downhole completion system: Liner hangers, tie back equipment, tubing accessories, tubing, casing/liner, reaming shoes, centralisers, running tools, plugs, cement, spacers.

·Evaluation services Wireline logging (including perforating), LWD, production testing, mud logging.

·Surface anti-vibration equipment: Soft torque rotary table/top drive/power swivel/false kelly drive. Fine feed off on drawworks. Mud pump pulsation damping (future).

·Surface instrumentation: KDS, extended scale drilling instrumentation.

·Fluids management: HP/HT testing equipment for mud and cement; waste stream measurement/reporting/treating; mud and cement mixing and dosing (especially of polymers): solids removal equipment monitoring and optimisation.

·Computer software: Planning: Hydraulics/mechanics while drilling, cementing, testing, etc.: Fluids management: Logistics: Evaluation data handling and reporting: Operational and financial control, etc.

Such a package would be transported in standard ISO 20 foot containers, with 40 foot containers for the tubulars.

3.6 Slim land rigs in mature areas

In a mature area, the main activity is probably going to be drilling production/appraisal wells and sidetracks from existing wells. Slim rigs will evolve from existing small rigs and workover hoists, which will sometimes be using parts of the "Retrofit" systems from major service companies to gain experience. As the specific application and technology for that area evolves, a consensus will emerge as to the optimal features for a slim rig and these will be incorporated into rig upgrading and new building if the market warrants the investment. For example, many medium- and small rigs in Canada are being retrofitted with small top drives to facilitate the drilling of horizontal wells and sidetracks, many of which are "slim".

The design and specification of such rigs will reflect the factors which affect operations in the proposed area. Thus, minimising environmental impact (noise, cuttings disposal, foot print, height, moving, etc.), minimising manning by mechanisation, automation and multi-functional equipment and crews, and a drilling and completion system tailored for a specific depth and geology, may determine the design process.

Rigs designed for infill drilling of a large number of production monobore wells may be specified as one half of a "team", the completion "member" being a coiled tubing unit. The rig would leave the well with the liner cemented but unperforated, the tubing run, and the Xmas Tree installed. The CT would then rig up, run its tubing to bottom, clean the well and coil, perforate under drawdown, and complete the well. The well would subsequently rarely be killed, intervention being by electric line or coiled tubing. Variations on this concept exist, all shifting more of the well construction process to the coiled tubing unit.

Many of the rigs will be equipped to drill underbalanced down through the productive interval, where this is permitted by regulation and borehole stability.

3.7 Rigs for operating in remote areas

One of the areas for which the potential of slim wells has been given a high profile is for remote area appraisal and exploration. Here the ability to bring in an integrated package including a slim hole rig, or small rig/hoist upgraded by use of a "Retrofit" package, can contribute to reducing well costs, while remaining cost competitive with equipment and procedures available locally. Alternatively the retrofit package can be used with the locally available rig. The optimum solution will vary according to circumstance and assessment criteria.

In areas where there is no local oilfield infrastructure, the logistics (largely a matter of tonnage) are a major part of the total campaign cost. Here also there are significant gains for a "slim" rig, whether new or "Retrofitted".

By their nature these ventures consist of a series of one or two wells, with fundamental inputs in to the rig selection, such as well depth, being determined only shortly before scheduled spudding date. There is naturally a considerable reluctance on contractor and operator side to commit to technology which is not "fully field proven".

A pragmatic approach would be to use the concept of a "Retrofit" package. This approach minimises the capital requirements of the contractors less and also the requirements for commitments from the operators. As such a package, with minor adaptation, can be used on practically any rig. It also offers a single concept to be used in both exploration and appraisal wells, and a valuable learning experience which can be incorporated into a new rig specification and design if a suitable long-term exploration and/or production programme should emerge.