Five different slim hole drilling approaches are discussed in this article. In continuous coring there was a strong drive to maximise the amount of geological information, while the shallow rotary and downsized conventional wells, with less geological involvement in the conceptual work, use conventional petrophysical evaluation. Sidetracks and coiled tubing drilling are mainly for production applications at present.
The five alternative approaches are as follows:
1 Continuous coring
The continuous coring technique depends on pipe which is almost externally flush, rotating at high speeds in a borehole only slightly larger than the pipe body. This allows the maximum ID for core recovery by wireline. Lateral vibrations are minimised by stabilisation of the pipe by the borehole, lubricated by a thin layer of drilling fluid. It also means that there is a very small annulus in which about 90% of the circulating pressure drop takes place, resulting in high ECDs. If a (gas) influx is taken a very small influx volume can dramatically reduce the hydrostatic pressure in the annulus, and this could in extreme cases, fracture the casing shoe. To avoid this it was originally proposed to kill the well dynamically.
The concept of dynamic killing was that by the time a kick was detected the gas level in the annulus would be too high to shut in without fracturing the casing shoe. The driller would therefore increase the pump speed from say 60 to 250 gal/min, and the additional pressure drop up the annulus would counteract the reduction in the hydrostatic head of the gas as it expanded on its way up the annulus. Drill string rotation also increases the pressure drop in narrow annuli and this effect could be utilised during killing operations. Once the gas influx is completely in the casing shoe the well may be killed conventionally.
This proposed technique gave rise to some operational concerns. The driller would be placed under high stress for about 2 hours while the kick was circulated out and the technique depends on running a computer program in real time to advise the driller as to circulating rates, with unpredictable consequences if the subsurface conditions were incorrectly assessed (e.g. if the hole were not in gauge, or if cross flow between two zones was taking place). The dynamic killing procedure depends on power being delivered to the top drive and pumps on an un-interruptable basis.
The total concept was operationally so different from the traditional "shut-in-and-think" well control procedures understood throughout the drilling community that, especially in an undeveloped area, communication problems were inevitable. For these reasons, and in the absence of any strongly expressed desire for continuous core from the exploration community, the concept of continuous coring is not be suitable for most applications.
Before embarking on a continuous coring programme it is essential to conduct a detailed technical and operational review of the drilling contractors and check with previous operators their actual performance in the field.
2 Shallow rotary
In the early 1980s the Swedish drilling contractor Microdrill developed a technology to drill shallow oil wells on the island of Gotland at a depth of 400 metres. The operator was Svenska Petroleum Exploration. They used a modified mineral exploration rig with a hook load of 11 tons. As these were shallow, normally pressured production wells the drilling was destructive, using standard mining drill rods, often in one-metre lengths. Most of the producing wells on Gotland were rod pumped using small hydraulically driven pump jacks through Microdrill's 54 mm OD ´ 47 mm ID (1.850" ID) tubing, which is only slightly smaller than the ID of 2 3/8" tubing.
Many of the tubular goods were more expensive than those of similar API size and specifications. BOPs were standard oilfield workover and wireline equipment with special rams to accommodate the mining rod sizes. Kick detection was by pit level indicator. The rig was a small hydraulic rig, with one engine and pump driving several motors, giving coupling between various loads (e.g. increasing pump rate, or starting the shale shaker, would slow down the power swivel).
Chevron report on a large number of slim well deepening operations using rotary drilling with 2 3/8" and 2 7/8" 8 round tubing work strings. These shallow wells were often highly corroded producers, initially drilled in the 1930s and 1940s, which were being converted into water injectors by deepening and installing a new conduit inside the old casing. Water, foam and mist were used as circulating fluids while drilling was often underbalanced. Pulling units equipped with Bowen power swivels were used as rigs.
3 Downsized conventional, or "retrofit"
Downsized conventional or retrofit concept was developed, considering the following requirements:
1.Minimal cultural change required
2.No reduction in safety standards for well control
3.Use of API tubular sizes, as these were cheaper and would interface with other API sizes, completion and test equipment, fishing tools, etc.
4.Use of conventional drilling techniques, equipment and practices for the upper part of the hole.
5.Phased testing and introduction of component parts of the system in mature areas to minimise development and testing costs and to build up confidence before going into remote areas.
6.Use of liners to minimise the amount of steel in the casing programme, take advantage of the higher burst pressure of the smaller upper casing strings, reduce wellhead stackup heights and costs, and minimise ECDs when drilling in the reservoir section.
7.Use of existing rigs. This was to avoid the upfront costs of having to mobilise/modify/build a special non-oilfield rig, which might have locked the project into a technological dead end
8.Use of downsized wireline logging tools.
9.Conventional production test/DST technology
10.Maximum depth was set at 3500 to 4000 metres to cover 95% of the existing oil reserves. Wells to be vertical.
11.Maximum temperature was set at 125°C for the electronics in the logging tools. Maximum mud weight was set at 1.6 SG.
12.To minimise interface problems when designing a well and to minimise the number of service companies on the rig site, as much as possible of the technology should come from one supplier, or group of suppliers.
13.Directional drilling was added to the requirements when the first field trial turned out to be a deviated well, with a TD at 5000 metres AHD
Initially work concentrated on the drilling and kick detection technology as this was where the greatest technical challenges were seen to lie. It was realised that the large DP/hole annulus which would be required for conventional "shut-in-and-think" kick control and reasonable ECDs would lead to vibration problems. Small bits were also seen as a technological gap. Therefore attention was concentrated on vibration reduction, strong small downhole tools and drillpipe using state-of-the-art mechanical engineering, small bits and kick detection equipment.
The "Retrofit" system evolved contains the following components, which can be assembled according to operational requirements:
1.PDC/TSD/impregnated diamond bits, often of anti-whirl design. 4 3/4", 4 1/8", 3 7/8" hard formation sealed-bearing roller cone rock bits are also available.
2.High power Moineau motors. As hole size declines the available mechanical power per unit of hole area increases. Motors are available for hole sizes 2 5/8" and larger. High temperature motors suitable for bottomhole temperatures of 180°C and above are available. In principle, motors of all sizes can be made steerable. 3 1/8" OD motors for drilling 3 7/8" horizontal holes using steering tools or 3 1/8" MWD for guidance are available.
3.Thruster to isolate the longitudinal and transverse vibrations in the drillstring from the BHA, to provide constant WOB and to minimise the drill collars required. Thrusters are available for hole sizes 2 5/8" and larger.
4.Soft torque rotary table or top drive to stop torsional vibrations being reflected back down the drillstring from surface.
5.For 4 1/8" and 4 3/4" hole a 2 7/8" DP string has been developed. This has conventional upsets for ease of handling and minimum ECD, but with advanced thread profile for maximum strength and fatigue resistance. Knotty pipe for running in compression instead of DC is also available. For 5 7/8" hole conventional 3 1/2" DP is used. For smaller hole Hydril or other tubing has been successfully used as DP.
6.Hydraulic drilling jars for 4 1/8" and larger holes. These can be dressed for 200°C and also used as fishing jars.
7.Slim line overshot fishing tools, initially a 4 1/16" OD overshot for picking up 3 3/4" BHA but the concept can be extended to other sizes.
8.Sensitive kick detection system (KDS) and hard shut-in procedure enabling the system to be used in conventional and HP/HT wells. This KDS has now been extended to operate on floating rigs.
9.3 1/2" diameter liner system, which can be reamed down to bottom and is designed for optimal cementing in 4 1/8" and 4 3/4" holes. This can be tied back to make a 3 1/2" or larger tubing string, thus making a monobore completion with a minimum ID of 2.81".
10.Directional and gamma ray MWD for 4 1/8", 4 3/4" and 5 7/8" holes at bottomhole temperatures of up to 200°C.
11.Extended scale surface instrumentation ("drillers box").
12.Anti-vibration, anti-rotation conventional coring system for improved primary core recovery in 4 1/8", 4 3/4" and 5 7/8" holes.
13.For critical HT/HP wells a range of high density brine-based drilling fluids with optimal viscosity and fluid loss properties at normal and elevated temperatures.
14.Awareness of other available equipment and services to cover drilling, evaluation and completion in slim holes (e.g. wellheads, BOPs, wireline logging tools, 3 1/2" completion equipment, etc.).
Further details of the various subsystems and the detailed rationale behind the design of them can be found in the following chapters.
4 Small horizontal sidetracks
Small horizontal sidetracks are very popular in the shallower oil fields of North America. Often drilling is underbalanced.
Numerically, re-entering old wells to drill small horizontal sidetracks is the biggest growth area in slim hole drilling, as it offers short-term additional production with rapid payout. Note that the thruster damps out stick-slip friction when operating a steerable mud motor in sliding mode. In a 6" medium radius horizontal hole it is possible to steer with a steerable motor in sliding mode at 1200 metres reach while without a thruster it is only possible to steer at 300 metres displacement. The ability to steer at greater reach has proved to be an application of the thruster giving significant production benefit to the operator.
Articulated 3 1/8" motors capable of drilling a 4 1/8" hole round a 12 metre radius are available. For conventional tubulars, an 8 3/8" hole has recently been drilled round a 40 metre radius which was subsequently lined with a modified 7" casing. This indicates that similar or greater build rates can be achieved with smaller holes.
Most of these sidetracks are left open hole or have a slotted liner installed.
Underbalanced drilling is becoming increasingly common in horizontal wells.
The nitrogen is injected either in the DP, or in the annulus via a "bubble tube", which is often a length of coiled tubing run strapped to the outside of the casing and cemented in place. The latter system has the advantage that the DP is always full of fluid, thereby permitting conventional mud pulse MWD to be used. A pressure relief valve has to be installed above the motor to stop the mud draining out during connections.
Differential flow kick detection systems and rotating BOP are often used.
The nitrogen consumption represents a considerable percentage of the well cost and several developments are in hand to develop an inert gas generator. This could substantially reduce the logistical difficulties and additional costs of drilling underbalanced. An alternative would be to run an ESP or Moineau pump driven by rod or electric motor in the well to provide drawdown.
5 Coiled tubing drilling
Coiled tubing drilling can be considered as an outstep from points 3 and 4 above.
This is a rapidly developing field, with substantial cross-fertilisation from other forms of slim hole drilling.