Wells can now be safely and effectively drilled or sidetracked, evaluated and completed in small diameters down to 4 1/8". The resulting slim wells can provide a total well cost reduction of 30%. At the same time, crew exposure, location size, cuttings and waste mud volumes are drastically reduced.
1. Introduction
A definition of a slim well is any engineered well with a hole size at total depth of 4 3/4" or less.
Many of the advances in technology now enable small diameter wells and sidetracks to be drilled safely, cheaply, effectively and with minimum borehole problems. Complementary technology enables these wells to be completed, produced, worked over and maintained with low costs for their total life cycle.
Advances in drilling technologies mean that new wells and sidetracks can now be safely drilled in small diameters. Small hole sizes should no longer be associated with low rate of penetration. Production rates of up to 50 MMscf/d (1.4 MMm3/d) gas or 4000 b/d (640 m3/d) oil are achievable in many cases through 3 1/2" liner in 4 1/8" hole.
A key element of slim wells is the monobore completion concept, which is a completion featuring full-bore access across the pay zone without diameter restrictions. The monobore concept has become more attractive with the emergence of coiled tubing and electric wireline as well maintenance tools in addition to the traditional slickline. For many formations the monobore offers greatly increased flexibility in managing the reservoir, especially during the later stages of its producing life. Coiled tubing intervention can often be substituted for a full workover. This has the added advantage that by avoiding well killing, impairment is reduced. In most cases the economically recoverable reserves attributable to that well should be increased. Equipment is now available for monobore completions in 3 1/2" final liner size to match the drilling capability of 4 1/8" and 4 3/4" holes.
2 Current technology
Two technologies that have developed for smaller diameter wells: downsized conventional drilling and continuous coring.
2.1 Downsized conventional drilling
Downsized conventional drilling is the use of a fit-for-purpose rig to drill conventionally a well with a reduced casing scheme ending in 4 3/4" or smaller open hole, possibly with a 3 1/2" production liner. The availability of purpose-built slimhole drilling rigs is currently. However, many of the advantages of downsized conventional drilling can be achieved using the retrofit downhole SHD system with a conventional rig. In this way a slim well can be drilled, evaluated and completed, with the associated benefits listed below but without those benefits resulting from a purpose-built rig.
Further reading: Downsized conventional drilling
2.2 Continuous coring method
The continuous coring method is the utilisation of modified small mining (or other) rigs capable of continuous coring using wireline retrievable core barrels, coupled with on-site evaluation of the cores (which is now available commercially).
Further reading: Continuous coring method
2.3 Advantages and disadvantages
Reduced well costs
Slim hole drilling with a small fit-for-purpose rig ensures lower costs of
. day rate
·wellhead
·casing
·mud products
·location preparation
·logistics
·manpower
·waste disposal.
HSE
Slim hole drilling also results in substantial HSE benefits:
·Smaller location and camp
·Smaller crew reduces exposure
·Less logistic support required
·40-70% reduction in cuttings and waste mud.
Technical advantages
·Smaller mud volumes allow more efficient mud cleaning or use of low solids (expensive) formate brines, resulting in a reduction in formation impairment.
·Technical possibilities increased by ability to drill, evaluate and complete through smaller casing strings.
Potential disadvantages
By drilling slim there are also potential disadvantages:
·High equivalent circulating densities (ECD) can limit mud weight
·Eliminate contingencies
·Limit petrophysical information obtained
·Limit completion options
·Limit production rates
·Limit potential for future sidetrack options
Early concerns about the robustness of well control procedures originally proposed for continuous coring have now been resolved in the light of field experience with improved computer-based kick detection system (KDS) and minor modifications to traditional well killing procedures.
2 Management
2.1 Challenges
The successful completion of a slim well project requires a co-ordinated effort from many different departments in the operator, drilling contractor, service contractors, and other organisations involved. The full benefit will not be obtained unless there is a strong commitment throughout the operation, based on a broad appreciation of the potential gains.
All contractors (drilling and service) have built up a substantial technical, operational, experience, and financial inventory over many years to drill, evaluate, complete, and service wells terminating in 8 1/2" or 6" hole.
Those companies whose revenues are related to consumables (e.g. casing, cement, tubing, trucking, waste disposal, pipe yard, shipping, mud) can expect a lower turnover to offset against their fixed and variable costs. Mechanical supply and rental companies (e.g. bits, wellheads, blowout preventers, completion equipment, downhole and surface tool rental, etc.) can expect the weight and hence cost of their equipment to decrease roughly with the square of hole diameter. Monobore completions and an increased use of liners will exacerbate these trends.
If the drilling process is to be efficient then land drilling contractors will have to:
1.either bid oversized rigs, in which case the full gains to the environment, infrastructure and cost of slim wells will not be realised,
2.or invest additional capital in building small rigs or upgrading workover rigs for drilling slim wells.
Provision of dedicated small rigs for slim wells, with the current financial outlook for drilling contractors, may require large financial commitment from the Company.
2.2 Opportunities
The potential applications of slim wells may also offer opportunities for management. For example, compared with conventional drilling techniques, slim production and exploration wells offer reduced costs and environmental impact, and will strengthen the Company's image as a high technology, environmentally responsible company.
If the geology is sufficiently well understood, the cheaper drilling method can be used to complement seismic exploration, by providing a cheap series of throwaway wells with wireline logging, occasional spot coring over the reservoir, and seismic check shots. The slower, continuous coring method can be used where there are substantial uncertainties over the geological model.
3 Environmental impact
Further reading: Environmental impact of Slim Hole Drilling
4 Safety
4.1 Well control
Established conventional well control standards can be applied with almost no modification to drilling using downsized conventional equipment, provided a sensitive kick detection system (KDS) and the hard shut-in procedure are used.
Early proposals for continuous coring included the concept of dynamically killing the well after a kick. This method has not yet found wide acceptance in the industry since it depends on the utilisation of poorly understood hydraulic phenomena, places higher demands on the driller, and has fewer redundancies than the industry standard methods, which are based on the shut-in-and-think concept. BP have developed and published a simple, practical modification to the industry standard well control method which may be suitable.
At this stage continuous coring is unsuitable for drilling overpressured formations. During well planning a decision tree should be made to show what is to be done when pore pressures much above hydrostatic are encountered, and when and how the well should be abandoned.
If a slim hole is drilled into a prolific formation and well control is lost, then a smaller hole will have a lower open hole production potential in the event of a blowout. This will reduce pollution and may make a surface kill and intervention by a snubbing unit easier. The downsized conventional approach incorporates full directional and homing-in capability and may be used for relief well drilling, but the maximum possible injection rate is less than that possible with conventional hole sizes.
4.2 Safety of personnel
Smaller wells mean smaller individual items of equipment and tubulars and a smaller total amount of material to be handled, which reduces the total exposure of personnel. Smaller rigs require smaller crews, and are easier to mechanise/automate than bigger rigs because the range of tubular size to be handled is smaller. They thus have the potential, in the longer term, of reducing total exposure even further, though consideration must also be given to the smaller dimensions of the workspace available.
Smaller rigs mean a reduction of the heaviest lift during rig move from about 30 tons to about 10 tons. The heaviest tubular to be handled can be reduced, for example from a 4-ton joint of 20" casing to a joint of 9 5/8" weighing 1 ton or less.
Where the smaller volume of materials enables a materials staging base to be eliminated, a step change in exposure can be realised. In the specific case of helicopter-supported operations, this lower total volume of materials can result in a significant reduction in total exposure as well as transport cost.
5 Evaluation of Slim wells
Conventional evaluation techniques such as mud logging, wireline logs, conventional coring, production testing, as well as on-site core evaluation, are now all available commercially for small boreholes.
Further reading: Evaluation of Slim Wells
5.1 Wireline and LWD logging
Wireline logging companies have HP/HT 2 3/4" diameter logging tools, and a range of 2 1/4" tools, including formation pressure sampling for normal pressures and temperatures. The major wireline service companies have a wide and sophisticated range of logging tools for 8 1/2"- 5 7/8" hole.
Horizontal holes of 3 7/8" and larger have been logged by wireline tools run on drillpipe (DP), tubing workstring, or coiled tubing.
All the interpretation charts and software are calibrated for conventional hole sizes. Smaller holes mean that all these charts, many of which are empirical, have to be extended, to ensure that they result in consistent correlation with existing larger sized wells.
5.2 Conventional coring
Conventional core barrels which are retrieved by tripping the whole drillstring are available in small sizes. One manufacturer has developed a non-rotating (inner barrel) low vibration 3 3/4" OD coring system for cutting a 2" core in 4 1/8" and larger hole. The design of these tools was carried out with the objective of cutting the core with the minimum of mechanical disturbance and fluid invasion. This is especially important in friable or fractured formations.
5.3 Continuous coring
Continuous coring is best utilised with on-site core evaluation, or inverse logging. Amoco have built and are operating a number of systems in house. BP and partners have developed a commercial system. It can handle 150 metres of core a day, giving gamma ray, density, porosity and permeability measurements, source rock evaluation, digitised video image, and geological description. Due to reduced rates of penetration (ROP) while coring, the less interesting parts of the geological column are often drilled destructively. Therefore wireline logs are usually run to get a continuous record, to ensure core-to-hole correlation by gamma ray, to estimate in situ saturations, and to tie the well to seismic data using the sonic log.
5.4 Production testing
Service companies have equipment for 3" open holes. A range of testing equipment for slightly larger hole sizes is available, including a fully enclosed burner which will allow location size to be reduced. For extended tests, a 3 1/2" monobore completion offers a cheap, reliable and flexible option. It also allows fine resolution of reservoir properties such as inflow capacity, skin, sand production, etc. Such vertical resolution will become increasingly important as field development by horizontal wells is becoming the norm.