Schlumberger has a set of 2 3/4" diameter tools offering all services for 4 1/8" holes. These tools have ratings of 175°C (350°F) and 137 MPa (20 kpsi) or better. The efforts to reduce micro-scanners and formation testers to less than 3 1/2" have met with some success. For holes down to 4 1/8" a slim hole 3 3/8" formation tester is available with sampling chambers of 2 3/8gallons and 450 cc. Schlumberger also offer their tools in 2 1/2" size to compete in the 3 7/8" hole market.

Halliburton Logging Services has a 2 3/4" set of tools available for hostile environments up to 260°C (500°F) and 172 MPa (25 kpsi).

A major slim hole objective is the 3 1/2" monobore completion. In a contingency hole with 2 5/8" diameter, only 1 11/16" tools can safely be run, severely restricting formation evaluation.

The application of slim hole technology is not restricted to exploration wells. The major market is in production wells and sidetracks. These wells are often highly deviated or horizontal, which introduces the problem of transporting light slim tools over horizontal intervals. Coiled tubing with wirelines permanently installed are currently the best choice despite the additional costs, which are however, minor when coiled tubing is used for drilling as well. Many of these holes will be drilled in 3 7/8" hole size out of 4 1/2" liners/casing. Many of them will be low cost sidetracks where a cost effective basic logging suit, often from relatively unknown logging companies, is the optimum economic solution. However, some of these companies may not be able to offer full combination capacity to minimise the number of runs required.

Logging while drilling (LWD) subs that can measure porosity and resistivity in addition to directional data are available for holes slimmer than 5 7/8".

Experience has demonstrated the risks of running 3 3/8" and 3 5/8" tools in deviated and vertical 4 1/8" holes, especially when solids laden muds are used at higher temperatures. The pad tools are the most prone to sticking. A 3/4" minimum clearance is accepted by contractors. In reality the downhole situation is too complex for clearances to be resolved by rules of thumb. The wellbore path and the combination of cable tension, wall condition, and dogleg severity will not only influence tool clearance but also the required flexibility of the tool string. Slim clearance overshots, continuous reading downhole tension metres and KDS systems which function while logging (with a winchman's display) are highly recommended.

Planning of slim hole logging jobs should take into account mobilisation time and charges. It is therefore important that petrophysicists participate in the planning of slim wells. The main restrictions are incompatible telemetry systems, micro-resistivity tools, dipmeter tools and fluid and formation sampling tools with high pressure and temperature ratings.

1 tools available for slim wells

Tools that are now not readily available for 4 1/8" holes: dipmeters, micro-resistivity scanners, and rock and fluid samplers. This is not surprising as these tools rely on moving mechanical parts in contact with the borehole wall, which are more difficult to miniaturise than electronics and transducers. An alternative for dipmeters and micro-resistivity scanning tools with pads is the use of acoustic scanning tools. Sidetracks from existing wells often have a high curvature, and the minimum radius determines the maximum acceptable rigid length.

It is emphasised that most slim hole services are not yet provided routinely, that most tools have to be mobilised, and that planning periods are accordingly much longer than for conventional jobs. The limited availability of fishing tools and smaller tensional strength of the tools should also be considered. Several operating companies have found that running 3 5/8" tools in 4 1/2" hole is often possible but sticking a tool and costly fishing jobs do no justify the additional risks. This has to be balanced against the substantial savings in overall well costs. A reliable set of 2 1/4" to 2 3/4" tools is required to routinely plan, drill and log 4 1/8" holes with 3 1/2" monobore completions. To log 2 5/8" contingency holes a combination of 1 11/16" tools i.e. slim borehole televiewer for dip, sonic tool for porosity, a slim C/O tool for shallow saturations and a laterolog could be used. Note that no tools with a radioactive source should enter into the slimmest holes.

Formate muds are practically free of solids. Their resistivity behaviour appears to be comparable to sodium chloride solutions of 100 g/l concentration. A tool string incorporating shallow and deeper reading resistivity tools is useful in quantifying depth of invasion and determining effective mud filtrate properties for the interpretation of logs from shallow-reading nuclear and sonic tools. Only for the high-density caesium formate mud, the density log may lose much of its sensitivity when mud filtrate invasion is deep, but neutron and sonic log readings will still be adequate.

2 Highly deviated and horizontal slim wells and sidetracks/MWD

As mentioned above the trend to drill more slim holes will not be restricted to exploration wells. Small accumulations near existing facilities can only be developed economically when slim production wells or slim sidetracks from existing wells are applied. Tubing conveyed logging in small holes depend on the availability of a small wet-connector

In or near producing fields with small targets, "geosteering" of wells is required. Vertical seismic profiling is more frequently used than before to ensure that wells are in the right interval. Three component seismic acquisition tools are especially important in naturally fractured reservoirs. Most companies have only single component tools available for these smallest holes.

3 Conclusions and recommendations

·For 4 1/8" open hole (3 1/2" monobore completions) with high pressure and temperatures, all regular services are available.

·For contingency holes smaller than 3" and moderate temperature and pressure conditions the use of 1 11/16" sonic scanner, sonic travel time and carbon oxygen tools is an alternative for dipmeter and radioactive porosity logging.

·A slim tension-compression sensor with continuous surface readout should be included in all tool strings to monitor logging operations for potential sticking, hang-up and swabbing on wireline.

·For horizontal wells with high build angles, radial clearance and length between knuckle joints need to be considered. A one-inch difference between tool and open hole diameter is sometimes recommended. For wells without high build angles smaller clearances can be used, especially those wells drilled with anti-vibration drilling systems.

·Fishing of slim tools is non-routine and requires dedicated equipment which should be specified and available before the tools are allowed in the hole.

·MWD equipment for porosity and resistivity measurements is available for most hole sizes.

·In formate muds most logging tools are expected to give adequate responses, with the exception of density tools in very high density muds.