Since the mid eighties there has been an interest in drilling wells with smaller hole sizes, typically 4-1/8", 4-3/4" or 5-7/8" hole sizes, with the smaller holes taking a 3-1/2" liner.
1. Slim hole surveying
There are a number of gyro survey tools and steering tools that have an OD of 1.75" and are used inside conventional drillstrings. These are directly applicable in slim hole drilling where the bore of the drillstring or casing inner diameter is sufficient. Scientific Drilling International (SDI) have the 'Super EYE' steering tool and magnetic multi-shot system that can be run in a 1.375" protective housing. The SDI Finder GWD is currently the only gyro that can be used as a wireline gyro steering tool and in its ruggedised form it can remain downhole while drilling. The smallest pressure barrel has an OD of 1.75".
When running wireline tools through diameters that are close to the tool diameter, where there is a small annular clearance, the wireline speed is important to get the tool to bottom. At low speeds in a deviated well, the tool drags along the low-side of the hole and the friction is quite high. As the speed increases, the fluid flow past the tool acts like a hydrodynamic bearing and the tool floats clear of the hole wall and the friction is reduced to a minimum. As the tool speed further increases, the fluid pressure drop along the tool length increases and becomes predominant. At this point the drag increases very rapidly with speed. In any one of the three regimes, the tool will stop running in the hole when the drag is greater than the downward force. When running in through small diameter hole, it may be necessary to try different wireline speeds to minimise drag and ensure that the tool gets to bottom. On the outrun, the drag acts to increase the wireline tension. Often the speed of the outrun will be limited by the maximum wireline tension, however, when there is open hole, care must be taken not to swab the well in.
Available 4-3/4" MWD systems for drilling a 5-7/8" hole are:
- Anadrill Schlumberger (Slim1- SEA-AD) (never tried)
- Halliburton Energy Systems (Datadrill) (never tried)
- Halliburton Energy Systems (BGD)
- Geolink (Orienteer)
- Baker Hughes Inteq (Navitrak)
2. Coiled tubing surveying
An extension of slim hole drilling has been into drilling with coiled tubing, both for deepening and sidetracking existing wells, as well as drilling new wells. Due to the small hole diameters (typically 3-3/4" or 3-7/8") new survey tools have been developed to monitor the directional drilling process.
Prior to shipping the coiled tubing unit to the well site, either a standard mono or multi-conductor cable is threaded through the bore of the coiled tubing.
When a survey is to be run on coiled tubing, the usual coiled tubing prediction programs need to be run to evaluate running conditions and possible pump rates for circulating fluid if necessary. This pre-job engineering which gives the tubing forces in both tension and compression, are compared with the actual results while running. Any deviation from the predictions are identified, monitored and checked to ensure that the safety limits of the tubing are not compromised.
There are many different sizes, but the four most common sizes of coiled tubing are, 1", 1-1/4", 1-1/2" and 1-3/4" and the capacity of a standard reel varies from 13,000 ft to 17,000 ft, depending on the coiled tubing diameter. The 1" coiled tubing has a maximum injection rate of 140 ft/min. For the larger size coiled tubing, this can increase to about 270 ft/min, dependent on the type of injector head. The coiled tubing injection rate determines the speed and duration of a survey, and this needs to be considered in the pre-job engineering.
The use of coiled tubing for directional drilling requires the toolface orientation to be controlled and measured. As coiled tubing cannot be rotated, a steerable drilling assembly is used with an 'orienting tool' on a non-rotatable coiled tubing drillstring. The orienting tool can orient the steerable bottom hole assembly relative to the coiled tubing on command. There are currently two options, either MWD or wireline steering tools, to measure the motor orientation (toolface).
3. MWD
There are a small number of companies that can offer small diameter MWD tools for use with coiled tubing, however this service is limited. The MWD tools are usually in 3-1/2" diameter, 15 ft long NMDCs.
The advantage of MWD over steering tools is that there is no wireline, however the data rate of the MWD is sometimes too slow for successful orientation. The MWD is also more sensitive to problems with debris and LCM.
Steering tools
The advantage of the steering tool over the MWD system is that it is shorter in length and gives a higher data rate that may be necessary for monitoring the toolface orientations. The disadvantage is the wireline and the connections required.
Running procedure
1.If not pre-installed, fit the wireline inside the coiled tubing.
2.Connect the wireline connector to the drilling assembly connector and make up to the coiled tubing.
3.Make up the BHA. Mount the steering tool in the NMDC and orient it relative to the mud motor.
4.Make up the wireline jumper between the steering tool and the drilling assembly connector. The wireline connection between the drilling assembly and the coiled tubing is made when the two housings of the drilling assembly connector are made up.
5.Make up the steering tool surface unit to the coiled tubing drum slip rings. Install the 'rig floor' read-out.
6.Test the wireline and steering tool are functional. Check the toolface orientation and steering tool readings.
Once rigged up, the taking of surveys and toolface orientations are similar to normal wireline operations.
4. Under-balanced drilling
The primary objective of under-balanced drilling is to reduce well impairment and to optimise early production. Drilling fluids used may be normal mud but can also be two phase flow while producing the well. In case of under-balanced drilling with normal mud standard mud pulse telemetry can still be applied. However, in case of two phase flow standard pulse telemetry as applied by conventional MWD tools is no longer possible. An alternative surveying method for mud pulse telemetry is the electromagnetic telemetry system offered by Geoservices (never tried). This system transmits direction survey data using electromagnetic signals to surface while drilling with the well shut-in. This can be used with under-balanced drilling with drillpipe and coiled tubing. Other methods of transmitting data to surface are to use wired coiled tubing or a wet connector.