Muds are a critical part of an optimised slim hole drilling system. The design of the mud for all slim wells must be closely integrated with the hydraulics program, which is in turn closely integrated with the mechanical design of the drilling, evaluation, and completion system. These interfaces are much more complex than with conventional holes.

1 Muds for continuous coring

In general, the mud requirements for a continuous coring system are much more severe than for a downsized conventional system.

The pipe is rotated at high speed in a small annulus. This means that any solids in the mud flowing down the inside of the drill rods tend to centrifuge out against the inside of the drill rod wall. The layer of solids prevents the inner core barrel being retrieved. It also turns the inner barrel into an excellent swab cup. Therefore either the well has to be drilled with a clear brine of the required density, or pigment grade barites has to be used as the weighting agent. Finely ground manganese oxide (Micromax), could be used as weighting material.

The mud should contain the minimum amount of solids at the pump suction. This can be achieved by utilising polymer-based additives for viscosity and fluid loss control. Polymers can also be used as friction reducers. If increased density or temperature resistance is required then brine-based systems will most probably have to be used.

From a technical viewpoint it is highly unlikely that a fit-for-purpose mud for narrow annulus drilling can be made using bentonite as a viscosifier. This is because bentonite-based muds give high gels and frictional pressure drops.

Drilled solids should be kept to a minimum:

• either by having and utilising a good suite of solids removal equipment. This would probably be based around fine screen shakers and decanting centrifuges;
• or by an aggressive programme of dumping, which implies a high consumption of mud additives.

Most of the circulating pressure drop is up the annulus giving rise to high ECDs. Pressure drop in the annulus is strongly dependent on the sizes of the drillpipe, tool joints, and hole, the degree of eccentricity, and the rotational speed.

Increases in ECD of up to 1 ppg have been recorded when speeding up the rotation of the drill rods.

High ECDs can give rise to numerous problems when drilling in permeable formations. These problems become more significant if the pore pressure starts to approach the formation strength. Kick/loss situations in a gas bearing formation would be of particular concern.

If the annular clearance is increased then drillstring vibrations will increase, which can lead to problems with the mining drill rod used for continuous coring. Similar effects are postulated if drilling is continued without returns, as the liquid in the annulus provides some lubrication and dampening.

2 Lost circulation for continuous coring

The small clearances between the inner and outer barrels of a wireline retrievable core barrel would possibly block up with conventional lost circulation material. Similarly the small annulus would inhibit the placement of LCM by reverse circulation. If a lost circulation zone were encountered one possible approach would be to pump down both annulus and DP with mud or water while pulling the inner core barrel on wireline. Once there is a clear path to the bit a (fibre) cement plug can be set.

Another method of coping with lost circulation when using the continuous coring system is to use silicate-based lost circulation material which is liquid during placement. It is pumped down the well between fresh water spacers. In front and behind the spacers, slugs of 10% calcium chloride solutions are pumped. (These are not required if the lost circulation zone contains a divalent cation brine). When the mixture has left the drill rods the silicate based LCM mixes with the calcium chloride and the mixture sets up to a firm gel in seconds.

An alternative approach is to use a cross-linked gel pill to combat lost circulation. Extensive understanding of these is available as a result of work done to improve fracturing technology. Such gels can be formulated to respond to different combinations of temperature, pH and time, thus permitting them to be used in a variety of different ways.

If the losses occurred while there was a zone capable of producing hydrocarbons open higher up the hole then the drill rod string in the hole can be used as an emergency casing string. This would call for acquiring special wiper plugs, one of which could be in the form of a latch down float shoe. To compensate for the absence of casing centralisers, rotating the drill rods/casing during the cement job to minimise channelling has been proposed. This would increase the annular pressure drop, hence restricting circulation rates. The collapse strength of the drill rod and the pressure integrity of the threads to external pressure should be checked prior to considering this option.