Slim Hole Drilling which, for the purpose of well control, means hole sizes of 4 3/4" and smaller, will impose certain restrictions on well killing techniques and will require well planning to take care of these limitations.

1 General

Annular friction losses are an important aspect in Slim Hole Drilling, especially with regards to well control. Higher than required bottom hole pressures may occur during well killing operations if the ECD is not taken into account. Also, during drilling/circulating operations, it is possible to have an unnoticed potential kick situation, when the ECD is higher than the formation pore pressure. As soon as circulation is stopped, the well may kick.

2 Kick detection

Small influx volumes will result in large influx heights due to the small annular clearances which are typical for Slim Hole Drilling. This will result in high pressures along the borehole, especially when a gas kick is experienced. A kick tolerance larger than circa 6 m3 (10 bbl) will make well design, blased on well control aspects, very limited. This emphasises the need to detect a kick as soon as possible. A highly sensitive and reliable kick detection system as well as constant maximum crew alertness is required to stay within such a small kick tolerance range.

Long and thorough flow checks should be standard practice in Slim Hole Drilling operations after having drilled a section and circulation is stopped, because the well may kick as the pore pressure gradient of the newly drilled formation could be higher than the static mud gradient.

Drilling breaks should be treated as potential kick situations. After each drilling break, the well should be closed-in immediately after circulation is stopped and the well should be observed for any pressure build-up. The closing-in procedure of a Slim Hole Drilling well should be designed to reduce the influx to a minimum. The hard shut-in method may be the best options.

When circulating bottoms-up, the well should be closely monitored for any kick indications (e.g. increased mud returns, pit level increase). When there are positive kick indications, circulation should be stopped and the well closed in after which the well can be killed.

Swabbing can be a major problem in Slim Hole Drilling. Due to the small annular clearances, the swabbing tendency is relatively high and a relatively small swabbed-in influx volume will still result in a large influx height. This may cause the well to become underbalanced.

Pumping out of hole (in open hole as well as inside the casing) should be a recommended practice in Slim Hole Drilling.

3 Kick control

Well control design should be based on kick situations which may occur during the drilling phase. The following main kick situations are considered:

• Kick whilst drilling into overpressured formation:
• Formation pore pressure > ECD.
• Kick after having drilled into overpressured formation and circulation is stopped:
• Mud weight < formation pore pressure < ECD
• Swab kick:
• Formation pore pressure < mud weight.
• Kick whilst drilling into overpressured formation

When the formation pore pressure is larger than the ECD, a kick will be taken during actual drilling. The influx will be dispersed in the mud of the annulus after the well is closed-in, because the kick was taken whilst pumping. This means that, due to the small annular clearances, a large part of the influx may be already inside the casing which results in lower than otherwise expected shoe pressures. Standard well control calculations, which are based on the assumption that the complete influx is on bottom, at the time the well is closed-in, are therefore producing highly conservative results which cannot be used for well control design purposes. Other, more realistic calculating methods cannot be used for well control design purposes either, because of the many (not accurately known) variables in the system (e.g. circulating rate, formation flow potential, method and speed of closing-in the well, etc.).

• Kick after having drilled into overpressured formation and circulation is stopped

Since the ECD is usually 5-10% higher than the mud weight in use, it is very unlikely to drill into formation pore pressures higher than the ECD when modern pore pressure prediction methods/tools are used. This means that in theory, during drilling operations, a kick may only be experienced when circulation is stopped. This results in a limited dispersion of the influx in the mud which makes the calculated shoe pressures using the "bubble on bottom calculation method" quite acceptable.

• Swab kick

If a well is swabbed-in, pressures acting along the open hole are usually lower as compared to pressures experienced during a kick whilst drilling.

4 Well control design

In principle, standard well control design is based on the relationship between formation strength at the shoe, mud density, influx volume, and possible kick situations. Slim Hole Drilling well control design criteria also include small annular clearances, the dispersion effect on the influx in the mud, and annular friction losses. The following kick conditions are considered for Slim Hole Drilling well control design:

• Kick after circulation is stopped: - mud weight < formation pore pressure < ECD.
• Swab kick: - formation pressure < mud weight.

Well control design calculations for Slim Hole Drilling are based on existing kick detection techniques applied in standard drilling operations. Calculating methods may be changed when better kick detection methods have been developed for Slim Hole Drilling operations. The following assumptions are made in the present calculating method:

• the "Drillers' Method" is used to circulate out the influx and to control the well;
• pore pressure gradients to be used in the calculations depend on the probability of encountering higher than expected pore pressures, but should not be higher than the ECD;
• the shoe strength should be high enough to withstand a large kick (larger than the annular open hole content).

5 Operational considerations

In order to minimise pressure control problems while drilling small hole sizes, the following should be observed:

• Minimise the use of DCs in slim hole wells. This would imply the use of PDC/TSD bits which require less WOB and the use of a hydraulic thruster/mud motor combination to replace some DCs.
• Tailor the mud rheology at downhole conditions in such a way as to maintain laminar flow, and hence lower ECDs in the annulus. During the planning and drilling phases, the computer program RHEOMUD should be used.

·Use very sensitive flowmeters on the standpipe and flowline (kick conditions are much faster identified with an accurate flowmeter system as compared to pit level indicators).

·Minimise the active mud system volume at surface in order to detect volume changes quicker.