In the past small holes could not be drilled safely and effectively. The article describes the main historical problems encountered when trying to introduce slim wells:
The tri-cone rock bit, rotated from surface by a rotary table, has a low aggressiveness which requires bit weight to make it drill. In small vertical holes this resulted in large numbers of collars (20 to 30) being run.
"Bit bounce", which increases the weight on bit momentarily, which in turn increases the torque on bit, generated torsional vibrations in the collars. As the collar size was selected to maintain clearance for fishing with an overshot (or for running a wash-over pipe), and stabilisers were often omitted, there was plenty of room for lateral vibrations to occur.
The small outside diameter meant that more collars were required. The small, single-shouldered drill collar threads were prone to over-torque due to the torsional vibrations. This then required high torque to break them on surface, which with manual tongs often permanently bent the collar between the box and the slips, resulting in a wobble when rotating downhole.
The high clearance between the relatively limber collars and hole wall meant that they buckled more easily. The buckled collar string slid, bounced and rolled against the borehole wall generating varying torques in the bottom hole assembly, which contributed greatly to the stick-slip torsional vibrations.
These vibrations were then transmitted up to the top of the drill collar string and into the more limber drillpipe. Due to the change in cross-sectional area, some of the vibrations were reflected downwards, back into the drill collars, adding another vibration source. The higher clearances between the drillpipe and hole wall and its more limber nature meant that the drillpipe vibrations were also of a violent nature, especially in vertical holes where damping was weaker. Vibrations were reflected up and down the string from each tool joint.
On top of the drillpipe is the rotary table, acting effectively as a large fly wheel with a high rotational inertia. This is driven by an electrical or mechanical drive system which also has a high rotational inertia.
The rotors of the motors, driving independent electrically driven rotary tables, have a high inertia due to their high speed. Many SCR systems are designed to be stiff, i.e. not to slow down when the required torque increases, thus also giving the rotary table drive system a high rotary inertia. The conventional rotary table and its drive system acts as an effective reflector for torsional vibrations, as do the hook, blocks, cables and mast for longitudinal vibrations.
Thus the design of the whole BHA tended towards a high vibrating system, prone to failure, fully justifying the large fishing and washover clearance between the collars and the borehole wall.
Sidetracking round a long fish in a slim hole was a slow operation, with all the problems of the original hole in addition to the plug back and kick-off. The kick-off, in pre-MWD days, was a slow and complex operation, involving much time with the drillstring stationary, resulting in a high risk of getting stuck. Thus the drilling strategy focused on contingency plans for backing-off as low as possible to retain the maximum amount of the original hole, and for having as many fishing options as possible.
Amongst this high level of vibrations in the BHA, it was difficult to see the surface torque spikes of the bit cones locking, so cones were either run off, presenting impossible fishes at depth, or bits pulled green. Control of weight on bit (WOB) was limited both by the string vibrations, which required that the weight indicator be heavily damped, and by the basic insensitivity of that instrument system design. The small size of the bits meant that the bearings were small and relatively weak, had a short life, and drillers were unwilling to run high weights or rotary speed. Thus penetration rates were low.
The lack of flexibility, which CAD and automated manufacturing have now given to rock bit manufacturers, meant that only a few bit types were available, reducing bit optimisation options.
The long slick string of collars, usually not grooved in order not to lose WOB, had a high propensity of becoming differentially stuck. Drilling jars were therefore run. These often had the same internal diameter (ID) as the drill collars so that survey tools and back off/cutting charges could be run. These jars were considerably more flexible and weaker than the surrounding collars and thus often twisted off.
The long collar string also gave high equivalent circulating densities (ECD), leading to kick/loss situations. The pressure drop in the narrow drill collar/hole annulus increased with rpm, thus raising the ECD further. The long, small IDs of the collars gave high pressure drops, restricting circulation rates and hence bit and hole cleaning.
If a weighted mud was used there could be a loss of solids carrying capacity at higher bottomhole temperatures due to the polymers degrading. Many operators stuck to bentonite as a viscosifier and filter loss control agent as it was much cheaper, and the thicker, softer filter cake did not seem to be a significant drawback. These muds were often used with poor surface solids removal, which in turn contributed to the thick filter cakes, which increased the possibility of differential sticking.
The high ECDs and use of pit level changes to detect kicks (which are slow to react and insensitive) resulted in high overpressures being used while drilling. This resulted not only in slow penetration rates, due to chip hold down effects and low circulation rates, but also in thick filter cakes and high overpressures which increased the chance of getting stuck. These overpressures also resulted in more rapid filtrate invasion into shales, increasing their propensity to slough in.
The high level of bit and drillstring vibrations not only contributed to drillstring failures, but also resulted in washouts and sloughing formation, which allowed the drillstring to vibrate even more. Buckling of the lower joints of the limber drill collar string allowed the bit to build angle in an uncontrolled manner, and these doglegs added to the overall borehole rugosity. The resultant high wall contact loads, especially when the string was stationary increased the chance of differential sticking, or getting stuck in a key seat.
The borehole rugosity increased the difficulties associated with interpreting the output of small wireline logging tools, especially before computerised wireline logging units became widely available. Washouts, doglegs, key seats, and thick filter cakes also increased the chance of getting stuck with small wireline logging tools.
Rugose boreholes made cementing more difficult. Mixing of small cement volumes on the fly produced a variable slurry quality. For example, pumping cement with a high fluid loss at high speed up a narrow annulus could generate pressure drops sufficient to separate the water from the cement, causing the de-watered cement to flash set. Small centralisers were unobtainable, so centralisation was a concept applied more in theory than in the field.
The intermittent nature of the demand meant that much of the equipment had been in storage for some time and in some cases its condition had deteriorated. On occasion, there was often no suitable equipment in an area.
The resulting mobilisations, often international operations by air, with the inevitable omissions and interface problems, resulted in increased costs and time.