When drilling downsized conventional holes, conventional core barrels can be used. The addition of the anti-vibration technology (soft torque, thruster, Moineau motor, etc.) will improve penetration rates and reduce downhole failures in a similar manner to destructive drilling. More importantly, reducing vibrations should reduce core damage and core jamming.
Experience have shown the detrimental effects of core barrel vibrations on core quality and recovery factors, especially in weak, friable, unconsolidated sandstones.
The shortcomings of conventional coring are:
·The only force stopping the inner barrel rotating is the torsional loading in the core, which is postulated to result in slip planes in the recovered core and occasional core jamming.
·The use of a face discharge core head, in soft friable formations, minimises the disturbance to the core by mud flowing through the throat of the core head. This is achieved by use of a rotating seal between the inner and outer barrels. This gives rise to additional torsional friction which has to be absorbed by the core. Viscous-dynamic seals have been developed to overcome this friction.
·Thus friable and highly fractured formations are difficult to core, and recovery rates are often low.
The integral coring system was developed as a result of this analysis. Although the basic idea is not new, the system incorporates virtually every known feature to optimise the coring process and provide minimum core disturbance, including low invasion coring, and it can be run with low invasion coreheads.
The system was designed to cut and recover a 2" core, the largest possible diameter of core which can be cut with a 3 3/4" core barrel in a 4 1/8" hole. The barrel is in 20 feet sections and has a maximum length of 80 feet. The core size was selected for two reasons, to maximise the geological information, and to provide the strongest core, which would maximise the length of core recoverable during one run. Fishing of the barrel in the event of a twist-off is by "Flexloc" SHOOT slim hole overshot, which limits the OD of a barrel that can be run in a given hole size. The inner barrel can be made of steel, aluminium, or glass fibre and is considered as disposable, i.e. a new inner barrel is used for each run. The design change to upgrade the barrel to provide oriented cores is relatively small, although the requirement of an MWD to provide a magnetic or gravity highside scribe line orientation would increase costs, and in most cases paleomagnetic orientation of cores in the laboratory is far cheaper.
During coring, the drillpipe is not rotated. The outer core barrel is rotated by an inverted Moineau motor. In this motor the outer member, which is normally the stator, rotates, and the inner member, which is normally the rotor, is attached to the drillstring and does not rotate. The bottom of the inner member is connected via a second flexible joint system to the inner core barrel, which therefore also does not rotate. The outer core barrel is connected to the outer member of the motor and therefore rotates with it. The core head can be of the low invasion type or the previously mentioned face discharge type so that the core is not washed away. The total system is run under a thruster to minimise vibrations.