A new drilling method uses fluid kerfing to minimize rock disintegration while increasing rate of penetration (ROP).

Jet kerf drilling tools cut thin concentric circles or notches in rock, weakening the structure. The slightest weight applied by a roller or other device results in a cracking failure below the rock face between kerf lines. This method causes a very low percentage of actual rock disintegration, while conventional bits such as roller cone and PDC bits crush and shear away a very high percentage of rock.

Of all the potential methods of forming a kerf in rock, fluid jet kerfing can minimize the width of the kerf. Another advantage to fluid kerfing is the already abundant use of fluids on today's rigs. Kerfing away only a small percentage of the rock reduces energy requirements by up to 75% and increases rates of penetration up to 400% over conventional methods. Acceptance of fluid jet kerf drilling has been slow, however, due to the high pressures necessary and the potential for nozzle erosion.

High-pressure fluid kerfing

Although most drilling rigs are not capable of high pressures, some drilling tools now available are capable of modified jet kerf drilling. These tools let tungsten carbide rollers assist in the kerfing process. Harvest Tool's jet roller drill bits have two thick, overlapping semi-circular blades with alternating tungsten carbide rollers and jet nozzles positioned on the leading edge of one blade; on the second blade, the opposite device follows in the same position. This allows each revolution of the tool to roller crush a thin concentric line and nozzle clean and potentially fluid kerf along the same concentric line. Each revolution of the tracking roller and subsequent blade's tracking nozzle causes kerfing, even without ultrahigh-pressure fluid. Thus, the hole advances more efficiently. As rigs and pumps become more sophisticated, higher pressures will be possible, and the jet nozzles undoubtedly will contribute more to the kerf.

In the past, the circulation medium has been used to mainly lift the cuttings out of the hole. Secondary benefits have been cooling of the drillstring and tools, and, if the rock was soft, perhaps some actual hole disintegration. The described tools also use the circulation medium to open the blades from a slimhole position (closed diameter) to the open position (full diameter) without pistons or cams. Another advantage of the thick, overlapping blades is their ability to collapse to the closed position in the event of a sloughing hole. The blades do not retract into any housing chamber and therefore can't get locked in the open position and become stuck in the hole when the housing gets filled with cuttings.

Nozzle erosion

Even with past documented increases in ROP, the drilling industry has been slow to adopt jet kerf drilling due to nozzle wear, downhole ultrahigh-pressure pump wear and resultant poor economics of constant tripping. The wear problem is not a major concern if the well is shallow or the total interval to cut is within the nozzle wear limits of the tool. However, wear concerns are heightened if the well is deep (bit trips are expensive) or the interval to be cut is long.

In the past, the search for ultra wear-resistant nozzles was the focus. A wireline retrievable drill bit has been designed that replaces not just the nozzles but also the cutting blades (but not the seat device) via wireline inside the drillstring or casing. This tool has cutting blades placed on a precision pin slightly smaller than the inside diameter of the drill string. The blades have jet kerf tracking nozzles and tracking rollers and are retrievable and deliverable via a latch tool on a wireline with no drill-pipe tripping. Pump pressure, rotation and rock face engagement cause the blades to open outward beyond the blade seat device diameter and beyond the diameter of the drillstring. When the nozzles are worn, the blades are simply pulled through the inside of the drill pipe or casing and replaced via wireline.

Fluid volume

With a small diameter nozzle to increase fluid velocity, the decrease in fluid volume and the inability to lift cuttings becomes a problem. One solution is multiple nozzles for each kerf line. For example, a jet roller underreamer, which is capable of underreaming below 7-in. casing to an open diameter of 72 in. or larger, has more than 80 nozzles. Most nozzles are placed in tracking kerf lines as a pair, where one nozzle is of a small diameter for cleaning and kerfing, and an additional nozzle at the same kerf point is of a larger diameter for cleaning and volume delivery. Even larger diameter nozzles are placed at the outer diameter concentric kerf lines to assist in the jet action opening of the tool.

Reduced torque

Actual field results have seen drillstring torques reduced by up to 50%. The reduced torque allows lower power requirements, extended drillstring life and less chance for drillstring failure and fishing jobs.
At present, certain niche applications of jet kerf drilling are of particular interest to operators: large-diameter underreaming used for target drilling for horizontal intersection; underbalanced horizontal drilling; and large-volume fluid collection downhole. Perhaps with further use and advancements, jet kerf technology will become more widely accepted.

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