High-pressure rotary jet drilling holds the promise of increased rate of penetration with reduced weight-on-bit, torque and vibration levels. A high-pressure rotary jet drill, pressure intensifier and gas separator have been developed to allow jet drilling using conventional surface pumping equipment and coiled tubing. High-pressure reaction turbine jet rotors have been developed for drilling holes ranging from 1–1/8" to 3–5/8". Jet drilling tests have shown that 70 MPa (10,000 psi) jets can effectively drill most conventional oil and gas producing formations. Conventional pumps, swivels and tubing operate at up to 28 MPa (4000 psi). A 2.5:1 pressure intensifier was developed to allow jetting at the pressure required for effective drilling. The intensifier can operate on two-phase flow using a downhole gas separator. In two-phase operation the separated gas is used to power the intensifier and the high-pressure water is provided to the jetting nozzles. The gas exhaust from the intensifier is ported to the drilling head to extend the range of the jets.
Tests have demonstrated that the jet drilling BHA is capable of cement milling but rates of penetration are lower than a motor and mill and the pumping pressures required are higher. The tools could find applications in situations where a motor cannot be used. For example the tools could power a small diameter lance jet drill through an ultra-short radius curve for lateral drilling. Well service applications include removal of hard scale without risk of damage to damage to downhole equipment.
Jet drilling is limited by the threshold pressure required to erode rock and by submerged fluid jet dissipation. The jet pressure delivered to the rock surface determines the ability of the jet to cut the rock. The jet power then determines the rate of drilling. The pressure that can be delivered to a jetting tool through coiled tubing (CT) is limited by fatigue limits of the coil and the pressure capabilities of available pumps. Approaches to jet drilling at the pressure available through coil include abrasives (Eslinger et al. 2000), and alternate fluids such as supercritical carbon dioxide (Kollé 2000) or acid (Moss et al. 2006). The consumables associated with these approaches add significant cost and complexity to the operation.
Another approach is to boost the pressure of the jets with a downhole intensifier. A downhole intensifier has been developed for jet-assisted drilling of 7–7/8" to 8–3/4" holes (Veenhuizen et al. 1995). The unit was designed to work with a conventional rotary drill string and to run on drilling mud. The intensifier area ratio was 14:1 - delivering 84 lpm at 200 MPa from mud supplied at 1260 lpm and 23 MPa. This system provided increased rate of penetration but required higher mud pressure and the economic benefit was marginal.
A coiled tubing downhole intensifier has been developed to boost fluid pressure by 2:1 to enable mineral scale milling with standard coil and pumps (Kollé et al. 2007). A rotary gas separator removes the nitrogen from the jetting fluid to allow jetting with a straight fluid jet. Dual passage rotary jetting tools port the nitrogen around the jets to enhance jet range. As discussed below, jet drilling of oil and gas producing formations requires a jet pressure of at least 70 MPa. A larger version of this tool with a higher intensification ratio for rock drilling is discussed here.
Typical CT pumping pressures range from 28 MPa for low pressure coil to 70 MPa for heavy wall, high strength coil. In areas where hydrogen sulfide is present, the maximum coil pressure will be reduced. The pressure differential available at the bottomhole assembly (BHA) may be 10 MPa lower than the pump pressure depending on flow rate, coil diameter and coil length. Underbalanced operations with commingled nitrogen and water reduce the bottomhole pressure and can increase the differential pressure available at the BHA relative to pump pressure.