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Deep hard-rock drilling
As the quest for new petroleum supplies has increased in the past few years, operators have been forced to drill deeper to find new reserves. Much of the higher cost of drilling deeper, especially onshore, is typically associated with decreased rate of penetration (ROP) caused by both harder rock and higher mud weights required to counter the overpressured reservoirs often associated with deeper drilling. The following discussion centers on technologies intended to enhance the deep drilling capability.
Industrial hammers for hard rock drilling have been around for some time, but most have been air operated and used in the mining industry. Historically, hammers have been thought to have limited capability in oil and gas drilling operations, with their use limited to air drilling. Because of “chip holddown” and erosion through the hammer when drilling mud is used, hammers were not considered for drilling operations involving drilling mud. As a result, hammers have never been seriously considered for most deep drilling where hammer energy might enhance ROP by helping to overcome increased rock strength.
In an effort to develop novel drilling technologies, the US Department of Energy (DOE) awarded a contract to Novatek to develop an “integrated” drilling system using a mud hammer as the primary engine (see Fig. 1). The previously referenced high-speed communication system for drillpipe was part of that development. Another part of that development was a mud hammer that incorporated a number of revolutionary concepts, as shown in Fig. 2. Most notably, the bit was a radical departure from typical hammer bits. It was essentially a five-bladed drag bit with polycrystalline diamond cutters (PDCs) specially manufactured by Novatek to allow the aggressive drag bit profile to be used in soft formations but still allow enhanced drilling in hard formations using the high-energy impacts of the optimized industrial hammer. In addition, the hammer piston is used to energize a series of high-pressure jets (≈ 5,000 psi) that exhaust directly in front of each PDC to achieve an unprecedented level of cleaning ahead of each cutter. The jets also energize fractures ahead of the bit to enhance ROP.
Directional steering is made possible by means of a directional control sub (see Fig. 3). The control sub causes preferential firing of the jet pulse on the side of the hole in the direction the operator wants to steer, as shown in Fig. 2.
The Novatek IDS hammer and other hammers were part of a test program funded by the DOE to provide a focused study program for investigation of mud hammer potential in deep hard-rock drilling environments. That program is being run by TerraTek with several industry participants. The first results of that program were published in the SPE Journal of Petroleum Technology Online.
In summary, the current status of mud hammer investigation is still unfolding, with improvements in a number of mud hammers being driven by the testing program at TerraTek. The promise provided by mud hammers is potentially far more extensive than simply enhancing ROP in deep hard rock, although that alone would be sufficient. Mud hammers provide extremely strong seismic energy coupling into the rock. It might be possible to incorporate a mud-hammer-based seismic imaging system into the previously discussed high-speed communications system to provide a “seismic look-ahead” capability that could allow navigation directly into the desired hydrocarbon target or sweet spot. Such a system would be a significant step forward in the exploration and development of fractured, unconventional reservoirs.
Another novel approach to enhancing ROP in deep mud drilled wells was developed by Tempress Technologies. Fig. 3 shows the basic principles used in this system. Chip holddown is a well-documented phenomenon associated with mud drilling, especially in deep environments. In essence, the fluid pressure of the mud inhibits rock chips made by the drill bit from being removed from the cutting face in front of the bit. The result is regrinding of cuttings and a slowing of ROP.
The mud-pulse drilling system comprises an oscillator valve in the drillstring, which momentarily interrupts flow of the drill mud around a velocity section on the outer wall of the pipe. This interruption in flow results in extreme depressurization pulses (> 1,500 psi) developing below the bit. Theoretically, this causes rapid decompression of the fluids in the rock ahead of the drill bit and results in an apparent decrease in rock strength ahead of the bit, which results in increased ROP. The system can be run with almost any drill bit.
- Tibbitts, G.A., Long, R.C., Miller, B.E. et al. 2002. World's First Benchmarking of Drilling Mud Hammer Performance at Depth Conditions. Presented at the IADC/SPE Drilling Conference, Dallas, Texas, 26-28 February. SPE-74540-MS. http://dx.doi.org/10.2118/74540-MS.
Noteworthy papers in OnePetro
Gill, J.A. 1983. Hard Rock Drilling Problems Explained by Hard Rock Pressure Plots. Presented at the IADC/SPE Drilling Conference , 20-23 February. 11377-MS. http://dx.doi.org/10.2118/11377-MS.
Bejarano, C.A. 2006. Case History - Application of a New PDC Bit Design in Deep Cretaceous and Jurassic Hard Formations in Southern Mexico. Presented at the First International Oil Conference and Exhibition in Mexico, 31 August-2 September. 102232-MS. http://dx.doi.org/10.2118/102232-MS.
National Energy Technology Laboratory (part of the US Department of Energy)