Drilling and Logging in Space;

Transcription

1 Drilling and Logging in Space; an oil-well perspective. Max Peeters 1, Brad Blair 2 Abstract Growing interest in extraterrestrial subsurface exploration has prompted an examination of advanced technologies for drilling slim holes and obtaining geophysical data in these holes. The borehole surveys with geophysical measurements called "logging", complement, and under favorable conditions, replace soil sampling. Very shallow drilling systems were used extensively during the Apollo lunar missions, and are in the planning stages for use on Mars. The prime objective is to gather scientific data, but these data could eventually provide a basis for the commercial use of space mineral resources. Given the strong scientific interest in water on Mars and the Moon, subsurface characterization with geophysical methods is attractive, because these methods can cover a much larger volume than soil sampling. Space technology has boosted the development of borehole geophysical instruments because both in space and in boreholes the instruments have to function in hostile environments, in confined spaces, and to be able to withstand large g-forces. This paper reviews oil industry drilling and geophysical borehole techniques that could be adopted for space applications. Coiled tubing drilling has many advantages because the surface facilities are compact, and an electrical cable in the tubing can transmit power and data. Moreover geophysical sensors can be embedded in the drill collars, which ensures that measurements are carried out while drilling, and this avoids risky reentry of geophysical tools in the hole. If kevlar is used for the coiled tubing, a laser beam could be transmitted via optic fibers in the coiled tubing wall. Using this beam to cut the rock would virtually eliminate mud and downhole motor requirements, and save a lot of weight. The quest for water and the strict requirements for redundancy, simplicity, and rugged instruments led to the selection of electromagnetic wave resistivity, natural gamma radiation, geophones, and induced epithermal neutron instruments as detectors. All these detectors can in principle be fitted into a coiled tubing string, and a combination of these measurements can provide quantitative information on the porosity, water-saturation, seismic velocity, and lithology of the Martian or Lunar soil. 1 Professor of Petrophysics and Borehole Geophysics, Colorado School of Mines, Golden, CO-80401, mpeeters@mines.edu. SEG, SPE, SPWLA, SCA, KIVI 2 Lunar Economic Development Authority, 1390 Adams St., Denver CO , bblair@mines.edu, MSc. AEG, SME.

2 Introduction Drilling systems were applied during the Apollo missions and soil probes were used during Mars' missions, but in both cases the investigations were restricted to very shallow depths (Blacic, 1989, Blair 1996, Podneiks et al 1993, Rostami et al 1998). Geophysical instruments that have been used for the exploration of the moon and the planets were either housed in spacecraft or confined to the surface of the celestial bodies. These instruments probed the subsurface with radar and seismic, but as is the case for the oil industry, only boreholes can conclusively demonstrate the presence of certain pore fluids, such as water. Growing interest in extraterrestrial subsurface exploration has prompted an examination of advanced technologies to obtain borehole geophysical data. Although scientific interest is the current driving force, the geophysical data could eventually form the basis for commercial use of space mineral resources. Space technology has boosted the development of borehole geophysical instruments because the instruments in space and deep down in boreholes both have to function in a hostile environment, in very confined spaces, and be able to withstand large g-force and temperature changes. Given the strong scientific interest in water on Mars's subsurface, it seems timely to close the circle and consider the use of advanced geophysical "logging" instruments in slim holes that are drilled with small rigs from the surface of this planet. These rigs should be two orders magnitude lighter in weight and volume than the typical oil-well rig shown in Figure 1.! " # % $ 1. Derrick, and drawworks 2. & 3. Casing strings 4. D rill p ip e 5. D rill b it Figure 1. Conventional rotary drilling rig 2 M. Peeters

3 Moreover the space-drilling rig should be fully automated, and maintenance free because 100,000 mile warranties have little value 150 million km from home. This paper reviews drilling and borehole exploration techniques that are used for petroleum exploration, with the objective of identifying operational procedures and geophysical measuring techniques that could be adapted for extraterrestrial boreholes. Drilling Rotary drilling with a bit attached to a series of detachable drill-pipes is considered impractical for planetary exploration, due to the complications of screwing and unscrewing the individual stands of the drillstring, the vulnerability of the hoist, and the large vertical dimension of the rig, as indicated in Figure 1. The mini-drilling rigs operating from the back of a pick-up truck (Wilson 1995) are more in line with requirements for space drilling. Large penetro-meters can be compact but they require an abundance of hydraulic power, and are limited to a range of some 100-ft, even in soft sediments. Moreover, the depth of penetration is equal to the length of the penetration cone, which again leads to unmanageable dimensions on the surface. Coiled tubing drilling underwent a rapid development in the nineties because this technique can be used to drill slim sidetracks from existing oil wells to reach bypassed oil (Gleitman 1996). Figure 2 shows the principles of this technique. Typical outside diameters of the tubing are 1"- 3", with an inner-diameter of ½"- 2". Figure 2. Coiled Drilling Principles 3 M. Peeters

4 This construction allows the drilling fluid to flow to the bit through the tubing and back to the surface via the annulus between the borehole wall and the coiled tubing. The fluid flow is also used to power the mud-motor, which in turn rotates the bit. This technique is depicted in Figure 4 for a conventional string. The reel and tractor, shown in Figure 2, which respectively store and propel the coiled tubing, could be restricted in size to a few m 3, if the Martian hole depth is limited to a few hundred meters. The bulky blow-out preventer, which consists of a stack of valves that are closed to prevent the uncontrolled production of fluids, will not be required in space, unless we expect to find high overpressured water accumulations. Small diamond (PCD) bits, down to 2" in diameter, have been applied to drill slim sidetracks away from existing oil wells. Drilling with a water base mud on Mars to drive the downhole turbine is not recommended, because the mud would have to be transported form earth, would be exposed to very low temperatures, and would interfere with the measurements to detect water. An attractive alternative is using "StarWars" laser technology (Graves, 1998 and O'Brien 1999). Successful feasibility tests have been performed, and a generic rig design is shown in Figure 3. Using a laser in a very lowpressure environment as encountered on Mars has the additional advantage that the heavy mud can in principle be replaced by circulating CO 2 or N 2 gas to remove the vaporized rock. Moreover the damage to the rock can in contrast to drilling with mud be limited to less than 0.2" away from the hole (O'Brien 1999), which would ensure that the geophysical instruments investigate pristine Martian rock. Figure 3. Laser Drilling 4 M. Peeters

5 Coiled tubing can accommodate an electrical cable that connects the surface equipment with the sensors down hole. This solves the data transmission and power supply problems of traditional drilling strings made up of individual stands, and simplifies the automation of the drilling process. For space applications low weight titanium is attractive, but for a depth of a few hundred meters man-made fibers like carbon / glassfiber epoxies with a kevlar armor can be used (Fowler 1997). This would reduce the weight by a factor 3 and allow that both the electrical power and glass-fiber optic lines can be embedded in the wall of the coiled tubing. The glass-fibers could then be used to transmit a laser beam to the "cutting edge" on the bottom of the hole (Figure 3). Sensors Beyond a depth of 10 to 30m it will be very difficult to retrieve soil samples even when bulky conventional drilling methods are used, because the entire process has to be automated. Similar to the oil industry the objective of geophysical measurements should therefore be to minimize or even replace coring. Moreover, the sensors can investigate a volume that is at least a factor 100 larger than the volume of a continuous core. Physical rock properties measured with geophysical "logging tools" include amongst others electron density, electrical resistivity, dielectric constant, natural gamma radiation, acoustic & EM propagation times, nuclear magnetic resonance, and neutron decay. The conventional drilling string depicted in Figure 4 contains density, neutron, gamma-ray and resistivity instruments. 5 M. Peeters

6 It should be realized that drilling fluid flows through or along these sensors. This severely restricts their dimensions, especially for borehole diameters smaller than 3", which is essential to restrain the coiled tubing reel dimensions. The density and thermal neutron tools in figure 4 use radioactive sources, which is undesirable for space applications. An alternative is the use of a small accelerator called a minitron as a source for high-energy (14 MeV) neutrons, based on the reaction : 2 3 reaction D1 + T1 He2 + n MeV 4 1 The minitron depicted in Figure 5 measures 3" by 1" and can be fitted into the drill collars shown in Figure 2. Figure 5. Minitron (Courtesy Schlumberger) A series of electrodes with increasing electrical potentials accelerates the deuterium (D) ions and guides them to the tritium (T) target, where the reaction shown above takes place. If the minitron is switched off no radiation is emitted. The density of the epithermal neutrons around the detectors is strongly affected by hydrogen and consequently very sensitive to the presence of water. coiled tubing & drill collars 1.5 outside view 3 bore-hole wall 1 epithermal detectors far detector near detector minitron Drilling fluid flow EWR :Electromagnetic Wave Resistivity Fluid flow to bit *) distances & positions are notional Figure 6. Generic design of coiled tubing geophysical sensors (Peeters, 1998) 6 M. Peeters

7 It is proposed to use He 3 proportional counters as epi-thermal neutron detectors, which are small (<1" diameter) and can also be embedded in the coiled tubing. Moreover, they are very rugged and have proven their viability in drill-strings. A generic design of the coiled tubing "logging" section is shown in Figure 6. Conventional thermal neutron instruments have the limitation that they are very sensitive to the salinity of the formation water and the presence of shale. Using epithermal neutrons as information carriers very significantly reduces the adverse effects of shale and salinity. If CO 2 or N 2 could be used as a drilling "fluid" in a closed system, the effects of the borehole would be negligible due to the absence of hydrogen. If a gas is used it will be necessary to compensate for adverse borehole effects by measuring not only the density of the epithermal neutrons but also the decay time. Combining these two parameters gives a porosity that is hardly affected by the borehole fluid (Peeters 1998) One measurement is seldom sufficient in oil well surveys to accurately determine a certain rock property such as water content. A combination of two or more independent measurements is usually required for a reliable evaluation. The same situation is expected for extraterrestrial borehole surveys. It is therefore proposed to use an Electromagnetic Wave Propagation Resistivity tool (EWR) together with the epithermal neutron instrument. The EWR tool consists of three or more wire coils, which can easily be fitted in the collars of the coiled tubing. One coil sends out a 2 MHz signal and the others are used to record the travel times of the electromagnetic signal through the rock parallel to the borehole wall. This travel time is related to resistivity of the soil, which in turn can be used to determine the water saturation using Archie's law (Brown 1999). One of the most important developments in the "logging" industry in the nineties has been the emergence of a reliable nuclear magnetic resonance (NMR) tool. The NMR borehole instrument employs rather large permanent magnets, which has prevented until now the incorporation of a NMR tool in a 6 drill string. It is unlikely that the technical problems, to fit this instrument into a coiled tubing collar with a diameter of 3", will be overcome in the near future. This is unfortunate because from all the geophysical borehole instruments that are used in the oil industry, this is the only one that can directly detect the presence of mobile hydrogen atoms, and would therefore be ideal for the location of water. Radar has been quite successful to determine the stratification of the first 30 feet of the subsurface, provided the soil is desiccated and contains little shale and pyrite (Olhoeft 1998). However, small boreholes can only accommodate small radar antennas and consequently should operate around 1 GHz. The depth of penetration of the radar signal at that frequency is restricted to less than one foot, and it will be difficult to fit the antennas in the wall of the drill-collars of a coiled tubing string. It is recommended to incorporate several natural gamma-ray detectors in the coiled tubing. These detectors consist of a scintillation crystal coupled to a photo-multiplier. The oil-industry has used these detectors for decades and they are sufficiently robust to 7 M. Peeters

8 use in drill-strings. The dimensions of small gamma-ray detectors are equivalent to the He 3 detectors and can therefore be inserted in drill collars. If the gamma rays are sorted by energy in a spectrum, the contributions from Uranium, Thorium, and Potassium can be unraveled. The ratios of the concentrations of these elements, which usually only occur as trace elements, can be used to estimate the mineralogical composition of the soil, and this will greatly enhance the determination of porosity and water saturation. It is finally recommended to include geophones in the coiled tubing string. The acoustic travel times measured with the geophones would be useful to calibrate surface seismic surveys, and assist in the determination of pore fluids and mechanical properties. Conclusions 1. If coiled tubing is used for drilling, surface equipment could be restricted to a reel, a tractor and a hydraulic power supply. This limits the volume of a rig to a few cubic meters provided Martian holes are only a few hundred meters deep. 2. Coiled tubing for this limited depth could consist of man-made fibers. This would allow incorporation of power and data communication lines in the coiled tubing wall. 3. Coiled tubing ensures an easy data communication and power transmission between surface equipment and the geophysical instruments in the drill collars near the bit. 4. Embedding optic fibers in the coiled tubing wall opens the possibility to use lasers as the drilling device. Experiments have demonstrated the feasibility of laser drilling. 5. Based on size, rigidity, and the sensitivity to water it is recommended to use a "minitron" high-energy neutron source, epithermal neutron & natural gamma-ray detectors, electro-magnetic wave propagation instruments. 6. The combination of the signals of the sensors can minimize or replace soil sampling, because they can quantify the porosity, lithology and amount of water in a much larger and undisturbed volume of the Martian soil than is possible with soil sampling. 7. The use of radar and nuclear magnetic resonance tools as water detectors is very desirable, but the current state of the art prevents the incorporation of these instruments in coiled tubing collars with a diameter of less than 3". References Blacic, J.D., Rowley, J.C., and Cort, G.E., "Surface Drilling Technologies for Mars", Proceedings Seminar on Planetary Excavation, Univ. of Maryland, 1989, pp Blair, B.R., "Fluidized Drilling for Lunar Mining Applications", Proceedings of Space '96, ASCE, M. Peeters

9 Boyd, R.C., "Drilling Systems on the Moon and Mars". Proceedings of the First International Symposium on Mine Mechanization and Automation, Colorado School of Mines, Golden, Colorado, 1991, pp / 18. Brown, G.A. "Analysis of non-linear resistivity index vs. saturation data using a binary Archie model." SPWLA, 40 th Annual Conference, Oslo, June Gleitman, D.D., Grantt, L.L., Hardin, J.R., Hightower, C.M., Smith, M.B "Newly applied BHA elements contribute towards mainstreaming of coiled tubing drilling applications". IADC/SPE 35130, 1996 Drilling Conference, March Fowler, H. "Update on advanced composite spoolable-pipe developments." SPE/ICoTA, Roundtable meeting Montgomery Texas, SPE 38411, April Graves, R.M., O'Brien, D.G. "StarWars laser technology applied to drilling and completing gas wells." SPE 49259, Annual Technical Conference, New Orleans September Olhoeft, G. R. "Ground penetrating radar on Mars." Proceedings of GPR'98, Seventh International Conference on Ground Penetrating Radar Lawrence KS, May O'Brien, D.G., Graves, R.M. "StarWars laser technology for gas drilling and completions in the 21 st Century." SPE 56625, Annual Technical Conference, Houston October Peeters, M. Epithermal neutron instrument for coiled tubing to determine porosity independent of borehole size, fluid type and salinity. (patent application pending). July Podneiks, E.R., and Siekmeier, J.A., "Terrestrial Mining Technology Applied to Lunar Mining." The High Frontier Conference XI, Space Studies Institute, Princeton, NJ, Rostami, J., Blair, B., Eustes, A., "Review of the Issues Related to Extraterrestrial Drilling." Proceedings of Space '98, ASCE, Wilson, D.D. "Federally funded innovative drilling demonstrations." Water well journal, Jan M. Peeters