Deep drilling projects

Deep drilling projects are ambitious attempts to investigate the origins and structure of the earth; another aspect of the projects is the location of resources that may someday be feasible to exploit. Among the important early deep drilling projects were the ill-fated American Mohole Project of the early 1960s and the Russian drilling project on the Kola peninsula, begun in 1970.

Background

Most of the earth is hidden from human view. Of its 13,000-kilometer diameter, people see only a thin rind. The deepest gold mines are slightly more than 3 kilometers deep; tunneling deeper meets heat greater than miners can stand, and increasing pressure causes frequent tunnel collapses. Drilling can go much deeper than tunneling because the narrow bore holes can better resist pressure. Also, drilling keeps people away from the heat, and the smaller amount of moved means considerably less cost to reach a given depth.

The central limitation to the usefulness of drilling is that bore holes are much smaller than mine shafts, and materials brought up can be no larger than the size of the hole. A drill hole provides cheap samples, but moving major amounts of material becomes expensive unless the material flows. However, even with that limitation, bore holes sample a vast world that cannot be found at the surface. For the geological and biological sciences, the surface holds the present, but the subsurface holds the previous millions of years. For mining, there seems to be tremendous potential at depths about two and a half times the maximum depth to which tunnel mines can reach. As far as tapping heat sources, the potential is many times greater than that available near the surface.

History

When prices of whale oil rose in the 1850s, Edwin Drake began mining for rock oil—petroleum. He dug at the location of a natural seep. When water flooded his digging pit, he drove a pipe down toward the expected oil. Along the way, he noted what materials were shoved up by his operation.

Generations of drillers continued such well logs of cuttings coming up from each depth, such as sandstone, shale, and limestone. Geologists used those logs to map the rise and fall of certain distinctive rock layers (strata), which showed bending and folding of the rocks. They in turn researched marker fossils that allowed the cuttings to show more precise dating and thus to show connections among the rock layers. Those connections allowed mapping of underground layers, making oil drilling more successful. More complex geological drilling methods were developed to obtain actual core samples (tubes containing rock just as drilled, rather than as cuttings). The cores allow detailed scientific studies, and for conventional mining they provide a comparatively cheap method of surveying ore bodies before tunneling.

The Mohole Project—A Glorious Failure

Nothing better illustrates the connections of to mining than the Mohole Project. Andrija Mohorovičić analyzed seismic waves from earthquakes and concluded that rock of the earth’s crust changes significantly about 16 to 40 kilometers below the surface as it changes to partially melted mantle. There was considerable speculation as to how rocks might be different at this Mohorovičić discontinuity, or “Moho,” and in the 1950s, researchers in a variety of fields from a number of countries discussed proposals about drilling to the Moho for samples. Such a project, it was understood from the outset, would be very expensive.

In the United States, the National Academy of Sciences proposed a government-funded drilling project in 1958; it was nicknamed the “Mohole.” Ocean drilling was to be undertaken because the crust is much thinner under the ocean floor. However, drilling into the ocean floor required three major innovations. First, the locations with a shallower Moho in the rocks were in deep water, so the drilling platform had to be an oceangoing vessel rather than a tower resting on the bottom or a simple barge. Second, because anchoring in those depths would be difficult, the drilling vessel had to actively maintain its position. Third, while out of sight of land, the drilling vessel crew had to navigate within a circle of a few tens of meters and return exactly to the drill hole after pulling out of the hole for any reason. This last, most difficult innovation, was managed by satellite position-finding and acoustic beacons on the seafloor.

The project did not attain its goal of reaching the Moho, and some observers considered the attempt an embarrassment to US science. Sufficient funding was not provided, and indeed the project began costing far more than anticipated. However, the project could be considered successful in that technologies developed were subsequently used for drilling throughout the oceans of the world. Oceangoing rigs allowed exploratory drilling in deep areas of the continental shelf before companies made multibillion-dollar investments in production platforms. Thus Mohole Project technology was a major factor in opening many offshore oil fields.

Deep Drilling after Mohole

To explore geology, the Joint Oceanographic Institutions for Deep Earth Sampling sent the Glomar Challenger on ninety-six voyages, drilling cores in the oceans. Some of the revolutionary discoveries that these cores contributed to were: similarities in rock on both sides of the Atlantic Ocean, lending credence to the concept of ocean spreading as part of continental drift; the fact that turbidites, deposits from undersea landslides and mudflows, cover large areas of the ocean floor; salt deposits in parts of the Mediterranean Sea from when sea levels were low enough to make the Straits of Gibraltar dry land and the Mediterranean a salty inland sea; few areas of geologically very old ocean floor, suggesting that much of the ocean floor has sunk back under other tectonic plates as other areas expanded; evidence that the dinosaur extinction may have been caused by a comet or asteroid striking the Earth; oil and natural gas deposits extending down the continental slope and perhaps out to the ocean floor, with vast economic implications; and indications of methane hydrate (natural gas frozen together with water) on much of the ocean floor, possibly holding more energy than all other fossil fuels combined. The Ocean Drilling Program, which began in 1984 as an international effort, started using the JOIDES Resolution vessel to complete 110 expeditions at two thousand drill holes. Between 2003 and 2013, such efforts were continued as part of the Integrated Ocean Drilling Program, which involved twenty-six countries and several different drilling platforms over more than fifty expeditions. The international collaboration for scientific discovery through deep drilling was expanded again in 2013 under the International Ocean Discovery Program.

Drilling on land has also yielded discoveries. The “ultradeep” drill hole in the Kola Peninsula of Russia (east of Norway) reached 12,261 meters despite steadily increasing problems of heat, pressure on the borehole, and logistics of moving samples up from such great depths. Cores from Kola confirmed that metal ores continue deep into the Earth. Also, changes in the returning drilling mud suggested that water and might continue to those depths.

In 1994, a German hole in Bavaria drilled to 9,100 meters confirmed abundant fluids at those depths. Applying advanced instruments, German scientists found brines with calcium and sodium salts twice as concentrated as those in the ocean. Furthermore, they found channels in the rock large enough for the fluids to move. Thus, brines can deposit metal ores at these depths, and the rocks are permeable enough to hold hydrocarbon reserves. Both these findings changed the previous belief that pressures below several kilometers squeeze the rock too tightly for fluids to exist.

Drilling around the Chicxulub depression, a buried crater in Mexico, has provided evidence that it was the place where a meteor hit the earth, which may have contributed to changes that led to the extinction of the dinosaurs. A 2016 offshore drilling expedition aimed to acquire the first samples from the crater's "peak ring," which would allow scientists to better understand the aftermath of the impact. Drilling has also been used to map major faults, such as the San Andreas in California. One of the prime objects of land drilling, particularly in Hawaii, is finding intrusions of molten rock (magma) near the surface. Their presence helps forecast where and when such rock may escape as lava. In areas such as the Salton Sea in the southeast of California, drilling has helped map hot water deposits for possible energy production.

Ultimately, energy might be the greatest resource from deep drilling. Theoretically, one can tap the heat of radioactive decay in the earth’s core by digging deeply enough from any spot on the surface. The Iceland Deep Drilling Project attempts to tap naturally occurring steam or hot water, which occurs much closer to the surface. Crews for the project had drilled a hole over fifteen thousand feet deep in the hopes of being able to run a geothermal power plant at the site. Research programs have attempted to inject fluids, such as water, to return heat to the surface. A limitation on this technology is the cost of drilling to the required depths compared with prices of competing energy sources. Cheaper drilling methods could lead to greater use of this energy source. Likewise, improvements in drilling technology could increase the amount and variety of minerals extracted by drilling. Drilling concluded in January 2017, with a final depth of 4,659 meters (15,285 ft).

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