About 30 years ago Sandia Labs developed polycrystalline diamond compact (PDC) bits for geothermal drilling. Today nearly two-thirds of the oil we use comes from wells drilled using the (PDC) bits. That’s technology gone astray, valuable – and still missing the original point. Now Sandia and the U.S. Navy recently brought the technology back full circle, showing how geothermal drillers might use the original PDC technology with added decades of improvements by the oil and gas industry.
Polycrystalline diamond compact cutters on the cutting faces of bits allow more aggressive drilling than bits traditionally used for geothermal drilling. PDCs are created by a sintering process. Starting with a tungsten carbine cutting face graphite powder is applied. The whole assembly of materials is compressed in three directions at pressures of 1 million pounds per square inch. When heated to a transition temperature, the graphite converts a to a 1-millimeter layer of synthetic diamond.
NOV Helios Drill Cutter
Three decades ago, Sandia played a large role in developing PDCs for geothermal drilling. That work focused on resolving issues with materials, devising laboratory tests and developing data and design codes that now form the basis of the rock drill bit industry. With new funding Sandia is out to improve the PDC bits, which should increase access to geothermal resources in the continental U.S. by enabling the drilling of deeper, hotter geothermal resources in hard basement rock formations.
Sandia principal investigator David Raymond explains because oil and gas drilling is generally less complicated than geothermal drilling; PDCs were first used to drill for oil and gas. “Oil and gas drilling is normally done in softer and less-fractured rock, resulting in fewer problems with fluid circulation to remove debris and cool the bit,” he said. “Oil and gas drilling also doesn’t usually involve the higher temperatures that geothermal wells exhibit.”
Over the past 30 years the petroleum exploration business has changed. The industry looks for new sustainable resources in deeper reservoirs encountering more difficult drilling conditions – similar to those found in geothermal drilling.
Raymond said, “Oil and gas drilling must now go deeper into the ground, into harder and sometimes fractured rocks, and in hotter environments.”
Geothermal resources are usually found in igneous and metamorphic rocks, base planetary crust rock that’s hard, tough and difficult to penetrate. Sedimentary rocks through which most oil and gas wells are drilled are compressed dirt, mud and sand. They’re “softer’, not so tough and penetrate much more easily.
The igneous and metamorphic rocks also can contain large amounts of abrasives such as quartz, which can cause vibration and accelerated wear that damages drill bits. When a cutting face meets different materials as it sweeps along, the different resistance slows and speeds the rotation, often at damaging frequencies. These types of rocks are often fractured, which can change the impact loading on drills and cause more damage. The gap of a fracture or crack releases cutting face pressure and when the face comes into fresh material again there’s an impact causing damage.
Raymond said, “Drilling for geothermal energy is still the most difficult drilling on a cost-per-foot basis. You have to go through the hardest rock, sometimes at high temperatures and pressures. The Department of Energy vision for advanced geothermal development is to drill to great depths, up to 30,000 feet, to access heat for geothermal.”
Another factor that took the three decades to come back to geothermal is tens of thousands of oil and gas wells are drilled per year. That gives the industry the resources and can invest significantly in research and testing to improve the ability to drill under increasingly difficult conditions.
Geothermal on the other hand, has advanced far more slowly. Geothermal drilling rig operators drill only a small number of new holes each year. That makes each well represent a substantial risk – so the geothermal drilling service industry finds it difficult and expensive to support innovation.
The Sandia/Navy demonstration project called for a test hole to evaluate geothermal resources that wouldn’t be detectable at the surface in the Camp Billy Machen/Hot Mineral Spa. The rock at the Chocolate Mountains site includes granite and andesite – formations typically encountered during geothermal drilling.
A key part of the demonstration project was to test and evaluate PDC bits and related technologies in a real-world drilling environment. Sandia worked with PDC bit manufacturer National Oilwell Varco (NOV) of Houston to find specific solutions for the company’s ReedHycalog PDC bits. NOV provided commercially available drill bits and on-site experts to counsel the drilling contractor during the demonstration drill runs.
Sandia worked with the Navy’s geothermal drilling contractor, Barbour Well Inc. of Henderson, Nev., in evaluating drilling technologies during production drilling.
Sandia also formed partnerships with Albuquerque’s Prime Core Systems, and the Barbour Well mud logging company, Prospect Geotech, to field instruments to monitor the Barbour drill rig during the drilling process.
In the tests, two bits drilled 1,291 feet of the overall well depth of 3,000 feet. The two bits were in the well just over four days, penetrating approximately 30 feet per hour throughout their drilling interval. That’s nearly three times better than standard roller bits used for comparison. The team retrieved and downloaded downhole data from both bits for analysis.
In a planned second phase of the project, Sandia will continue work with NOV to evaluate drill performance and improve the bit design and materials.
30 years is a long wait. Geothermal is a nearly inexhaustible resource. So far geothermal doesn’t look economically viable. Something has to give – and it must be the capital cost, much of which is just getting access to the heat. Somehow technology has to get down deep on the cheap. Getting Sandia back to work and the petroleum industry bringing its experience with the new needs they have for very deep and very hot work is going to help.
It’s too soon to say if the new work on PDC faces will get to low enough cost – still, every savings of time, materials and inputs will help.
There’s a long way to go . . .
By. Brian Westenhaus