In the far north of Alaska, near the giant Prudhoe Bay oil field, an international research consortium has been studying the potential of an altogether different energy source. In late December of 2018, drilling operations confirmed the existence of two high-quality reservoirs that were fully saturated with a potential alternative fuel source: gas hydrate.
Test Well Confirms Two Gas Hydrate Reservoirs in Alaska North Slope
The test well also provided critical geologic and reservoir data on gas hydrate
Fire Frozen in Ice
Gas hydrate, also known as methane hydrate, is made up of solid crystalline structures filled with methane gas. Because it requires very specific temperatures and pressure to exist, gas hydrate is usually found either in permafrost regions or beneath the sea in sediments of the outer continental margins. Because of its potential as an energy source, gas hydrate has garnered increasing attention as governments and companies seek to explore its potential.
Focus of Research in the Far North
The number of technical discoveries regarding gas hydrate has advanced at a rapid pace in recent years. As a leading voice in international gas hydrate research, the U.S. Geological Survey has contributed substantially to the discourse. The recent revelations from the test well near Prudhoe Bay are the latest step in a productive partnership between the USGS, the Department of Energy (DOE) through the National Energy Technology Laboratory, and the Japan Oil, Gas and Metals National Corporation, also known as JOGMEC.
The Alaska North Slope has long been a focus for gas hydrate research. In 2008, the USGS published the very first assessment of technically recoverable gas hydrate resources there, estimating 85 trillion cubic feet of natural gas that could be recovered from hydrate-bearing formations throughout the region.
Then, in 2015, the DOE-USGS-JOGMEC team, with assistance from the Alaska Department of Natural Resources, identified a promising site within the Prudhoe Bay area where an existing, but unused, gravel pad adjacent to an existing road was identified overlying a promising gas hydrate accumulation. Over the next two years, the team developed an operational drilling plan that enabled the needed science to be conducted in a manner that would not disrupt industry’s ongoing field operations in the area.
Confirmation of Potential, and a few Firsts as Well
After an exhaustive planning effort, the research partnership moved forward with a stratigraphic test well, which was drilled and completed in December 2018 by BP Exploration (Alaska) Inc. (BP) as the Prudhoe Bay Unit Operator. BP drilled the well using the Parker 272 rotary drilling rig through a Drilling Services Agreement executed with Petrotechnical Resources of Alaska (PRA) in association with a contract between DOE and PRA. The Stratigraphic Test Well confirmed the occurrence of highly-saturated gas hydrate-bearing reservoirs, which were designated Unit B and Unit D.
Unit B, the deeper of the two reservoirs at 844 meters (about 2770 feet) below the surface, is comprised of well-sorted, very fine-grained sand to coarse-silts. The hydrate resides in the spaces between the grains of sand and silt (aka the porosity), and was interpreted to be filling 65 percent to more than 80 percent of that porosity in the upper 40-feet of the unit.
Unit D, the shallower of the two reservoirs at 700 meters (about 2300 feet) below the surface, has similar saturation ranges as Unit B. In addition, Unit D has a water-bearing section at its base, which could provide opportunities to investigate additional scientific and well design issues as a potential follow-on to testing in Unit B.
The stratigraphic test well also collected pressurized core samples from the wall of the borehole. Special equipment was inserted into the wellbore, sent down to the depths of the gas hydrate reservoir, and drilled into the borehole wall to collect samples of sediment and gas hydrate. The samples were kept at the same pressure and temperature they had underground, which is important when studying gas hydrate, which tends to break down when the pressure decreases.
This test well is the initial phase of a planned three-well program designed to conduct an extended duration test of the response to gas hydrate reservoirs to controlled depressurization. Depressurization is an essential element of potential future production of natural gas from the hydrate.
In addition to confirming the gas hydrate reservoirs for future testing, the well drilled in December was instrumented to serve as a monitoring well during future field operations.
The pressurized sidewall core samples have been sent to labs in Japan and United States for analysis of reservoir sediment properties.
The ultimate goal is for the project partners to finalize the design of the remaining wells, surface production facilities, and testing procedures. Should the project progress to the next phase, these plans will allow the implementation of efficient and safe scientific production test that will address a range of questions associated with the response of gas hydrate bearing reservoirs to depressurization.
Start with Science
Although exciting, this success is just the next step in a more than 35-year cooperative research program devoted to the evaluation of the resource potential of gas hydrate in Alaska. As a founding member of this research effort, USGS will continue to provide its partners with valuable science for planning and operation of the planned gas hydrate production test in Alaska.
In addition to the Alaska North Slope research, the USGS Gas Hydrates Project has been participating in and sometimes managing large-scale drilling projects that investigate the resource potential of gas hydrate in Alaska, offshore India, and the Gulf of Mexico; providing unique electron microscopy imaging capabilities and special high-pressure laboratory facilities to study hydrate in conditions close to its natural state; producing the first assessment of technically-recoverable gas hydrate resources; characterizing the physical properties of hydrate bearing sediments to constrain reservoir properties; unraveling the possible synergies between gas hydrate breakdown and environmental change; and acquiring data to image the distribution of these deposits on the U.S. Atlantic, Gulf of Mexico, and Beaufort Sea margins.
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