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Gas hydrates are an important potential source of unconventional natural gas. Significant progress has been made with regard to understanding geologic and engineering limitations of the ultimate energy potential of gas hydrate; however, more work is required. The complete paper reviews the results of gas hydrate engineering and production testing studies associated with northern Canada and Alaska. The results of the marine gas hydrate producing testing efforts in Japan’s Nankai Trough and in the South China Sea are also summarized.
Mallik (Canada)
The Mallik gas hydrate research site in northern Canada has been the focus of three important gas hydrate field tests (in 1998, 2002, and 2007–08). The Mallik 2L-38 gas hydrate research well (part of the 1998 testing project) was drilled to evaluate the geologic controls on the occurrence of gas hydrate and to acquire specialized core and well data needed to characterize reservoir properties.
During the 2002 project, gas hydrate was produced for the first time by both depressurizing and heating the reservoir. Depressurization alone appeared to be the most-feasible method for producing gas hydrates. However, because of the limited nature and duration of the 2002 tests, it was determined that a longer duration test would be required, leading to the 2007–08 research program.
A 12-m-thick sand-rich hydrate-bearing reservoir was tested at a downhole flowing pressure of approximately 7.3 MPa. The fact that gas hydrates can be produced by depressurization techniques was demonstrated. The winter 2006 operations included a 6-day depressurization flow, which was able to establish a sustained and stable gas flow rate averaging approximately 3000 m3/d. The total volume of gas and water produced over the duration of the test was approximately 13 000 and 100 m3, respectively.
Alaska North Slope (US)
The occurrence of gas hydrate on the Alaska North Slope is associated closely with well-characterized petroleum systems. The Mount Elbert gas hydrate test well, in 2007, showed a mobile water phase within hydrate-bearing reservoirs even at very high gas hydrate saturations. The recognition of the presence of a mobile water phase within gas hydrate reservoirs was an important development in that it provided the means, or pathway, to draw the pressure down on hydrate-bearing reservoirs. The PBU L-Pad area was identified as the optimal site for the subsequent Iġnik Sikumi gas hydrate production test.
The 2011 Iġnik Sikumi field test involved the drilling of a single near-vertical test well and conducting wireline logging of targeted hydrate-bearing reservoirs. In 2012, the Iġnik Sikumi field testing program included a carbon dioxide (CO2)/methane (CH4) hydrate production test and an extended duration depressurization flowback test.
One of the notable scientific accomplishments of the trial was the identification of a specific mixture of nitrogen (N2) and CO2 gas that prevented the formation of secondary CO2 hydrate in the reservoir, which, in turn, allowed for the injection of CO2 into the reservoir being tested. The Iġnik Sikumi test demonstrated successfully that CO2 could be injected into a water-bearing reservoir under conditions that would usually form secondary CO2 hydrates, CH4 was then produced from the reservoir, and N2/CO2 exchange technology was shown to be technically feasible.
Nankai Trough (Japan)
In 2000, the Japan Oil, Gas and Metals National Corporation led an effort to drill a gas hydrate prospect to confirm the occurrence of gas hydrate in the Nankai Trough. Drilling, downhole logging, and coring at the test site documented the presence of several thick hydrate-bearing coarse-grained reservoir sections.
In 2001, the Ministry of Economy, Trade and Industry launched a new project with a similar goal of assessing the energy resource potential of gas hydrates offshore Japan. However, this project was intended to go much further, with the goal of developing the technology required to produce gas hydrates commercially. The multiwell drilling project was conducted in early 2004. A total of 16 sites (32 wells) was established in water depths ranging from approximately 700 to 2000 m. The occurrence of pore-filling gas hydrate in turbidite sand reservoirs was confirmed from the analysis of downhole logs and cores.
The MH21 program in 2013 completed the first test of gas production from marine gas hydrates after a test site, named the AT1 site, was established. At the site, a production test well, two monitoring wells, and a core well were established. An extensive logging-while-drilling (LWD) and wireline-logging program was conducted in the AT1-MC well to characterize gas hydrate reservoir properties and to select the stratigraphic section to be tested later.
The gas hydrate production flow test was conducted at the AT1 site in the spring of 2013. The AT1 production test started on 12 March and was completed on 19 March, earlier than planned because of sand-production problems and deteriorating weather conditions. The cumulative volume of gas and water produced during the 6-day test at AT1 was estimated at approximately 120 000 and 1300 m3, respectively. Subsequent test results are detailed in the complete paper.
South China Sea (China)
The Guangzhou Center for Gas Hydrate Research was established in 2004 to conduct energy-focused laboratory and field studies offshore China. In June 2007, the Guangzhou Marine Geological Survey (GMGS) successfully completed a deepwater gas hydrate drilling and coring program (GMGS1) in the South China Sea. In 2015 and 2016, two additional geoscience expeditions (GMGS3 and GMGS4) were conducted in the Shenhu gas hydrate region. Both drilling expeditions featured LWD implementation, with 19 sites drilled in the area during GMGS3 and 11 during GMGS4.
During GMGS3, pore-filling, strata-bound gas hydrates were discovered above the bottom simulating reflector at Site W17. The occurrence of relatively higher gas hydrate saturations at Site W17 is controlled by contribution of thermogenic gas sources, gas migration into the gas hydrate stability zone from below, and the physical properties of the sediments hosting the gas hydrates.
In 2017, the China Geological Survey conducted an industrial pilot gas hydrate production test in the Shenhu area. The water depth at the test site was 1266 m, and the hydrate-bearing reservoir section was at a depth of 203–277 m below the seabed. The production test lasted for a total for 60 days, recovering 309 046 m3 of gas, at a mean daily production rate of 5151 m3/d.
Challenges
Gas hydrate energy resource studies being conducted in Korea, India, Canada, China, Japan, and the United States have made significant contributions to our understanding of gas hydrates. Scientific and industrial drilling has confirmed that gas hydrates are an abundant potential resource. However, fully understanding the role that gas hydrates may play as a future energy resource will require more work.
The emergence of other unconventional gas resources and more-traditional sources of energy represents a significant challenge to the goal of commercial production of gas hydrates. In most settings, the potential volume of gas associated with a given gas hydrate accumulation is unknown and the technology to produce gas hydrates is unproven. Gas hydrates generally occur in deep marine and Arctic environments, where high operational cost represents a significant challenge to the commercial production of gas hydrates.
Limited economic modeling has shown that the commercial production of gas hydrates may be possible; however, many unknowns still exist. The effect on gas hydrate commercialization of specific national interests and local motivations needs to be considered, including taxation and climate-change policies, development of industry and government partnerships, the design of purpose-built gas hydrate drilling and production systems, local industrial use of produced gas, and access to other energy resources.
Major technical challenges and potential opportunities that will need to be dealt with on the path to the commercial production of gas hydrates include the following:
Gas Hydrate Resource Characterization
- Refine current gas hydrate resource assessments, with a focus on moving from mostly in-place gas volume assessments to technically recoverable assessment and eventually to reserve estimates
- Develop and integrate gas hydrate system modeling, laboratory studies, and field surveys
- Develop, test, and deploy new field-characterization tools to address important gas hydrate research requirements, along with the development of new gas hydrate prospecting procedures
Gas Hydrate Production Technology
- Advance the development of new gas hydrate production models that incorporate advanced macro- and pore-scale mechanical models
- Conduct laboratory, modeling, and field-scale analysis of stimulation techniques that may enhance gas hydrate production
- Review and apply to gas hydrate production existing and new completion technologies, including horizontal completions and multilateral drilling
- Characterize potential drilling, completion, and production concerns associated with producing gas hydrates
- Assess the effect of gas hydrate production on the physical and mechanical properties of gas hydrate reservoir systems
