Timothy S. Collett

Natural Gas Hydrates: A Review

A strong upward trend exists for the consumption of all energy sources as people throughout the world strive for a higher standard of living. Someday, possibly soon, the earths store of easily accessed hydrocarbons will no longer satisfy our growing economies and populations. By then, an unfamiliar but kindred hydrocarbon resource called natural gas hydrate may become a significant source of energy. Approximately 35 years ago, Russian scientists made what was then a bold assertion that gas hydrates, a crystalline solid of water and natural gas and a historical curiosity to physical chemists, should occur in abundance in the natural environment. Since this early start, the scientific foundation has been built for the realization that gas hydrates are a global phenomenon, occurring in permafrost regions of the arctic and in deep-water parts of most continental margins worldwide. The amount of natural gas contained in the worlds gas-hydrate accumulations is enormous, but these estimates remain highly speculative. Researchers have long speculated that gas hydrates could eventually be a commercial producible energy resource, yet technical and economic hurdles have historically made gas-hydrate development a distant goal instead of a near-term possibility. This view began to change in recent years with the realization that this unconventional resource could possibly be developed with the existing conventional oil and gas production technology. The pace of gas-hydrate energy assessment projects has significantly accelerated over the past several years, but many critical gas-hydrate exploration and development questions still remain. The exploitation and potential development of gas-hydrate resources is a complex technological problem. However, humans have successfully dealt with such complicated problems in the past to satisfy our energy needs; technical innovations have been key to our historical successes.

Relationship of gas hydrate concentration to porosity and reflection amplitude in a research well, Mackenzie Delta, Canada

Abstract Well logs acquired at the Mallik 2L-38 gas hydrate research well, Mackenzie Delta, Canada, reveal a distinct trend showing that the resistivity of gas-hydrate-bearing sediments increases with increases in density porosities. This trend, opposite to the general trend of decrease in resistivity with porosity, implies that gas hydrates are more concentrated in the higher porosity. Using the Mallik 2L-38 well data, a proportional gas hydrate concentration (PGHC) model, which states that the gas hydrate concentration in the sediments pore space is linearly proportional to porosity, is proposed for the general habitat of gas hydrate in sediments. Anomalous data (less than 6% of the total data) outside the dominant observed trend can be explained by local geological characteristics. The anomalous data analysis indicates that highly concentrated gas-hydrate-bearing layers would be expected where sediments have high proportions of gravel and coarse sand. Using the parameters in the PGHC model determined from resistivity–porosity logs, it is possible to qualitatively predict the degree of reflection amplitude variations in seismic profiles. Moderate-to-strong reflections are expected for the Mallik 2L-38 well.

Chapter 8 – Methane Hydrates

Gas hydrate is a solid, naturally occurring substance consisting predominantly of methane gas and water. Recent scientific drilling programs in Japan, Canada, the United States, Korea and India have demonstrated that gas hydrate occurs broadly and in a variety of forms in shallow sediments of the outer continental shelves and in Arctic regions. Field, laboratory and numerical modelling studies conducted to date indicate that gas can be extracted from gas hydrates with existing production technologies, particularly for those deposits in which the gas hydrate exists as pore-filling grains at high saturation in sand-rich reservoirs. A series of regional resource assessments indicate that substantial volumes of gas hydrate likely exist in sand-rich deposits. Recent field programs in Japan, Canada and in the United States have demonstrated the technical viability of methane extraction from gas-hydrate-bearing sand reservoirs and have investigated a range of potential production scenarios. At present, basic reservoir depressurisation shows the greatest promise and can be conducted using primarily standard industry equipment and procedures. Depressurisation is expected to be the foundation of future production systems; additional processes, such as thermal stimulation, mechanical stimulation and chemical injection, will likely also be integrated as dictated by local geological and other conditions. An innovative carbon dioxide and methane swapping technology is also being studied as a method to produce gas from select gas hydrate deposits. In addition, substantial additional volumes of gas hydrate have been found in dense arrays of grain-displacing veins and nodules in fine-grained, clay-dominated sediments; however, to date, no field tests, and very limited numerical modelling, have been conducted with regard to the production potential of such accumulations. Work remains to further refine: (1) the marine resource volumes within potential accumulations that can be produced through exploratory drilling programs; (2) the tools for gas hydrate detection and characterisation from remote sensing data; (3) the details of gas hydrate reservoir production behaviour through additional, well-monitored and longer duration field tests and (4) the understanding of the potential environmental impacts of gas hydrate resource development. The results of future production tests, in the context of varying market and energy supply conditions around the globe, will be the key to determine the ultimate timing and scale of the commercial production of natural gas from gas hydrates.

ANALYSES OF PRODUCTION TESTS AND MDT TESTS CONDUCTED IN MALLIK AND ALASKA METHANE HYDRATE RESERVOIRS: WHAT CAN WE LEARN FROM THESE WELL TESTS?

Pressure drawdown tests were conducted using Schlumberger’s Modular Formation Dynamics Tester™ (MDT) wireline tool in the Mallik methane hydrate (MH) reservoirs in February 2002 as well as in the Mount Elbert (Alaska) MH reservoirs in February 2007, while a production test was conducted applying a depressurization method in one of the Mallik MH reservoirs in April 2007. All of these tests aimed at measuring production and bottomhole pressure (BHP) responses by reducing BHP below the MH stability pressure to estimate reservoir properties such as permeability and MH dissociation radius. We attempted to analyze the results of these tests through history matching using the numerical simulator (MH21-HYDRES) coded especially for gas hydrate reservoirs. Although the magnitude of depressurization and the total duration spent for these tests were almost identical to each other, the simulation studies revealed that there existed significant differences in what could be inferred and could not be inferred from test results between a MDT test and a production test. The simulation studies mainly clarified that (1) the MDT tests were useful to estimate initial effective permeability in the presence of MH, (2) when BHP is reduced below the MH stability pressure at MDT tests, the pressure and temperature responses were significantly influenced by the wellbore storage erasing all the important data such as those indicating a radius of MH dissociation and effective permeability after partial MH dissociation, and (3) history matching of production tests tended to result in multiple solutions unless establishing steady flow conditions. This paper presents the results of history matching for the typical MDT and production tests conducted in Mallik and Alaska MH reservoirs. This paper also discusses the parameters reliably estimated through MDT and production tests, which should provide many suggestions on future designs and analyses of short-term tests for MH reservoirs.

PRELIMINARY REPORT ON THE ECONOMICS OF GAS PRODUCTION FROM NATURAL GAS HYDRATES

Economic studies on simulated natural gas hydrate reservoirs have been compiled to estimate the price of natural gas that may lead to economically viable production from the most promising gas hydrate accumulations. As a first estimate, large-scale production of natural gas from North American arctic region Class 1 and Class 2 hydrate deposits will be economically acceptable at gas prices over

Relative Permeability Measurements of Gas-water-hydrate Systems

CDN2005 10/Mscf and

Methane Hydrate Field Program. Development of a Scientific Plan for a Methane Hydrate-Focused Marine Drilling, Logging and Coring Program

CDN2005 17/Mscf, respectively, provided the cost of building a pipeline to the nearest distribution point is not prohibitively expensive. These estimates should be seen as rough lower bounds, with positive error bars of

Geomechanical response of permafrost-associated hydrate deposits to depressurization-induced gas production

5 and

Regional long-term production modeling from a single well test, Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

10, respectively. While these prices represent the best available estimate, the economic evaluation of a specific project is highly dependent on the producibility of the target zone, the amount of gas in place, the associated geologic and depositional environment, existing pipeline infrastructure, and local tariffs and taxes. Class 1 hydrate deposits may be economically viable at a lower natural gas price due largely to the existing free gas, which can be produced early in project lifetimes. Of the deposit types for which hydrates are the sole source of hydrocarbons (i.e. Class 2, 3, and 4 deposits), theoretical simulation studies imply that Class 2 deposits may be the most likely to be economically viable (with all else equal) due to assistance that removal of the underlying free water will provide to depressurization; thus

Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope: Overview of scientific and technical program

CDN2005 17/Mscf can be seen as a lower bound on the natural gas price that may render hydrate deposits economically acceptable in the absence of free gas. Results from a recent analysis of the production of gas from marine hydrate deposits are also considered in this report [6]. On a rate-orreturn (ROR) basis, it is approximately