Gas hydrates are recognized as a massive source of fossil fuel that could be far in excess of conventional hydrocarbon resources. The evaluation of formations that contain gas hydrates is therefore receiving renewed emphasis through contemporary petrophysical technology. A key factor is the use of logging-while-drilling (LWD) to sense hydrate-bearing intervals before drilling-induced thermal invasion and thence hydrate dissociation take hold. Recent advances in LWD technology have brought most of the potentially diagnostic tools onto the drill string, so there is little disadvantage in not having a wireline database. Moreover, modern tools have a sharper spatial resolution and a greater capability for differential depths of investigation. Petrophysical models have to be capable of distinguishing hydrates from ice in permafrost regions: this complication does not exist subsea. In general, pristine hydrates are characterized by high resistivity, low sonic transit time, and low density, possibly in conjunction with gas shows from mud logs. High neutron porosity can also be diagnostic away from permafrost. Other tools with a role to play include dielectric tools, for distinguishing ice from methane hydrate; electrical imagers, for identifying the mode of hydrate occurrence; and magnetic resonance tools, for contributing to estimates of hydrate volume by difference, because of hydrate invisibility. These matters are important, because a hydrate-supported structure will not produce as well as a framework-supported structure due to pore collapse with dissociation. The evaluation process is guided by some laboratory measurements of recovered and synthetic hydrates and sand/hydrate mixtures. The methodology is illustrated by field examples. Although formation evaluation for gas hydrates remains largely semi-quantitative, current interests are driving towards interpretation protocols that target estimates of producibility. Indicators are provided as to how this objective might be best approached.
Gas hydrates or clathrates were discovered in 1810 by the British chemist Sir Humphry Davy. They were first sampled in deep ocean sediments in the Black Sea (Yefremova and Zhizhchenko, 1974) and subsequently through the Deep Sea Drilling Project (DSDP) (Kvenvolden and Barnard, 1983). Today, they are recognized as being present in all continents. Indeed, it has been estimated that gas hydrates contain hydrocarbon resources, typically methane, considerably in excess of the global fossil-fuel energy base. Their potential energy value has assumed a high profile, because it brings together issues of Earth, sea, environment, climate and society.
Because it is technically difficult and expensive to sample preserved gas hydrates for laboratory measurement, well logs are especially important in the evaluation of gas-hydrate-bearing intervals. The use of well logs to evaluate gas hydrate formations has received attention over the past 25 years in a manner that was naturally conditioned by the logging technology of the day. Mathews (1986) set out the log signature of methane hydrate using information from the DSDP and the North Slope of Alaska. Subsequent analyses include Prensky (1995), Collett (1998a), Lovell et al. (2002), and Murray et al. (2005).
This paper provides a contemporary overview of the petrophysical evaluation of formations that contain gas hydrates with a view to their exploitation. The description adopts the terminology of the Petroleum Resource Management System (PRMS) published by the Society of Petroleum Engineers and others (March 2007). Thus, for example, the term "hydrocarbon volume" quantifies a gas volume in place referred to standard conditions of 60 °F and one atmosphere. On the other hand, the term "resources" relates to that portion of an in-place volume (at standard conditions) that is considered to be recoverable and that may be undiscovered or discovered. Yet again, the term "reserves" relates to resources that are discovered, remaining (in the subsurface), recoverable, commercial, and for which there exists a development plan.
