Petrophysical evaluation of gas-hydrate formations

Abstract

ABSTRACT 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 in the subsea environment. 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 logs, for distinguishing ice from methane hydrate; electrical imagers, for identifying laminated hydrate formations; and magnetic resonance logs, for contributing to estimates of hydrate volume by difference, because of hydrate invisibility to these tools. The mode of hydrate formation is especially important, because a hydrate-supported structure will not produce as well as a framework-supported structure due to pore collapse with dissociation. A proposed workflow for the petrophysical evaluation of gas hydrates is guided by field examples. This is set against the backdrop of a hydrate categorization scheme, which brings together the type mode of hydrate formation at the pore scale, the type class of hydrate occurrence at the reservoir zonal scale, and the resolvability of mesoscale hydrate-bearing layers by contemporary logging tools. Although the formation evaluation of gas hydrates remains largely semi-quantitative, current interests are driving towards data-driven interpretation protocols that target estimates of producibility. Indicators are provided as to how this objective might be best approached.