Capillarity-controlled displacements in sediments with moveable grains: Implications for growth of methane hydrates

Authors: Maša Prodanovic (speaker), Steven L. Bryant

Venue: SPE Annual Technical Conference and Exhibition, Denver, Colorado, 21-24 September, 2008. http://www.spe.org [external site].

Abstract: We consider immiscible displacements when fluid/fluid interfaces are controlled by capillary forces. The progressive quasistatic (PQS) algorithm based on the level set method readily determines the geometry of these interfaces at the pore level. Capillary pressure generally exerts a net force on grains supporting an interface. We extend PQS to implement a kinematic model of grain displacement in response to that force. We examine the changes in the drainage curve caused by this coupling. We compute the interfacial area associated with the bulk water phase, anticipating preferential growth of methane hydrate there. Gas invasion of sediments is one mechanism by which methane hydrates are believed to form. In unconsolidated ocean sediments the capillary pressure exerted by an accumulated gas phase below the hydrate stability zone can be large enough to move grains apart. This motion alters the pore throat sizes which control subsequent drainage of the sediment. A model for the dynamics of this process is useful for assessing the competition between drainage (controlled by capillary forces) and fracturing (controlled by pore pressure and earth stresses). This in turn provides insight into the possible growth habits within the hydrate stability zone. When grains can move in response to net force exerted by the gas phase, small variations in an otherwise uniform distribution of pore throat sizes quickly lead to self-reinforcing, focused channels of gas phase. In contrast to behavior in stationary grains, the drainage curve exhibits no clear percolation threshold. Displacements in materials with broad throat size distributions also exhibit self-reinforcing channels. Behind the leading edge of the displacement front, the net force exerted on the grains tends to push them together. This effectively seals off these regions from subsequent invasion. Thus hydrate growth tends to be localized along the channel of displaced grains. This is the first quantitative grain-scale study of the drainage behavior when grains can move in response to invasion events. The coupling leads to qualitatively different displacement patterns. The method presented for studying this behavior is applicable to any granular material and to other applications, such as sand production.

Related NETL Project
This presentation is related to the NETL project DE-FC26-06NT43067, “Mechanisms Leading to Co-Existence of Gas and Hydrate in Ocean Sediments.” The goal of this project is to quantitatively describe and understand the manner in which methane is transported within the Hydrate Stability Zone (HSZ).

NETL Project Contacts
NETL – Robert Vagnetti (Robert.Vagnetti@netl.doe.gov or 304-285-1334)
University of Texas at Austin – Steven Bryant (Steven_Bryant@mail.utexas.edu or 512-471-3250)