The Change in Geomechanical Properties of Gas Saturated Methane Hydrate‐Bearing Sand Resulting From Water Saturation

Abstract

AbstractLaboratory tests were carried out on gas‐saturated hydrate‐bearing sand (HBS) specimens that were subsequently water saturated to determine the influence of water saturation on their small‐strain (stiffness) and large‐strain (strength) behavior. Hydrate formed using the “excess gas” method led to significant increases in stiffness and strength of the sand. The magnitude increase in stiffness was largest for the lowest hydrate saturation tests and was sensitive to the distribution of hydrate within the sand. In contrast, increases in strength were directly related to increases in hydrate saturation, and to a lesser extent, on the applied confining stresses. Subsequent water saturation of these gas‐saturated HBS (achieved by flushing up to three times the pore volume of water through the HBS specimen) led to ∼90% and ∼70% reduction in stiffness and strength, respectively, along with a 37% reduction in hydrate volume (initial hydrate pore saturation ∼21.5%). Specimens with higher initial hydrate saturations (∼48%) saw slightly smaller reductions in stiffness and strength associated with ∼17% reduction in hydrate saturation. Changes in behavior appear to be related to the reduction of “cementing” hydrate at particle contacts through dissociation/dissolution of the hydrate, with remaining hydrate being more “pore filling.” Processes that may induce fluid flow in a natural gas‐saturated HBS, such as during the depressurization of the HBS reservoir during hydrate production, may lead to changes in hydrate morphology and affect the geomechanical properties of the HBS even when the HBS are still under hydrate stability conditions.