Coupling Geomechanics and Transport in Naturally Fractured Reservoirs

Document Type : Research Paper


University of Waterloo, Waterloo, Ontario, Canada


Large amounts of hydrocarbon reserves are trapped in naturally fractured reservoirs which are
challenging in terms of accurate recovery prediction because of their joint fabric complexity and
lithological heterogeneity. Canada, for example, has over 400 billion barrels of crude oil in fractured
carbonates in Alberta, most of this being bitumen of viscosity greater than 106 cP in the Grosmont
Formation, which has an average porosity of about 13-15%. Thermal methods are the most common
exploitation approaches in such viscous oil reservoirs which, in the case of steam injection, are associated
with up to 275-300°C temperature changes, leading to considerable thermoelastic expansion. This
temperature change, combined with pore pressure changes from injection and production processes, leads
to massive effective stress variations in the reservoir and surrounding rocks. The thermally-induced
(thermoelastic) stress changes can easily be an order of magnitude greater than the pore pressure effects
because of the high intrinsic stiffness of the low porosity limestone and bounding strata. Study of these
stress-pressure-temperature effects requires a thermo-hydro-mechanical (THM) coupling approach which
considers the simultaneous variation of effective stress, pore pressure, and temperature and their
interactions. For example, thermal expansion can lead to significant joint dilation, increasing the
macroscopic, joint-dominated transmissivity by an order of magnitude in front of and normal to the
thermal front, while reducing it in the direction tangential to the heating front. This leads to strong
induced anisotropy of transport processes, which in turn affects the spatial distribution of the heating
arising from advective heat transfer.