Mesoscale structure, mechanics, and transport properties of source rocks' organic pore networks

Abstract

Organic matter is responsible for the generation of hydrocarbons during the thermal maturation of source rock formation. This geochemical process engenders a network of organic hosted pores that governs the flow of hydrocarbons from the organic matter to fractures created during the stimulation of production wells. Therefore, it can be reasonably assumed that predictions of potentially recoverable confined hydrocarbons depend on the geometry of this pore network. Here, we analyze mesoscale structures of three organic porous networks at different thermal maturities. We use electron tomography with subnanometric resolution to characterize their morphology and topology. Our 3D reconstructions confirm the formation of nanopores and reveal increasingly tortuous and connected pore networks in the process of thermal maturation. We then turn the binarized reconstructions into lattice models including information from atomistic simulations to derive mechanical and confined fluid transport properties. Specifically, we highlight the influence of adsorbed fluids on the elastic response. The resulting elastic energy concentrations are localized at the vicinity of macropores at low maturity whereas these concentrations present more homogeneous distributions at higher thermal maturities, due to pores' topology. The lattice models finally allow us to capture the effect of sorption on diffusion mechanisms with a sole input of network geometry. Eventually, we corroborate the dominant impact of diffusion occurring within the connected nanopores, which constitute the limiting factor of confined hydrocarbon transport in source rocks.

Keywords: electron tomography; fluid transport; mechanics; mesoscale; porous media.

Fig. 1.

Morphological characterization of the organic mesoporous networks imaged in electron tomography. Aperture maps of the organic pore networks of (A) LEF (φ_meso = 9.1%), (B) HAY (φ_meso = 19.3%), and (C) MAR (φ_meso = 16.9%). (D) Pore-size distributions from aperture maps calculation. (E) In-pore CLDs. (F) Specific surface areas (A_s) calculation from Eq. 1 as a function of the cutoff distance (r_c) of the in-pore C.L.D.

Fig. 2.

Elastic simulations of the two porous mesostructures originating from LEF and MAR under two types of agent: (a) imposed strain boundary, and (b) adsorbed fluid with free strain boundary. The averaged strain of the simulation box is $\langle {\mathit{ϵ}}_{ii}\rangle$ = 0.01 for all cases. The size is 700 × 700 pixels with a resolution of 0.42 nm for LEF and 0.35 nm for MAR. Mesopore phase is in white. (Top) Elastic energy density fields (10⁶ J/m³). (Bottom) Distribution of the elastic energy density (10⁶ J/m³) for each simulation.

Fig. 3.

Homogenized fluid transport properties of the mesoscale lattice models. (A) Homogenized diffusion coefficient of the MAR reconstruction scaled by $D_{nano}$ as function of the diffusion contrast $r_D=D_{meso}/D_{nano}$ for different concentration ratios $r_{\rho }={\rho }_{meso}/{\rho }_{nano}$ indicated by colors. The corresponding curves stand for the effective medium theory (Eq. 2) where the value of $\gamma {\left(r_{\rho }\right)}^2$ is $\overline{D}/D_{nano}$ when $D_{meso}=D_{nano}$ (i.e., $r_D=1$). (B) Concentration-dependent obstruction factor $\gamma {\left(r_{\rho }\right)}^2$ as function of the probability $p\left(r_{\rho }\right)$ for a random walker to cross the interface from the nano- to the mesoporous phase (red: LEF, green: HAY, blue: MAR). The corresponding curves display the analytical model (Eq. 4) where ${\gamma }_0^2$ is simply taken as ${\gamma }_0^2=\gamma {\left(p=0\right)}^2$. (C) Homogenized diffusion coefficient of methane for LEF (red) and MAR (blue) as function of the bulk fluid pressure. The dashed curves correspond to the diffusion in the nanoporous phase only, while the solid lines represent the homogenized model with mesoporosity. Standard geological conditions (T = 400 K and lithostatic pressure of 25 MPa) have been considered.