Scale-dependent fracture patterns and flow in low-enthalpy geothermal targets: the role of diagenesis and contingent nodes

Fracture network patterns in the Earth may have a significant effect on the success of geothermal energy optimization in sedimentary rocks. To develop simulation tools and characterize, quantify, and eventually manipulate Earth properties, accurate descriptions and conceptual models of networks are needed across wide scale ranges.

Contingent nodes. Forstner & Laubach 2022

Our studies show that under diagenetic conditions between ca. 50 ℃ to 250 ℃ the systematics of cement precipitation and differential infill makes network porosity, and thus permeability and strength, scale and thermal history dependent.

For example, using examples of regional opening-mode fractures in sandstones from the Cambrian Flathead Formation, Wyoming, a low-enthalpy geothermal outcrop analog, Forstner & Laubach (2022) show that quartz deposits preferentially fill fractures up to ca. 0.05 mm wide with a transition from mostly sealed to mostly open fractures over a narrow size range of opening displacements from 0.05 to 0.1 mm.

In this example, although networks have trace connectivity, effective connectivity for fluid flow is greatly reduced by quartz cement. Near some faults, trace connectivity increases as initially wide porous fractures preferentially shear and wing cracks form, increasing fracture intersections (Y-nodes). However, pore space is lost due to the development of quartz-cemented microbreccia. Macro-scale trace connectivity increases, but porous connectivity diminishes and thus potential for fluid flow is markedly lower.

Scale- and diagenesis- dependent connectivity can be described using use rule-based node descriptions to rapidly measure diagenesis sensitive connections within the context of current field practice.

Ongoing work in large outcrop analogs in the western US and New York show how diagenesis-sensitive contingent nodes can be used with diagenesis information or models to extrapolate permeability estimates to locations having different thermal histories. The outcrop analog data sets include quantitative 2D spatial analysis (Corrêa et al., 2022; Shakiba et al. 2023) and use diagenesis information to reconstruct fracture patterns through time.

Preliminary results suggest the types of patterns that develop are also sensitive to mechanical and cement interaction (Lee et al., 2022).

Plans are being made for field-based seminars of the Wyoming and New York outcrops.

Corrêa, R.S.M., Marrett, R., Laubach, S.E., 2022. Analysis of spatial arrangement of fractures in two dimensions using point process statistics. Journal of Structural Geology 163, 104726. | view at publisher

Forstner, S.R, Laubach, S.E., 2022. Scale-dependent fracture networks. Journal of Structural Geology 165, 104748. doi.org/10.1016/j.jsg.2022.104748 | view at publisher

Lee, B., Olson, J.E., & Laubach, S.E., 2022. Coupling diagenesis and mechanics for more realistic simulation of natural fracture pattern development, Second International Meeting for Applied Geoscience & Energy, AAPG IMAGE 2022. | view at publisher

Shakiba, M., Lake, L.W., Gale, J.F.W., Laubach, S.E., Pyrcz, M.J., 2023. Multiscale spatial analysis of fracture nodes in two dimensions. Marine & Petroleum Geology 149, 106093. doi.org/10.1016/j.marpetgeo.2022.106093 | view at publisher