Patterns and Failure Modes of Fractures Resulting From Forced Folding of Cohesive Caprocks – Comparison of 2D vs. 3D and Single-vs. Multi-Layered Analog Experiments

Abstract

Knowledge of the formation mechanisms and geometries of fracture systems in sedimentary rocks is crucial for understanding local and basin-scale fluid migration. Complex fracture networks can be caused by, for instance, forced folding of a competent sediment layer in response to magmatic sill intrusion, remobilisation of fluidized sand or fluid overpressure in underlying porous reservoir formations. The opening modes and geometries of the fractures mainly determine the bulk permeability and sealing capacity of the folded layer. In this study, we carried out laboratory analog experiments to better comprehend patterns and evolution of the fracture network during forced folding as well as differences of the fracture patterns between a 2D and 3D modelling approach and between a homogenous and a multi-layered cover. The experimental layering consisted of a lower reservoir layer and an upper cover, which was either a single high-cohesive layer or an alternation of low- and high-cohesive layers. The two configurations were tested in an apparatus allowing quasi-2D and 3D experiments. Streaming air from the base of the model and air injected through a needle valve was used to produce a regional and a local field of fluid overpressure in the layers. The experimental outcomes reveal that the evolution of the fracture network undergoes an initial phase characterized by the formation of a forced fold associated with dominantly compactive and tensile fractures. The second phase of the evolution is dominated by fracture breakthrough and overpressure release mainly along shear fractures. Structures observed in 2D cross sections can be related to their expressions on the surface of the 3D respective experiments. Furthermore, the experiments showed that the intrusion network is more complex and laterally extended in the case of a multi-layered cover. Our results can be instructive for detecting and predicting fracture patterns around shallow magmatic and sand intrusions as well as above underground fluid storage sites.

Fractures and faults across intrusion-induced forced folds: a georesource perspective

“…n is the number of fracture traces mapped. Exp1B and Exp2B (Warsitzka et al 2022) and SPCTIN06 (Poppe et al 2019), showing its evolution can be described by power-law or exponential relationships. Reductions in connectivity may be attributed to resurfacing of the forced fold by erupted products (see Fig.…”

“…However, there are some uncertainties in the measurement of forced fold length and amplitude: (1) we can rarely establish the original surface topography prior to emplacement and folding, so often cannot accurately constrain true amplitudes; (2) syn-or post-emplacement deposition of sediments or resurfacing by lavas may alter apparent forced fold heights or regional base levels (e.g. Dobb et al 2022;Warsitzka et al 2022) ; (3) fold crests may have been eroded (e.g. Hansen and Cartwright 2006); and/or (4) measurements from 2D seismic reflection data, which are rarely depth-converted and decompacted (Magee et al 2019), or physical model crosssections may not intersect forced fold maximum amplitude or length (e.g.…”

“…The 3D seismic reflection survey (T38) used to map 'Fold B' has a vertical and horizontal resolution of up to ~68 m, if we consider each is equivalent to a quarter of the seismic wavelength (λ/4) (Hansen and Cartwright 2006;Brown 2011). I also generate fold and fracture maps for physical experiments using select published images (see Supplementary Material) (Montanari et al 2017;Henriquet et al 2019;Poppe et al 2019;Montanari et al 2020;Warsitzka et al 2022).…”

“…Castro et al 2016;Chadwick Jr et al 2019). I compare these mapped fold and fracture geometries to forced folds recognised in seismic reflection data (Hansen and Cartwright 2006) and generated in physical experiments (Montanari et al 2017;Henriquet et al 2019;Poppe et al 2019;Montanari et al 2020;Warsitzka et al 2022). With these data, I aim to constrain fracture network characteristics of intrusion-induced forced folds.…”

Analysis of influencing factors on wellbore instability for high-pressure gas well during testing and production

“…Wellbore instability failure is usually divided into two categories, one is a shear failure, and the other is a tension failure. The current research focuses on shear failure (Zhao et al, 2021;Hou et al, 2022b;Hou et al, 2022c;Warsitzka et al, 2022), while the strength of extension is frequently used in analyzing tensile failure for wellbore instability during testing and production (Tan et al, 2021;Hou et al, 2022d;Sun et al, 2022;Hou et al, 2022e;Huang et al, 2023).…”