Experimental Study on Fracturing Fracture Deformation Mechanism of Shale Reservoir

After large-scale sand fracturing of horizontal wells in shale gas reservoir, fracturing fractures will deform in the production process. However, fracture deformation will lead to the decrease in fracture conductivity and then cause the decrease of gas well productivity. Therefore, in order to evaluate the fracturing fracture deformation mechanism of shale reservoirs, the shale proppant-supported fracture deformation evaluation experiments were carried out under different proppant types, particle sizes, sanding concentrations, and closure pressure conditions, respectively, and the variation curves of fracture width was calculated by a stereomicroscope under different experimental conditions. Then based on the experimental results, the fracture sensitivity factors and fracture deformation mechanism were analyzed, and the deformation mechanisms of fracturing fractures affected by proppant embedding and crushing were studied emphatically. The analysis results of fracture sensitivity factors indicate that the larger the particle size and hardness of proppant, the lower the sand concentration, proppant embedded on the shale rock surface. Moreover, the deeper the proppant is embedded, the faster the fracture conductivity decreases. In addition, the greater the closure pressure, the more serious is the proppant embedment, and the faster the fracture width decreases. The analysis results of fracture deformation mechanism show that, on the on hand, under variable closure pressure, the proppant with larger hardness and larger particle size is used for fracturing, and the proppant embedded in the fracture surface is the main cause of fracture deformation. However, if only the sand concentration of the proppant in the fracture is changed, the fracture deformation is jointly dominated by the embedding and crushing of the proppant. On the other hand, under constant closure pressure, the main mechanism of fracture deformation is that the proppant is embedded into the fracture surface when the closure pressure is low, but if the closure pressure is high, the main mechanism of fracture deformation is the crushing and compaction of proppant.

In Situ Deformation Analysis of a Fracture in Coal under Cyclic Loading and Unloading

The deformation analysis of fractures is vital for advantageous development of oil and gas fields, especially the coalbed methane (CBM) reservoir, since the change of fracture parameters can be directly evaluated through fracture deformation analysis. Then the flow capacity of CBM and the effect of various stimulation methods can be analyzed. In this study, X-ray CT image analysis is used to quantitatively characterize the deformation of a coal fracture in situ, and the evolution of fracture aperture under cyclic loading is presented. Furthermore, dimensionless permeability at different confining pressures by the Lattice Boltzmann method is simulated. The current results indicate that the fracture deformation changes significantly under cyclic loading. A dramatic change is observed in the initial loading stage, in which the coal is strongly compacted, and the fracture aperture and permeability are reduced to 13.9% and 0.1%, respectively, when the confining pressure is loaded to 10 MPa. When unloading to 0 MPa, the fracture aperture and dimensionless permeability are far less than that of the initial 0 MPa. It is worth noting that the deformation of the second cycle fracture is weaker, and the change range of permeability and aperture of coal fracture becomes smaller, but when unloading to 0 MPa in the second cycle, the fracture permeability can be restored to 50.8% compared with 0 MPa of the loading stage. Additionally, a special phenomenon has been observed that under cyclic loading, even when the confining pressure reaches 10 MPa, some areas of the fracture are still not closed. We infer that there are some large pore structures in fracture space, and high confining pressure leads to fracture closure, but the deformation of the pore structure is not prominent compared with the fracture space. These characteristics of fracture deformation are of great significance for the production of CBM and are worthy of further study.

Efficient Modeling of Depletion Induced Fracture Deformation in Unconventional Reservoirs

The change of fracture conductivity during reservoir depletion significantly affects the well performance and stress evolution in unconventional formations. A common practice is to model fracture deformation using the traditional finite element method with very dense unstructured grids representing complex fracture geometries. However, the associated computational cost is high, so previous studies mainly use empirical correlations to catch the fracture conductivity loss or neglect fracture deformation during the production period. This work proposes a novel coupled flow and geomechanics model with embedded fracture methods to capture the fracture deformation accurately yet efficiently in unconventional reservoirs. Under a single set of structured grids, an embedded discrete fracture model (EDFM) is employed to characterize fluid flow through discrete fractures by introducing non-neighboring connections, and an extended finite element method (XFEM) is applied to simulate discontinuities over fracture walls by adding phantom nodes. In addition, a modified proppant model is incorporated to represent interactions between proppants and hydraulic surfaces, and an iterative coupling scheme is implemented to link the fracture-related fluid flow and solid mechanics. Being validated against the classical benchmark problem, the coupled model is used to investigate the impacts of proppant strength, closure stress, and bottomhole pressure on fracture deformation, well production, and in-situ stresses. Numerical results indicate that weaker proppant support induces more fracture aperture and production losses, resulting in greater stress changes and higher residual pressure in the depletion region. In comparison, the fracture deformation for a well-propped scenario is modest and barely affects the well performance and stress redistribution. Less stressed formation corresponds to lower closure stress on fracture walls, which triggers limited fracture closure and stabilizes well production. Moreover, a moderate bottomhole pressure decline rate avoids significant fracture closure while preserves relatively high initial production rates. The coupled flow and geomechanics model with embedded fracture methods resolves computational difficulties in modeling complex fracture deformations and delivers more insights on production forecast and stress changes crucial to refracturing and infill operations.

Importance of fracture deformation throughout hydraulic testing under in-situ conditions

Summary In this work, we propose a hydro-mechanical simulation model to study the strong interaction of fluid flow and fracture deformation under in-situ stress conditions. The general model is reduced under physics-based assumptions to provide an efficient numerical approach for inverse analysis of experimental studies and is applied to experimental field data obtained from hydraulic tests conducted at the Grimsel Test Site (GTS), Switzerland. The present set of hydro-mechanical measurement data provides not only valuable information about the transient pressure and flow evolution but also the transient change of fracture deformation. We aim to introduce a strongly-coupled hydro-mechanical model to numerically characterize the fractured reservoir based on experimental data below the limit of hydraulically induced irreversible changes of the reservoir’s properties. Insights into the leading mechanisms of flow processes throughout hydraulic testing under in-situ conditions are then gained by best numerical fits of the measurement data. Based on the experimental and numerical findings, this study emphasizes the importance of a consistent consideration of local and non-local fracture deformation throughout inverse analysis of hydraulic testing data to a) better understand hydro-mechanical flow processes in fractured reservoirs and b), to increase the estimation quality of hydraulic properties of tested fractures.

Non-normal flow rules affect fracture angles in sea ice viscous–plastic rheologies

The standard viscous–plastic (VP) sea ice model with an elliptical yield curve and a normal flow rule has at least two issues. First, it does not simulate fracture angles below 30∘ in uni-axial compression, in contrast with observations of linear kinematic features (LKFs) in the Arctic Ocean. Second, there is a tight, but unphysical, coupling between the fracture angle, post-fracture deformation, and the shape of the yield curve. This tight coupling was identified as the reason for the overestimation of fracture angles. In this paper, these issues are addressed by removing the normality constraint on the flow rule in the standard VP model. The new rheology is tested in numerical uni-axial loading tests. To this end, an elliptical plastic potential – which defines the post-fracture deformations, or flow rule – is introduced independently of the elliptical yield curve. As a consequence, the post-fracture deformation is decoupled from the mechanical strength properties of the ice. We adapt Roscoe's angle theory, which is based on observations of granular materials, to the context of sea ice modeling. In this framework, the fracture angles depend on both yield curve and plastic potential parameters. This new formulation predicts accurately the results of the numerical experiments with a root-mean-square error below 1.3∘. The new rheology allows for angles of fracture smaller than 30∘ in uni-axial compression. For instance, a plastic potential with an ellipse aspect ratio smaller than 2 (i.e., the default value in the standard viscous–plastic model) can lead to fracture angles as low as 22∘. Implementing an elliptical plastic potential in the standard VP sea ice model requires only small modifications to the standard VP rheology. The momentum equations with the modified rheology, however, are more difficult to solve numerically. The independent plastic potential solves the two issues with VP rheology addressed in this paper: in uni-axial loading experiments, it allows for smaller fracture angles, which fall within the range of satellite observations, and it decouples the angle of fracture and the post-fracture deformation from the shape of the yield curve. The orientation of the post-fracture deformation along the fracture lines (convergence and divergence), however, is still controlled by the shape of the plastic potential and the location of the stress state on the yield curve. A non-elliptical plastic potential would be required to change the orientation of deformation and to match deformation statistics derived from satellite measurements.

A generalized stress correction scheme for the MEB rheology: impact on the fracture angles and deformations

A generalized damage parameterization is developed for the Maxwell Elasto-Brittle (MEB) rheology that reduces the growth of residual errors associated with the correction of super-critical stresses. In the generalized stress correction, a decohesive stress tensor is used to bring the super-critical stresses back on the yield curve based on any correction path. The sensitivity of the simulated material behaviour to the magnitude of the decohesive stress tensor is investigated in uniaxial compression simulations. Results show that while the decohesive stress tensor influences the short-term fracture deformation and orientation, the long-term post-fracture behaviour remains unchanged. Divergence first occurs when the elastic response is dominant followed by post-fracture shear and convergence when the viscous response dominates – contrary to laboratory experiment of granular flow and satellite imagery in the Arctic. The post-fracture deformations are shown to be dissociated from the fracture process itself, an important difference with classical Viscous Plastic (VP) models. Using the generalized damage parameterization together with a stress correction path normal to the yield curve brings the simulated fracture angles closer to observations (from 40–50° to 35–45°, compared to 20–30° in observations) and reduces the growth of errors sufficiently for the production of longer-term simulations.

Ground-Based Radar Interferometry of Sea Ice

In light of recent Arctic change, there is a need to better understand sea ice dynamic processes at the floe scale to evaluate sea ice stability, deformation, and fracturing. This work investigates the use of the Gamma portable radar interferometer (GPRI) to characterize sea ice displacement and surface topography. We find that the GPRI is best suited to derive lateral surface deformation due to mm-scale horizontal accuracy. We model interferometric phase signatures from sea ice displacement and evaluate possible errors related to noise and antenna motion. We compare the analysis with observations acquired during a drifting ice camp in the Beaufort Sea. We used repeat-scan and stare-mode interferometry to identify two-dimensional shear and to track continuous uni-directional convergence. This paper demonstrates the capacity of the GPRI to derive surface strain on the order of 10−7 and identify different dynamic regions based on sub-mm changes in displacement. The GPRI is thus a promising tool for sea ice applications due to its high accuracy that can potentially resolve pre- and post-fracture deformation relevant to sea ice stability and modeling.