Quantitative 3D Analysis of Fracture Propagation Resulting from Composite Layering Effect in Unconventional Reservoirs

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2019-05-08

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Abstract

Before designing and creating a hydraulic fracture, it is important to understand the parameters that determine the fracture geometry and whether the fracture extends beyond the payzone. By fixing other mechanical parameters (i.e. Stress, Young’s modulus, Poisson’s ratio, and Fracture toughness), FracPro software was used in the first part of the study to investigate the effect of composite layering on the fracture geometry. Composite layering plays a major role in the fracture geometry and design process. Also, this study investigates how the lithology barriers and the Composite Layering Effect (CLE) of the targeted zone affect the fracture geometry. Based on the methodology of the study, the study consists of two major parts; the first part involved developing an equation for CLE, while the second involved the application of the equation through experimental work. The effect of permeability on fracture geometry at several CLE values was examined using three different scenarios (no permeability, low permeability, and high permeability). FracPro was used in the first part of the study for investigating the effect of composite layering on the fracture geometry. Several simulation cases were run with different CLEs (1, 10, 20, and 100), and the height-to-length ratios of the fracture were estimated. The generalized equation which was developed calculates the CLE from the height-to-length ratio of the fracture in the required range for the targeted zone. In order to inspect how fracture-free the shale and sandstone samples’ were, three of each were CT scanned and the images were produced using ImageJ software. Then the samples were partially fractured using Uniaxial Compressive Strength (UCS), in which a compressive-strength machine initiates fractures by applying uniaxial load and stopping automatically at a certain pre-determined load. Following that, a post-UCS CT scan was done and the resulting images were fed to ImageJ to read and visualize those images. After the samples were fractured; X-ray Powder Diffraction (XRD) and X-ray Fluorescence (XRF) techniques were executed to measure the percentage of minerals and elements in the core samples respectively. As the study used the CLE equation, it has proved that the equation is valid and applicable to similar lithology. Also, CLE has a significant impact on fracture behavior. Both the XRD and XRF results revealed the impact that minerals and elements of the rock composition on fracture dimensions. The existence of quartz and clay minerals having the strongest impact on the CLE value which is ascribed to the brittleness of quartz and ductility of the clay minerals (nacrite). Moreover, the highest CLE value was recorded for the shale sample that has a preexisting fracture.

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Keywords

Composite Layering Effect, height-to-length ratio, fracture geometry, X-ray Computed Tomography (CT), X-ray Powder Diffraction (XRD), X-ray Fluorescence (XRF).

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