Laboratory investigation of process zone stress

Abstract

This dissertation has as its main objective the study of the phenomena known as Process Zone Stress (PZS), which in the past, has been of high concern in the petroleum engineering industry during hydraulic fracture operations in conventional permeability reservoirs and has had a rebirth for tight shale reservoirs. In order to study PZS, a comprehensive review of fracture tip mechanisms, specifically of process zone stress was performed. The literature review provided a path towards the required experimentation to characterize this phenomena. Due to the subsurface nature of the oil and gas industry, a scaled methodology was applied to represent field operations in a laboratory dimensions for two lithologies; one a homogeneous tight sandstone, the Lyons sandstone, and the other a high clay, high organic material mud rock, the Niobrara shale. Besides the study of the two lithologies, the use of two different fluid viscosities and two different wellbore directions were studied to perform comparisons that provide valuable insight and complement past investigations. Achievement of fracture propagation under an anisotropic stress environment was accomplished for the two different rock types, using a normal faulting stress regime while inducing fractures of tensile nature (Mode I). A total of 20 tests were performed of which 14 provided reliable results that gave insight in the characterization of PZS in a number of conditions and environments. The experimental setup provided accurate monitoring of fracture propagation under anisotropic stress environments, that was then observed on a magnified view (thin sections) to characterize the crack layer at its passive and active zones with the task of observing the development of PZS. In terms of PZS it was found that the Lyons sandstone behaved as an homogeneous, isotropic material validating the application of Linear Elastic Fracture Mechanics (LEFM). The characteristics of PZS for the Lyons sandstone showed measured Fissure to Grain Ratio (FGR) density increased where connected to the main crack and decreased when moving away. The microcracks had an azimuth parallel to the main crack. The view at the tip showed the expected mushroom cloud of heterogeneous micro fissures that provide an energy dissipation of the fracture's propagating strength. For the case of the Niobrara shale, this mud rock provided a different behavior of the crack and its tip that contradicts LEFM but provides a sense of how mud rocks crack and what is developed around them. The observations seen in the Niobrara bring to the analysis the relevance of rock fabric in terms of fracture propagation and tip mechanisms. We were able to observe a different phenomena not seen in any other studied rock type. The expected pressure breakdown values for the Lyons sandstone was within the expected as well as for the less stiff Niobrara shale. The use of pressure curves combined with the derivative of pressure in terms of time was used to identify fracture initiation to then estimate the time for the material to fail at pressure breakdown.