Drilling with drag bits (PDC bits) simultaneously involves fragmentation of rock by the cutters and frictional contact on the cutter wear flats. While there is reasonable understanding of the forces arising from the cutting process, knowledge of the factors affecting the contact forces on the wear flats is still fragmentary. This paper focuses on determining the parameters that influence the mean normal stress σ on the cutter wear flats, and on mapping the dependence of σ on these parameters, by analyzing the idealized problem of a slightly inclined rigid slider moving on the surface of a Mohr-Coulomb elastoplastic half-plane.
The evolution of enhanced geothermal systems (EGS) entails spatially and temporally evolving permeability fields. During non-isothermal fluid injection, thermo-elastic stress and fluid pressure changes act upon partially open or hydrothermally altered fracture sets to enhance formation permeability. The physical couplings that drive this behavior are non-linearly dependent upon one another to varying degrees. To explore these interactions we are developing a simulator capable of coupling the dominant physics of shear stimulation using a variety of methods, allowing flexibility in the use of monolithic or staggered numerical schemes. The new simulator uses standard Galerkin and control-volume finite elements to balance fluid mass, mechanical deformation, and thermal energy with consideration of local thermal non-equilibrium and/or dual-porosity heat exchange between fluids and solids or fractures and intact rock. Similarly, changes in mechanical stress and fluid pressure can be rigorously coupled in single or multiple continua. Permeability is allowed to evolve under several constitutive models tailored to both porous media and fractures, considering the influence of thermo-hydromechanical stress, creep, and elasto-plastic shear and dilation in a ubiquitously fractured medium. From this basis we explore the coupled physical processes that control the evolution of permeability during shear stimulation and long-term evolution of a geothermal reservoir.
Conventional stability analysis of rock slopes during earthquakes is often confined to vertically propagated ground motions. One of the main reasons for the limitation is due to the lack of a rational yet simple way to apply an input motion of a specific incident angle. In this study a method of ground motion input was implemented for this application in analyzing a jointed rock slope. In a nutshell, it involved extending a problem domain by wrapping it with an elastic region and converting the incident waves into nodal forces in addition to make a boundary non-reflecting. The approach is general, but here only the plane strain problem was tackled and thus only two dimensional P-wave was considered. A jointed rock is defined by an elastic modulus, a Poisson ratio, the joint orientations and the strength of the joints. A 1:1 jointed rock slope of 30 m in height was used as the base rock slope. An El Centro record of 1979 was employed as the base ground motion. Three incident angles were considered in the analysis. Results obtained indicated that incident angles do have a significant impact on the stability of a jointed rock slope, and the severity of the impact depended on the relative orientation of the incident wave with respect to that of the joints.
The geological storage of Co2 and the extraction of geothermal energy from deep saline aquifers are promising methods, of mitigating global warming and obtaining a valuable source of renewable energy. In such processes the porosity and permeability of the reservoir rock are the two governing factors. Porosity can be easily predicted and is conventionally used to identify permeability properties. However, such identification is inaccurate, because the presence of irregularities such as bedding and authigenic quartz significantly reduces the permeability of even highly porous rock. This paper investigates the effect of bedding and the presence of authigenic quartz on brine permeability in sandstone using two types of sandstones with similar porosities, Warwick white (WWS) and Warwick golden (WGS), from the Queensland Basin, Australia. Permeability tests were conducted on 38mm diameter and 150mm long sandstone samples for a range of injection pressures (4 – 7MPa) under 10 MPa confining pressure at 30°C temperature, using a highly precise core flooding apparatus. The experimental data provide important data for the geological storage of Co2 and the production of geothermal energy from deep saline aquifers.
The complex fracture network or stimulated reservoir volume (SRV) can be induced by hydraulic fracturing of the unconventional reservoirs. The SRV dimension is one of the main drivers in a horizontal well performance after the hydraulic fracturing operation. It is of great importance to simulate the SRV dimensions to identify the optimum hydraulic fracturing treatment parameters. In this research, a new analytical model is proposed to accurately simulate the SRV dimension created from hydraulically fractured horizontal wells in unconventional reservoirs. More specifically, a SRV dimensional model is developed to simulate SRV dimensions using effective stresses, injected slurry volume and other reservoir and pumping data during the generation of the hydraulic fracture network. The SRV dimensional model is calibrated using microseismic data from 6 stages of a hydraulic fracturing job in a horizontal well penetrating the Glauconite formation in Hoadley field, Alberta, Canada. The calibrated SRV dimensional model can serve as an optimal fracture spacing estimator for future hydraulic fracture job designs. The average simulated SRV width is smaller than the average fracture port spacing and therefore for this study it is suggested to have the fracture port spacing tighter and equal with the simulated SRV width for optimum production.
The crack damage progression in crystalline rocks is approximated in laboratory by means of rigorous strain measurement and/or monitoring of Acoustic Emission (AE) activity. When both means are used, they are treated independently for quantification of damage in the rock. This paper is investigating a new method to combine the AE and strain data in a unified function to calculate the balance of stored and released energy in the rock due to loading (strain energy) and micro-cracking respectively. This method introduces a new solution for measurement and quantification of crack damage in rock and also provides a tool to investigate the brittleness of different rock types. Unconfined Compressive Strength (UCS) testing of six different rock types with strain measurement and AE monitoring was performed for this study. The application of the new method to the data collected from the UCS tests indicates the difference between the behaviour of the various rock types in terms of sudden energy release at the onset of CI threshold and the difference in the storability of strain energy before and after CI and CD thresholds.
In order to construct mountain tunnels rationally and safely, observational construction method has been employed. The necessity for such observation relating to the behavior of tunnel and surrounding ground means that there are so many uncertainties in the tunnel construction process. Uncertainties are inaccuracy in the geological survey before construction start, many assumptions in the design of tunnel supports, difference between design and constructed result and so on. Especially rock mass classification is important, because a design of tunnel supports and tunneling method is carried out based on the pre-investigated result under the condition of budget and construction period. However the item in pre-investigation might be limited. And rock mass class selected in the design is often different from that of actual condition. In this paper, rock mass classification result based on the advancing boring from tunnel face is compared with actually constructed rock mass class. Authors also discussed about rock mass classification process with geological engineers who have had many experiences in pre-investigation and advancing boring. As a result, it was suggested that rock mass classification table given in the standard cannot exactly provide a good result. So rock mass classification process should be examined in detail and a new scheme to have more reliable classification results should be discussed.
We present a numerical model for the simultaneous initiation and subsequent propagation of multiple transverse hydraulic fractures from a horizontal wellbore. In particular, we investigate the efficiency and robustness of the multistage hydraulic fracturing technique. We restrict the created hydraulic fractures to remain radial and planar but fully account for the stress interaction between fractures, the fluid flow in the wellbore and across the different perforation clusters which are modeled via a classical relation between the friction pressure drop and the flow rate entering a given fracture. The initiation is modeled from a radial notch of given initial length using linear elastic fracture mechanics. The solver models the complete pressurization of the wellbore, the initiation of the different fractures and their propagation and interactions. The split of the fluid between the different clusters is part of the solution at each time-step. We present some validations and a case study investigating the effect of a number of heterogeneities (in-situ stress etc.) on the robustness of the limited entry technique.
When a falling rock block strikes the ground, the block does not bounce much. This energy loss can be simply represented by the coefficient of restitution for mass point motion. In this paper, dependence of the tangential component of the coefficient of restitution on the incoming angle and the surface friction angle is examined under the normal component being kept constant. A small scale laboratory test is reported to get a better understanding during/at impact. Under the condition in which the normal component has set to be (roughly) constant, it has been recognized that the tangential component of the coefficient restitution Ret is a roughly constant when the incoming angle is small and then increases with the increase in the slope angle. This nature of impact is explained by ‘sticking’ behavior and the transition to slippage. In the transition zone, the ratio of Er/Ev (rotational/translational energy) has been decreased, however, we cannot conclude that this transition behavior result in the apparent friction reduction with the momentum or sticking with some slippage during the contact.
Single Polycrystalline Diamond Compact (PDC) cutter tests were conducted with cutters of different diameters and chamfer sizes on Carthage Marble rock samples at pressurized conditions. The cutter size does not seem to affect the frictional response; however, the chamfer size has a considerable influence on the friction and aggressiveness of the cutter. Unlike the normal force, the cutting force was observed to be insensitive to the chamfer size for the same cut depth, in the tested conditions. Data analysis was performed and resulted in introducing an empirical relationship between the cutter back rake angle and the friction angle. This was then used to obtain the equivalent back rake angle for each chamfered cutter as a function of cut depth. The empirical relationship was also used to clarify the relation between the shear angle and back rake angle. The relationship was then combined with a recently developed analytical model for rock cutting to develop a combined model that accounts for the complex relationship between back rake angle and friction angle on the face of cutter. The combined model demonstrates decent predictability for Shale.
This study has significant implications in cutter design, as it provides a more robust semi-empirical basis to account for the cutter geometry. It is also useful in enhancing current PDC bit simulations.