ABSTRACT: Canada has a long history of underground hard rock mining. As mines deplete current resources, new technologies enable deeper deposits to be extracted economically. With increased depth, Canadian mines typically experience an increase in both quantity and severity of mining-induced seismicity. A Canada Wide Seismic Monitoring Survey was used to gather data pertaining to the current state of seismic monitoring in Canadian hard rock mines. This paper presents the initial survey results with a focus on: microseismic monitoring systems and data quality.
Seismic monitoring has existed in Canadian mines since the 1980's. Over the past four decades, significant advances in on-site microseismic and macroseismic monitoring have enabled technical and operational personnel to integrate the use of seismic data into daily operations. When a seismic event or rockburst occurs, details regarding size and location are available within minutes for communication with underground personnel.
For some Canadian mining operations mine seismicity has been a concern for a decade or more. Other sites are only now beginning to deal with workplace safety concerns resulting from dynamic rock mass failure. Seven of the twenty-five seismic monitoring systems currently operating in Eastern Canada have been installed within the past five years (Hudyma et al., 2016). In 2014, the Ontario Ministry of Labour released a list of mining related hazards ranked by an expert group of industry and worker representatives. The occurrence of a rockburst was risk ranked number one overall (Ontario Ministry of Labour, 2014).
The future of Canadian mining is deep. More than ten Canadian hard rock mines are currently operating at depths greater than 1500 metres below surface. Of these, four are currently mining more than 2000 metres below surface. With increased depth, typically comes elevated stress and an increased potential for seismicity and rockbursting.
The Canada Wide Seismic Monitoring Survey was designed to gather data pertaining to the current state of seismic monitoring in Canadian hard rock mines. The survey was provided as a Microsoft Excel workbook, to better enable site personnel to quickly and easily complete and electronically return the survey. Figure 1 provides an example of the survey format.
ABSTRACT: Wellbore instability problems play major rule in the increasing of the non-productive time (NPT) during drilling processes. In most cases, this cost can be reduced by designing a suitable operational window using geomechanical models. Several wellbore instability problems have been encountered during drilling Nahr Umr formation in an oil field in southern Iraq. These problems are including, but not limited to, mechanical stuck, caving, and tight holes. Data from more than twenty wells (vertical and deviated wells) are investigated to reveal the major factors that control the instability problems and to design an optimum mud window. In the present work, a 1-D mechanical earth model (MEM) was constructed using numerous field data for Nahr Umr formation. Based on the offset well data, open hole wireline logging measurements (e.g., density logs, gamma ray (GR) logs, sonic logs, formation micro-imager (FMI) logs, porosity logs, resistivity logs, drilling records, and mud logs (master logs)) the magnitude and orientations of the in-situ principal stresses, pore pressure, and rock mechanical properties were estimated. The 1-D-MEM was verified and calibrated using all the available data (i.e., drilling observations, caliper logs, results of image data interpretation, repeated formation test (RFT), hydraulic fracturing, and laboratory rock mechanical properties, etc.) such that it robustly and accurately predicts shear failure around given wellbores. The generated model was then coupled with three failure criteria (i.e., Mohr-Coulomb, Mogi-Coulomb, and Modified Lade) to analyze the existing wellbore stability problems for wells with directional profiles and to determine the appropriate mud weight to drill a well in any desired direction. Our analysis shows that the majority of the wellbore instability problems are mainly caused by; rock failure around the wellbore due to high stresses and low rock strength, and inappropriate drilling practice.
Despite the modern advancements and the usage of new technology in the oil and gas industry, wellbore instability remains one of the most challenging aspects in terms of the cost to drill and complete a well. Eight billion dollars are spent each year due to wellbore instability problems (Peng, 2007), causing an increase in the drilling budget by 10% (Aadnoy, 2003). Therefore, wellbore stability is considered to be one of the major stages of well planning and has been studied extensively (Bell, 2003; Bradley, 1979; Ding, 2011; Zhang et al., 2003; Zhang et al., 2009; Gentzis et al., 2009; Alsubaih et al., 2017; Abbas et al., 2018).
ABSTRACT: Cyclic steam stimulation has become an important method for enhancing the recovery of heavy oil reservoir. Accurate prediction of changes in pore pressure and stress within the near wellbore region during the stimulation is of critical importance for wellbore stability and sand production analyses. However, cyclic steam stimulation is a complicated process which involves complex interactions among multiphase fluid flow, heat transfer and elastoplastic deformation of the formation rock. In this present work, we developed a fully coupled thermo-hydro-mechanical model for simulating the cyclic steam stimulation of heavy oil reservoirs. A three-phase, two-component formulation is employed for characterizing the flow of oil and water/steam within the pore space. Elastoplastic deformation of the heavy oil reservoir rock is treated with the Mohr-Coulomb model. In addition to thermal conduction, thermal convection is also considered due to the high permeability frequently featured by heavy oil reservoirs. Mathematical equations governing these coupled physical processes are discretized and solved with the finite element method in a fully coupled manner. Validations of the model against analytical solutions to some simple problems have been performed, which demonstrate the capability of the model to capture the coupling behavior between pore fluid flow, heat transfer and deformation of the reservoir rock. As an example of application, the model has been applied to simulate the cyclic stream stimulation performed in horizontal wells drilled in a heavy oil reservoir in the Bohai oilfield of China. The sizes of the heated zones as well as the stress, temperature and pore pressure distributions around the wellbore under different injection parameters were predicted and some implications for wellbore stability have been presented.
Heavy oil is abundant around the world. However, it is hard to be extracted from the reservoir due to the extremely high viscosity at the reservoir temperature. Various thermal recovery methods have been proposed for significantly reducing the viscosity of heavy oil and realize economic production, among which Cyclic steam stimulation (CSS) has become an extremely successful and widely used technique for enhancing recovery of heavy oil (Vittoratos et al., 1990). CSS generally involves several days of injection of steam into a well drilled in the heavy oil reservoir, followed by a short period of soaking process and a long period of hot production of the heated oil from the same well. This injection-soaking-production cycle is usually repeated many times, and the amount of steam injected into the reservoir usually increases from one cycle to the next in order to extend the unheated zone.
ABSTRACT: Wellbore collapse as a result of severe borehole breakouts represents a major problem in many cases. In order to quantify the risk associated to wellbore collapse a reliable estimate of the collapse volume is necessary. In this study, a novel approach determining the depth/area/volume of collapse failure by using image processing approach is presented. Since image processing can be applied to any result set, the proposed approach is independent of any failure criterion (such as Mohr Coulomb, Mogi-Coulomb and Modified Lade criteria) and very versatile. For hydrocarbon fields where Mechanical Earth Modeling (MEM) approaches capable of predicting the spatial distribution of horizontal stresses exist, the presented image processing approach is utilized to generate an automated log of collapse volume while drilling. Based on this log, mud pressure adjustments can be undertaken while drilling a new well based on collapse volume. The main contribution of this work is the estimation of a real-time collapse volume log while drilling. It can help the drilling engineers in evaluating the mud weight effect on the hole cleaning efficiency to avoid stuck pipe problems. In addition, knowledge of the collapse volume provides better estimates on the required mud and cement volumes.
The obvious goal for drilling operators is to drill economical, safe, and stable wells by reducing non-productive time (NPT) due to wellbore stability problems such as borehole collapse and associated stuck pipe, and borehole breakdown and associated loss of circulation. A key issue for successful drilling operations in geomechanically challenging zones is considering all relevant factors including-formation strength properties, in-situ stresses, pore pressure, and applied pressure by the drilling mud. If collected, these data sets of rock strength and stress can be used to generate a Mechanical Earth Model (MEM; e.g., Abbas et al., 2018; Goodman and Connolly, 2007; Kristiansen, 2007; Gholami et al., 2014). Once the MEM is validated, it can be used to predict applications such as wellbore stability (Cheatham, 1984; Kaushik et al., 2016; Alkamil et al., 2017).
Swyer, M. W. (AltaRock Energy Inc.) | Cladouhos, T. T. (AltaRock Energy Inc.) | Forson, C. (Washington Geological Survey) | Steely, A. N. (Washington Geological Survey) | Davatzes, N. C. (Temple University)
ABSTRACT: Three geothermal prospect areas in Washington State were modeled using Poly3D, a boundary element code that simulates fault slip and volcanic magma chamber deflation which causes local stress perturbation within an elastic half-space. This work was done as a part of Play-Fairway Analysis for Washington State by the Washington Geological Survey, AltaRock Energy Inc., and Temple University. The region has complex tectonics which causes areal and 3D complexity in the crustal stress-strain field. Geodetic strain rate tensors from GPS velocities that represent surface block rotation as opposed to deep subduction were used to constrain the remote stress. 3D fault geometries were created for seismogenic faults using the earthquake catalog from the Pacific Northwest Seismic Network for the faults located at Mount St. Helens and the Wind River Valley. A magma chamber deflation model was used for Mount Baker that was best fit to a campaign geodetic survey on the volcano. The results of the modeling give insight into natural processes that cause dilational stress favorable for geothermal resources and are also used for siting temperature gradient holes.
The Washington State Geothermal Play-Fairway Analysis (PFA) is a project by the Washington Geological Survey, a division of the Washington State Department of Natural Resources. The project is funded by the US Department of Energy, with the main goal of developing a geothermal exploration methodology by adapting techniques for the oil and gas industry. The purpose of PFA is to minimize risk associated with geothermal resource development through geothermal favorability models that can be used to site exploratory drilling targets. The PFA incorporated simulations of fault and magma chamber deformation to address the complex tectonic setting of the Cascade Range, which can make fault and fracture permeability highly elusive in this region. Three prospect areas were selected from a prior statewide geothermal resource assessment by the Washington Geological Survey (Boschmann et al., 2014): 1) the Mount St. Helens Seismic Zone, 2) the Wind River Valley, and 3) Mount Baker (Fig. 1). Phase 1 of the PFA was a desktop study of publicly available data to assess the geothermal heat potential and fracture permeability of each prospect area (Norman et al., 2015). In Phase 2, geological and geophysical data were collected in the most favorable areas identified in Phase 1. The favorability maps were refined to a higher resolution for siting temperature gradient (TG) holes to be drilled in Phase 3 (Forson et al., 2017). Critical sources of modeled crustal deformation include subduction of the Juan De Fuca Plate under the North American Plate, crustal block rotations that manifest as slip on transtensional to transpressional strike-slip faults, and magma chamber contraction which causes local extensional stress. Superimposed brittle deformation from faults and magma chambers create zones of highly localized crustal dilation needed for hydrothermal fluid circulation. Distinguishing zones of compression and dilation and the areal distribution of these zones can be difficult to determine for non-idealized fault geometries and tectonic settings.
ABSTRACT: The estimation of the in-situ stress state is required for the design and execution of deep engineering operations related to Enhanced Geothermal System (EGS). Borehole failures, often referred as borehole breakouts, which are controlled by local stress concentration around the wellbore, are recognized being a useful indicator to assess in-situ stress conditions. However, breakouts evolve with time and this may affect our ability to use them for quantifying the stress state. We use a unique data set from the deep geothermal well of Rittershoffen GRT-1 in order to verify the hypothesis concerning wellbore breakout geometrical evolution. In GRT-1 wellbore, imaging has been acquired 4 days, 348 days and 946 days after drilling completion. Thermal, hydraulic and chemical stimulations have been performed between the first and the second image acquisition. Using this data set, we were able to describe in-situ the breakout evolution with time. We show increase in the extension of breakouts along the well. Contrary to the common assumptions, we also show that breakout widen, but within the limit of the accuracy of our analysis they do not deepen. The consequences of the breakout evolution for stress characterization are significant and add up to other important uncertainties in such analyses like the estimation of strength parameters.
A large amount of energy is available at depth. This energy can be extracted by circulating fluids between boreholes through the hot rock mass, but this requires that sufficient permeability is present at depth. As permeability tends to decrease with depth (Manning and Ingebritsen 1999), it is necessary to target deep structures with locally higher permeability (e.g. fault zones) and/or to perform permeability enhancement operations. The later approach is referred as Enhanced Geothermal Systems (EGS). The principle underlying this technology consists of increasing the hydraulic performance of the reservoir through different types of stimulations so that commercially interesting flow rate can be achieved. The stimulations consist of high-pressure injection (hydraulic stimulation), cold water injection (thermal stimulation) or chemical injection (chemical stimulation). In the two first cases, the permeability increase is obtained by inducing a thermohydromechanical perturbation to the rock mass which reactivates existing structures or create new ones. The in-situ stress state is central to understand the response of the rock mass to injections and to design such operations.
ABSTRACT: We use an evolutionary geomechanical model to study stress and deformation in sediments during the emplacement of a frontal-rolling salt sheet. We show that overturned-roof sediments develop high differential stresses and plastic strains. We illustrate that these high plastic strains may allow roof layers to overturn and fold below the advancing salt. Sediments fail during roof overturn but regain strength as they get buried below salt. We discuss that sediment strength and failure depend on the overall evolution of the salt system. We show that the salt-base geometry can provide a first order estimation of the level of shear as well as of the decrease in least principal stress below salt. We build our large strain models in the finite element program Elfen. We model salt as solid viscoplastic and sediments as poro-elastoplastic materials. Overall, our evolutionary models provide insights into the mechanics of salt-sheet emplacement, identify potential drilling hazards and help understand stress and deformation of basin sediments near salt.
A salt sheet is an allochthonous salt body sourced from a salt diapir, and whose breadth is several times greater than its maximum thickness (Jackson and Hudec, 2017). Salt sheets are a common feature in salt systems around the world, including a very strong presence in the Gulf of Mexico (Jackson and Hudec, 2017).
Salt is a solid viscous rock that cannot sustain deviatoric stresses over geologic time (Urai and Spiers, 2007). When subjected to differential stresses, for example by differential sedimentation or tectonic loading, salt flows until its stress state is uniform. During this process, it may form diapirs and salt sheets. In particular, salt sheets commonly form when the upward rise of the salt in a diapir is much faster than the local sedimentation rate (Hudec and Jackson, 2006).
The advance of a salt sheet imposes strains onto roof and underlying-basin sediments; hence, it perturbs their state of stress. Indeed, shear zones and high pore pressures are often encountered below salt (Dusseault et al, 2004, Harrison et al, 2004, House and Pritchett, 1995, O’Brien and Lerche, 1994, Willson et al, 2003, York et al, 2009, Zhang, 2013). As a result, exiting the base of a salt sheet is one of the most dangerous moments in subsalt drilling. There is a lot of uncertainty in evaluating the present-day stress, pore pressure, and deformation state of subsalt sediments because seismic imaging is poor immediately below salt (Israel et al, 2008, Jackson and Hudec, 2017, Perez et al, 2008). Under these conditions, one of the most plausible ways to predict stress and pressure anomalies subsalt is to understand how salt was emplaced and how it interacted with basin sediments during its emplacement.
ABSTRACT: Helium and argon are represented by known amounts in air. Helium is 5.2 ppm by volume in the atmosphere and primarily the result of the natural radioactive decay of heavy radioactive elements. Argon is the third most abundant gas in the Earth's atmosphere, 9340 ppm; radiogenic argon-40, is derived from the decay of potassium-40 in the Earth's crust. The isotopic signature of noble gases found in rocks is vastly different than that of the atmosphere which is contributed by a variety of sources. Geogenic noble gases are contained in most crustal rock at inter and intra granular sites, their release during natural and man-made stress and strain changes represents a signal of deformation. When rock is subjected to stress conditions exceeding about half its yield strength, micro-cracks begin to form. As rock deformation progresses a fracture network evolves, releasing trapped noble gases and changing the transport properties to gas migration. Thus, changes in gas emanation and noble gas composition from rocks could be used to infer changes in stress-state and deformation. An experimental system we developed combines triaxial rock deformation and mass spectrometry to measure noble gas flow real-time during deformation. Geogenic noble gases are released during triaxial deformation and that release is related to volume strain and acoustic emissions. The noble gas release then represents a signal of deformation during its stages of development. Gases released depend on initial gas content, pore structure and its evolution, and amount of deformation imposed. Noble gas release is stress/strain history dependent and pressure and strain rate dependent. Sensing of gases released related to both earthquakes and volcanic activity has resulted in anomalies detected for these natural processes. We propose using this deformation signal as a tool to detect subterranean deformation (fracture).
Noble gases present in crustal rocks are derived from groundwater (atmospheric origins), magmatic activity (mantle origins), and radioactive decay of natural radioactive elements (e.g. U, Th, and K). Radiogenic noble gases are produced within minerals and retained to different degrees within the minerals. The composition of gases within the mineral phase is a function of the mineralogical composition, the rock matrix and the thermal, tectonic, erosional and depositional history of the formation. These gases migrate to the adjacent pore fluid and/or fracture networks over geological time periods via diffusion, recoil and chemical dissolution. Transport of gases occurs within the rock grain, along grain boundaries, in the pore fluid and within the micro to macro fracture network. The transport is a function of the stress state of the rock and its control on chemical processes and the physical configuration of the grain and fracture networks. The work we present investigates the release of these gases as a function of the systematic change in stress state by application of stresses sufficient to fracture the rock. We hope to use the noble gas release and subsequent detection to signal deformation.
ABSTRACT: In northern North America, a huge amount of tight gas is trapped in relatively impermeable rock formations at great depths. Physical fracturing of these formations by fluid injection, i.e., hydraulic fracturing, could enhance the overall permeability of these formations, and thus improve tight gas extraction. One of the outstanding issues in hydraulic fracturing in tight formation is to determine the magnitude of the injection pressure for initiating cracks in intact formation, i.e., criterion for crack initiation in intact rock. This study extends Griffith theory to determine the crack initiation criterion in brittle rocks containing minute pores of different shapes under biaxial compression and pore pressure elevation. Analytical results reveal that the pore aspect ratio has a significant impact on the crack initiation near the pore tip. In addition, the combined effect of applied total stresses and internal pore pressure dictates if the initial cracks near the tip would become stable or grow by tensile or shear fracturing. These results have practical implications on permeability enhancement in tight gas shale by hydraulic fracturing.
The gas production rates from tight gas shale formations are extremely low due to its ultra-low permeability in a range of 10-19 m2 (Aguilera, 2014). Multiple hydraulic fracture treatments in horizontal wellbores are becoming a viable technology to economically develop unconventional resources in shale reservoirs. Hydraulically induced fractures radiate outward from the wellbore intersecting the nearby existing natural fractures. The resulting fracture network increases the drainage area and reduces the drainage paths within the stimulated rock volume (SRV). The total production and its rates are dependent on the extent of SRV and the fracture spacing. If the fracture spacings are large, the rates are still low. The production rate will be much enhanced if cracking could be induced in the intact tight shale inside SRV.
Micro cracks and fractures are detected in triaxial compression tests on intact rock specimens in laboratory and underground excavations in intact rock formation in the field. Griffith theory (1920, 1924) provided a starting framework to explain the cracking process from existing flaws, defects or pores in intact rocks. However, there is no or limiting studies on crack initiation and propagation in intact rock under pore pressure elevation in hydraulic fracturing treatment.
ABSTRACT: The seismic response to mining can vary significantly throughout a rock mass. Seismicity may be primarily driven by local mining activities (commonly blasting), or other factors such as geological discontinuities and tectonic loading. During regular mine operation, it can be challenging to determine the extent to which rock mass failure is generated from blast-induced stress change. Mine shutdown periods provide a unique opportunity to examine the seismic response in the absence of mine blasting. This paper focuses on utilizing total radiated seismic energy to differentiate between different rock mass failure mechanisms surrounding a mine shutdown. Examples are provided from a single deep Canadian hard rock mining operation.
Seismic events, or dynamic rock mass failure, are a common occurrence in many deep and high stress underground mining environments. Mining activities, such as blasting, cause stress redistribution throughout the rock mass that can generate induced and triggered seismic responses. Induced seismicity refers to seismic events that occur as a direct result of a causative activity (i.e. mine blast). When mine blasting generates a stress change of similar magnitude to the in-situ shear stress acting on a fault (McGarr et al., 2002), the failure is considered induced. Mining-induced seismic events are closely related, both in space and time, to mine blasting. Triggered seismicity refers to seismic events that are not attributed directly to a specific causative activity (mine blast). In other words, the stress change resulting in the seismic event is relatively small compared to the in-situ stress (McGarr et al., 2002). Triggered seismic events may therefore be distant to blast locations and/or blast times.
The seismic response to mining, commonly induced or triggered, varies throughout the rock mass and is a product of the complex interaction between factors such as rock mass geological and stress conditions. Understanding, and to some extent anticipating, a specific seismic response can play a critical role in mine operation procedures and decisions. Induced seismic events pose significantly less risk to underground mining operations, as these events are typically associated directly with mine blasting, and the related risk can be mitigated using isolation techniques such as exclusion zones and re-entry protocols. Triggered seismicity however, typically occurs distant to mine blasts and at unrelated times. As a result, it is very challenging to proactively isolate these areas within a mine, resulting in a larger risk to the operation.