Geomechanical Factors Research Articles

An investigation of the effects of important grouting and rock parameters on the grouting process

ABSTRACTToday, grouting is used as an aid in ground improvement in most civil and mining engineering projects. Groutability and grout penetration depth are among the most important issues that are considered in grouting operation. Various parameters such as in-situ stress, pore water pressure, joint geometric and geomechanical characteristics, grout properties (viscosity and yield stress) and technical factors such as grouting pressure and flow rate affect the groutability and grout penetration depth in a jointed rock mass. Knowledge of the effect of these parameters has advantages in the prediction of grouting results. Typically, cement-based grout is used in jointed rock masses. Unlike water, stable cement-based grout usually acts as a Bingham fluid. In this study the effect of important parameters on grouting process in a jointed rock mass was investigated numerically using the DEM method. In the conducted study, the problem geometry represents a horizontal section in a regularly jointed rock mass with two joint sets. The analyses results show that the grout penetration depth and intake increase as joint aperture, normal stiffness and grouting pressure increase and in-situ stress and pore water pressure decrease. Increase in joint spacing does not have any effect on the grout penetration depth but decreases the grout intake. The effect of joint orientation on grouting process is strongly dependent on in-situ stress state. On the other hand, increase in grout yield stress decreases the grout penetration depth and intake, while grout viscosity does not have any effect on maximum grout penetration depth and intake. To further investigate the above mechanisms, the grouting process conducted in Gotvand dam-Iran was simulated numerically.

Effect of Parameters of Selection and Replacement Drilling Bits Based on Geo-Mechanical Factors: (Case Study: Gas and Oil Reservoir in the Southwest of Iran)

The purpose of the present study was to explore the effect of drill bit selection parameters on enhanced oil and gas recovery fin Iranian oil and gas fields. In this study, many methods and parameters were discussed. These parameters are practically tested on many wells in southwestern field of Iran. Out of common drill bit selection methods, length unit drilling cost is the most popular one owing to the direct investigation of economic parameters related to the performance of the drill bit. However, previous studies conducted in one of the southern gas fields of Iran showed that this method’s results do not yield the optimal drill selection and geology since it fails to evaluate drill bits used in various drilling conditions. To address this problem, some methods have been introduced as complementary of this method which allow comparing various drill bits. In the present work, it was illustrated that using geo-mechanical factors are ideal complements for the method of length unit drilling cost in Iran.

Intense rockburst impacts in deep underground construction and their prevention

In the last few decades, in both mining and civil engineering projects, deeper excavations have been carried out than in the past. With this increase in depth, rocks may overstress and rock failures can occur during excavation. When competent rock strata are encountered under high stress conditions, these failures can vary from superficial spalling to explosive rockburst. Intense rockbursts may cause fatal injuries to workers and significant loss of equipment and time. The occurrence of rockbursts is always difficult to predict and special steps and measures must be taken to control them. First, burst-prone zones must be predicted by an early exhaustive geological study and by the assessment of in situ stress level and orientation. Second, basic design parameters, e.g., shape, size, and excavation method, should be modified and adapted to the expected conditions to minimize rockburst risk. Third, in situ pre-conditioning methods, e.g., destress blasting, can be applied to decrease the capacity of the rock mass to store energy. Finally, special rock support and reinforcement systems, i.e., yielding systems, must be installed after excavation to ensure total stability of the opening. This paper reviews the geological and geomechanical factors that provoke and influence rockbursts in overstressed rock masses and the engineering measures taken to control them.

A new effective method and new materials for high sealing performance of cross-measure CMM drainage boreholes

This study focuses on improving the sealing performance of the cross-measure CMM (coal mine methane) drainage boreholes. The sealing of cross-measure CMM drainage boreholes is not only related to the gas emissions but also closely related to the CMM drainage efficiency and further affects the safety of the coal miners. However, the sealing performance of the traditional sealing method becomes increasingly poor with increasing mining depth. The three main shortcomings of the traditional sealing method were revealed based on the examination of the geomechanical factors and characteristics of the traditional sealing materials. To eliminate these shortcomings and obtain an effective sealing method, 10 groups of sealing materials were tested; an automatic grout pump and two types of rubber bottom subs were developed. Next, a new sealing method, which is characterized by performing grouting two or more times with high grouting pressure was developed and compared with the traditional sealing method in engineering. The cement grout with a cement-to-water ratio of 1:0.8 is a suitable sealing material from both engineering applicability and economic perspectives. The new sealing method has a better sealing performance of the cross-measure CMM drainage boreholes than that of the traditional sealing method based on the engineering test results.

Heat transfer and mechanical stability analyses to determine the aspect ratio of rock caverns for thermal energy storage

Thermal stratification in solar thermal storages is used to improve the efficiency of solar heating systems because a high degree of thermal stratification in the storages increases the thermal performance of the systems. It has been demonstrated that better thermal stratification can be achieved by increasing the aspect ratio (height-to-width ratio) of the heat storage containers. However, a high-aspect-ratio design may lead to mechanical (structural) instability of the storage space because of its tall, narrow shape. Therefore, heat storage containers should be designed to provide good thermal performance, while considering the mechanical stability of the storage space. This is an important issue in the design of thermal energy storage (TES) spaces, particularly the underground rock caverns used for TES, because the stability of rock caverns depends largely on geomechanical factors, such as rock properties and in-situ stresses. To address this issue, we present a numerical approach for determining the aspect ratio of underground TES caverns that considers both thermal performance and mechanical stability. This approach is based on a thermal performance evaluation in terms of thermal stratification using heat transfer analysis and a mechanical stability assessment that calculates the factor of safety using finite element analysis combined with a shear strength reduction (SSR) method. The applicability of our approach is demonstrated in the preliminary design of a silo-shaped rock cavern used to store hot water for district heating. The results of the numerical analyses under various design conditions are presented and discussed in detail, and we propose an aspect ratio for the rock cavern.

Geomechanical modeling for CO2 storage in Nisku aquifer in Wabamun Lake area in Canada

During CO2 injection, increasing pore pressure and temperature reduction create geomechanical deformation of both the reservoir and surrounding rocks. One of the most important concerns with respect to the long term CO2 storage is that stress changes caused by injection could lead to formation fracturing or reactivation of fracture networks and fault movements which could potentially provide pathways for CO2 leakage through previously impermeable rocks.Nisku geological formation located in Wabamun lake area was chosen as storage target based on storage capacity, expected ease of injectivity, low leakage risks and lack of interference with current petroleum production in the area. In order to determine whether the injection-induced stress changes affect the viability of this target formation to act as an effective storage unit, the geomechanical assessment model of the injection process has been developed, which couples the thermal flow model and geomechanical model covering all layers from the basement to the surface, and incorporates the modeling of dynamic injection-induced fracture. This work established the methodology for investigating the increase of well injectivity by allowing fracturing, and assessing the effect of the injection temperature.The pressure and stress variations were modeled during and after CO2 injection utilizing a coupled flow and geomechanical software. Since injected CO2's temperature may be considerably lower than the formation temperature, thermal effects were incorporated into the models. In order to increase the well injectivity, the case of allowing fracture initiation and propagation in Nisku was considered. The modeling shows that total minimum in situ stress is not constant and can be dramatically reduced by injection of cold CO2, thus leading to spontaneous fracturing. The cases of isothermal and thermal injection (without containment) show that operating above the fracture pressure in Nisku will increase the well injectivity (from 1Mt/year to at least 2Mt/year) but creates potentially long fractures and also the possibility of fracturing the caprock.Decreasing temperature due to CO2 injection will result in stress reduction and smaller surface deformations compared to the isothermal model. Thermal effects of cold CO2 injection will also reduce the fracture (injection) pressure and enhance the horizontal fracture propagation through Nisku. However, to make recommendations on a design that does not jeopardize isolation capacity of the containment requires site specific geomechanical and in situ stress data for all overlaying strata, in order to assess the risk of vertical propagation more accurately.This study predicts that CO2 injection in the Nisku zone is not likely to cause any significant surface heave and is not likely to have any environmental impact associated with surface deformations.In summary, the geomechanical factors put significant constraints on CO2 storage in Nisku in terms of injection pressures that may be limited by fracture propagation, injection rates that can be achieved without compromising the seal and excessive fracture propagation, and the CO2 placement affected by a fracture. The demonstration of the importance of thermal effects on CO2 storage is the most important contribution of this work.

Using a fully coupled flow and geomechanical simulator to model injection into heavy oil reservoirs

SUMMARYIn this paper, the geomechanical factors that may affect injection processes in heavy oil recovery are investigated. To accurately capture the geomechanical effects, we employed a numerical formulation that allows fully coupling of nonlinear geomechanical deformation and multicomponent porous media flows. Two salient features of this new coupling formulation are the following: (1) all flow and geomechanical equations are solved implicitly in one single matrix equation, and (2) it allows reuse of matrices from both a traditional fully implicit multicomponent reservoir simulator and a nonlinear geomechanics simulator. The former feature ensures stable coupling between the reservoir flow and geomechanics, and the latter significantly reduces the programming work. Numerical examples are given to demonstrate the accuracy and convergence performance of the new formulation.The proposed formulation is then applied to model injection into heavy oil reservoirs. The numerical investigation revealed that geomechanical factors, such asin situstress anisotropy and the uneven deformation of reservoir rock and attached impermeable rock, can result in skewed or nonuniform plastic strain and, hence, alter the sweep of the injected fluid. Coupled geomechanics simulation also gives rather different transient pressure response from that of uncoupled simulation. Copyright © 2012 John Wiley & Sons, Ltd.

Technology Focus: Hydraulic Fracturing (March 2010)

Technology Focus Edward de Bono (physiologist and lateral thinker) said: “You can’t dig a new hole by digging the same one deeper.” The “same hole” refers to current beliefs or work processes and a “new hole” refers to new ideas or concepts. This quote alludes to the “psychological inertia” that tells us to stay in the hole we have been digging since “how could we possibly let it go to waste?”* In our fast-paced world it can seem more acceptable to continue digging the same hole than be seen “sitting back” considering where to dig next. This is general societal commentary, but how does the fracture-stimulation industry compare? The diagram shows the proportion of papers reviewed for the 2010 hydraulic-fracturing feature and which phase of the project cycle they focus on (i.e., initiation, planning, implementation, or review). These data imply that we spend the least amount of dedicated effort conducting detailed look-back reviews of our work. It begs the question—Are we doing our best to learn from the past (i.e., using existing information to reflect on where to “dig”), or are we happy just to “dig the same hole deeper?” Hydraulic Fracturing additional reading available at OnePetro: www.onepetro.org SPE 123586 • “Proper Evaluation of Shale-Gas Reservoirs Leads to a More Effective Hydraulic-Fracture Stimulation” by Donald Kundert, SPE, Halliburton, et al. (See JPT, July 2009, page 59) SPE 124327 • “Coupled 3D Numerical Investigation of Hydraulic-Fracture Cleanup for Both Slickwater and Gelled Fluids” by Hector A. Wills, SPE, Colorado School of Mines, et al. SPE 124227 • “Evaluation of Haynesville Shale Vertical-Well Completions With a Mineralogy-Based Approach to Reservoir Geomechanics” by Brian LeCompte, SPE, Baker Hughes, et al. SPE 122315 • “ Geomechanical Factors Affecting the Hydraulic-Fracturing Performance in a Geomechanically Complex, Tectonically Active Area in Colombia” by J.G. Osorio, SPE, BP plc, et al. SPE 122185 • “The Use of Saturated Fracturing Fluid To Cope With Hydrate in Gas Wells Located in Ultradeep Waters” by C.A. Pedroso, SPE, Petrobras, et al. *Mann, D., www.triz-journal.com/archives/1998/08/b/index.htm, accessed 27 January 2010.

Shallow casing shoe integrity interpretation technique

Unlike the leak-off testing (LOT) in deep wells, the LOT plots from shallow well sections subsea are often inconclusive due to the lack of an obvious point of deflection from their initial linear behavior. In the paper, simultaneous effects of geo-mechanical factors and fall-off mechanism have been combined to explain and analyze the tests in shallow marine sediments (SMS). Based on theoretical analysis, three main stages of leak-off plot from SMS, pressure buildup, stabilization and fall-off are identified and explained. Non-linearity of the pressure buildup and mechanisms controlling pressure stabilization are explained and used to develop a method to analyze LOT. The presented interpretation procedure considers separately the pressure stabilization and the pressure buildup and fall-off sections of the LOT plot. The procedure combines well data with pressure testing record. Well data includes overburden pressure — calculated from correlations specific for SMS, and the maximum cementing pressure at the time of slurry placement. The stabilized LOT pressure analysis identifies the maximum strength of the casing shoe controlled by the rock stress (overburden pressure) and the failure by rock fracturing. Also presented is a graphical procedure for comparing the plots of actual and equivalent pressure fall-off. The graphical procedure provides a pattern-recognition method to distinguish the mechanism of cement–rock parting from the fluid loss mechanism.

The effect of groundwater fluctuations on the velocity pattern of slow-moving landslides

Abstract. Slow-moving landslides show complex mechanical and fluid interactions. They show among others non linear intrinsic viscosity of the shear zone, undrained loading effects and the generation of excess pore water pressure. The parameterization of hydrological and geomechanical factors by field and laboratory tests to describe the movement pattern of these landslides is difficult. It is a challenge to simulate accurately the de- and acceleration of these landslides and particularly, to forecast catastrophic surges. In this paper the relation between groundwater fluctuation and landslide velocity for two deep-seated landslides of the Trièves Plateau (the Monestier-du-Percy landslide and the Saint-Guillaume landslide) is analysed. Inclinometer measurements, showing the displacement in depth after 1–2 months periods, showed on both landslides shear band deformation within 1 m. At the Monestier-du-Percy landslide, depending on the position, the shear band depths vary between 25.0 m and 10.0 m. At the Saint-Guillaume landslide, the inclinometers detected several slip surfaces inside the clays, at respectively 37.0 m, 34.5 m, and 14.0 m depth. Two simple geomechanical models are developed to describe these displacements in depth in relation to measured groundwater fluctuations. Calibration of the models using the friction angle delivered no constant values for different measuring periods. It appeared that calibrated (apparent) friction values increase with increasing groundwater levels. The paper discusses the possibility of the generation of negative excess pore water pressures as a feed back mechanism, which may explain the complex displacement pattern of these landslides developed in varved clays.

Geomechanical problems in high-rise construction

Fundamental and determining geomechanical factors that ensure the safety of high-rise construction are discussed. Characteristics used in selecting a geomechanical model for the beds of high-liability structures, and a mechanical model for the bed soils and surrounding mass are analyzed.

Geomechanical Factors Affecting Geological Storage of CO2 in Depleted Oil and Gas Reservoirs

Abstract A key to the success of long-term storage of CO2 in depleted oil or gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. This paper provides a review of the geomechanical factors affecting the hydraulic integrity of the bounding seals for a depleted oil or gas reservoir slated for use as a CO2 injection zone. Potential leakage mechanisms reviewed include fault reactivation, induced shear failure of the caprock, out-of-zone hydraulic fracturing, and poorly sealed casing cements in enlarged, unstable boreholes. Parameters controlling these mechanisms include the upper and lower bounds of pressure and temperature experienced by the reservoir, the orientation and mechanical properties of existing faults, rock mechanical properties, in situ stresses, and reservoir depth and shape. Approaches to mitigate the likelihood of geomechanics-related leakage include the identification of safe upper limits on injection pressures, preferred injection well locations, review of historical records for reservoir pressures, temperatures and stimulation treatments, drilling program design to mitigate rock yielding in new wells, and assessment of wellbore integrity indicators in existing wells. Introduction In order to achieve significant reductions in the atmospheric release of anthropogenic greenhouse gases, the implementation of technologies to capture carbon dioxide (CO2) and store it in geological formations will be necessary. Deep saline aquifers have the largest potential for CO2 sequestration in geological media in terms of volume, duration, and minimum or null environmental impact(1). The first commercial scheme for CO2 sequestration in an aquifer is already in place in the Norwegian sector of the North Sea, where 106 tonnes of CO2 are extracted annually from the Sleipner Gas Field and injected into the 250 m thick Utsira aquifer at a depth of 1,000 m below the sea bed(2). In light of the economic benefits of enhanced oil recovery (EOR) derived from CO2 injection in oil reservoirs(3), these types of reservoirs will be attractive CO2 injection targets and, most likely, CO2 storage in depleted oil and gas reservoirs (or in conjunction with EOR) will be implemented before CO2 storage in aquifers. An advantage of CO2 storage in depleted oil or gas fields is the fact that much of the infrastructure for fluid injection (e.g., wellbores, compressors, pipelines) is already in place. The Weyburn CO2 Monitoring and Storage Project in Saskatchewan, Canada(4) is an example of a large-scale application of EOR operations using anthropogenic CO2, in which the oil reservoir is being evaluated for subsequent use as a long-term storage zone. A key to the success of long-term storage in depleted oil and gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. The initial integrity of this "bounding seal" system is governed by geological factors. A considerable amount of effort has been devoted to the development of procedures for assessing fault seal capacity in potential hydrocarbon reservoirs(5).

When Is It Important to Consider Geomechanics in SAGD Operations?

Abstract This paper generalizes the typical reservoir conditions for which SAGD is being implemented or considered in order to parametrically analyze the influence of geomechanical factors on the startup and production phases of SAGD projects. Numerical simulation of the SAGD process using a thermal reservoir program and a geomechanical program is used to assess the relative influence geomechanics may have on SAGD operations. While variations to the initial dual well SAGD process are becoming numerous, this study presents analysis results for only dual well SAGD geometries. A primary focus of this research is to clearly define the role of pore volume change (compressibility or shear-induced) on the basis of fundamental geomechanical parameters and correct an ongoing misconception that formation dilation can be simulated based on injection pressure alone. Dilation is a complicated process controlled by significantly more parameters than just injection pressure. Clear, definable guidelines are presented to aid SAGD project designers in determining the relative importance of the geomechanical response of their particular reservoir. The major geomechanical/ reservoir factors studied include:initial in situ effective stress state;initial pore pressure;steam injection pressure and temperature; and,process geometry variables, such as well spacing and wellpair spacing. Introduction The geomechanical response of an oil sands/heavy oil reservoir is complex, reacting to both near and far field temperatures and pore pressures. To aid in elucidating fundamental geomechanical principles affecting the steam assisted gravity drainage (SAGD) rocess and to gain insight into a reservoir's response to thermal loading and pore pressure change, a parametric analysis of the SAGD process within three separate but similar reservoir settings was completed. Since the objective of this paper is to highlight how the geomechanical response affects the SAGD process, twodimensional analyses of reservoir cross sections with basic treatment of the inherent complex geology have been conducted. Thenalysis results presented herein are not intended to portray a history match of any particular SAGD operation. Fully coupled thermal-stress-fluid flow analyses are extremely difficult to conduct. While fully coupled mathematical formulations exist(1–3), the computational effort in their solution is onerous and continues to be an area of active research. Coupled solutions that consider single phase flow only have become common(4, 5) and are routinely utilized in both advanced reservoir simulations and geotechnical/hydrogeological simulations. Consequently, a decoupled approach was adopted for the analyses presented herein. The decoupled approach consisted of conducting a reservoir simulation of SAGD using STARS (an advanced process and hermal reservoir simulator developed by CMG in Calgary) and utilizing the temperatures and pore pressures as input to an effective stress geomechanical simulation of the formation response to SAGD. In agreement with Tortike(3), that while the removal of "feedback" to the fluid flow model would not allow conclusions to be drawn regarding the fluid solution, this decoupled approach would permit conclusions to be drawn and inferences to be made concerning the likely response of the formation to the SAGD process.

Geologic Factors Affecting Coal Mine Roof Stability in the Eastern United States

Roof falls in underground coal mines are a continuing source of trouble for mine operators. Fatalities and injuries are a prominent concern, but lost time due to cleanup and disruption of mining operations presents an economic problem as well. Many geologic and geomechanical factors contribute to mine roof instability. These may be seen in rock core during exploration, or in the roof as mining progresses. Once these features are recognized, remedial action can then be taken. Geologic parameters attributed to roof falls in underground coal mines include both lithologic and structural discontinuities. The roof may be weakened where different lithologic materials are in contact with one another, such as paleochannels, scours, rider seams, clay veins, or crevasse splay deposits. Fractures, i.e., joints and faults, also contribute to roof falls. The trend, dip angle, and density of fractures, and the presence of slickensides are important in determining roof stability. Stress fields underground, both tectonic and mining-induced, not only can determine whether failure will occur, but also the mode of such failure. Detrimental features of a more localized nature include kettlebottoms, coal balls, and moisture fluctuations. The use of paleoenvironmental studies is important in determining hazardous roof conditions. Particular hazards are moremore » likely to be present in one paleoenvironment than another. When that environment is determined in advance of mining, precautions can be taken during the mine-planning phase.« less

Results of pressurized-slot measurements in the G-tunnel underground facility : Zimmerman, R M; Mann, K L; Bellman, R A; Doods, D J Rock Mechanics as a Guide for Efficient Utilization of Natural Resources: Proc 30th U.S. Symposium, Morgantown, 19–22 June 1989P697–704. Publ Rotterdam: A A Balkema, 1989

A rock-mechanics field-testing program is underway at Sandia National Laboratories (SNL) as part of the YMP. SNL has the responsibility for assessing the repository design and performance as well as characterizing the geomechanical behavior of the rock. SNL has conducted field experiments in G-Tunnel in Rainier Mesa at the NTS, where tuffs similar to those at Yucca Mountain, the potential repository site, are found. Later experiments are planned as part of the YMP Exploratory Shaft investigations at Yucca Mountain. Major geomechanical factors in repository developments are determinations of the stress state and the deformability of the rock mass (described by the modulus of deformation). One feature of SNL`s rock-mechanics program was the development of a testing program for cutting thin slots in a jointed welded tuff and utilizing flatjacks for pressurizing these thin-slots on a relatively, large scale. Objectives in the pressurized-slot testing in G-Tunnel have been to apply and possibly improve methods for (1) utilizing the flatjack cancellation (FC) method for measuring stresses normal to the slot and (2) measuring the modulus of deformation of the jointed rock surrounding the slot. This paper discusses the results of field measurements in and around a single slot and evaluates potential applicationsmore » and limitations. 10 refs., 1 fig., 4 tabs.« less

Effect of some engineering and geomechanical factors on the stability of closely spaced tunnels

Effect of some engineering and geomechanical factors on the stability of closely spaced tunnels