Laser perforating is a new approach to the generation of uniform holes in oil and gas reservoir wells at a selected pitch to improve the permeability of rocks. Laser drilling in rocks is a very complex phenomenon that its performance depends on many factors. Since it is not possible to consider all of these factors in the laboratory, numerical modelling is used. In this study, a finite element code (FEM) has been taken to model the thermal and mechanical stresses induced by ND: YAG laser drilling in the hydrocarbon reservoir rock samples. For this purpose, the software ABAQUS was used to analyze the thermal and mechanical stresses induced by laser. It is found that Numerical models show good agreement with the actual observation of holes drilled by the laser. During laser drilling, the rock temperature quickly increases in a few seconds and immediately reduces, thus instantaneous heating and cooling process cause thermal stresses around the hole. Also, the maximum value of thermo-mechanical stress exceeds the strength of the limestone rock and consequently, the formation of cracks and fractures in the wall of the hole are unavoidable.
Laser perforating is a new scientific way to the creation of uniform holes in petroleum reservoir wells to increase the permeability of rocks (Ahmadi et al., 2011). Thermal stress generated by differential thermal expansion of minerals and high-temperature gradient, cause to break the bonds between the grains. In this range of temperature, physical and chemical changes occur that are associated with the process of spallation. A primary physical change associated with this process is due to the thermal expansion of the grains of the rock. For example, the expansion of quartz and plagioclase grains in sandstone lead to a sudden temperature increase in it (Gahan et al., 2004).
As closely-packed grains in the matrix expand with a rapid rise in temperature, they develop thermal stress fractures and cracks within the grains, as well as break the cementation of adjacent grains. As a result, an affected grain will begin to break free from one another (Salehi et al., 2007). Laser effects appear in two steps in rocks, firstly, the creation of a hole in the rock and secondary include melting, evaporation, laser beam gases and micro fractures.
Kazemi, Alireza (K. N. Toosi University of Technology) | Mahbaz, SeyedBijan (University of Waterloo) | Soltani, Madjid (K. N. Toosi University of Technology / University of Waterloo) | Yaghoubi, Ali A. (Tarbiat Modares University) | Dusseault, Maurice B. (University of Waterloo)
ABSTRACT: One of the basic design factors in porous media enhanced geothermal systems exploiting hot (warm) saline liquids is the time that the system starts to operate until significant thermal breakthrough occurs, i.e., the reservoir lifetime. This study is focused on a sedimentary enhanced geothermal system (SEGS) located in the Williston Basin, a sub-basin of the Western Canada Sedimentary Basin (WCSB). Variables such as the doublet distance and the injection/production flow rate are investigated to assess the effect of the growth of the heat transfer area with and without a high-permeability fracture. For instance, the modeling results illustrate that with short doublet distances, the production temperature of a fractured reservoir with high injection rates is higher than that of a nonfractured system with very same properties, whereas decreasing the injection rate leads to opposite outcomes. The stress changes are also estimated as fluid flows over time at the reservoir scale, since they impact fracture aperture in a highly non-linear manner. The dominant phenomenon is tension development in the entire domain, except in the injection well. The computational platform used is a finite element based model and a 30-year project timescale is considered.
Kim, H. M. (Sejong University) | Yazdani, M. (Tarbiat Modares University) | Lee, J. W. (Korea Institute of Geosicence and Mineral Resources) | Park, E. S. (Korea Institute of Geosicence and Mineral Resources)
ABSTRACT: A numerical analysis of coupled viscous fluid flow and joint mechanical deformation was developed to estimate penetration length of cement grout. The coupled analysis was established using discrete element method (DEM) such that parameters of both individual joint and discrete network can be implemented without difficulty in estimating grout penetration in a jointed rock mass. For a verification of the established numerical analysis, the results of penetration length in 1-dimensional linear flow analysis was compared to previous analytical solution. We also showed that the estimated penetration radius in 2-dimensional radial flow analysis was well comparable to our experimental observation of cement grout injection in a rock joint replica. Our simulations of the coupled flow-deformation analysis showed that grout injection-induced joint opening may enhance a penetration and produce a non-linear injection pressure profile within a joint. The effect of parameters on grout penetration length, which investigated in this study, can be usefully informative in designing a cement grout injection in a jointed rock mass.
Cement grout to be injected in a subsurface jointed rock mass flows mainly through connected individual joints. Hence, the grout flow behavior should be first clearly understood in a single rock joint to study grouting behavior in the jointed rock masses which corresponds to the network of discrete individual joints. Although, the theory of grouting in rock joints is well established on a basis of good characterization of jointed rock mass, it is still difficult to accomplish a grouting exactly as designed and obtain the grouting performance as estimated, compared to homogeneous soil grouting (Zheng et al., 2016).
Grout penetration in a discrete joint has been extensively investigated analytically, experimentally or numerically and reported that penetration is subject to mainly grout material properties and geometry of joint (Xiao et al, 2017, Sui et al., 2016, Tani, 2012). One, however, of the parameters that is not sufficiently investigated is the effect of joint deformation during injection. Due to a combined influence of injection pressure and in-situ stresses acting on the joint, it may result in either opening or closure of the joint, which may change the penetration behavior of cement grout consequently.
ABSTRACT: Dynamic uniaxial compressive strength of Pennsylvania blue sandstone was investigated using split Hopkinson pressure bar both physically and numerically. A hybrid finite-discrete element code called CA3 was employed to simulate the physical tests. The incident and transmitted bars were modeled using finite elements while the rock specimen was represented by a bonded particle discrete system. The incident stress pulse measured in the physical test was utilized as the input for the numerical simulation and was applied to the free end of the incident bar. Analysis of the numerical results suggests an underestimation of the dynamic rock strength; the effect of axial and circumferential inertia of the specimen didn’t manifest the strength value consistent with the physical observation. Therefore, a parameter called rock strength enhancement coefficient was introduced which increases the bond strength between the particles as a function of the relative velocity of particles at the contact points. A much better match between the physical and numerical results is observed if this coefficient is applied in the numerical simulation.
Most of the operations on the rock like materials from mining and road structures to dam foundations usually include the dynamic application of the load to the rock. Rocks are pressure sensitive and rate-dependent materials and show a drastically different behavior under dynamic loading. Since the dynamic loading of rocks is applied in a variety of loading rates, it is essential to study the dynamic strength parameters of the rocks and fracture properties over a wide range of loading rates.
There are three main methods for testing the rock materials under dynamic loading conditions which have been suggested by the International Society for Rock Mechanics . These methods include dynamic compression test, dynamic Brazilian test, and dynamic notched semi-circular bend (NSCB) test. All these tests are performed using the split Hopkinson pressure bar (Kolsky bar).
The split Hopkinson pressure bar (SHPB) was first designed for testing of ductile materials such as metals . In the last couple of decades, SHPB has been widely used for evaluating various parameters for brittle materials like rock, concrete, and ceramics [3, 4].
Along with the physical tests, many numerical methods have been utilized to investigate the characteristics of quasi-brittle materials such as rock and concrete in dynamic loading. Various constitutive models were used in ABAQUS to study different parameters which affect the dynamic strength of rock like materials in the FEM method [5, 6, 7]. As previous studies show, discrete element method (DEM) provides an alternative and reliable solution for discontinuum materials like rocks [5, 8, 9]. However, this method is computationally time-consuming when it comes to large scale domains.
ABSTRACT: When rock samples are loaded, Acoustic Emission (AE) occurs when stress reaches a level greater than that which the rock has previously experienced. This phenomenon, known as Kaiser Effect, which has been used as an indirect method of in-situ stress measurement in rock mass that can be one of the most important parameters in designing of underground structures. Several physical parameters have effect on Kaiser Effect such as porosity, sample size, moisture content and so on. In this research, effect of porosity as a physical property of rock on in-situ stress measurement using AE method is studied. Porosity, creating a heterogeneous environment in the rock sample and causes effects on propagation of acoustic signals. In order to omit interfering with other parameters artificial rock samples (Ferro Cement Mortar), with fixed grading concrete and no memory of previous stress are used so the only variable factor is air cavities in the samples (porosity). Several samples with different porosity percentages ranging %14 to %20 were compressed uniaxialy. The results shown that porosity have effect on Kaiser Effect and in-situ stress amount perdition. Ultimately, amongst various AE parameters, energy and count parameters are better for determination of pre stress.
Knowledge of an amount and direction of in-situ stress in many rock structures such as; an underground space, rock slopes is essential. There are various methods to estimate the in situ stress in field and laboratory. Most of these methods are time consuming and require high costs. In the laboratory, several methods have been proposed. All experimental methods are based on the drilled cores. When the depth is high, the measurement will be based on core drilling. Since core-based methods are very easy, cheap and less time. So this method is interested by rock mechanician. One of the core-based techniques is using of Acoustic Emission (AE) method. This method is based on Kaiser Effect. Kaiser Effect is emission of acoustic when rock undergoes stress. In this method, the maximum stress pre applied on the rock sample is equal to the starting point in AE activity. This phenomenon is due to the lack of acoustic signals at levels lower than the maximum stress which rock has experienced before (Seto et al. (1999), Tuncay & Ulusay (2008), and Tuncay & Obara (2012)). To verify the accuracy of Kaiser Effect, Felicity ratio is used. Felicity ratio takes different values for different stress levels and at different behavior stages of rock.
The present study aims to employ modern intelligent method to predict intact rock strength parameters. This method can be used for intact rock strength parameters prediction of different extraction projects. Mechanical rock excavation projects need uniaxial compressive strength (UCS) and static modulus of elasticity (E) of the intact rock material. Many parameters affect to strength parameters, but some of them are applicable to empirical or analytical equations and the other difficulty, high-quality core samples of appropriate geometry are needed to find out these parameters. In this regard, models predicting UCS and E based on rock index tests and intact rock properties could be useful methods. This paper aims to employ Gene Expression Programming (GEP) to predict E and UCS. Out of the 44 sets of the data, 22 sets (50% of the data) were considered for training and the remaining 22 sets of the data (50%) were considered for testing. The intelligent method has been studied on the basis of data obtained from 44 different excavation projects all over the world. These parameters were collected from previous research data. The values of UCS and E are predicted by using quartz content (Q), dry density (γd) and porosity (n) of the rocks. 22 datasets (50% of the data) were utilized for modeling and the remaining 22 sets of the data (50%) were considered for evaluating theirs performance. For this purpose, writing a code was necessary, as some of the proposed relations were complex. The obtained results of this study are presented within a computer-based format in order to be easily accessible too every experts. With respect to the accuracy of the GEP method, it may be recommended for predicting intact rock strength parameters for future excavation design purpose.
In many cases the geomechanical properties of rock are required to make decisions in rock engineering projects. These properties could be unit weight, uniaxial compressive strength, tensile strength, modulus of elasticity, etc. High-quality core samples are required for laboratory tests if reliable results are desired. Such cores are not available always or it could be a time and money consuming procedure to prepare them. To overcome this difficulty encountered during the core sample preparation, some intelligent predictive models could be introduced to use the simple index parameters such as point load, block punch, Schmidt hammer and other easy accessing properties of rock to evaluate the desired properties. Among them, the models introduced by different researchers [1, 3, 5, 7, 13, 16] could be named.
Moradi, B. (Institute of Petroleum Engineering, University of Tehran) | Pourafshary, P. (Sultan Qaboos University) | Farahani, F. Jalali (University of Tehran) | Mohammadi, M. (Tarbiat Modares University) | Emadi, M. A. (National Iranian Oil Company)
Difficulties and constrains of water & gas injection, like unfavorable mobility ratio, gravity override and underride, and early fingering of injected fluid have led to application of water alternating gas (WAG) injection method instead of water flooding. Nanotechnology can be applied to enhance the performance of WAG process by changing microscopic and macroscopic sweep efficiencies of the process.
In this study, several experimental works have been completed to investigate the improvement of the efficiency of the WAG method by application of nano-particles in the aqueous phase. The developed water-based nano fluid alternating gas injection effects on the oil recovery were studied by different core flooding tests in carbonated samples and the results were compared to the conventional WAG approach. The medium crude oil sample and plugs were provided from a mature oil field in the Middle East. Silica nano-powder (SiO2) with particle size of 11-14 nm was used to prepare the water-based nano fluid. Several characterization experiments such as Interfacial tension (IFT) and contact angle measurements were performed to study the influences of the new method on the IFT and wettability alteration during flooding.
Experimental results demonstrated more than 20% incremental in recovery factor by Nano-WAG process in comparison to the WAG process. Our study showed that adsorption of SiO2 nano particles on the rock surface changed the wettability of the rock from the oil wet to strongly water wet which affects the recovery. In addition, reduction of IFT occurred due to the placement of nanoparticles on the interface of oil and water phases. Our experiments indicated that water-based nano fluid alternating gas injection is a novel potentially feasible process for enhanced oil recovery and can be used in the field applications to overcome the problems of the WAG and enhance the oil recovery.
Behbahani, Seyed Saleh (Islamic Azad University) | Moarefvand, Parviz (Amirkabir University of Technology) | Ahangari, Kaveh (Islamic Azad University) | Goshtasbi, Kamran (Tarbiat Modares University) | Iseley, Tom (Louisiana Tech University)
For keeping an open-pit mine operational while a giant sliding mass exists and is flowing on the benches and minerals like debris, at this time, it is necessary to unload the sliding mass (debris). For achieving this, monitoring of sliding mass should be done along with unloading so that displacement and velocity values do not exceed a certain limit and do not cause a serious incident. One of the suitable software which is able to model this sliding mass and unload it, is PFC (Particle Flow Code) which is based on Discrete Element Method. This paper will describe how PFC was used to model a sliding mass in an open-pit mine and unloading it in seven stages has been done. During the unloading of sliding mass, maximum velocity and displacement among the particles have been obtained. Also, in this article the Angooran mine, which is the largest metal mine in Iran and also one of the most economical lead and zinc mines in the world, has been studied. Sliding mass volume that occurred in the Angooran mine was about 12 ×106 m3, i.e. 25×106tons. The sliding mass moved about 100 m horizontally and 45 m vertically.
Slope stability is one of the most important issues in the construction and mining activities and any mistake in analysis can lead to irreparable damage. Utilizing the subsurface geological model, and results which are obtained from the field investigation and lab testing can help for analyzing slope stability by using suitable modeling methods. Slope movements might be minor and be limited to falling a small boulder or might be huge and catastrophic. In many cases, advancing failure or the slope rupture can be prevented while in other cases, improvement methods cannot be remedial. One of the example of this huge slide is the Angooran mine, one of the most economical lead and zinc open-pit mines in the world. Angooran mine is located in the northwest of Iran and has experienced a large scale slope failure in northern wall. Sliding mass volume that occurred in the Angooran mine was about 12 ×106 m3, i.e. 25×106 tons (Fig. 1). Generally, the slope movements can affect engineering structures and human activities. Slope movements are different in many ways such as hugeness, speed of incidence, and predictability. Heavy or prolonged rainfall, melting snow, earthquakes, blasting in mining can be causes of slope instability. Creep, tensile cracks, and water leakage locations are surficial signs of instability. The Fig. 2 shows a sample of tensile cracks in the Angooran mine.
The interaction of the loading machine and rock specimen is studied in this paper. The rock is modeled as a bonded particle discrete element system, while the machine is discretized using finite elements. Three aspects of rock-machine interaction including the effect of machine stiffness, end platen friction coefficient, and sample aspect ratio will be investigated. It is illustrated that the machine stiffness can drastically change the post-peak stress-strain curve; a soft machine underestimates the slope of the post-peak curve. The numerical results suggest that by increasing the sample aspect ratio, the effect of end platen friction can be reduced. Furthermore, it is observed that end friction is the main reason for the phenomenon of deterministic size effect and this size effect can be effectively removed if the end friction can be reduced to zero.
Understanding of the mechanics of rock fracture plays an important role in the solution of many engineering problems that involves rock structures. In fracture mechanics, Stress Intensity Factor (SIF), K, is used to predict the stress state (stress intensit y) near the tip of a crack caused by a remote load, which were introduced by Irwin . For each mode of loading, the crack tip stress and displacement fields can be determined by the stress intensity factor. K is dependent on far field stress and geometry of crack , which has been studied by several investigators. Wong et al.,  performed laboratory tests to study the pattern of crack coalescence of sandstone-like material and the normalized peak strength.