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Abstract A semi-analytical model is presented in this paper to predict the surface deformation given the reservoir pressure or volume changes in an inhomogeneous media. The difference in mechanical properties between the reservoir and overburden or overburden โlayeringโ is accounted for using the moduli perturbation method. Essentially a center of dilatation point source solution in an inhomogeneous half-space is adopted and replaces the existing center of dilatation Greenโs function in homogeneous half-space. With the moduli perturbation method, the effect of the modulus contrast on deformation is transformed into integration over the interface between different mechanical properties, which makes it sufficiently versatile to implement for different field geometries other than horizontal layers. As a case demonstration, the proposed method is used to calculate the vertical displacement for a sample problem in the literature and compare with the result from a geomechanically coupled reservoir simulator. The semi-analytical model presented in this paper can be used as a forward model that accounts for the effects of inhomogeneous mechanical properties in any inversion program to obtain more accurate volume and pressure changes in reservoir. It can be used as a calibration tool to more accurately history-match reservoir parameters from the measured surface deformation. INTRODUCTION Surface deformation-based reservoir-monitoring technologies such as Tilt, GPS and InSAR have been successfully applied to monitor fluid flow or pressure changes in the reservoir. Both, subsidence due to production and uplift due to injection can be measured to assure conformance, containment, and effectiveness of the technologies being applied. In the oil field, these technologies often are waterfloods or enhanced recovery processes such as thermal recovery, but drill cuttings injection, water disposal and CO2 injection are also good candidates for surface deformation measurements. The nucleus of poroelastic or thermoelastic strain type of solution [1, 2] is usually adopted as the forward model to relate reservoir activity to surface deformation [3-10], primarily due to its simplicity, but very often because sufficient information about the overburden is unavailable. However, with the recent trend of the increasing use of coupled geomechanical and flow simulation and improved modeling, such information is increasingly being measured and becoming available. Not accounting for differences in mechanical properties between the reservoir and overburden or overburden โlayeringโ can potentially affect the predicted deformation at surface. The most common approach to account for the effect of mechanical property differences is the application of the Finite Element Method (FEM). However, in order to obtain meaningful results, it usually requires adequately characterizing complex constitutive models, applying reasonable boundary conditions and developing a stable mesh. Another set of methods is analytical or semi-analytical algorithms. One particular semi-analytical model was developed to account for mechanical layering in subsidence modeling by adding different types of point source solutions together to satisfy the boundary condition at the layer interface [11]. Another semi-analytical method that falls in between the FEM and analytical method is the type curve or correction factors method (Morita et al. [12]).
- North America > Canada (0.29)
- North America > United States (0.28)
- Africa > Middle East > Algeria > Tamanrasset Province > Ahnet-Timimoun Basin > Krechba Field (0.99)
- Africa > Middle East > Algeria > Ghardaia Province > Ahnet-Timimoun Basin > Krechba Field (0.99)
INTRODUCTION Abstract Arapuni Dam, a 64 m high curved concrete gravity structure founded on Quaternary-age ignimbrites, has experienced several seepage flow incidents since its construction between 1925 and 1927. After grouting of the latest flow in 2001, the longer-term future performance of the dam was investigated by triple-tube coring of the foundations, laboratory strength testing, weir and piezometer monitoring of responses to induced changes in pressures in the foundations, and characterization of water chemistry and isotope signatures as well as temperature. The paper describes the use of these techniques to identify four subvertical zones (fissures) controlling the pattern of foundation seepage along joints formed in the Ongatiti Ignimbrite, many of which were either open or infilled with erodible nontronite clay. Recognition of the four fissure zones has enabled the design and construction of four discrete 90 m high cut-off walls through the dam and into its foundations, rather than treatment across the whole dam foundation. Continued safe performance of dams requires ongoing surveillance in conjunction with an active dam safety assurance program, in particular as they age. The 64 m high Arapuni Dam on the Waikato River in New Zealand (Figs. 1 & 2), which was constructed between 1925 and 1927 [1], provides such an example. During its operational life seepage flows have suddenly increased several times without apparent cause (e.g. [2]). The latest incident was repaired in late 2001 by targeted grouting under full reservoir conditions [3]. The dam is an integral element of the chain of eight hydro electric power stations along the Waikato River owned and operated by Mighty River Power (MRP). The oldest of the dams, Arapuni impounds the reservoir for a 186 MW powerhouse at the end of a 1 km long headrace channel downstream from the dam on the left bank. Following the 2001 grouting, evaluation of the safe performance of the dam in light of possible future leakage incidents culminated in construction of cut-off walls through the dam and into its foundations with the reservoir still in service [4]. This paper describes foundation investigations aimed at assessing the stability of the dam and the design of the foundation cut-off walls. (Figure in full paper) GeologyArapuni is located in an area of flat-lying ignimbrite flows originating from volcanic eruptions over the last 1.2 million years. Three flows (Ongatiti Ignimbrite, Ahuroa Ignimbrite, Manunui Ignimbrite) are present in the narrow Waikato River gorge in which the dam is located (Fig. 2). They are dominantly sinter (point) welded so that their unconfined compressive strength (qu) values are generally <10 MPa. Defects (joints) in the ignimbrite rock masses, which formed during cooling of the ignimbrites after their emplacement, typically have subvertical attitudes. The dam footprint is founded on the lower 40 m of the Ongatiti Ignimbrite where there is some variation in welding, in particular in the more welded โhard zoneโ immediately below the base of the cut-off wall (Figs. 3 & 5).
- Oceania > New Zealand (0.86)
- North America > United States (0.68)
- Geology > Geological Subdiscipline > Volcanology (0.68)
- Geology > Geological Subdiscipline > Geomechanics (0.47)
- Geology > Mineral > Silicate > Phyllosilicate (0.35)
Modeling Variation of Stress And Permeability In Naturally Fractured Reservoirs Using Displacement Discontinuity Method
Tao, Q. (Department of Petroleum Engineering, Texas A&M University) | Ehlig-Economides, C.A. (Department of Petroleum Engineering, Texas A&M University) | Ghassemi, A. (Department of Petroleum Engineering, Texas A&M University)
Abstract Fractures are the main channels of production in naturally fractured reservoirs, therefore the fracture permeability is a key parameter to production optimization and reservoir management. The perturbation of effective stress acting on a fracture can change the fracture aperture, thereby changing the fracture permeability. Pressure depletion in a naturally fractured reservoir can result in effective stress change that, in turn, can change fracture permeability. The displacement discontinuity method is a boundary element method with the ability to handle the rock discontinuities and fractures. The coupled poroelastic displacement discontinuity method also involves into the interaction of fluid flow and the discontinuity. A nonlinear mechanical model is applied to represent the fracture deformation including normal and shear deformation. In this work we apply displacement discontinuity method combining with the fracture deformation model to model the variation of stress and fracture aperture and resulting permeability variation during production in naturally fractured reservoirs. INTRODUCTION The effect of stress on reservoir permeability as a change in reservoir porosity has been well investigated [1,2,3,4,5]. The effective stress increases with the decrease in pore pressure due to production from wells. The increase of effective stress compacts the reservoir rock and reduces the reservoir porosity, thereby reducing the reservoir permeability. But the effect of stress on the fracture aperture and fracture permeability in naturally fractured reservoirs has not been studied in sufficient detail. The fracture aperture and permeability are often treated as constant during production. However, A few investigations have shown that stress had an important effect on the fracture aperture and permeability. Min et al [6] investigated the fracture aperture change of a complicated fracture network in an impermeable material using a two-dimensional distinct element method program UDEC (universe distinct element code). Their numerical modeling shows that hydrostatic stress tends to reduce the fracture aperture, and thereby fracture permeability, and that the deviatoric stress tends to increase the aperture of those fractures oriented parallel or close to the maximum principal stress direction (compression is positive), and reduce the aperture of those fractures oriented perpendicular to the maximum principal stress direction. The interface flow between the fracture and matrix can affect the pore pressure inside fracture, and therefore the fracture aperture. However, UDEC does not model permeable porous media, and so cannot account for the effect of production from matrix to the fracture. Asgian [7,8] applied the displacement discontinuity method to model fracture aperture changes while injecting fluid into a fracture network in a naturally fractured reservoir. The matrix was assumed impermeable and an elastic displacement discontinuity solution was applied. Still the affect of flow from matrix to natural fractures was not addressed. We apply a poroelastic displacement discontinuity method fully coupling the fracture change with stress and pore pressure in both matrix and fractures to investigate stress dependent fracture aperture and permeability under two-dimensional single phase flow conditions. The fluid is compressible and the fluid density is pressure dependent. Fractures are deformable and a nonlinear mechanical model is applied to represent the fracture deformation due to stress change.
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Naturally-fractured reservoirs (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization (1.00)
Characterizing Internal Macropores Using Cross-specimen Acoustic Tomography: Verification of Two Dimensional Results
Sherman, C.S. (Montana Tech of The University of Montana) | MacLaughlin, M.M. (Montana Tech of The University of Montana) | Link, C.A. (Montana Tech of The University of Montana) | Hudyma, N. (University of North Florida)
Abstract The engineering properties of a geologic material are greatly affected by the presence of macropores. Previous research has demonstrated that the size, location, and proximity of macropores influences both the strength and stiffness of specimens. Knowledge of the distribution of macropores in a specimen prior to testing would be useful for a number of reasons. We are currently developing a non-destructive method called cross-specimen acoustic tomography (CSAT) to determine the number, location, and size of the macropores in a laboratory specimen. The CSAT method uses a set of piezoelectric sensors that generate and receive high frequency acoustic waves. We measure the travel times of the acoustic waves through a specimen and then use a commercially available tomography software package to invert the data. The inverted velocity model is in turn used to locate the voids within the specimen. The verification of two dimensional (cross-sectional) results from plaster specimens containing large macropores of Styrofoam show the technique is promising and worthy of further development. INTRODUCTION Previous research has shown that the presence of macropores (large void spaces) has a significant effect on the structural behavior of geologic materials. Prior studies have used laboratory produced brittle specimens [1], numerical modeling [2], and real rock specimens in attempts to quantify the effects of large voids on engineering properties [3]. These studies showed that at the same level of macroporosity there can be large differences in strength and stiffness. This implies that the size, shape, and location of macropores within a specimen control the engineering properties of the specimen. As such, determining the size, shape, and location of macropores in a rock specimen is important for evaluating the laboratory test results. There are a number of common imaging techniques available to characterize the internal structure(s) within geological specimens. These techniques include X-ray computed tomography [4], magnetic resonance imaging [5], ultrasound [6], and computerized axial tomography [7]. A comprehensive discussion of such techniques is beyond the scope of this paper. However it is important to note that the above techniques are medical imaging techniques which require expensive equipment utilizing highly skilled personnel for operation. The motivation for this work is to develop a simple yet robust non-destructive method to characterize macropores within laboratory specimens prior to destructive testing. Characterizing the macropores would include quantitatively determining the size, shape, and location of macropores. The technique that is being developed has been named cross-specimen acoustic tomography (CSAT). The goal is to be able to use the CSAT method to characterize the macropores and use the information to gain an understanding of how the macropores influence the variability of the engineering properties of macroporous rock. For this initial study, we used three plaster of Paris cylindrical specimens (15.2 cm diameter by 30.5 cm length) containing a known number and size of Styrofoam inclusions. THEORY Elastic wave tomography is a technique that has been used in structural and geotechnical engineering to determine the locations of inclusions and defects in materials such as concrete [6].
- North America > United States > Montana (0.28)
- North America > United States > California (0.28)
- Energy > Oil & Gas > Upstream (1.00)
- Health & Medicine > Diagnostic Medicine (0.89)
Abstract In this study we have measured the undrained pore pressure response of Berea sandstone and Indiana limestone in response to changes in mean and deviatoric stresses. In particular, we have performed tests up to a confining pressure of 70 MPa and differential stress raging from 0 to 150 MPa. For Berea SS, the pore pressure responded to confining pressure (also mean stress because there was no differential loading) in the usual manner corresponding to ?p=B?sm, with B ranging from 0.3 to 0.55. Similarly, for Indiana limestone, B was measured to be 0.15 to 0.46. On the other hand, when the mean and deviatoric stresses were both increased during the test, a B in the range of 0.37 to -0.55 was measure for the Berea sandstone. This reduction in pore pressure increase with deviatoric loading, is suggestive of volumetric deformation under deviatoric loading. At high deviatoric stress levels and undrained conditions, the pore pressure response consists of both elastic and inelastic volumetric strain. For Indiana Limestone, the pore pressure was measured before and after failure. Skempton factor A and B values for different confining pressure and differential axial loadings in elastic and post peak regions for Indiana Limestone also have been measured. After yielding, the inelastic response was eliminated by repeated stress cycling, to capture the reversible elastic component. The sensitivity of pore pressure to deviatoric stress measured at constant confining pressure was found to decrease with increasing deviatoric stress level resulting in a smaller value for A. INTRODUCTION Knowledge of pore pressure and the pore variation in rock is important in a number of reservoir geomechanics problems including wellbore stability and reservoir compaction. Conventional pore-pressure estimation methods are based on one-dimensional compaction theory and depend on a relationship between porosity and vertical effective stress. However Strike-slip or reverse faulting environments especially require a different way of estimating pore-pressure, since the overburden is not the maximum stress. Rock deformation changes the pore volume and can change the pore pressure. The latter can be very significant under undrained condition. Usually, the pore pressure variation in response to volumetric deformation is analyzed within the framework of isotropic linear poroelasticity. As a result, a pore pressure change is predicted only by changes in the mean stress. However, as shown by Skempton and more recently by Wang (1997), deviatoric stress also causes the pore pressure to change. (Equation in full paper) Where, B is also called the isotropic Skempton coefficient. Wang [3] studied the reversible pore pressure response of rocks to changes in deviatoric stress for Indiana limestone at a confining pressure of 27.6 MPa. In his study, tests at zero deviatoric stress yielded B = 0.53 when fit by Eq. 2. He then increased both the mean and deviatoric stress by applying axial load. In this case, the best fit by Eq. 2 resulted in B of 0.34 and 0.39 for the two reported tests. This represented a remarkable change in poroelastic response for a rock under reversible loading conditions.
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Abstract Engineering properties of rock have been shown to be influenced by defects including porosity (Talesnick et al., 2001, Avar et al., 2003, Gates, 2008). Strength and stiffness of rock is affected by macroporosity as demonstrated in previous studies (Avar et al., 2003, Gates, 2008, Al-Harthi et al., 1998). The quantified effects of macropore spacing on the unconfined compressive strength of synthetic rock analog material and the effect of macropore spacing on failure mode are described in this investigation. Fifty-four 4โ cubic specimens made of HydrocalTM and Plaster of Paris were tested in unconfined compression for strength, and the failure mode was observed. The cubic specimens have cylindrical voids extending from the front of the specimen through the back. The laboratory results are used as validation of 2D numerical simulation of unconfined compression testing of square specimens with circular holes. Strength data appear to fall between two bounds: an upper bound that displays increasing specimen strength as distance between macropores increases, and a lower bound that suggests decreasing strength followed by increasing strength as distance between macropores increases. There is also a trend observed in the failure mode of specimens, showing that as macropore spacing increases, the failure mode changes from tensile cracking to tensile cracking accompanied by shear failure. At the minimum macropore spacing the macropores act as a single โmega-macroporeโ and at maximum macropore spacing peak strengths are relatively high and specimens fail as macopores react to loading individually. INTRODUCTION Understanding the effect of macropores on the engineering behavior of rock is important to practical engineering applications. Macroporosity has been shown to influence strength and stiffness of rock as demonstrated in previous studies (for example Avar et al., 2003, Gates, 2008, Al-Harthi et al., 1998). In general, as rock porosity increases, strength decreases (Goodman, 2003, Sowers, 1996). Investigating the influence of macroporosity on rock behavior is not just an academic endeavor; a significant construction project which encounters macroporous lithophysal tuff, is that of the Yucca Mountain deep geologic disposal for nuclear waste (US DOE, 2003). Buildings can settle as the result of compaction if constructed over porous media, such as that of the Miami Limestone (Sowers, 1996). In the limestone of Kuala Lumpur, Malaysia, the installation of foundation pilings can create the potential for failure if pile foundations rest on macroporous rock cavity roofs (Tan, 2004). It is often assumed that microscopic porosity is a uniformly distributed characteristic of rock. There is a positive linear correlation between increasing percent porosity and homogeneity of void space distribution. Uniform loading of specimens with microscopic pores creates stress field interactions that consistently produce smooth shear failure curves. Increases in microporosity are indicated by decreases in Youngโs modulus and specimen strength. The failure of macroporous rock is less predictable. Void space distribution of macroporous rock is typically non-homogeneous. This change in void space distribution accompanied by an increase in the ratio between void size and specimen size from the microscopic to the macroscopic condition produce non-uniform stress field interactions.
- North America > United States > Montana (0.29)
- North America > United States > Nevada > Nye County (0.24)
- North America > United States > Gulf of Mexico > Eastern GOM (0.24)
- Asia > Malaysia > Kuala Lumpur > Kuala Lumpur (0.24)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.45)
- North America > United States > Wyoming > Mountain Field (0.98)
- North America > United States > Utah > Mountain Field (0.98)
Experimental Study On Reinforcement Mechanism of Rock Bolts to Jointed Rock Mass
Yang, W.M. (Geotechnical and Structural Engineering Research Center of Shandong University) | Li, X.J. (School of Civil Engineering, Shandong Jianzhu University) | LI, S.C. (Geotechnical and Structural Engineering Research Center of Shandong University) | Wen, N.D. (Geotechnical and Structural Engineering Research Center of Shandong University)
Abstract The present study aims to determine the effect of rock bolts on the behavior of a rock joint through physical experiments. A rock analog material and bamboo were selected to simulate the rock material and rock bolts respectively. Considering different anchoring angles, anchoring position and anchoring density, a series of uniaxial experiments were carried out in the laboratory. On the basis of these test results, several laws were found. (1) Installing bolts in the middle of the crack was more efficient in improving peak strength than at the end; (2) It was better to install bolts perpendicular to the loading direction; (3) When the number of bolts reached a certain amount, the anchoring effect would not be further enhanced. INTRODUCTION Jointed rock masses are very common in rock engineering [1,2]. With the excavation, the original cracks propagate, coalesce and loosen the rock mass. Therefore, the surrounding rock mass may fail and threaten the safety of engineering structures on and in rock [3-5]. Fortunately, rock bolts can mitigate this problem efficiently. Rock bolts bind the rock mass into an integral unit which interacts through the so called suspension function, combination beam function and dowel function [6, 7]. Besides, the normal stress and shear stress near the cracks provided by rock bolts, they also prevent cracks from propagating [8, 9]. So, the bearing capacity of the rock mass is promoted. Compared to the wide use and efficient anchoring effect observed in practice, mechanistic studies of rock bolts are meager. So far, the anchoring mechanism can not be interpreted very well. To utilize the rock bolts better, it is necessary to make the mechanism clear. In this paper, physical tests were performed to study the characteristic of anchored samples with different anchoring angles and anchoring positions. CHOICE OF ANALOGOUS MATERIALS AND PREPARATION FOR SAMPLES Choice of rocky material To simulate the rock material, several principles should be considered. Choice of grout material The grout material was mixture of barite powder and rosin alcohol solution. The weight ratio was barite: rosin: alcohol=1:0.1:0.2. Preparation of samples Brittleness Density Ratio of tensile strength to compressive strength The sample was in parallelepiped shape. The crack runs through the sample and with an angle of 45 degree to the loading axes.With such consideration, a kind of mixture was chosen, which was developed by Shandong University of China. This kind of material was composed of barite powder, ferrous powder, quartz sand, rosin, alcohol and plaster powder. (Table in full paper) Choice of rock bolt material The choice of rock bolt material was always a key point, because it was hard to meet all the analogy theory for the mechanical parameters. With reference to literature and trial testing [10-13], a kind of bamboo was selected to simulate the rock bolt. The mechanical parameters of the bamboo were E=10.10GPa,st = 132.05MPa. The diameter of bolt and hole were 2.5mm and 3.5 mm respectively.
- Research Report > New Finding (0.51)
- Research Report > Experimental Study (0.41)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral > Sulfate > Barite (0.66)
ABSTRACT The flow-coupled DEM simulations are performed to better understand the hydraulic fracturing mechanism and the influence of fluid viscosity. The simulation results were in good agreement with the actual experimental results which containing the AE measurement data. As the results, the followings were found. When the low viscous fluid is used, the fluid is infiltrated into the fracture instantaneously. On the other hand, when the highly viscous fluid is used, the fluid is infiltrated slowly into the crack after the fracture elongates first. Although the tensile cracks are dominantly generated in the simulation, the energy released from a tensile crack becomes small because the tensile strength of rock is obviously small compared with the compressive strength. Such a small AE is easily buried in a noise and hard to be measured in an experiment. Therefore, in AE measurement experiment, shear type AE with large energy is dominantly observed, as many previous researches have indicated. INTRODUCTION To better understand the mechanics of hydraulic fracturing, a considerable amount of research has been carried out in the past few decades. Conventional theory suggests that hydraulic fracturing is formed by tensile crack generation [1]. On the other hand, most of the acoustic emission (AE) events recorded during the laboratory and field hydraulic fracturing experiment classified as the shear type mechanisms [2]. Ishida et al. carried out a laboratory hydraulic fracturing experiment with low viscous water and high viscous oil. The source mechanisms of AE events indicate that shear type mechanisms are dominant when low viscous fluid is injected, and tensile type mechanisms are dominant when high viscous fluid is injected [3]. Thus, the hydraulic fracturing mechanism has not been sufficiently clarified. Another approach to reveal the hydraulic fracturing mechanisms is numerical simulation. Various numerical analysis techniques have been developed and applied to various problems of rock engineering. Among these techniques, the distinct element method (DEM) can directly represent grain-scale microstructural features of rock, such as pre-existing flaws, pores, microcracks and grain boundaries by considering each grain as a DEM particle [4,5]. These grain-scale discontinuities in the DEM model induce complex macroscopic behaviors without complicated constitutive laws. This suggests that the DEM model may be a strong tool to understand the fracture behavior of the rock. In this research, the flow-coupled DEM code was developed and simulations of hydraulic fracturing are performed to better understand the hydraulic fracturing mechanism and the influence of the fluid viscosity. SIMULATION METHODOLOGY Formulation of mechanics of bonded particlesIn two dimensional DEM, the intact rock is modeled as a dense packing of small rigid circular particles. Neighboring particles are bonded together at their contact points with a set of three kinds of springs as shown in Fig. 1 and interact with each other. (Equation in full paper) SIMULATION CONDITION Fig. 5 illustrates the rock model and loading condition for the hydraulic fracturing. The rock model is expressed by the assembly of particles bonded with each other. The particle radius was chosen to have a uniform distribution between maximum radius and minimum radius.
Abstract Drillability of a rock is often expressed in terms of a large number of parameters; however, the industry hardly uses any. Quite often these are not well understood or communicated to the end users. As a compromise, the present work describes drillability in terms of eight simple physical, mechanical, and micro-structural properties, which are displayed visually and are available from either log data or from laboratory core testing. The relevant rock properties are density, porosity, compressional and shear wave velocities, unconfined compressive strength, Mohr friction angle, mineralogy, and grain sizes. These are compiled and normalized in a scale of 1 to 8; value of 1 represents very soft rock and a value of 8 represents hard rock, ideally. The real rock is in between depending upon the rock type. The plot is called a โspider plot,โ which characterizes drillability fully in simple enough parameters for use in the industry, yet detailed enough to describe drillability issues to a great extent. Further, this gives an excellent tool to optimize the bit and drilling process for a given rock formation while depicting its physico-mechanical and micro-structural properties as a signature plot. INTRODUCTION Drillability may be defined as an ease of drilling or rate of penetration (ROP), achieved using specific cutter-metallurgy type cutters or compacts with the given cutter-bit design parameters and operating parameters; an efficient cutting removal system for the drilling environment using a particular drill rig type; and an outcome of unchanged cutter /bit dullness or balling condition. The drillability is assumed to be indicative of unconfined compressive strength (UCS) at atmospheric drilling [1,2] or confined compressive strength (CCS) together with a factor of efficiency [3,4,5]. The UCS is considered fundamental as it not only indicates atmospheric stress and strain at failure but also is related to elastic behavior (compressional and shear wave velocities, Youngโs, bulk and shear modulii), which doesnโt change much under confinement. It also indicates a co-efficient of energy transfer and the extent of vibration in an efficient drilling process. Drillability of rocks, by an expert, is defined by a large number of parameters [6,7,8,9,10,11,12,13,14,15,16], which are not well understood or communicated to the end user. As a result, the industry uses hardly any. However, quite often, rock properties like UCS and abrasiveness have historically been used to define rock drillability and help decide bit selection, drive mechanism and drilling parameters. These rock properties are typically estimated using electric log data [17,18,19,20,21] using a known calibration or mea in the laboratory in absence of logs. Further, it has been found that these estimates do not often provide the full picture of drillability of rocks, which have complicated mineralogy and microstructures that go through complex digenesis process. In addition, it is also well known in the industry that different rocks such as limestone, anhydrite, shale, and sandstone with similar UCS values have very different drilling characteristics and need completely different sets of bit designs/drilling parameters.
- North America > United States > Texas (0.93)
- Asia (0.68)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.56)
- Geology > Mineral > Silicate > Phyllosilicate (0.47)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock (0.36)
- Geophysics > Borehole Geophysics (1.00)
- Geophysics > Seismic Surveying > Seismic Processing (0.54)
- North America > United States > Texas > Travis Peak Formation (0.99)
- North America > United States > New Mexico > San Juan Basin > San Juan Basin Field > Mancos Formation (0.99)
- North America > United States > Mississippi > Travis Peak Formation (0.99)
- (3 more...)
- Well Drilling > Wellbore Design > Rock properties (1.00)
- Well Drilling > Drilling Operations (1.00)
- Well Drilling > Drill Bits (1.00)
- (3 more...)
Hydromechanics of a Virtual Rock Core
Liu, Jianxin (School of Mechanical Engineering, the University of Western Australia) | Liu, Jishan (School of Mechanical Engineering, the University of Western Australia) | Liu, Keyu (CSIRO Petroleum) | Elsworth, Derek (Department of Energy and Mineral Engineering, Penn State University) | Wang, Jianguo (School of Mechanical Engineering, the University of Western Australia)
ABSTRACT In this study, numerical tests of hydromechanics were conducted on a virtual rock core. A series of 2D digital images is captured through X-ray CT scanning of the rock sample. Each 2D CT image is then processed to ascribe a map of pixel values. The map of pixel values is thresholded to relate pore density that defines grain or pore to porosity. The same number of 2D CT images that represent the porosity distribution (map) are then used to construct the 3D virtual rock core. Through interpretations, the porosity for each voxel within the virtual rock core is defined. Through these steps, a 3D porosity map within the virtual rock core was created. Maps of other physical properties such as elastic modulus and permeability were produced through their empirical relations with the porosity. These property maps were used as direct inputs for the hydromechanical numerical tests. The numerical experiments are completed through the development and application of a porosity-based finite element hydro-mechanical model. In this hydromechanical model, the porosity changes with the volumetric strain and pore pressure. Because both elastic modulus and permeability are defined as a function of porosity, all these three important physical properties change with time. Results from these numerical tests demonstrate the impacts of rock heterogeneity on the hydromechanical performance (without considering the heterogeneity, the fluid flux could be overestimated by 130%). Through comparing the results from numerical experiments with those of the ideal homogeneous rocks, these heterogeneous impacts were quantified. INTRODUCTION In earth sciences, the term hydromechanics refers to the physical interactions between hydraulic and mechanical processes, which are important in influencing the effectiveness of enhanced oil and gas recovery from stress-sensitive reservoirs. Over the last three decades, substantial efforts have been made in theoretical and experimental studies of the effects of coupling between hydrologic flow and mechanical deformation in geological systems and on the implications of hydromechanics coupling for engineering applications. The geological deformation of oil and gas reservoirs has been of concern to petroleum engineers ever since the discovery in the mid-1980s of a large subsidence bowl in the Ekofisk field in the North Sea. Understanding of seafloor subsidence due to sediment decompression remains unresolved despite large investments and input of resources. Lewis et al. [1] presented a worldwide review of reservoir subsidence phenomena and applied coupled continuum modeling of multiphase poroelasticity to three-dimensional numerical simulations of oil and gas reservoirs. Hydromechanical coupling exists in a multitude of problems related to oil and gas exploration and extraction, including poroelastic stress redistribution around a borehole, reservoir compaction and subsidence, as well as the mechanics of hydraulic fracturing [2], borehole stability and shale mechanics, enhanced oil recovery [3], and stress-sensitive permeability changes during production [4], etc. The assessment of the influence of these various phenomena, caused by human or nature, on the behavior of the system, requires the development of realistic conceptual models. At the centre of such conceptualizations there lies development of phenomenological models comprising of phenomena formulation and solution techniques.
- Europe > Netherlands (0.34)
- Europe > Norway > North Sea > Central North Sea (0.24)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.35)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Tor Formation (0.99)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Ekofisk Formation (0.99)