Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
Results
Slotted Boreholes for Improved Well Stability and Sand Management
Addis, M.A. (Shell International Exploration and Production) | Khodaverdian, M.F. (Shell International Exploration and Production) | Lee, C.A. (Shell International Exploration and Production) | Fehler, D.F. (Shell International Exploration and Production)
ABSTRACT ABSTRACT: Borehole and perforation instability commonly occur during the life of a hydrocarbon producing field as a result of the increased effective stresses which accompany reservoir depletion. The use of slots cut into the sides of the boreholes has been investigated as a method of providing long term stability, using polyaxial block tests and numerical analyses. The slots are shown to redistribute the stress concentrations away from the borehole wall to the tips of the slots, for the polyaxial block tests, which results in improved stability. Numerical analyses demonstrate that the slots are expected to be more effective in increasing the strength of the borehole for field conditions, than in the laboratory. INTRODUCTION Wells drilled into high porosity sandstone formations commonly experience failure of the borehole walls due to excessive stress, normally high tangential (hoop) stresses. Fluid flow into the well during production tends to exacerbate the problem, ultimately leading to sand production in hydrocarbon producing reservoirs [1-7]. The stability of the borehole changes with time as the effective stresses in the reservoir increase in response to reservoir depletion. One approach for limiting borehole instability relies on increasing the formation strength: e.g. through injecting resins. This also normally stiffens the near wellbore region. The subject of our investigation addresses increasing the stability of boreholes by reducing the stresses acting in the vicinity of the well, by reducing the stiffness of the near wellbore region.
- North America > United States (0.46)
- Europe > Netherlands (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.55)
ABSTRACT ABSTRACT: In naturally fracture formations, injected fluid could flow into the existing fractures or induce a new fracture. It is important to predict conditions where opening of natural fractures happens and to determine the best strategy to avoid this, if one desires to pump proppant. We studied this with experiments on fractured blocks and a numerical model that incorporates transmissive discontinuities INTRODUCTION When injecting fluid at high pressure into naturally fractured formations the fluid has a choice between flowing into the existing fractures and creating a new fracture. If one desires to inject proppant, the latter is best because the fracture pressure can induce a sufficiently wide channel for proppant entry whereas a fracture network will show obstructions to proppant transport. We studied this with experiments on fractured blocks and a numerical model that incorporates transmissive discontinuities. Both reservoir and treatment parameters influence the hydraulic fracture geometry in naturally fractured reservoirs [1,2]. Practically, it is difficult to change the reservoir parameters, like in-situ stress, but the treatment parameters like the fluid viscosity and injection rate can be changed by an order of magnitude. Currently, it is believed that near-wellbore multiple fractures are mitigated by pumping viscous fluid, both during initiation and in later injections [3]. However, numerical simulations showed that the opposite effect might occur when high fluid pressure would open natural discontinuities, which deviate from the preferred fracture plane. The objective of our research is to investigate whether the measures to mitigate fracture tortuosity are also valid in fractured reservoirs. The fluid viscosity and injection rate influence the characteristic time scales of the fracture process. Pressurization rate is determined by injection rate and wellbore storage. Infiltration rate into the rock is determined by permeability (including natural fractures), porosity and fluid viscosity. Finally, pressure drop inside the fractures (natural or hydraulic) is determined by flow rate and viscosity.
- Asia (0.46)
- Europe > Netherlands (0.29)
- North America > United States (0.28)
ABSTRACT: Calcium oxide (CaO)-based expansive cement has been widely used to generate fracture in massive rocks and concrete structures. When water is added to expansive cement, calcium oxide reacts with water, and changes into calcium hydroxide expanding 1.96 times larger in volume with generating heat. The authors made an experiment to fracture a mortar specimen of 300 mm cube by setting expansive cement in a 50 mm diameter hole bored in the center of the specimen. Located AE (acoustic emission) sources clustered in the middle of a lateral surface and the hole rather than the vicinity of the hole, just before macroscopic cracks was generated. In a previous test applying only expansive pressure, located AE sources clustered only around hole and cracks extended from the hole. On the other hand, in a previous test applying only heat, AE source distribution indicated that macroscopic cracks extend from outside of the specimen to the center hole. Thus, it can be concluded that the thermal stress induced by the heat plays important role, although previous researchers focused on only the expansive pressure due to the volume increase. INTRODUCTION Calcium oxide (CaO)-based expansive cement, also known as "non-explosive demolition agents", has been widely used in open pit mines and in civil engineering to generate fracture in massive rocks and concrete structures. When water is added to expansive cement, calcium oxide reacts with water, and changes into calcium hydroxide expanding 1.96 times larger in volume [1] with generating heat along the following chemical equation, "CaO + H2O -> Ca(OH)2 + heat (16.5 kcal/mol)". In previous researches, for examples [2,3,4,5], although the expansive pressure due to the volume increase has been focused, effect of thermal stress induced by the heat has not been carefully discussed. In this meaning, the fracture mechanism of the expansive cement has not been clarified. Previously, the authors made an experiment to fracture a mortar specimen of 200 mm cube by setting calcium oxide-based expansive cement in a 55 mm diameter hole bored in the center of the specimen. AE (acoustic emission) sources clustered in outside of the specimen rather than vicinity of the hole just before macroscopic cracks generated, suggesting that macroscopic cracks originated at the outside of the specimen and then extended inward [6]. However, in the previous experiment, the size of the specimen was small in comparison with the diameter of the hole. Thus, to make sure the fracture mechanism, we newly conducted the same experiment using the larger mortar specimen of 300 mm cube with 50 mm diameter hole. In this paper, the results of the experiment using the larger specimen will be described. Moreover, the fracture mechanism will be discussed comparing to results.
ABSTRACT: An underground research tunnel (URT) is planned for construction at KAERI for the in situ studies related to the validation of a HLW disposal system. For the safe construction and long-term researches at the URT, the geological characteristics of the site were investigated and adequate design could be made. Mechanical stability analysis using a threedimensional analysis code with the rock mass properties determined from various tests was carried out. In the modeling, the actual topography, erosion effect, tunnel geometry and slope, sequential excavation, and geological conditions were considered. From the analysis, it was possible to predict the rock deformation, stress concentration, and plastic zone developed before and after the excavation. INTRODUCTION The Korea Atomic Energy Research Institute (KAERI) is developing a disposal system for the permanent disposal of the spent fuel generated from 19 nuclear power plants in Korea. In order to develop a reasonable disposal system, it is necessary to validate the system in a rock similar to the host geological formation. In Korea, a small scale underground research tunnel (URT) for validating the design of the underground high-level waste (HLW) repository is being planned. The construction of the URT will be started in 2004 and completed in 2006. During the construction as well as after the construction, various in situ experiments related to the HLW disposal will be performed in the access tunnel and the research modules. The URT will be a major infrastructure for the HLW disposal program and be used for various in situ studies such as rock behavior, in situ stress change, influence of discontinuities, excavation disturbed zone (EDZ) study, fluid flow in rock mass, heater test at different scales, Thermo-Hydro-Mechanical (THM) experiment, generation and migration of colloids, and the validation of the processes for the transportation, emplacement, retrieval operation, and closure will be carried out. Research items which can be carried out at the URT are listed in Table 1.
Rotary Percussion Sounding System for In Situ Rock Mass Characterization
Chitty, D.E. (Applied Research Associates, Inc) | Blouin, S.E. (Applied Research Associates, Inc) | Zimmer, V.L. (University of California) | Thompson, P.H. (Defense Threat Reduction Agency) | Tremba, E.L. (Defense Threat Reduction Agency)
ABSTRACT: The Rotary Percussion Sounding System (RPSS) was developed as an innovative approach to in situ rock mass characterization. It is based on a crawler-mounted rotary percussion drill rig, with modifications to provide digital records of relevant drilling parameters. In essence, the system uses modern instrumentation and computer technology to quantify the readily observable phenomenon that drilling speed decreases as rock strength increases. Empirical correlations have been derived to provide quantitative estimates of rock properties from the measured drilling parameters. The system includes direct measurements of drill head vertical position, hydraulic pressure input to the percussion hammer, blow rate of the percussion hammer, downward force on the bit, hydraulic pressure input to the bit rotation motor, rotation speed, and drilling fluid flow rate. This paper presents descriptions of the drill rig, as well as the special-purpose instrumentation and data acquisition systems that were developed for rotary percussion sounding. Data recording and reduction approaches appropriate to rotary percussion sounding are described. The primary focus of this paper is the development of an analysis methodology based on drilling inputs and the resulting drill advance rate that produces an empirically defined metric, the Percussion Index (รซp), which is independent of drilling inputs and can be correlated to rock strength. The analysis methodology was developed from a dataset derived by drilling in specially prepared concrete test articles having unconfined compressive strengths ranging from approximately 3 to 100 MPa (400 to 15,000 psi), and from measurements of the stress pulse amplitude made on strain-gaged drill rods. INTRODUCTION The measurement of in-situ rock mechanical properties has long been a challenge to engineers and geologists. Traditionally employed methods include borehole sampling, laboratory testing, seismic surveys, geophysical borehole logging, electromagnetic surveys, and geological mapping. There have been some efforts to extract mechanical properties information from drilling parameters [1,2], but the methodology and analysis methods are neither widely accepted nor commonly used. The Rotary Percussion Sounding System (RPSS), described in this paper, is based on a rotary percussion drill rig that has been instrumented to record drilling parameters and the resulting advance rate of the drill bit. The drill rig is also equipped for rock coring, and this capability has been used to provide samples for laboratory testing to support development of correlations with the sounding data. For various reasons, no one method is currently able to provide a cheap, low-cost, comprehensive in-situ mechanical property assessment. Seismic and electromagnetic surveys have relatively low resolution. Laboratory testing is relatively expensive, and is typically limited to intact rock samples. Geophysical logging requires boreholes, incurs high costs and is time-consuming. Geological mapping and examination of borehole samples are not quantitative enough. The RPSS was developed in an effort to address the shortcomings of the existing characterization methods for rock sites.
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Borehole Geophysics (0.88)
ABSTRACT: A numerical simulation of coupled multiphase fluid flow, heat transfer, and mechanical deformation was carried out to study coupled thermal-hydrological-mechanical (THM) processes at the Yucca Mountain Drift Scale Test (DST) and for validation of a coupled THM numerical simulator. The ability of the numerical simulator to model relevant coupled THM processes at the DST was evaluated by comparison of numerical results to in situ measurements of temperature, water saturation, displacement, and fracture permeability. Of particular relevance for coupled THM processes are thermally induced rock-mass stress and deformations, with associated changes in fracture aperture and fractured rock permeability. Thermally induced rockmass deformation and accompanying changes in fracture permeability were reasonably well predicted using a continuum elastic model, although some individual measurements of displacement and permeability indicate inelastic mechanical responses. It is concluded that fracture closure/opening caused by a change in thermally induced normal stress across fractures is an important mechanism for changes in intrinsic fracture permeability at the DST, whereas fracture shear dilation appears to be less significant. Observed and predicted maximum permeability changes at the DST are within one order of magnitude. These data are important for bounding model predictions of potential changes in rock-mass permeability at a future repository in Yucca Mountain. INTRODUCTION The Yucca Mountain Drift Scale Test (DST) is a multiyear, large-scale, underground heating test conducted by the U.S. Department of Energy at Yucca Mountain, Nevada. The DST is designed to study coupled thermal-hydrological-mechanicalchemical (THMC) processes in unsaturated fractured and welded tuff. The DST evolution has the same processes operating over a similar range of thermal conditions as that of a future Yucca Mountain repository. Therefore, DST data are used to validate models of those processes such that the models can be shown to be useful for modeling the post-closure behavior of the system. Pre-test predictions of coupled thermal-hydrological (TH) and thermal-mechanical (TM) processes at the DST and validation of TH and TM models have previously been conducted as part of the Yucca Mountain site characterization project. These predictions included three-dimensional simulations of TH processes conducted by the Lawrence Berkeley National Laboratory using the TOUGH2 code [1, 2, 3] and the Lawrence Livermore National Laboratory using the NUFT code [4]. Coupled TM processes have been simulated by the Sandia National Laboratories using the JAS-3D code [5] and by the Lawrence Livermore National Laboratory using the 3-DEC code [6]. However, no fully coupled THM analysis of the DST was performed until recently, when Rutqvist et al. [7, 8] applied a model for the analysis of coupled THM processes under multiphase flow conditions. This paper presents the current results of such a coupled THM analysis of the DST. Experience from the earlier modeling studies of coupled TH and TM processes at Yucca Mountain has been very valuable for development of the coupled THM model applied in this paper. In the pre-test prediction of coupled TH processes at the DST, Birkholzer and Tsang [1, 2] developed a three-dimensional numerical model based on previous experience in simulating the Yucca Mountain Single Heater Test (SHT) in the same formation. The modeling of the SHT had shown that an overlapping continuum model, is appropriate for modeling fracture and matrix interactions with multiphase, multicomponent fluid flow and heat transfer.
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.88)
- North America > Canada > Alberta > Mountain Field > Amoco Chiefco A-1 Sterco 16-25-47-21 Well (0.98)
- North America > United States > Texas > Permian Basin > Delaware Basin > Millard Field (0.93)
ABSTRACT: It is important to estimate both the absolute values and stress and its orientation at seismogenic depths, because it controls earthquake generation. To overcome the problem that we do no have a single method, with which we can estimate both stress values and orientation, we need to combine all the possible methods and compare the results to establish realistic and reliable methods to estimate stress at great seismogenic region. We will review the results at two site: 1) Stress measurements at the region with very shallow earthquakes and and 2) Stress measurements after the 1999 Chi-Chi earthquake (Taiwan). INTRODUCTION The state of stress provides us insights into understanding the mechanisms of generation and propagation of large earthquakes. Several methods have been proposed to estimate stress state at depths: In-situ stress measurements, core stress measurements and stress estimates from earthquake focal mechanism analysis. It is possible to estimate stress state at seismogenic depths (deeper than 10 km) by earthquake focal mechanism analysis. However, it is still necessary to estimate absolute value of stress at great depths, because earthquake focal mechanism analysis provides us only stress ratios and orientation. In-situ measurements and core stress measurements have advantages and disadvantages. Hydraulic fracturing stress measurements will be difficult to be performed at great depths due to breakouts and has fundamental problem regarding maximum stress. Core stress measurements need to have physical basis on the assumption on the stress estimates. To overcome the problem that we do no have a single method, with which we can estimate both stress values and orientation, we need to combine all the possible methods and compare the results to establish realistic and reliable methods to estimate stress at great seismogenic region. We will review the results at two site: 1) Stress measurements at the region with very shallow earthquakes and and 2) Stress measurements after the 1999 Chi-Chi earthquake (Taiwan).
ABSTRACT: Fracture heterogeneities have been measured by air-permeability tests in five niches located in two welded tuff units at Yucca Mountain. Niches are room-size excavations along underground drifts. The air permeability along borehole intervals at each niche site exhibits strong local variability, while the mean values of four niches in the middle nonlithophysal unit agree within ~0.4 orders of magnitude. After niche excavations in this fractured unit, the air permeabilities above the niche ceilings increase by 0.6 to 1.6 orders of magnitude. In comparison, the lower lithophysal unit has distinctly higher permeabilities, but with less effects from the niche excavation (0.2 to 0.4 orders of magnitude). Permeabilities are measured in boreholes both above the ceiling and on the side away from the fifth niche to evaluate stress release effects. INTRODUCTION This paper describes the air-permeability fieldtesting results pertaining to unsaturated zone processes in underground drifts at Yucca Mountain, Nevada. Air-permeability tests were undertaken at various locations to characterize the potential fluid flow paths in the rock. The rock consists predominantly of unsaturated, fractured welded tuff. Airflow occurs mainly through the fractures. Therefore, air-permeability tests characterize the fracture network and may be utilized to study fracture heterogeneity. Because the fracture network is highly permeable, the pressure field returns to ambient conditions quickly, generally within minutes, after air injection is stopped. As a result, many tests can be performed quickly, allowing measurements of fracture heterogeneity. Air permeability is measured from pressure change and flow rate of injected air into packed borehole interval [1]. Data collections and analyses have been conducted in other fractured test sites, including alcoves at Yucca Mountain [1, 2] and the tuff site of Apache Leap, Arizona [3]. In the niche study, we use 0.3 m (1 ft.) as the length of the packed intervals, comparable to the width of a wet feature (likely a flow path) observed within a brecciated zone at the end of the first niche [4]. The same borehole-interval length was used in liquidrelease seepage tests [5].
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.73)
ABSTRACT: Conventional nonlinear finite element analyses suffer from non-symmetric tangential stiffness matrices due to the non-associative behavior of geomaterials. In order to overcome this drawback, an implicit integration algorithm for the Drucker-Prager elastoplastic model has been proposed in this paper. The algorithm not only has the advantages of the good accuracy, high efficiency, and unconditional stability; but also can simulate the real behavior of geomaterials with the arbitrary degree of nonassociativity. The validity and accuracy of the algorithm are also demonstrated by numerically analyzing an axisymmetric borehole problem in an isotropic, elastic-plastic geomaterial. INTRODUCTION The algorithm for implementing the nonlinear constitutive relations of geomaterials plays a central role in analyzing and predicting the real behavior of geomaterials by using nonlinear finite element methods. Recent advances in theoretical studies and numerical analyses about the nonlinear constitutive integration algorithms have been achieved for metal materials [1-3]. It has been shown that the implicit integration schemes have the advantages of better accuracy, higher efficiency and unconditional stability [1-2]. The consistent tangent operators in conjunction with the implicit integration schemes of the elastic-plastic constitutive relations have also been presented for associative models [3-4]. But the non-associativity of geomaterials leads to non-symmetric consistent tangent operators and non-symmetric stiffness matrices for conventional explicit tangent stiffness methods [6-8]. Although a symmetric stiffness formulation for the tangential stiffness method has been presented for non-associated geomaterials by defining new equivalently associated materials [5], this results in increased complexity of the constitutive model expressions for geomaterials. In this paper, an implicit stress integration scheme of the Drucker-Prager's elastic-plastic model with an arbitrary degree of non-associativity, in conjunction with an accelerated initial stiffness method for nonlinear equilibrium iteration, has been presented. The scheme has been implemented in a nonlinear finite element procedure. The accuracy, efficiency, and convergence of this algorithm are demonstrated by analyzing an axi-symmetric plane strain borehole problem. The effects of the nonassociativity of geomaterials on stresses and deformations of a borehole are also discussed in detail.
- North America > United States (0.47)
- Asia > China (0.29)
ABSTRACT: This study focuses on using borehole fracture traces to estimate the size and aspect ratio of long and narrow subsurface fractures in sedimentary rocks. The method is well suited to fractures with length greater than borehole diameter and width less than borehole diameter, or more specifically apparent width as projected into the plane perpendicular to the borehole axis. Rectangles are assumed to be the shape of fractures in sedimentary rocks. Fracture traces associated with long and narrow (elongate) fractures are often caused by fractures piercing through either one side (single piercing) or two sides (double piercing) of the borehole. In such cases, a fracture trace map or borehole image will contain characteristic traces. An approach is described to estimate the size and aspect ratio of fractures based on borehole diameter and the intersection counts of singly and doubly piercing fractures. The method is directly applicable to large diameter cylinders, such as shafts and tunnels, with a few modifications. INTRODUCTION Fracture shape, size and orientation have a great impact on the permeability of fractured rock oil and gas fields and also affect the geomechanical properties of rock masses. Since fractures are embedded in an opaque rock mass, these parameters are commonly estimated from rock mass outcrops, boreholes, tunnels, or other exposures, or from geophysical imaging [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12]. Recently Wang et al. [13] proposed an approach to estimating fracture size in sedimentary rocks by using borehole or tunnel data, in the special case where all fractures are of constant size. In this paper we focus on estimating the size and shape of piercing-type fractures in sedimentary rocks. Here, the piercing-type fractures refer to long and narrow fractures with apparent length greater than borehole diameter and apparent width less than borehole diameter (Fig. 1).
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock (0.95)