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ABSTRACT Abstract Laboratory data on the time-dependent compaction of porous rocks are presented, and various attempts to model this behavior are described. These lead to a discussion of the problems that arise in using laboratory data to estimate long-term effects. A method is suggested for combining laboratory data with information from the geologic record to estimate long term effects. Introduction The phenomena of reservoir compaction and subsidence as a result of fluid withdrawal are well known. Particularly well-documented examples include the excessive subsidence at Long Beach, California, resulting from oil production from a thick sequence of sand [1,2] and subsidence at the Wairaki geothermal field in New Zealand where withdrawal of hydrothermal fluids from volcanic rocks is the driving mechanism.[3] Many straightforward attempts to model and predict this behavior have been made,[4,5] none of which have been particularly successful. Typically, such models assume that the reservoir materials are linearly elastic, and compressibilities are obtained from data taken in short-term (1 day) tests on small (typically 50 mm diameter x 100 mm length) samples. Field evidence suggests that the assumptions of elasticity are erroneous. Thus at Long Beach, for example, we can cite the irreversibilty of compaction and subsidence, while at Wairaki, and at many other sites, there is strong evidence that the process of compaction is time-dependent as well. In an earlier study, Schatz, Kasameyer and Cheney[6] indicated a time-dependence in geothermal field compaction based upon a comparison of laboratory compressibilities with those deduced from field porosity measurements. There are a number of factors which will affect the applicability of compressibility measurements in the laboratory to predictions of field values. These include sample disturbance, the effects of fluid/rock interaction if the saturating fluids in the laboratory and in the field are not identical, the temperature and pressure of tests and the fact that small samples may not be representative of a formation which may be heterogeneous and discontinuous on a scale larger than the sample. While these are undoubtedly important factors, we choose to concentrate in this discussion of the aspect of time-dependence of formation rocks. Over the last several years, several investigators have observed time-dependence of porous materials in the laboratory. Some of these observations have been made in relation to the stability of soil-based structure. [8,9] Others have been made in relation to compaction of reservoir formations.[10-17] In this paper we will concentrate on observations made by ourselves and co-workers in relation to three rocks - a siltstone which occurs as a lignite overburden in central Texas, a geopressured-geothermal reservoir sand from the Gulf Coast of Texas, and geothermal reservoir sands from Southern California and Northern Mexi co. A major thrust of the work involved was the development of constitutive relations which would allow the prediction of long-term compaction. In doing this, we have approached the development of constitutive relations both from an empirical standpoint and from the point of phenomonological modelling. A major result of this work has been to highlight the strong possibility of increased compaction occurring over long time periods not usually observable in the laboratory.
- North America > United States > Texas (0.89)
- North America > United States > California > Los Angeles County > Long Beach (0.24)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.34)
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Renewable > Geothermal > Geothermal Resource (0.54)
ABSTRACT ABSTRACT A seismic fissuration index (K) is defined as the ratio of the difference in the P and S wave velocities obtained when a dry rock specimen is tested both under a load equal to half the uniaxial compressive strength of the rock (Vo) and without an applied load (Vd) to the velocity obtained without an applied load (Vd), as K = (Vo - Vd)/Vd. A similar seismic fissuration index is also proposed for classifying the dry rock mass in terms of field seismic velocities. The field equation is extended by including the intact rock seismic velocities. K is found to be related to the porosity of rocks. The proposed equation can equally be used for characterizing saturated rocks, using S-wave velocities. However, the time average equation should be used for characterizing saturated rocks, using P-wave velocities, even though this equation gives upperbound estimates of the rock material and fracture porosities. INTRODUCTION Geophysical seismic techniques are generally employed to characterize and determine the dynamic properties of rocks. As these tech- niques are non-destructive and easy to carry out, they are increasingly being used in geotechnical engineering. The seismic methods, in general, give an overall estimate of the rock mass velocity and they can be carried out from the ground surface, underground, or in boreholes. Seismic surveys are generally carried out in the early stages of site investigation in order to delineate the zones of interest and areas where further investigation is required. Excavatability of the rock mass is suggested by the equipment manufacturers in terms of their seismic velocities (Anon, 1980). Attempts have also been made to assess grouting, rockbolt reinforcement and blasting efficiencies in the rock mass by the seismic velocity determinations (Knill, 1970; Price et al., 1970 and Young et al., 1985). Some other interesting applications of seismic techniques have included the prediction of rock mass deformation, stress and the extent of fracture zones developed around underground openings (Onodera, 1963; Gladwin, 1982; Hudson et al., 1980). The seismic properties of the rock mass are influenced by the intrinsic rock material properties and external factors such as temper- ature, pressure, pore fluids and fractures. The influence of these factors is best studied in the laboratory under controlled conditions. Such a study would enable the influence of jointing or weathering to be determined by comparing the seismic velocity of the rock material at the pressure, temperature and moisture content corresponding to the rock mass conditions. Variation in seismic velocity may not only be due to quality variation in the rock mass, but also a change of rock type, depth, and the presence of groundwater. In this paper, initially the factors influencing seismic velocity and the suggested methods for characterizing the rock mass are briefly presented. Then a new method for characterizing rocks in terms of seismic velocity is given. According to this method a seismic fissuration index has been defined as the ratio of the difference in velocities obtained when a dry rock specimen is tested under a load equal to half the uniaxial compressive strength and without an applied load to the velocity obtained without an applied load.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type (0.89)
ABSTRACT INTRODUCTION The inelastic stress-strain behavior of rocks has urged engineers to seek for the possible application of plasticity to engineering projects such as tunnels, oil wells, dams, etc. for a long time. Early efforts in this direction have tentatively used the classical theories to solve some axisymmetric problems, in which the rocks were treated as perfectly plastic and nonfrictional materials (Talobre, J. A., 1957). Meanwhile, one of the fundamental assumptions is that the volume strain of elastoplastic material is purely elastic. Drucker and Prager (1952) developed a perfectly plastic and frictional model with a generalized form of Mohr-Coulomb law, which serves as both failure criterion and yield criterion. Although the associated flow rule gives unacceptably large dilatancy, this model is a very important step to deal with the frictional behavior of geological materials. As a further step, Roscoe et al (1963) and Schofield and Wroth (1968) proposed a Cam-clay model, which accounts for the yielding before the material reaches a failure envelope and reflects the plastic volume strain as a strain hardening factor, and the open end of the Drucker-Prager envelope is capped by a family of yield surfaces, the envelope is made conforming with critical void ratio line. The proposed shapes of the cap have been based on experimental data and on convenience of mathematical descriptions. Drucker, Gibson and Henkel (1957) introduced a spherical cap for soils, while in Cam-clay model the cap is semi-logarithmic yield function derived from the result of triaxial tests. DiMaggio and Sandlet (1971) proposed an elliptical model for sand, in which the hardening function depends exponentially on plastic strain. Sandlet et al (1976) used a plane cap model to express the behavior of a wide range of geological materials of which the nonlinear hysteretic nature is significant. Mizuno and Chen (1982) illustrated the physical meaning of cap models and their adaption to finite element calculation. In recent years, Carroll et al (1972, 1980) have studied the compaction of porous materials under hydrostatic pressure and discussed the mechanism of the nonlinear response thoroughly. Analytical and numerical analysis of the hollow sphere model by Curran and Carroll (1979) predicted an initial yielding surface, dependent on the initial porosity, and showed a strong coupling between hydrostatic and deviatoric effects. All these main features from the abovementioned theoretical research are visualized by the experimental data on Boise sandstone (1984). The combination of such previous work would be logically the construction of a plasticity model for porous rocks which, following the tradition of assigning the name of birthplace to a special model, is called Cal-Rock model (Carroi, M. M., 1984). The results of triaxial tests along different loading paths reveal the compaction characteristics for deviatoric and hydrostatic stress separately. the fitting of the nonlinear stress-strain curves leads to a construction of yield surfaces. Moreover, an uniaxial strain test gives the strength envelope which is the critical state line. The whole model is composed in the frame of plasticity theory. The concept and representation of plasticity used here are in the spirit of Naghdi- Casey theory (Casey and Naghdi, 1984 and 1984, Naghdi and Trapp, 1975).
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.62)
- Geology > Geological Subdiscipline > Geomechanics (0.48)
ABSTRACT Abstract. Two separate studies investigated the compressive strength of basic igneous rocks and its relation to index properties such as porosity, density and sound-wave velocity. The rocks tested were weathered dolerite from S. W. England and vesicular lavas from California. In both cases strength was affected considerably by porosity. In the dolerite, strength was also affected by fissures (microcracks) which were formed in the parent rock by shearing and later accentuated by weathering. While test results show that porosity, density and sound velocity are not reliable indices of rock strength, these properties are useful in interpretation of strength test results. Index testing can be used to classify samples according to similar behavior and to recognize the influence of fissures. INTRODUCTION It is well known that the strength and deformability of so-called "intact rock" are influenced both by pores and by fissures. Fissures and microcracks are distinguished from spherical pores by their narrow, elongated shape and their high length-to-width ratios, typically of the order of 103 (Walsh & Brace, 1966). They are formed by stress, and result from tectonic shearing or from the expansion of minerals during weathering or alteration. In some cases, the crack or fissure pattern may be related to the macroscopic joint pattern in the rock mass, while in others it forms part of a separate shear fabric. Microcracks and fissures can range in length and spacing from the microscopic to a few cm, and can be distinguished according to the terminology proposed by Londe (1973): micro- cracks are features with a spacing of less than 1 cm, fissures have a spacing of less than 10 cm and joints have a spacing greater than 10 cm. In much of the literature, however, the adjective "fissured" refers to both microcracks and fissures. As a consequence of their shape, microcracks and fissures are further distinguished from pores by their compressibility and preferred orientation. Their effects on the mechanical properties of "intact" rock, as reviewed by Jaeger (1972) and Goodman (1976), include the dependence of strength on specimen size, the change in slope of the stress-strain curve as cracks are closed with increasing stress, the stress-dependence of permeability, and hysteresis in load cycling. Cracks and fissures also influence static elastic modulus values, sound wave velocity, direct tensile strength, resistivity and conductivity. In all these cases, the effects of cracks and fissures are quite distinct from the effects of spherical pores, which are noncompressible and which do not have preferred orientation. This paper describes two separate studies of engineering properties of porous and fissured basic igneous rocks. The first study examined properties of weathered dolerite (diabase) from S. W. England and had the original objective of using simple index tests to aid in classification of the degree of weathering. The effects of weathering in these rocks are to create pores by leaching of relatively soluble constituents and to accentuate tectonic microcracks which occur in varying degrees in the unweathered rocks. The second study reported in the paper examined engineering properties of vesicular (i.e., porous) lavas from eastern California.
- Europe > United Kingdom > England (0.89)
- North America > United States > California (0.55)
- Research Report > New Finding (0.48)
- Research Report > Experimental Study (0.34)
- Geology > Mineral (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Igneous Rock > Basalt (0.41)
ABSTRACT ABSTRACT The Nuclear Waste Policy Act of 1982 (NWPA) assigns the U.S. Department of Energy (DOE) the responsibility of locating, constructing, operating, closing and decommissioning a nuclear waste repository. Prior to submittal of a licensee application to the U.S. Nuclear Regulatory Commission, the DOE is required by 10 CFR part 60 to conduct a program of site characterization for the site to be described in such application. Site characterization includes an exploration and research program in the field and laboratory to determine the geologic conditions and ranges of those parameters which are necessary for determining the suitability of the site as a geologic repository. The information thus collected should be able to establish with reasonable assurance that the public and environment will be adequately protected from the hazards associated with a repository at the site. This paper demonstrates by example a geostatistical approach which, on the basis of the existing limited data base, can be used to evaluate (1) whether existing information on a certain parameter is adequate to be considered representative, (2) if additional information is required, where should the additional sampling or testing be performed, and (3) whether the newly acquired data along with the existing information constitute representative data. INTRODUCTION The Nuclear Waste Policy Act (NWPA) of 1982, Public Law 97-425, assigns the Department of Energy (DOE) the responsibility of locating, constructing, operating, closing, and decommissioning the geologic repository. The U.S. Nuclear Regulatory Commission (NRC) has statutory responsibility for reviewing DOE's license application and site investigation programs. The NRC Rule, 10 CFR 60.10, requires that prior to the submittal of an application for license DOE shall conduct a program of site characterization with respect to each site to be described in such application. DOE must conduct an exploration program both in the laboratory and in the field to establish the geologic conditions and ranges of those parameters which are necessary for determining suitability of the site as a geologic repository. The information collected should be suitable to establish with reasonable assurance that the public and the environment will be adequately protected from the hazards associated with a repository at the site. The intended purposes of site characterization testing are to: (1) obtain information needed to assess the suitability of particular site; (2) provide data for key parameters for design of a repository; (3) ensure the representativeness of key parameter values throughout the site; and (4) provide adequate information to support findings on important issues which affect the repository performance, prior to the licensing hearing. The problem of ensuring the representativeness of the host rock parameter values throughout the site arises because of sampling constraints imposed by 10 CFR Part 60.10 on site characterization activities. To limit the adverse effects of site characterization testing on the long term performance of the geologic repository, 10 CFR 60 requires that: (1) the number of boreholes and shafts be limited to the extent practical; (2) exploratory boreholes and shafts in the geologic operations area be located where shafts are planned for, or where large unexcavated pillars are planned to the extent practical; and (3) subsurface exploratory drilling, excavation and in situ testing be coordinated with the repository operations area design and construction.
- Government > Regional Government > North America Government > United States Government (1.00)
- Energy > Power Industry > Utilities > Nuclear (1.00)
Rock Mechanics Modelling Of The Ekofisk Reservoir Subsidence
Barton, N. (Norwegian Geotechnical Institute) | Harvik, L. (Norwegian Geotechnical Institute) | Makurat, A. (Norwegian Geotechnical Institute) | Chryssanthakis, Barton (Norwegian Geotechnical Institute) | Vik, G. (Norwegian Geotechnical Institute) | Bandis, S.C. (NTNF) | Christianson, M. (Itasca Inc)
ABSTRACT Abstract The large jointed chalk reservoir situated at 3 km depth in the North Sea's Ekofisk field is undergoing major compaction after nearly 15 years of oil and gas production. Approximately 150 km of the overlying sediments, mostly shales, are involved in the subsidence. A maximum central subsidence of nearly 3 meters, and a maximum present rate of 45 cm per year has set in motion numerous studies of the phenomenon. Non-linear finite element (FEM) and non- linear distict element (DEM) analyses of the compaction and large scale subsidence were performed. Consistent differences between the continuum and discontinuum analyses were noted. In the latter, slip on hypothetical bedding planes and subvertical or vertical faults was allowed, which possibly gives a more realistic simulation of the real processes of subsidence in such a large body of rock. Laboratory studies of the reservoir joints included roughness measurement, JRC and JCS characterization, direct shear tests while saturated in heated Ekofisk oil, and coupled closure-shear-flow tests with heated oil followed by heated sea water. Discontinuum modelling using Cundall?s UDEC was performed on representative jointed assemblies (two sets of steeply dipping conjugate joints) to investigate the effect of a major reduction of pore pressure within the deformable matrix and along each joint. It was found that the large shrinkage deformation of the matrix allowed joint shearing to occur despite the constraint of uniaxial strain. Relative mass bulking due to small but widely distributed joint shear possibly explains the observed maintenance of excellent productivity despite large vertical strains in the reservoir. Introduction The Ekofisk field which is operated by Phillips Petroleum Company, is one of several hydrocarbon reservoirs associated with the Central Graben in the southern North Sea. The Maastrichtian and Paleocene (Tot and Ekofisk) chalks form an extensively jointed gentle anticlinal-domal structure, 300 meters in thickness at 3 km depth. The reservoir is pear- shaped in plan, with maximum dimensions of approximately 9 km (EW) by 14 km (NS). The higher porosity chalks (30 to 45 %) which are undergoing non-linear deformation have caused a central compacting zone measuring approximately 30 km in area (approx. 4 by 7 km). The area of seabed presently affected by the subsidence appears to be more circular in shape (approx. 7 by 9 km) and covers an area of approximately 50 km . Numerical modellets are therefore faced with the problem of predicting the subsidence of some 150 km of overburden (mostly shales), using laboratory samples of the chalk and shale as small as 15-30 cm ; a discrepancy in volume of sixteen orders of magnitude. Compaction modeling using a non-linear continuum model Non-linear finite element continuum analyses of the compaction process were performed with the CONSAX code (D'Orazio and Duncan, 1982). This program uses eight-noded isoparametric elements. Known distributions of porosity and fluid pressure time histories were modelled. A modified Cam Clay Model was used to simulate the non-linear void ratio-log effective stress curves, which represent the pore collapse behavior of the most porous chalk.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.65)
- North America > United States > California > Los Angeles Basin > Wilmington Field (0.99)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Tor Formation (0.98)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Ekofisk Formation (0.98)