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SPE Members Abstract Accurate knowledge of static rock properties is required to study sand production control, fracturing, casing design, and depletion of reservoirs. A high pressure cell allows simultaneous measurements of acoustic velocities, strains, and pore volume changes of cores under varying hydrostatic or biaxial stresses. Rock properties computed from static measurements are a function of the stress history. Theories allowing the correlation between dynamic and static properties are suggested. Introduction Today's economic conditions emphasize the need for better engineering designs in well completion and reservoir production activities. Some of the least known reservoir data are the elastic/mechanical rock properties: bulk or pore compressibility, Young's modulus, and Poisson's ratio. These properties have a wide range of application in areas such as sand control, hydraulic fracture treatment, wellbore stability analysis, perforation design, subsidence studies, prediction of reservoir behavior, and analysis of pressure transient tests. Two sources of data are available for the determination of elastic/mechanical rock properties. Stress-strain relations or pore volume changes measured on a core, yield what are called .. pore volume changes measured on a core, yield what are called .. static" rock properties. Velocities of compressional (longitudinal or "P") and shear (transversal or "S") waves yield "dynamic" rock properties. properties. While in-situ dynamic properties can be computed from a suite of logs, static rock properties better represent situations encountered in well completion or reservoir studies. This is because the rates of static stress loading are orders of magnitude lower than rates of stress imposed by wave propagation. Also, the magnitudes of reservoir stress are more in the order of the static tests than of the dynamic tests. Independent determination of dynamic and static properties is routinely done in the laboratory, but most of the time the properties measured using both techniques do not agree. For properties measured using both techniques do not agree. For example, static bulk compressibilities are reported to be one to ten times greater than the dynamic bulk compressibilities. This discrepancy decreases for rocks with a large Young's modulus or at high confining pressures. One of the first challenges in attempting to relate static and dynamic properties is to define an effective stress concept which will allow comparison of properties determined from different testing conditions. Wave velocities are a function of the pressure applied in the direction of propagation, while some static rock properties, such as strength, are best described with differential properties, such as strength, are best described with differential pressure (Tresca's criterion). From theoretical considerations, pressure (Tresca's criterion). From theoretical considerations, the mean pressure should be used. Mean pressure is defined as: (1) where i = x, y, z are principal directions in Cartesian coordinates. Another consideration is fluid saturation.
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Borehole Geophysics (0.95)
ABSTRACT: Acoustical data represent a valuable source of information in rock mechanics, in particular in cases where direct measurements of rock mechanical parameters are impossible. By using proper interpretation techniques, acoustic measurements may reveal information about stress state, static elastic properties and strength, as well as large scale inhomogeneities as fractures and joints. 1. INTRODUCTION Rock or soil mechanical parameters are required for a wide range of engineering applications, from establishing slope or foundation stability on the earth surface to predicting borehole instabilities at several kilometers depths in drilling or production of petroleum. Such parameters, like static elastic coefficients and rock strength, are often unavailable by direct measurements. Acoustic waves, being by nature mechanical disturbances, clearly may give information about rock mechanical features. Surface seismic recordings give wave velocities at depth, and when a borehole has been drilled, VSPs (Vertical Seismic Profiles) and sonic logs contribute more detailed analyses. However, the acoustic waves provide only indirect information, so there is a need for interpretation methods by which the rock mechanical parameters can be obtained. Current methods of mechanical properties logging do not take proper account of the difference between static and dynamic rock mechanical behaviour. Further, rock strength is often deduced from the elastic moduli. These assumptions clearly have severe limitations: In particular, in weak rocks, the difference between static and dynamic moduli may be several hundred %. This paper will discuss the basis of these assumptions, and review different theoretical and empirical models that have been put forward in order to solve the mechanical properties logging problem. The discussion will be accompanied by experimental results, where acoustic measurements have been performed during failure tests in various rock materials. An interesting feature of the acoustic behaviour, is the development of stress-induced acoustic anisotropy. This anisotropy is related to the generation of an oriented distribution of microcracks, which again relates to the failure mechanism itself. In the following, we will discuss how acoustics can be applied in estimates of earth stresses (Section 2), elastic moduli (Section 3), and rock strength (Section 4) in " intact" rock. In Section 5 we will briefly discuss how acoustic waves also may be applied to explore fractured/jointed rock masses, which is of practical importance for optimization of oil & gas production in low permeability reservoirs, for preventing leakage into tunnels or mines or from waste storage reservoirs. 2. ROCK STRESS AND ROCK ACOUSTICS Experimental experience has repeatedly proved that acoustic wave velocities are stress dependant. Besides being a manifestation of non-linearity in rock mechanical behaviour, this also offers a potential of indirect stress monitoring through acoustic measurements. The effect of an increasing hydrostatic stress is to increase p- and s-wave velocities. As a general trend, the effect is more pronounced at low than at high stress levels. This is exemplified in Fig. 1, showing the pressure dependence of acoustic wave velocities in a relatively weak sandstone. Pore pressure is generally thought to affect the sound velocities in accord with the effective stress principle.
- Europe > Norway (0.46)
- North America > United States (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.36)
Keshen reservoir is a deep, tight gas sandstone reservoir under high tectonic stress with reservoir pressure over 16,000 psi (110 MPa) and temperatures up to 165 °C. Development wells for this field are in excess of 6500m in true vertical depth. Stimulation is required to provide production rates that compensate for the high cost of drilling and completing wells. Hydraulic fracture design and execution must be optimal to ensure economic production. To effectively stimulate a more than 200 m thick sandstone reservoir with consistently high performance, it is necessary to understand the mechanical behaviour of the reservoir, especially mechanical properties and in-situ stresses as the two control initiation and propagation of each hydraulic fracture. The mechanical behaviour is complicated by high tectonic stresses, significant compaction, and high overpressure. To gain an in-depth understanding of the mechanical properties and in-situ stresses of Keshen reservoir, an integrated geomechanical evaluation was conducted. The evaluation used core from two wells, KS205 and KS207, and log data obtained from 15 wells including the wells with core evaluation in the field. A laboratory testing program to investigate the mechanical behavior of the reservoir sandstone under realistic in-situ stresses, pore pressures, and temperature was performed. The description of mechanical behavior obtained from the laboratory testing was used to calibrate and augment mechanical earth models (MEMs) constructed from well log data. The reliability of the completed MEMs was validated through comparison between wellbore stability predictions with observation of borehole failure from the borehole microresistivity image. The geomechanics information was delivered to the stimulation engineering team. Hydraulic fracture design and execution was conducted based on this information. The outcome of hydraulic fracturing was very encouraging. This study demonstrated that successful stimulation of tight reservoir in high pressure, high temperature relies on integrated geomechanical analysis.
- Europe (1.00)
- Asia > China (1.00)
- North America > United States > Texas > Harris County > Houston (0.28)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geophysics > Borehole Geophysics (1.00)
- Geophysics > Seismic Surveying > Borehole Seismic Surveying (0.34)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- Asia > China > Xinjiang Uyghur Autonomous Region > Tarim Basin > Keshen Field (0.99)
- Asia > China > Xinjiang Uyghur Autonomous Region > Tarim Basin > Dabei Field (0.99)
- North America > United States > Kansas > Panoma Field (0.94)
Abstract A comparison of Young's moduli and Poisson's ratios obtained from ultrasonic laboratory measurements with static moduli obtained under identical stress conditions shows that the Young's moduli are 1 to 6 times higher under ultrasonic loading conditions. A comparison of these two laboratory measured quantities with log derived moduli measured at 20 KHz indicates that E-ultrasonic > E-sonic >E-static. The clay content and porosity of the samples varied from 1% to 54.5% and 3% to 17.5%, respectively. This clearly suggests that a wide variety of sandstones behave in a viscoelastic manner. The magnitude of the variation with frequency is a function of the clay content, grain size, shape of intergranular contacts, mineralogy and fluid saturations. A model is presented that describes this observed viscoelastic behavior. The measured static moduli are a function of the sample length. This effect is investigated in some detail. The reported static moduli were obtained at L/d ratios of 2. When comparing log measurements to core, two inherent problems were encountered: depth correlation and sample size, These effects need to be properly accounted for when comparing logs with cores. Introduction Two methods are commonly used to measure elastic moduli of materials: static stress-strain measurements and the measurement of acoustic velocity. The stress-strain measurements yield static elastic moduli whereas the velocity measurements provide dynamic moduli. Early studies on this matter report non-linear stress-strain relations and dynamic elastic moduli that are greater than static moduli. This was ascribed to the existence of microcracks in the body and was thus related to the microcrack density. Static moduli are affected by existing microcracks while dynamic moduli appear to be less affected by them. At large overburden stresses, most cracks are closed and the values of static and dynamic moduli approach each other. The literature discussing the correlation between static and dynamic moduli of tight gas sandstones is very limited. Jizba and Nur and Jizba et al. reported that the relation between static and dynamic moduli in tight gas sandstones is controlled by stress and lithology. P. 299^
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
A comparative study of the stress-dependence of dynamic and static moduli for sandstones
Wang, Yang (University of Houston) | Han, De-Hua (University of Houston) | Li, Hui (Xi’an Jiaotong University) | Zhao, Luanxiao (Tongji University) | Ren, Jiali (University of Houston) | Zhang, Yonghao (China Petroleum Logging Co., Ltd)
ABSTRACT Understanding the differences between the static and dynamic elastic moduli of reservoir rocks is essential for the successful exploration and production of hydrocarbon reservoirs. However, the controlling factors on the dynamic-static discrepancy for sandstones remain ambiguous. Consequently, we have purposely selected three outcrop sandstone samples with large porosity contrast to investigate the effects of the stress state, magnitude, and load-unload history on the dynamic and static moduli through laboratory measurements. The results suggest that the dynamic moduli are systematically larger than the static moduli at almost any hydrostatic or deviatoric stress magnitude. In contrast, the static moduli are much more sensitive to the stress variations than the dynamic ones, leading to the decreasing dynamic-static difference upon hydrostatic loading and the increasing dynamic-static difference upon deviatoric loading. When the maximum stress in a cycle is initially reversed, the dynamic-static ratio tends to approach one, whatever the bulk modulus under hydrostatic pressure condition or the Young’s modulus under triaxial stress condition. Under the subsequent unloading process, the static bulk modulus is always higher than that derived during loading. However, the unloading static Young’s modulus is larger than the loading Young’s modulus only at a relatively high deviatoric stress magnitude greater than 30 MPa, while showing an opposite trend at a low-stress condition of less than 25 MPa. From the microstructural viewpoint, it is believed that the static tests accumulate the elastic, viscoelastic, and nonelastic properties within a certain stress or strain range. In contrast to the dynamic modulus, the static modulus exhibits greater sensitivity to the pressure or stress changes under hydrostatic and deviatoric stress conditions. The strong stress dependence makes it important to consider the in situ stress conditions when establishing dynamic-static modulus relations.
- Asia > China (0.68)
- North America > United States > Texas (0.28)
- North America > United States > Idaho (0.17)
- Research Report > New Finding (0.34)
- Research Report > Experimental Study (0.34)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Borehole Geophysics (0.68)
- North America > United States > Michigan > Michigan Basin (0.99)
- 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)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)