Static Elastic Parameters Research Articles

Determining the geomechanical units using rock physics methods

The stability of the drilled wellbores in carbonate formations is of great importance. This paper is a new approach to determine the elastic properties of the wellbore based on rock physics methods using the Hashin-Shtrikman bounds to determine the wellbore stability. Initially, the static elastic parameters were determined by two different methods, namely the rock physics method and DSI log, followed by clustering through Multi-Resolutional Graph-Based clustering (MRGC) and calculating geomechanical-units (GMUs) of each method separately. The obtained results from DSI log and rock physic methods were then compared followed by determining the wellbore stability. The results showed that the correlation between shear and compressional wave velocity obtained from the petrophysical method with the measured values of shear and pressure wave velocity in the well was 0.94 and 0.90, respectively, which shows that the petrophysical method can estimate these two logs with high accuracy, and it can be a suitable alternative instead of using the Dipole Shear Sonic Imager (DSI). It was observed that the geomechanical units of elastic parameters calculated by proposed rock physics method are in good agreement with those obtained from DSI log. Rock-physic method can be a good alternative for when expensive DSI logs are missing.

Study on dynamic and static elastic moduli of shale oil by different loading methods

Experiments on the dynamic and static elastic parameters under distinct loading conditions help clarify the effects of loading rate and strain amplitude on dynamic and static elastic moduli, and gain further insight into the moduli. Significant scientific guidance are provided for fracturing transformation of oil shale reservoirs. AutoLab 1500, servo-controlled triaxial equipment with ultrasonic transducers, is used in an elastic parameter test on shale oil samples. The test concludes that the loading rate moves the dynamic elastic modulus little, with a maximum change rate of 0.8%, while it significantly affects the static elastic modulus by 31.7% at most. In the loading or unloading phase, the larger the initial differential stress (or strain), the higher the dynamic Young's modulus. The static elastic modulus, on the other hand, presents a significant negative correlation with the stress amplitude. To be specific, the smaller the strain amplitude, the higher the measured modulus; the larger the strain amplitude, the smaller the modulus. Given the loading rate, the smaller the differential stress, the smaller the difference between the dynamic and static elastic moduli. This study can provide a reference base for fracturing construction in oil shale.

Measurement of multi-stage triaxial compression test and static elastic parameters of shale samples from the Talangakar formation for a feasibility study of shale gas in the South Palembang sub-basin, South Sumatra

Shale gas is an unconventional gas source that has great potential to be developed and produced in the future. One area that has great potential in Indonesia is in the South Sumatra basin. For good hydrofracturing planning, we used samples from core rock specimens from field well B in the Talangakar formation at a depth of 2289 m with shale lithology located within the South Palembang sub-basin in the South Sumatra basin. We tested the sample by multi-stage triaxial (MST). MST is a compression test requiring only one rock specimen to be tested in three cycle stages with different confined pressures From this test, the curve of Stress-Strain, yield point, elastic zone and plastic zone is obtained. Then it can also be generated Mohr-Coulomb curve, elastic parameters, i.e: Young’s modulus, Poisson’s ratio, etc. The results of this measurement are also useful for predicting the minimum horizontal stress which is very important in the design of hydraulic fracturing in shale gas.

Study on Mechanical Properties of Volcanic Breccia in Mahu Area, Xinjiang

The rock mechanics experiments of dynamic and static elastic parameters, brittleness index and fracture toughness of Mahu volcanic breccia in Xinjiang are carried out. The transformation relationship of dynamic and static elastic parameters, the distribution of brittleness index and the prediction model of fracture toughness are established, which is of great practical significance to the exploration and development of Mahu oilfield. The results show that: (1) The relationship between deformation characteristics of volcanic breccia and confining pressure is studied. The radial strain and transverse strain increase, and the stress at failure increases with the increase of confining pressure. The axial strain is larger than the radial strain under the same confining pressure. (2) The dynamic elastic parameters are larger than the static ones under the same confining pressure. The transverse and longitudinal wave velocity, the dynamic and static elastic parameters increase with the increase of confining pressure. (3) The brittle index distribution of breccias in Xinjiang is established, ranging from 12 to 68. Based on the fracture toughness test results, regression analysis was performed on density, tensile strength, elastic modulus, and Poisson's ratio. A prediction model for fracture toughness of the volcanic breccia in Mahu, Xinjiang was established, and its calculation results are highly correlated with the measured values. It provides basic parameters for the study of rock mechanical properties in Mahu oilfield, Xinjiang, and provides reference for the formulation of oilfield exploration and development construction scheme.

Laboratory investigation of the relationship between static rock elastic parameters and low field nuclear magnetic resonance data

The static rock elastic parameters are important in the petrophysical evaluation of unconventional reservoirs since many of them need the fracturing technology for development. However, it often fails to establish models of the static rock elastic parameters through geophysical well logging data. We developed a preliminary research to explore the relationship between the static elastic parameters and the low field Nuclear Magnetic Resonance (NMR) parameters based on joint measurements of the NMR responses and rock mechanical properties. The geometric mean, the arithmetic mean of the transversal relaxation time, the cutoff value, as well as the bin porosities are considered to investigate their relationships with the static rock elastic parameters. The result revealed that the static Young's modulus is strongly correlated with the NMR parameters, whereas the static Poisson's ratio is slightly influenced by the pore size properties. This study provides a new perspective in the application of the low field NMR data.

A New Model between Dynamic and Static Elastic Parameters of Shale Based on Experimental Studies

During the exploration and development of shale gas resources, elastic parameters provide crucial geomechanical information for shale gas extraction and hydraulic fracturing projects. Static elastic parameters are usually adopted in rock mechanics analyses and hydraulic fracturing designs and are generally obtained from experimental tests using core samples collected from the subsurface. However, experimental core data could be discontinuous despite the high costs. Dynamic methods, including well logging and seismic survey, are applied in practice. Given the difference between static and dynamic elastic parameters, dynamic elastic parameters gained from those aforementioned dynamic methods need to be converted to static elastic parameters. Currently, static and dynamic elastic parameter test data are often directly used in converting dynamic elastic parameters to static elastic parameters without consideration of the influence of rock mineral composition on the difference between elastic and dynamic elastic properties. Consequently, the conversion results cannot fully reflect reservoir geomechanical conditions. In this research, the rock mineral composition is analyzed, and the effects of various mineral components on static and dynamic elastic parameters of shale are discussed. A new conversion model is established by taking into account the rock mineral composition. The new model is capable of providing improved conversion results and thus can be applied in shale gas extraction and hydraulic fracturing projects at the target reservoir. Therefore, this research has both theoretical and practical impacts.

Fracture Mechanical Properties of Damaged and Hydrothermally Altered Rocks, Dixie Valley‐Stillwater Fault Zone, Nevada, USA

AbstractDamaged and hydrothermally altered rocks are ubiquitous in fault zones, with the degree of damage and type and intensity of alteration varying in space and time. The impact of damage and alteration on hydromechanical properties of fault zones is difficult to assess without characterizing the associated changes to rock and fracture mechanical parameters. To evaluate the mechanical properties of fault rocks from different alteration regimes, we conducted (1) double‐torsion load‐relaxation tests to measure mode‐I fracture toughness (KIC) and subcritical fracture growth index (SCI), (2) uniaxial testing to measure unconfined compressive strength (UCS) and static elastic parameters, and (3) mineralogic and textural characterization of rock from four sites in the footwall of the Dixie Valley‐Stillwater fault zone. Alteration at these sites includes acid sulfate alteration and silicification associated with active fumaroles, intense silicification after calcite and chlorite alteration in an epithermal setting, quartz‐kaolinite‐carbonate alteration from an intermediate‐depth system, and a calcite‐chlorite‐hematite assemblage containing abundant unhealed damage. Silicification is associated with high KIC, SCI, UCS, and increased brittleness, and in precipitation‐dominated settings produces fault cores that are as strong or stronger than adjacent damage zone material. Calcite‐chlorite‐hematite assemblages containing abundant unsealed microfractures are approximately 4–5 times weaker than the granodiorite protolith. Mechanical properties are not predicted by mineralogical composition alone; a key control is the accumulation of damage and degree of healing. Measures of strength increase when mineral precipitation reduces microfracture porosity to <10–15% of total microfracture area. These results show that fault‐proximal weakening or strengthening is influenced by hydrothermal setting.

Brittleness characteristics of tight oil siltstones

Rock brittleness directly affects reservoir fracturing and its evaluation is essential for establishing fracturing conditions prior to reservoir reforming. Dynamic and static brittleness data were collected from siltstones of the Qingshankou Formation in Songliao Basin. The brittle–plastic transition was investigated based on the stress–strain relation. The results suggest that the brittleness indices calculated by static elastic parameters are negatively correlated with the stress drop coefficient and the brittleness index B2, defined as the average of the normalized Young’s modulus and Poisson’s ratio, is strongly correlated with the stress drop. The brittleness index B2, Young’s modulus, and Poisson’s ratio correlate with the brittle minerals content; that is, quartz, carbonates, and pyrite. We also investigated the correlation between pore fluid and porosity and dynamic brittle characteristic based on index B2. Pore fluid increases the plasticity of rock and reduces brittleness; moreover, with increasing porosity, rock brittleness decreases. The gas-saturated siltstone brittleness index is higher than that in oil- or water-saturated siltstone; the difference in the brittleness indices of oil- and water-saturated siltstone is very small. By comparing the rock mechanics and ultrasonic experiments, we find that the brittleness index obtained from the rock mechanics experiments is smaller than that obtained from the ultrasonic experiments; nevertheless, both decrease with increasing porosity as well as their differences. Ultrasonic waves propagate through the rock specimens without affecting them, whereas rock mechanics experiments are destructive and induce microcracking and porosity increases; consequently, the brittleness of low-porosity rocks is affected by the formation of internal microcrack systems.

Numerical investigation of dynamic and static properties of reservoir rocks

A 3D geomechanical model describes the elastic and mechanical properties of rock as well as underground stresses. The static elastic parameters of rock are required to build a model. However, the elastic properties resulting from wireline logs, dynamic experiments and seismic inversion are dynamic and must be converted. Implementing an accurate conversion is an essential part of any 3D geomechanical model. The static and dynamic moduli can be obtained by numerical and experimental methods. Laboratory experiments are known to provide more realistic outcomes, but this method has its constraints such as availability of samples, time constraints and limitation of experimental resources. Other approaches such as numerical modelling can be used supplementarily to compute the mechanical behaviour and elastic parameters of sandstone. This paper provides a literature review on past numerical modelling efforts to examine dynamic and static parameters of rocks. This is followed by an explanation of grain and core scale model and research methodology. A discrete element model-based numerical simulation is then carried out using Itasca’s particle flow code in 3D. The digital plug scale specimen was calibrated to replicate the experimental findings and was then used to establish a broad sensitivity analysis on the important parameters. The simulation results were in good agreement with experiments on sandstone specimens. The present study forms a foundation for building a more reliable 3D geomechanical model and consequently better field development, reducing risks and lowering costs.

Linking dynamic elastic parameters to static state of stress: toward an integrated approach to subsurface stress analysis

Stress is the most important parameter to understand basin dynamics and the evolution of hydrocarbon systems. The state of stress can be quantified by numerical geo-mechanical modelling techniques. These techniques require static elastic parameters of the rocks as input, while tectonic and gravitational forces are given as explicit boundary conditions to compute the local state of stress at different scales. We developed a technique to determine the density and elastic constants at seismic frequencies using full Zoeppritz inversion on angle-dependent seismic reflection data. The dynamic elastic parameters as obtained from seismic data differ from their static equivalents, which are necessary to determine the static state of stress. The dynamic elastic parameters are related to their static equivalents through experimentally obtained relations. In these rock-physics experiments, the static and dynamic elastic parameters are measured simultaneously during different external loading conditions. The experiments used here are all carried out in a tri-axial pressure machine under equal axial stresses. Then pre-stack seismic data analysis in combination with the relation between the static and dynamic elastic parameters, from the rock-physics experiments, provides the input parameters for geo-mechanical modelling.

Residual stresses in oscillating thoracic arteries reduce circumferential stresses and stress gradients

The purpose of this paper is to examine the effects of residual stresses and strains in the oscillating arteries on the stress distribution in the vascular wall. We employ a static theory of large elastic deformations for orthotropic material (Chuong and Fung, 1986, J. biomech. Engng 108, 189–192) with the acceleration term added to make the theory dynamic. We use the static elastic parameters of residual stresses in our analysis because the dynamic parameters are not available in the literature. Our analysis reveals that the effect of considering the residual stresses is to decrease the very large circumferential stresses at the inner wall by 62% and reduces the stress gradient through the arterial wall by 94% compared to the case when residual stresses are ignored. Thus, because the arteries do contain residual stresses, the consequent lower stresses at the inner wall and the reduced stress gradient may reduce the progression of atheroma. Our computations show that the stress gradients do not depend on the heart rate.

Seismic interpretation theory for an elastic earth

The seismic interpretation problem for an isotropic spherical earth is analyzed on the basis of elastic theory, under the assumption that the three independent elastic parameters are unknown continuous functions of the depth. It is shown that solutions for these functions may be obtained in the form of Taylor’s series. The problem is treated for three types of symmetrical excitation conditions on the free surface: (1) a shear source of type p rϕ only; (2) a pressure distribution with vanishing surface shear stress; (3) an excitation consisting of pressure in combination with surface shear stress of type p rθ . In each case the excitation functions are arbitrary functions of time. It is assumed that the associated components of surface displacement over the sphere are known from available observations, as functions of time. Thus, the complete information contained in seismic records is used in the proposed interpretation process, without need of selecting, identifying and assigning arrival times to specific events on the records. The two static elastic parameters may theoretically be determined from observations at a single frequency, including the frequency zero, or static case. The determination of the dynamic elastic parameter requires the use of at least two frequencies. Algebraic checks are obtained by comparing the general solutions with the corresponding results for two special cases in which the elastic parameters vary in a prescribed manner in the interior of the sphere. In both these cases treatment by the classical ray-path method of interpretation is excluded, because the wave velocity decreases with depth. Furthermore, the ray-path method (which is essentially a method of geometrical optics) would fail to distinguish between the two examples in any case, since the velocity function is the same in both, although the elastic parameters differ. In contrast to the valuable ray-path method, the analytical procedures in the present solution of the elastic problem are prohibitively cumbersome. Practical application of elastic theory to the direct interpretation of seismograms requires further development of the theory with probable utilization of modern high-speed computing methods.