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Collaborating Authors
Chapman, Mark
Water Saturation Effects on Wave Velocity and Attenuation in Porous Rock With Aligned Fractures
Amalokwu, Kelvin (National Oceanography Centre) | Best, Angus I. (National Oceanography Centre) | Sothcott, Jeremy (National Oceanography Centre) | Chapman, Mark (University of Edinburgh) | Minshull, Tim (University of Southampton) | Li, Xiang-Yang (British Geological Survey)
Summary Elastic wave velocity and attenuation are known to be sensitive to the presence of aligned fractures (anisotropy) and to partial gas saturation but their combined effects are poorly understood. Using synthetic, silica-cemented, sandstones with aligned penny-shaped voids to simulate the effect of fractures in the Earth according to theoretical models, we conducted laboratory ultrasonic experiments to investigate the effects aligned fractures would have on velocity and attenuation under partial saturation conditions. Our results for the non-fractured rock agree with published data, but the fractured rock exhibits significantly different behavior, pertinent to fluid saturation estimation from remote seismic measurements in fractured rocks.
- Geophysics > Borehole Geophysics (1.00)
- Geophysics > Seismic Surveying > Seismic Processing (0.96)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (0.69)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Faults and fracture characterization (0.93)
Sensitivity of Azimuthal Seismic AVO Responses to Fluid-Saturated Fractures in the Bakken Formation
Guo, Zhiqi (Jilin University) | Zhang, Feng (China University of Petroleum) | Li, Xiangyang (China University of Petroleum) | Ren, Yingrui (University of Edinburgh) | Chapman, Mark (University of Edinburgh) | Shen, Ye (CNOOC Research Institute)
Summary The presence of vertical aligned fractures in the Bakken Formation results in azimuthal seismic AVO responses. In this paper, we design a realistic geologic model of the fracture zone in the Bakken Formation, and investigate corresponding azimuthal seismic AVO responses. In the model, based on results from formation images, we assume that fractures and cracks cut through all seven major geologic units in the Bakken Formation with a specific ratio of fracture intensity about 3:3:2:1:2:1:1. Corresponding anisotropy parameters are calculated based on Hudson's theory. We then employ the reflectivity method to generate elastic seismograms. AVO curves picked for four interested geologic units indicate azimuthal variations in amplitudes especially at far offset. The decrease in amplitude at far offset corresponds to the increase in azimuth, and the AVO gradient also presents a linear variation with azimuth. However, because picking of the exact amplitudes is difficult due to interference and the presence of overlying fractures, it may be appropriate to treat the formation as an integrated part and investigate azimuth variations in RMS amplitudes. Comparison results indicate that the stacked RMS amplitudes act as a better indicator of the variations in crack density and fluid saturation than the corresponding AVO gradient.
- North America > United States > South Dakota (1.00)
- North America > United States > North Dakota (1.00)
- North America > United States > Montana (1.00)
- (2 more...)
- Geology > Geological Subdiscipline > Geomechanics (0.37)
- Geology > Rock Type > Sedimentary Rock (0.32)
- North America > United States > South Dakota > Williston Basin > Bakken Shale Formation (0.99)
- North America > United States > North Dakota > Williston Basin > Bakken Shale Formation (0.99)
- North America > United States > Montana > Williston Basin > Bakken Shale Formation (0.99)
A Rock Physics Workflow for the Modeling of the Effect of Kerogen Content and Maturity Level in Shales
Guo, Zhiqi (Jilin University) | Li, Xiangyang (China University of Petroleum) | Ren, Yingrui (University of Edinburgh) | Chapman, Mark (University of Edinburgh) | Shen, Ye (CNOOC Research Institute)
Summary Kerogen content plays a significant role in elastic properties due to its low density, and hydrocarbon-filled pores in organic matter generated during different maturity levels will enhance the effect of kerogen in shales. In this study, we develop a shale rock physics model by incorporating Kuster and Toksöz theory and the self-consistent approximation method to quantify the effects of such factors on elastic properties in shales. Modeling results show that both kerogen content and kerogen-related porosity decrease velocities and density of shales, but kerogen content tends to affect elastic properties more profoundly, and the effect of kerogen-related porosity on the elastic properties only becomes more obvious as kerogen content goes up beyond about 0.1. We also find that the increase in kerogen content increases Poisson's ratio, while the variation in kerogen-related porosity has little effect on Poisson's ratio. Finally, for the reflector model designed in this study, the variations in kerogen content and kerogen-related porosity result in significant and predictable variations in AVO intercept and gradient.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Summary Azimuthal amplitude-versus-offset (AVOZ) data can contain abundant information on fracture properties, lithology and fluid saturation, but procedures for interpretation in terms of such properties remain relatively poorly developed. This paper develops recently developed rock physics model based inverse schemes with a particular emphasis on accounting for the frequency-dependence of anisotropy, which is believed to be important in fractured reservoirs. Frequency-dependent anisotropy is expected to be influenced by fluid mobility, and determining this parameter from seismic data would be advantageous. The method is applied to a range of models, and it is shown that under relevant conditions it is theoretically possible to obtain fluid mobility information from AVOZ.
- Geology > Geological Subdiscipline > Geomechanics (0.50)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.31)
Summary The reliable estimation of gas saturation from seismic data is a difficult and longstanding problem. Rocks saturated with gas often show high attenuation and frequency-dependence of seismic properties, and previous work has indicated that such effects can be detected by considering frequency-dependence of reflectivity. Theoretical studies show that such analysis has the potential to improve estimates of gas saturation. This paper develops a Bayesian inversion scheme which allows probability distributions for porosity and saturation to be derived from pre-stack seismic data. We apply the method to well-log and 3D seismic data from the Vienna Basin. Analysis at the calibration well demonstrates the potential power of the method, and probability distributions for other zones of interest are derived.
- Geology > Geological Subdiscipline > Geomechanics (0.52)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.47)
Abstract It is well known that seismic amplitudes contain important information which can be related to fluid saturation. Most interpretation is based on Gassmann's theory and studies of amplitude variations with offset (AVO) with the Zoeppritz equations. However, this traditional AVO technique is often unable to make quantitative estimation of the gas saturation, hence unable to distinguish between commercial and non-commercial gas deposits. Recent attention has focussed on if the frequency response of reflections can also be used to reveal fluid information through the application of frequency-dependent AVO (FAVO), although many of these studies are based on empirical relationships and are lack of a thorough understanding of the underline mechanism and rock physics principles. In this study, we extend the "squirt-flow" model to include the effect of gas saturation on seismic attenuation and dispersion. Combining with Wood's formula, the FAVO response can be calculated as a function of porosity and gas saturation. Therefore a model-based inversion scheme can be established by matching the calculated FAVO response to the observed ones for quantitatively estimating the gas saturation. There are three technical contributions in this approach:a theoretical frame work for modelling the FAVO response and the effects of gas saturation on seismic attenuation and dispersion; an efficient spectral decomposition algorithm for extracting the FAVO response from real data; an optimization work flow for estimating the gas saturation. In our examples, the numerical modelling is able to predict the FAVO response to changing gas saturation which is seen in the physical modelling data, and the 3D P-wave field seismic data. Consequently we argue that with a careful model-based approach, it is possible to invert the FAVO response in terms of gas saturation. This may have important implications in the exploration of tight and unconventional gas resources.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.31)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.30)
Summary Rock brittleness plays a significant role in effective hydraulic fracturing for shale gas production, and is often related to mineralogy, mechanical properties, and microstructure features in shales. We construct a rock physics workflow to link elastic properties of shales to complex constituents and specific microstructure attributes. Multiple compositions and various pore geometries are considered using a self-consistent approximation (SCA) method. The laminated textures due to the preferred orientations of clay particles and possible laminated distribution of kerogen are considered using Backus averaging to model the anisotropy (transverse isotropy) of shales. Results based on the analysis of the rock physics templates reveal that the degree of clay lamination significantly affects Vp/Vs of shales, whereas it has little impact on acoustic impedance of shales along the vertical direction. An increasing degree of clay lamination will increase Vp/Vs, and therefore the Poisson’s ratio. With increasing porosity, the variation of mineralogy has less impact on acoustic impedance than on Vp/Vs, which illustrates that Vp/Vs is a better indicator for lithology detection. On the other hand, acoustic impedance is a more suitable parameter to discriminate porosity compared with Vp/Vs. Our rock physics model is calibrated on the well log data from the Barnett Shale and is used to find reasonable parameters to characterize the Barnett Shale. Based on the model, we generate rock physics templates for the interpretation and prediction of shale rock brittleness, mineral constituents, and porosity from elastic properties of shales. Seismic AVO analysis based on modeling data from the top and base of the Barnett Shale illustrates that AVO intercept and gradient have predictable trends according to the variation of brittleness index, mineralogy, and porosity, which means that we can predict variations of such factors in space from seismic responses.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > Texas > Haynesville Shale Formation (0.99)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- North America > United States > Louisiana > Haynesville Shale Formation (0.99)
- North America > United States > Arkansas > Haynesville Shale Formation (0.99)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Shale gas (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
Summary The effectiveness of fracture stimulation techniques depends on the microstructural features which control the rock strength. We analyze brittleness index, fractures, and microstructure of the Barnett Shale for a better understanding of the correlation between mechanical properties, mineralogy, and pore geometry. The complexity of multiple minerals, pore geometries, and pore inclusions are modeled using the self-consistent approximation (SCA) model, with consideration of statistical distributions of pores and cracks in shales. The method is applied to core samples and well log data from the Barnett Shale to invert the aspect ratio (ratio of short axis to long axis) of pores, and to estimate crack density, and the proportion of stiff pores and cracks in the Barnett Shale. The inverted crack density gives an average estimate of the pore space geometry. Results show that the aspect ratio for the Barnett Shale varies between 0.01 and 1 and has a dominant value of 0.1. Analysis on the core data indicates that both quartz and carbonate minerals contribute to increased crack density. Comparison reveals good correlations between the brittleness indices defined in terms of ?+2µ)/? and Poisson’s ratio. While the brittleness index defined by Young’s modulus is not consistent with the definition in terms of (?+2µ)/? and Poisson’s ratio, Young’s modulus is a good indicator for the variation of crack density. Results of pore-type inversion show that the variation of pore types coincides with inverted crack density.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Petroleum Play Type > Unconventional Play > Shale Play > Shale Gas Play (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > Texas > Haynesville Shale Formation (0.99)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- North America > United States > Louisiana > Haynesville Shale Formation (0.99)
- (2 more...)
Technology Update A new ceramic proppant has made detection possible without placing radioactive material downhole. The new detection method makes use of a high thermal neutron capture compound (HTNCC) incorporated into the ceramic proppant. Traditional fracture-height or proppant-placement evaluation after hydraulic fracturing relies on the detection of radioactive tracers pumped downhole with the proppant. Although this technique is useful, it raises environmental, safety, and regulatory issues. The HTNCC is added to the proppant during its manufacture and is included in concentrations low enough not to affect the proppant’s strength or conductivity. The proppant is detected using standard compensated or pulsed neutron tools, with detection based on the high thermal neutron capture of the compound relative to the surrounding downhole constituents. This new detectable proppant was used in the T sand of the Villeta and Caballos formations of the Juanambu field in the Putumayo Basin of Colombia. Two detection methods use a comparison of before-fracture log count rates and after-fracture count rates, with reduced after-fracture count rates observed in zones containing proppant. Another detection method, especially useful when formation gas saturations change, uses only the after-fracture log. The HTNCC method has advantages over the common radioactive particle method. First, the HTNCC tagging material is incorporated in very small quantities into each proppant grain during the manufacturing process. Because it is present in every particle of the fracture treatment, the detection of all propped fractures is insured. With traditional radioactive tracers, which are blended into the slurry at extremely small ratios compared with total proppant volume, segregation can occur, which can lead to misinterpretation of fractures in which no radioactive particles are contained near the wellbore in the propped fracture section. A related but opposite problem also can occur in situations in which a stray radioactive particle is located in an area that is not a propped fracture (e.g., a casing collar or perforation). These false positives are eliminated by the new method because the small quantities of HTNCC in a few stray pellets will not create a log response. A second advantage, and more important in many cases, is that the new method contains only inert materials, thereby eliminating the need for the special requirements or permitting necessary for handling, transporting, pumping, or flowing back of hazardous materials associated with traditional radioactive tracers. This new method provides intrinsic value to operators by providing an environmentally friendly and virtually hazard-free alternative to radioactive tracers. A third advantage is that the HTNCC is inherently stable and permanently incorporated within the proppant. The HTNCC can be logged at any time in the future to evaluate remedial operations or determine whether proppant has flowed back from any interval. Typical radioactive tracers experience radioactive decay, and the detectability declines as a function of the isotope half-life, which prevents the accurate identification of proppant location after a few months.
- South America > Colombia > Putumayo Department > Putumayo Basin (0.99)
- South America > Colombia > Caballos Formation (0.99)
Estimating Seismic Dispersion From Pre-stack Data Using Frequency-dependent AVO Inversion
Wu, Xiaoyang (Edinburgh Anisotropy Project, British Geological Survey) | Chapman, Mark (Edinburgh Anisotropy Project, British Geological Survey) | Wilson, Adam (Edinburgh Anisotropy Project, British Geological Survey) | Li, Xiang-Yang (Edinburgh Anisotropy Project, British Geological Survey)
Summary Fluid-saturated rocks are generally expected to have frequency-dependent velocities, and it is attractive to try to use this property to discriminate different fluids with seismic data. Previous work has demonstrated how to combine spectral decomposition techniques with AVO inversion to obtain direct estimates of dispersion from prestack data. In this paper, we present the application of frequency-dependent AVO inversion to real seismic data. The Wigner-Ville distribution based method is used for spectral decomposition in order to achieve high resolution. Numerical studies and a real example illustrate the potential of this method for detection of seismic dispersion due to fluid saturation. We may also avoid stacking related “frequency-shadows” as spectral decomposition is performed on pre-stack data. Introduction Frequency dependent seismic attributes are of fundamental interest because they are believed to be directly related to the scale length of heterogeneities, rock permeability and saturating fluid. Particular interest has focused on frequency dependence of the time delay between split shear-waves and azimuthal variations in compressional wave attenuation. Recently, laboratory studies show that seismic velocities are frequency-dependent at a relative high frequencies regime and appeared to be caused by fluid mobility (Batzle et al., 2006), which is defined as the ratio of rock permeability to fluid viscosity. Theoretical investigation (eg. Jakobsen and Chapman, 2009) is also conducted to understand this frequency-dependence. Being able to measure frequency-dependence of velocity from reflection data would greatly assist fluid discrimination efforts. Chapman et al. (2005) performed a theoretical study of reflections from layers which exhibit fluid-related dispersion and attenuation, and showed that in such cases the AVO response was frequency-dependent. Application of spectral decomposition techniques allows the behavior to be detected on synthetic seismograms. Wilson et al. (2009) extended this analysis, introducing a frequency-dependent AVO inversion concept aimed at allowing a direct measure of dispersion to be derived from pre-stack data. In this paper, we combine this frequency-dependent AVO inversion with a high-resolution Smoothed Pseudo Wigner- Ville distribution. Numerical studies and field data application from North Sea show that this method has the potential to be useful for fluid detection. Smoothed Pseudo Wigner-Ville Distribution We use Wigner-Ville Distribution (WVD) based method for spectral decomposition. WVD is well-recognized as an effective method for time-frequency analysis of nonstationary signals (Cohen, 1995). Numerical Example We consider a two-layer Class III AVO model presented by Chapman et al. (2005), where the top elastic layer had Pand S-wave velocities of 2743ms-1 and 1394ms-1. For the dispersive model, the lower layer is defined as a material under water-saturation then substituted with gas by changing the fluid bulk modulus from 2GPa to 0.2GPa. For the elastic model, the P- and S-wave velocities of lower layer were calculated from elastic tensor for the dispersive model at low frequency. Eleven traces for each model are generated using 40Hz Ricker wavelet as the source. The trace space is 100m. Figure 1 displays the synthetic gathers of the elastic and dispersive models at the interface respectively, both of which the amplitudes increase with the offset gradually.
- Europe > United Kingdom > North Sea (0.25)
- Europe > Norway > North Sea (0.25)
- Europe > North Sea (0.25)
- (2 more...)