Simultaneous Inversion of Layered Velocity and Density Profiles Using Direct Waveform Inversion (DWI): 1D Case

To better interpret the subsurface structures and characterize the reservoir, a depth model quantifying P-wave velocity together with additional rock’s physical parameters such as density, the S-wave velocity, and anisotropy is always preferred by geologists and engineers. Tradeoffs among different parameters can bring extra challenges to the seismic inversion process. In this study, we propose and test the Direct Waveform Inversion (DWI) scheme to simultaneously invert for 1D layered velocity and density profiles, using reflection seismic waveforms recorded on the surface. The recorded data includes primary reflections and interbed multiples. DWI is implemented in the time-space domain then followed by a wavefield extrapolation to downward continue the source and receiver. By explicitly enforcing the wavefield time-space causality, DWI can recursively determine the subsurface seismic structure in a local layer-by-layer fashion for both sharp interfaces and the properties of the layers, from shallow to deep depths. DWI is different from the layer stripping methods in the frequency domain. By not requiring a global initial model, DWI also avoids many nonlinear optimization problems, such as the local minima or the need for an accurate initial model in most waveform inversion schemes. Two numerical tests show the validity of this DWI scheme serving as a new strategy for multi-parameter seismic inversion.

Linearized Frequency-Dependent Reflection Coefficient and Attenuated Anisotropic Characteristics of Q-VTI Model

Seismic wave exhibits the characteristics of anisotropy and attenuation while propagating through the fluid-bearing fractured or layered reservoirs, such as fractured carbonate and shale bearing oil or gas. We derive a linearized reflection coefficient that simultaneously considers the effects of anisotropy and attenuation caused by fractures and fluids. Focusing on the low attenuated transversely isotropic medium with a vertical symmetry axis (Q-VTI) medium, we first express the complex stiffness tensors based on the perturbation theory and the linear constant Q model at an arbitrary reference frequency, and then we derive the linearized approximate reflection coefficient of P to P wave. It decouples the P- and S-wave inverse quality factors, and Thomsen-style attenuation-anisotropic parameters from complex P- and S-wave velocity and complex Thomsen anisotropic parameters. By evaluating the reflection coefficients around the solution point of the interface of two models, we analyze the characteristics of reflection coefficient vary with the incident angle and frequency and the effects of different Thomsen anisotropic parameters and attenuation factors. Moreover, we realize the simultaneous inversion of all parameters in the equation using an actual well log as a model. We conclude that the derived reflection coefficient may provide a theoretical tool for the seismic wave forward modeling, and again it can be implemented to predict the reservoir properties of fractures and fluids based on diverse inversion methods of seismic data.

Application of Wide-Azimuth Anisotropy Inversion Predicting Carbonate Fracture Intensity and Direction

Fractures in carbonate rock are both storing reservoirs and migrating channels for oil and gas, so such fractures are one of the key targets for oil exploration and development. Traditional fracture prediction methods by seismic data include ant tracking cube, coherence cube and other seismic attributes. Fractures predicted by these methods are less accurate. This paper introduces a wide-azimuth anisotropic inversion method to effectively predict the fracture density and direction in carbonates. a wide-azimuth seismic anisotropy inversion workflow is established to predict the fractures in carbonates, and consequently the fractured zones in the target layer. The key steps include: (1) carry out quality control and optimization of wide-azimuth seismic gathers; (2) conduct pre-stack simultaneous inversion of pre-stack seismic data at partial sub-offsets and sub-azimuths to obtain the Vp/Vs of the azimuths; (3) use Azimuthal Fourier Coefficient to calculate the anisotropic gradient and direction. Based on the anisotropic intensity and direction and elastic parameters in the study area, the density and direction of fractures are obtained. The prediction results show that in the study area, nearly SN-striking fractures are developed, which are chiefly tectonic fractures, and consistent with the imaging logging results. It has been proved that the method is reasonable and feasible, and the accuracy of fracture prediction is improved.

Migration to Cased Hole Petrophysics: Are We Ready?

In the present Oil & Gas business context, the uncertainties reduction for hydrocarbon production increase in an operational costs and risk reduction scheme, are among the main drivers in several operating companies in the northern region of South America (Colombia & Ecuador). Electrical logging and drilling tools stuck events due to differential pressures, fishing operations, high wellbore tortuosity, difficult geometries and unconsolidated formations affecting wellbore stability, are among the main problems increasing non-productive time and operating costs. Minimizing open hole operations with a full migration to cased hole data acquisition, providing inputs for petrophysical uncertainty reductions without jeopardizing well completion decisions or initial reservoir characterization, would constitute an attractive solution for operators. Following those initiatives, we start by achieving a complete open hole formation evaluation and then migrating to case hole data acquisition and petrophysical assessment while benchmarking against open hole results. Low and variable formation water salinity, complex mineralogy's affecting resistivity and radioactive minerals, are common petrophysical challenges in our reservoirs. We had to implement Archie and salinity-independent formation evaluation solutions with cased hole technologies and in absence of open hole logs. The open hole petrophysics consist on simultaneous assessment of matrix and fluids saturations, while evaluating the oil mobility and water cut with the incorporation of multi-depth of investigation sensors in single logging runs (spectroscopy, dielectric dispersion, and magnetic resonance). We then moved to cased hole formation evaluation, with spectroscopy & nuclear-based petrophysics in gas, light oil, and heavy oil-bearing reservoirs. By implementation of non-archie fluids volumetric computation (that relies on conversion of dry weight total carbon to oil saturation and fast neutron cross section to gas saturation- done through a simultaneous inversion by solving matrix-porosity-fluids volumes into an elemental analysis), we obtained a representative formation saturation range behind casing. We then discussed on the different scenarios were migrating to cased hole is sustainable and its potential limitations.

Deep Structure and Dynamics of the Central Balkan Peninsula from Seismic Data

Abstract—Analysis of P- and S-receiver functions for 19 seismic stations on the Balkan Peninsula has been performed. Half of the stations are in Bulgaria. The crustal thickness varies from 28–30 to 50 km. The ratio of longitudinal and shear wave velocities in the upper crust reaches 2.0 in some places. In the southwest of the study area, the 410-km seismic boundary is uplifted by 10 km relative to nominal depth. The elevation may be caused by hydration and/or cooling of the mantle transition zone under the influence of the Hellenic subduction zone. A low S-wave velocity layer related to the 410-km boundary may be located atop this boundary. In the northwestern part of the study area this layer is present in spite of the absence of the 410-km boundary. A similar paradox has been previously noted in central Anatolia. Indications of a low-velocity layer are present at a depth exceeding 410 km. The simultaneous inversion of the receiver functions of the two types (P and S) and the Rayleigh wave phase velocities reveals a large (7–9%) decrease in the S-wave velocity in the upper mantle of southern Bulgaria and northern Greece. The thickness of the low-velocity layer (asthenosphere) is about 50 km. The lithosphere-asthenosphere boundary (LAB) is at depths of 40 to 60 km. In terms of tectonics, this zone is characterized as the South Balkan extension system. To the north of 43° N, the S-wave velocity in the upper mantle is usually at least 4.4 km/s and the LAB is not detected or is detected at a depth of over 80 km. The SKS analysis of azimuthal anisotropy reveals lateral zoning in the upper mantle that is correlated to velocity zoning. Probably, the mechanically weak low-velocity mantle of the South Balkan system is easily deformed, and the azimuth of the fast direction of anisotropy (20°) indicates the direction of extension. At the northern stations, the fast direction (about –30°) may be a reflection of an older process.

Transdimensional simultaneous inversion of velocity structure and event locations in downhole microseismic monitoring

In downhole microseismic monitoring, the velocity model plays a vital role in accurate mapping of the hydraulic fracturing image. For velocity model uncertainties in the number of layers or interface depths, the conventional velocity calibration method has been shown to effectively locate the perforation shots; however, it introduces non-negligible location errors for microseismic events, especially for complex geological formations with inclinations. To improve the event location accuracy, we exploit the advantages of the reversible jump Markov chain Monte Carlo (rjMCMC) approach in generating different dimensions of velocity models and propose a transdimensional Bayesian simultaneous inversion framework for obtaining the effective velocity structure and event locations simultaneously. The transdimensional inversion changes the number of layers during the inversion process and selects the optimal interface depths and velocity values to improve the event location accuracy. The confidence intervals of the simultaneous inversion event locations estimated by Bayesian inference enable us to evaluate the location uncertainties in the horizontal and vertical directions. Two synthetic examples and a field test are presented to illustrate the performance of our methodology, and the event location accuracy is shown to be higher than that obtained using the conventional methods. With less dependence on prior information, the proposed transdimensional simultaneous inversion method can be used to obtain an effective velocity structure for facilitating highly accurate hydraulic fracturing mapping.