Recent advances in theory and technology of oil and gas geophysics

Oil and gas are important energy resources and industry materials. They are stored in pores and fractures of subsurface rocks over thousands of meters in depth, making the finding and distinguishing them to be a significant challenge. The geophysical methods, especially the seismic and well-logging methods, are the effective ways to identify the oil and gas reservoirs and are widely used in industry. Due to the complexity of near surface and subsurface structures of new exploration targets, the geophysical methods based on advanced computation methods and physical principles are continuously proposed to cope with the emerging challenges. Thus, some new advances in theory and technology of oil and gas geophysics are summarized in this editorial material, especially focusing on the geophysical data processing, numerical simulation technology, rock physics modeling, and reservoir characterization.Document Type: EditorialCited as: Wang, Y., Liu, Y., Zou, Z., Bao, Q., Zhang, F., Zong, Z. Recent advances in theory and technology of oil and gas geophysics. Advances in Geo-Energy Research, 2023, 9(1): 1-4. https://doi.org/10.46690/ager.2023.07.0

Amplitude variation with incident angle inversion for fluid factor in the depth domain

The development of Pre−stack depth migration makes the imaging of the subsurface structure in the depth possible, which set a foun− dation for the study of amplitude variation with incident angle (AVA) inversion. This leads to the increasing demanding of the seismic inversion methods in the depth domain for guiding reservoir characterization. However, the conventional seismic inversion methods in the time domain are not suitable in the depth domain due to the seismic wavelet in the depth domain is depth−variant and depending on medium velocity. To address this issue, we proposed a pragmatic seismic inversion method for fluid factor in the depth domain with amplitude variation with incident angle gathers by using a true−depth wavelet on the process of seismic inversion. This wavelet is es− timated by converting the time wavelet to the depth wavelet with seismic velocity. To guide the fluid discrimination, the proposed method directly estimates the fluid factor from the pre−stack seismic data and all the process of the method is implemented in the depth domain. To deal with the weak nonlinearity induced by the velocity, the Bayesian inference, the prior information and model constraint are in− troduced in this seismic inversion method. Tests on synthetic data show that the fluid factor can be well estimated reasonably even with moderate noise. The field data example illustrates the feasibility and efficiency of the proposed method in application and the estimated fluid factor and shear modulus are in good agreement with the drilling results

Amplitude Variation with Offsets and Azimuths Simultaneous Inversion for Elastic and Fracture Parameters

Azimuthal elastic inversion or AVO/AVA analysis has proven to be effective for fracture description and stress evaluation in unconventional resource plays. Fracture weakness including normal and tangential weakness from linear slip theory bridge the seismic data and fracturing parameters as intermediate parameters. However, the stability of the azimuthal elastic inversion methods available for anisotropic parameters or fracture parameters in field data remains challenging. This study explores a practical azimuthal simultaneous elastic inversion method in heterogeneous medium for fracture weakness estimation. Taking the heterogeneity and anisotropy of fracture media into consideration, and based on perturbation theory and stable phase approximation, the fracture medium can be considered as the superimposition of background medium and perturbation medium, and then the seismic scattering coefficient of fracture media can be derived. This equation establishes the relationship between seismic data and fracture weakness together with elastic parameters like P-wave and S-wave moduli and weaknesses. With this equation, a heterogeneous inversion method is proposed. This method implements the estimation of P-wave and S-wave moduli and fracture weaknesses simultaneously, and the constraint from initial model and multi-iterations enhances the stability of this method. In this approach, the parameters of the perturbation medium are initially estimated, and then they can be superposed to the parameters of the known background medium as the renewal parameters of the background medium in next iteration. We can yield the final estimation of the parameters in heterogeneous medium after several iterations when the last two estimated results are similar. Model test and field data examples verify the feasibility and potential of the proposed approach

Analytical seismic wave attenuation and velocity dispersion in layered double-porosity media

Due to differences of rock properties such as porosity, permeability and compressibility between different regions in the porous media, pressure gradients are induced between those different regions and lead to local fluid flow. When seismic wave propagates in the porous media, the local fluid flow process is a main cause of wave attenuation and velocity dispersion. The local fluid flow mechanism of the layered porous model has been studied by many authors in the numerical approaches without analytical wave equations and solution for this kind of rock physics models. In this study, we first establish a layered double-porosity model saturated with a single fluid and derive the wave equations. According to the derived novel wave equations, then we calculate the phase velocity and quality factor in the layered double-porosity media based on plane wave analysis. The results demonstrate that there are three kinds of wave modes named as the fast P-wave and two slow P-wave in layered porous media when P-wave propagates through the model perpendicularly. Finally, we study the effects of local fluid flow on the mesoscopic loss mechanism by analyzing the attenuation and the velocity dispersion of seismic waves in the low frequency range

The Characteristics of Reflection and Transmission Coefficients of Porous Medium Saturated with an Ideal Fluid

The underground rock is composed of rock skeleton and pore fluids. When seismic waves propagate in underground medium, it will show complicated change influenced by pore and pore fluids in rocks. It is very important to study the characteristics of reflection and trans- mission coefficients of seismic waves at the interface and to analyze the properties of the lithology and pore fluids of porous medium, which can reveal the oil and gas bearing in underground medium. Based on the relationship among wave functions, displacement and stress in porous medium, the equation of reflection and transmission coefficients at the interface of porous medium saturated with an ideal fluid is derived. A geological model with sandstone porous medium in the top layer and mudstone porous medium in the bottom layer, the rock skeleton parameters of which vary with porosities, is established. Based on the equation and the model, the variation of reflection and transmission coefficients with the incident angle at the interface of porous medium is studied under the conditions of dif- ferent pore fluids filling and different porosities. The study shows that the existence of the pore and pore fluids will impede the reflected and transmitted abilities of seismic waves at the interface of porous medium. Combining with the theory of rock physics and well data, the porosities and pore fluids of porous medium can be identified qualitatively by studying the variation of the reflected fast P- and SV- waves with incident angle. The values of reflection and transmission coefficients of slow P-wave are very small, but the variation of that is relatively large due to the influence of the pore and pore fluids

Seismic wave attenuation and dispersion induced by fluid flow within various cracks and a small amount of bubbly fluid

Hydrocarbon reservoirs usually contain pores and various cracks, which may contain a small amount of bubble fluid. In this study, based on the wave theory of basic porous medium, the influence of various factors on fluid pressure are investigated, and then the stress-strain relationship and Lagrange’s equations with a dissipation function is utilized to derive the elastic wave equation of porous medium containing various cracks and a small amount of bubbly fluid initially. This elastic wave equation describes the influence of squirt flow induced by various cracks on seismic wave attenuation and dispersion, and the influence of the bubble linear vibration on seismic wave attenuation and dispersion effectively. Then, the seismic wave attenuation and dispersion of a given model is estimated and the matching of rock physics parameters are obtained in different frequency bands. The numerical results illustrate that the proposed approach is compatible with previous theory to explain the mechanics of the seismic wave attenuation and dispersion induced by fluid flow and can better describe the propagation of elastic waves in actual rock medium especially for the mechanics of the seismic wave attenuation and dispersion induced by fluid flow within various cracks and a small amount of bubble fluid

An Analysis of the 8.85- and 4.42-Year Cycles in the Gulf of Maine

In the background of global warming and climate change, nuisance flooding is only caused by astronomical tides, which could be modulated by the nodal cycle. Therefore, much attention should be paid to the variation in the amplitude of the nodal cycle. In this paper, we utilize the enhanced harmonic analysis method and the independent point scheme to obtain the time-dependent amplitudes of the 8.85-year cycle of N2 tide and the 4.42-year cycle of 2N2 tide based on water level records of four tide gauges in the Gulf of Maine. Results indicate that the long-term trends of N2 and 2N2 tides vary spatially, which may be affected by the sea-level rise, coastal defenses, and other possible climate-related mechanisms. The comparison between Halifax and Eastport reveals that the topography greatly influences the amplitudes of those cycles. Moreover, a quasi 20-year oscillation is obvious in the 8.85-year cycle of N2 tide. This oscillation probably relates to a 20-year mode in the North Atlantic Ocean