Luanxiao Zhao

Quantitative geophysical pore-type characterization and its geological implication in carbonate reservoirs

This paper discusses and addresses two questions in carbonate reservoir characteriza- tion: how to characterize pore-type distribution quantitatively from well observations and seismic data based on geologic understanding of the reservoir and what geolog- ical implications stand behind the pore-type distribution in carbonate reservoirs. To answer these questions, three geophysical pore types (reference pores, stiff pores and cracks) are defined to represent the average elastic effective properties of complex pore structures. The variability of elastic properties in carbonates can be quantified using a rock physics scheme associated with different volume fractions of geophys- ical pore types. We also explore the likely geological processes in carbonates based on the proposed rock physics template. The pore-type inversion result from well log data fits well with the pore geometry revealed by a FMI log and core informa- tion. Furthermore, the S-wave prediction based on the pore-type inversion result also shows better agreement than the Greensberg-Castagna relationship, suggesting the potential of this rock physics scheme to characterize the porosity heterogeneity in carbonate reservoirs. We also apply an inversion technique to quantitatively map the geophysical pore-type distribution from a 2D seismic data set in a carbonate reservoir offshore Brazil. The spatial distributions of the geophysical pore type contain clues about the geological history that overprinted these rocks. Therefore, we analyse how the likely geological processes redistribute pore space of the reservoir rock from the initial depositional porosity and in turn how they impact the reservoir quality.

Characterizing the effect of elastic interactions on the effective elastic properties of porous, cracked rocks

Elastic interactions between pores and cracks reflect how they are organized or spatially distributed in porous rocks. The principle goal of this paper is to understand and characterize the effect of elastic interactions on the effective elastic properties. We perform finite element modelling to quantitatively study how the spatial arrangement of inclusions affects stress distribution and the resulting overall elasticity. It is found that the stress field can be significantly altered by elastic interactions. Compared with a non-interacting situation, stress shielding considerably stiffens the effective media, while stress amplification appreciably reduces the effective elasticity. We also demonstrate that the T-matrix approach, which takes into account the ellipsoid distribution of pores or cracks, can successfully characterize the competing effects between stress shielding and stress amplification. Numerical results suggest that, when the concentrations of cracks increase beyond the dilute limit, the single parameter crack density is not sufficient to characterize the contribution of the cracks to the effective elasticity. In order to obtain more reliable and accurate predictions for the effective elastic responses and seismic anisotropies, the spatial distribution of pores and cracks should be included. Additionally, such elastic interaction effects are also dependent on both the pore shapes and the fluid infill.

Frequency‐ and angle‐dependent poroelastic seismic analysis for highly attenuating reservoirs

We extend the frequency- and angle-dependent poroelastic reflectivity to systematically analyse the characteristic of seismic waveforms for highly attenuating reservoir rocks. It is found that the mesoscopic fluid pressure diffusion can significantly affect the root-mean-square amplitude, frequency content, and phase signatures of seismic waveforms. We loosely group the seismic amplitude-versus-angle and -frequency characteristics into three classes under different geological circumstances: (i) for Class-I amplitude-versus-angle and -frequency, which corresponds to well-compacted reservoirs having Class-I amplitude-versus-offset characteristic, the root-mean-square amplitude at near offset is boosted at high frequency, whereas seismic energy at far offset is concentrated at low frequency; (ii) for Class-II amplitude-versus-angle and -frequency, which corresponds to moderately compacted reservoirs having Class-II amplitude-versus-offset characteristic, the weak seismic amplitude might exhibit a phase-reversal trend, hence distorting both the seismic waveform and energy distribution; (iii) for Class-III amplitude-versus-angle and -frequency, which corresponds to unconsolidated reservoir having Class-III amplitude-versus-offset characteristic, the mesoscopic fluid flow does not exercise an appreciable effect on the seismic waveforms, but there exists a non-negligible amplitude decay compared with the elastic seismic responses based on the Zoeppritz equation.

Combined Bayesian AVO inversion with rock physics to predict gas carbonate reservoir

Summary A better understanding of fluid effects on carbonate rock properties is considered to be key issue in applying AVO inversion to carbonate reservoir characterization. Analysis of well log data in a gas carbonate reservoir of Southwestern China indicates that gas does influence carbonate’s elastic rock properties, and low VP/VS ratio and low acoustic impedance can be treated as calibration for the interpretation of seismic inversion result. We invert prestack seismic data using Bayesian linearized AVO inversion to estimate elastic properties and assess their uncertianty. We also show how to combine a credible seismic inversion result with rock physics analysis to identify gas carbonate reservoir.

An Effective Reservoir Parameter for Seismic Characterization of Organic Shale Reservoir

Sweet spots identification for unconventional shale reservoirs involves detection of organic-rich zones with abundant porosity. However, commonly used elastic attributes, such as P- and S-impedances, often show poor correlations with porosity and organic matter content separately and thus make the seismic characterization of sweet spots challenging. Based on an extensive analysis of worldwide laboratory database of core measurements, we find that P- and S-impedances exhibit much improved linear correlations with the sum of volume fraction of organic matter and porosity than the single parameter of organic matter volume fraction or porosity. Importantly, from the geological perspective, porosity in conjunction with organic matter content is also directly indicative of the total hydrocarbon content of shale resources plays. Consequently, we propose an effective reservoir parameter (ERP), the sum of volume fraction of organic matter and porosity, to bridge the gap between hydrocarbon accumulation and seismic measurements in organic shale reservoirs. ERP acts as the first-order factor in controlling the elastic properties as well as characterizing the hydrocarbon storage capacity of organic shale reservoirs. We also use rock physics modeling to demonstrate why there exists an improved linear correlation between elastic impedances and ERP. A case study in a shale gas reservoir illustrates that seismic-derived ERP can be effectively used to characterize the total gas content in place, which is also confirmed by the production well.

Geophysical Pore Type Characterization from Seismic Data in Carbonate Reservoir

Pore geometry in carbonates control the fluid flow properties and geological story, the purpose of our work is to predict pore type distribution from well observations and seismic data based on geologic understanding of the reservoir. Based on developed rock physics model which can take into account volume fraction of pore type quantitatively, we bridge the three defined geophysical pore type with extracted seismic properties from seismic data. The approach is evaluated on real well log data and seismic data from offshore Brazil carbonate reservoir, we obtain the distribution of reference pores, stiff pores and cracks which can be used to predict reservoir permeability.

Petrophysical Characterization of Pore Type in Tight Gas Carbonates of Southwestern China

Exploration and production in tight gas carbonates reservoir requires a comprehensive petrophysical characterization of rock microstructure in low-porosity and low-permeability tight carbonates. The complex pore system in tight gas carbonate makes pore shape must be taken into account for estimating elastic properties in rock physics modeling, differential effective medium(DEM) theory can be used to study the relationship between changing pore geometries and elastic properties. In this article, detailed rock physics modeling steps was put proposed, and on this basis pore type inversion algorithm was developed according to the concluded velocity-porosity relationship. Finally, Geological knowledge, core data, and well log data are integrated to discuss the pore shape effect on elastic properties of carbonate rocks in tight gas carbonates reservoir of Southwestern China, and the given pore type inversion result fits well with features of pore geometry which was revealed by thin section of cores and FMI image of local area, indicating this method can be can be effective to characterize complexity of pore system in tight gas carbonates.