R. Galvin

Fluid substitution, dispersion, and attenuation in fractured and porous reservoirs - Insights from new rock physics models

The importance of natural fractures for development and production of hydrocarbon reservoirs requires little justification. While in clastic reservoirs fractures can cause permeability anisotropy and thus affect field development, in carbonates and tight sands they are often critical for reservoir production. If open fractures have a preferential direction (which is almost always the case), they cause azimuthal seismic anisotropy, making seismic a powerful tool for the characterization of fractured reservoirs.

Frequency-dependent anisotropy of porous rocks with aligned fractures

Naturally fractured reservoirs are becoming increasingly important for oil and gas exploration in many areas of the world. Because fractures may control the permeability of a reservoir, it is important to be able to find and characterize fractured zones. In fractured reservoirs, the wave-induced fluid flow between pores and fractures can cause significant dispersion and attenuation of seismic waves. For waves propagating normal to the fractures, this effect has been quantified in earlier studies. Here we extend normal incidence results to oblique incidence using known expressions for the stiffness tensors in the low- and high-frequency limits. This allows us to quantify frequency-dependent anisotropy due to the wave-induced flow between pores and fractures and gives a simple recipe for computing phase velocities and attenuation factors of quasi-P and SV waves as functions of frequency and angle. These frequency and angle dependencies are concisely expressed through dimensionless velocity anisotropy and attenuation anisotropy parameters. It is found that, although at low frequencies, the medium is close to elliptical (which is to be expected as a dry medium containing a distribution of penny-shaped cracks is known to be close to elliptical); at high frequencies, the coupling between P-wave and SV-wave results in anisotropy due to the non-vanishing excess tangential compliance.

Seismic Attenuation from VSP and Well Log Data: Approaches, Problems and Relative Contribution of Scattering

All methods for quantitative interpretation of seismic data are based on the analysis of amplitudes of seismic (reflected) waves. Seismic attenuation along the ray path of a wave significantly affects this amplitude information. As such, understanding of this phenomenon has a huge impact for the industry. For the last sixty years vertical seismic profiling (VSP) was an obvious method of choice for exploring this phenomenon in-situ. A large number of different approaches for attenuation estimation were introduced. We have tested a large number of these methods and developed a reasonably robust workflow for attenuation estimation based on the modified centroid frequency shift method. Seismic attenuation measured from seismic data (so-called apparent attenuation) comprises two different components, namely, transfer of the energy into heat (absorption) and scattering. We employ seismic modelling using finely-layered model of the medium obtained from the log data as a part of the workflow to estimate relative contribution of scattering. In order to investigate causes and mechanisms of seismic attenuation we select ~70 wells from several areas in Australia (primarily from NW Shelf) with publically-available high-quality well log, VSP data and geological information. In this presentation we show some preliminary results from this study.

A Simple Approximation For Seismic Attenuation And Dispersion In a Fluid-saturated Porous Rock With Aligned Fractures

Physical properties of many reservoir rocks can be modelled using the concept of poroelasticity. Many reservoir rocks, in addition to the network of pores, contain larger fractures or cracks. Galvin and Gurevich (2006) solved the single scattering problem for a crack in a poroelastic medium and then estimated the effective properties for a distribution of cracks. However this problem requires the solution of a Fredholm integral equation of the 2 kind which in general has no analytical solution for intermediate frequencies. We propose a simple analytical approximation of this solution using the branching function approach. Quantitative comparison shows good agreement between the two solutions. Our analytical solution exhibits a relaxation peak at a frequency where the fluid diffusion length is of the order of the crack

Estimation of Scattering Attenuation from Zero-offset VSP Data: CO2CRC Otway Project Case Study

Seismic attenuation consists of anelastic absorption and scattering loss. Due to the dominance of stratification, the scattering attenuation in the sedimentary crust is dominated by 1-D scattering. In this study we applied an integrated workflow for estimation of attenuation from ZVSP and log data to a comprehensive dataset acquired at Otway basin. Both 1D reflectivity modeling and application of generalized O’Doherty-Anstey theory to the Otway log data shows that the 1-D scattering component of attenuation gives Q of over 200. At the same time, average Q estimated from field VSP data value is close to 60. Hence we conclude that scattering plays a relatively minor role in the study area. Further research is required to understand whether this conclusion holds in other areas. In particular, scattering attenuation might be larger in environments with larger variability of elastic properties between layers, such as in areas with laminated coal layers.

Seismic Attenuation from VSP and Well Log Data - NW Shelf Australia Case Study

Seismic attenuation is an important contributor to the overall amplitude decay of the seismic wave travelling through the medium. Seismic attenuation anomalies in the overburden can both mask amplitude changes at the target horizons and carry important information about presence of free gas or overpressure. In order to understand the pattern and principal causes of the attenuation in one geographical area, specifically, one of the Australian North-West shelf’s basins, we examine a relatively large number of wells coming from this area using one consistent workflow. The dataset includes recent wells with high quality zero-offset (rig) VSP and good well log coverage.

Frequency dependent anisotropy of porous rocks with aligned fractures

In fractured reservoirs, the wave induced fluid flow between pores and fractures can cause significant dispersion and attenuation of seismic waves. For waves propagating normal to the fractures this effect has been quantified in earlier studies. Here we extend normal incidence results to oblique incidence using known expressions for the stiffness tensors in the low- and high-frequency limits. This allows us to quantify frequency-dependent anisotropy due to the wave-induced flow between pores and fractures and gives a simple recipe for computing phase velocities and attenuation factors of P, SV and SH waves as functions of frequency and angle.

Two Models for Seismic Attenuation and Dispersion in Fractured Porous Reservoirs

Natural fractures in hydrocarbon reservoirs can cause significant seismic attenuation and dispersion due to wave induced fluid flow between pores and fractures. We present two theoretical models explicitly based on the solution of Biot’s equations of poroelasticity. The first model considers fractures as planes of weakness (or highly compliant and very thin layers) of infinite extent. In the second model fractures are modeled as thin penny-shaped voids of finite radius. In both models attenuation exhibits a typical relaxation peak around a normalized frequency of about 1. This corresponds to a frequency where the fluid diffusion length is of the order of crack spacing for the first model, and the crack diameter for the second. This is consistent with an intuitive understanding of the nature of attenuation: when fractures are closely space, the waves reflected/scattered by cracks interfere with each other, with the interference pattern controlled by the fracture spacing. Conversely, if fracture length is smaller than spacing, then fractures act as independent scatterers and the attenuation resembles the pattern of scattering isolated cracks.

Attenuation and Dispersion in Fluid-Saturated Rocks Due to Circular Cracks

We consider interaction of a normally incident time-harmonic longitudinal plane wave with a circular crack imbedded in a porous medium governed by Biot’s equations of dynamic poroelasticity. The problem is formulated in cylindrical coordinates as a system of dual integral equations for the Hankel transform of the wave field, which is then reduced to a single Fredholm integral equation of the second kind. The solution of this equation yields elastic wave dispersion and attenuation in a medium containing a random distribution of aligned cracks. These dissipation effects are caused by wave induced fluid flow between pores and cracks.