Seismo-electric conversion in shale: experiment and analytical modelling

The development of seismo-electric exploration techniques relies critically upon the strength of the seismo-electric conversion. However, there have been very few seismo-electric measurements or modelling on shales, despite shales accounting for the majority of unconventional reservoirs. We have carried out seismo-electric measurements on Sichuan Basin shales (permeability 0.00147–0.107 mD), together with some comparative measurements on sandstones (permeability 0.2–60 mD). Experimental results show that the amplitudes of the seismo-electric coupling coefficient in shales are comparable to that exhibited by sandstones, and are approximately independent of frequency in the seismic frequency range (<1 kHz). Numerical modelling has also been used to examine the effects of varying (i) dimensionless number, (ii) porosity, (iii) permeability, (iv) tortuosity and (v) zeta potential on seismo-electric conversion in porous media. It was found that while changes in dimensionless number and permeability seem to have little effect, seismo-electric coupling coefficient is highly sensitive to changes in porosity, tortuosity and zeta potential. Numerical modelling suggests that the origin of the seismo-electric conversion in shales is enhanced zeta potentials caused by clay minerals, which are highly frequency dependent. This is supported by a comparison of our numerical modelling with our experimental data, together with an analysis of seismo-electric conversion as a function of clay mineral composition from XRD measurements. The sensitivity of seismo-electric coupling to the clay minerals suggests that seismo-electric exploration may have potential for the characterization of clay minerals in shale gas and shale oil reservoirs

Modelling and Simulation of Heterogeneous and Anisotropic Formations using Advanced Fractal Reservoir Models

Energy and carbon-efficient exploitation, management, and remediation of subsurface aquifers, gas and oil resources, CO2-disposal sites, and energy storage reservoirs all require high quality modelling and simulation. The heterogeneity and anisotropy of such subsurface formations has always been a challenge to modellers, with the best current technology not being able to deal with variations at scales of less than about 30-50 m. Most formations exhibit heterogeneities and anisotropy which result in variations of the petrophysical properties controlling fluid flow down to millimetre scale and below. These variations are apparent in well-logs and core material, but cannot be characterised in the inter-well volume which makes up the great majority of the formation. This paper describes a new fractal approach to the modelling and simulation of heterogeneous and anisotropic aquifers and reservoirs. This approach includes data at all scales such that it can represent the heterogeneity of the reservoir correctly at each scale. Advanced Fractal Reservoir Models (AFRMs) in 3D can be produced using our code. These AFRMs can be used to model fluid flow in formations generically to understand the effects of an imposed degree of heterogeneity and anisotropy, or can be conditioned to match the characteristics of real aquifers and reservoirs. This paper will show how 3D AFRMs can be created such that they represent critical petrophysical parameters, as well as how fractal 3D porosity and permeability maps, synthetic poro-perm cross-plots, water saturation maps and relative permeability curves can all be calculated. It will also show how quantitative controlled variation of heterogeneity and anisotropy of generic models affects fluid flow. We also show how AFRMs can be conditioned to represent real reservoirs, and how they provide a much better simulated fluid flow than the current best technology. Results of generic modelling and simulation with AFRMs show how total hydrocarbon production, hydrocarbon production rate, water cut and the time to water breakthrough all depend strongly on heterogeneity, and also depend upon anisotropy. Modelling with different degrees and directions of anisotropy shows how critical hydrocarbon production data depends on the direction of the anisotropy, and how that changes over the lifetime of the reservoir. Advanced fractal reservoir models are of greatest utility if they can be conditioned to represent individual reservoirs. We have developed a method for matching AFRMs to aquifer and reservoir data across a wide range of scales that exceeds the range of scales currently used in such modelling. We show a hydrocarbon production case study which compares the hydrocarbon production characteristics of such an approach to a conventional krigging approach. The comparison shows that modelling of hydrocarbon production is appreciably improved when AFRMs are used, especially if heterogeneity and anisotropy are high. In this study AFRMs in moderate to high heterogeneity reservoirs always provided results within 5% of the reference case, while the conventional approach resulted in massive systematic underestimations of production rate by over 70%

The effect of rock permeability and porosity on seismoelectric conversion: experiment and analytical modelling

The seismoelectric method is a modification of conventional seismic measurements which involves the conversion of an incident poroelastic wave to an electromagnetic signal that can be measured at the surface or down a borehole. This technique has the potential to probe the physical properties of the rocks at depth. The problem is that we currently know very little about the parameters which control seismoelectric conversion and their dependence on frequency and permeability, which limits the development of the seismoelectric method. The seismoelectric coupling coefficient indicates the strength of seismoelectric conversion. In our study, we focus on the effects of the reservoir permeability, porosity and frequency on the seismoelectric coupling coefficient through both experimental and numerical modelling. An experimental apparatus was designed to record the seismoelectric signals induced in water-saturated samples in the frequency range from 1 kHz to 500 kHz. The apparatus was used to measure seismoelectric coupling coefficient as a function of porosity and permeability. The results were interpreted using a micro-capillary model for the porous medium to describe the seismoelectric coupling. The relationship between seismoelectric coupling coefficients and the permeability and porosity of samples were also examined theoretically. The combined experimental measurements and theoretical analysis of the seismoelectric conversion has allowed us to ascertain the effect of increasing porosity and permeability on the seismoelectric coefficient. We found a general agreement between the theoretical curves and the test data, indicating that seismoelectric conversion is enhanced by increases in porosity over a range of different frequencies. However, seismoelectric conversion has a complex relationship with rock permeability, which changes with frequency. For the low-permeability rock samples (0-100×10‾15 m²), seismoelectric coupling strengthens with the increase of permeability logarithmically in the low frequency range (0-10 kHz); in the high frequency range (10-500 kHz), the seismoelectric coupling is at first enhanced, with small increases of permeability leading to small increases in size in electric coupling. However, continued increases of permeability then lead to a slight decrease in size and image conversion again. For the high-permeability rock samples (300×10‾15 m² - 2200×10‾15 m²), the seismoelectric conversion shows the same variation trend with low-permeability samples in low frequency range; but it monotonically decreases with permeability in the high frequency range. The experimental and theoretical results also indicate that seismoelectric conversion seems to be more sensitive to the changes of low-permeability samples. This observation suggests that seismic conversion may have advantages in characterizing low permeability reservoirs such as tight gas and tight oil reservoirs and shale gas reservoirs

The Effect of Rock Permeability and Porosity on Seismoelectric Conversion

The seismoelectric coupling coefficient indicates the strength of seismoelectric conversion. In our study, an experimental apparatus was designed to record the seismoelectric signals induced in water-saturated sandstones in the frequency range from 10K to 500 KHz. The results were interpreted using a micro-capillary model for the porous medium to describe the seismoelectric coupling. The relationship between seismoelectric coupling coefficients and the permeability and porosity of sandstones were also examined theoretically. A general agreement between the theory and the test data indicates that seismoelectric conversion is enhanced by increases in porosity over a range of different frequencies. However, seismoelectric conversion has a complex relationship with rock permeability, that changes with frequency. In the low frequency range (0-50KHz), seismoelectric coupling strengthens with the increase of permeability logarithmically. In the high frequency range (50K-10000KHz), the seismoelectric coupling is at first enhanced, with small increases of permeability leading to small increases in seismoelectric coupling. However, continued increases of permeability lead to a slight decrease in size and image conversion again. The experimental and theoretical results indicate that seismoelectric conversion seems to be more sensitive to the changes of low-permeability samples. This suggests that seismic conversion may have advantages in characterizing low permeability reservoirs

The Seismoelectric Coupling in Shale

Seismic explorations are not sensitive to the mineral properties of shale reservoirs. The seismoelectric conversion induced based on the mechanism of electric double layer can reveal the mineral properties. We investigated 14 shale samples (Sichuan Basin, China) based on seismoelectric coupling for the first time. Combining the acoustic velocity and XRD method, we analyzed the relationship between minerals, shale anisotropy and seismoelectric coupling. This study shows that extremely low-permeability shale (0.00038-0.00495mD) can produce seismoelectric signals as strong as those of high-permeability sandstone (0.91412-58.74323mD). The effects of clay minerals and non-clay minerals on seismoelectric coupling indicate that clay minerals can enhance seismoelectric conversions in shale. The illite in clay minerals has the greatest influence on seismoelectric conversion. Seismoelectric coupling is also used to characterize the shale anisotropy caused by clay minerals. The anisotropy presented by seismoelectric conversion is in proportion to that characterized by acoustic velocities, it increases from approximately 0 to 0.25 with anisotropy parameters ranging from approximately 0.1 to 0.5 for ε and 0.15 to 0.35 for γ. This feature of seismoelectric effects, which can reflect both the clay minerals and the anisotropy, is helpful for intuitive understanding of the anisotropy caused by clay minerals in shale reservoirs