Jianxin Wei

Fluid-dependent anisotropy and experimental measurements in synthetic porous rocks with controlled fracture parameters

In this study, we analyse the influence of fluid on P- and S-wave anisotropy in a fractured medium. Equivalent medium theories are used to describe the relationship between the fluid properties and the rock physics characteristics in fractured rocks, and P-wave and S-wave velocities and anisotropy are considered to be influenced by fluid saturation. However, these theoretical predictions require experimental measurement results for calibration. A new construction method was used to create synthetic rock samples with controlled fracture parameters. The new construction process provides synthetic rocks that have a more realistic mineral composition, porous structure, cementation and pressure sensitivity than samples used in previous research on fractured media. The synthetic rock samples contain fractures which have a controlled distribution, diameter, thickness and fracture density. In this study, the fracture diameter was about 4 mm, the thickness of fractures was about 0.06 mm, and the fracture density in the two fractured rock samples was about 3.45%. SEM images show well-defined penny-shaped fractures of 4 mm in length and 0.06 mm in width. The rock samples were saturated with air, water and oil, and P- and S-wave velocities were measured in an ultrasonic measurement system. The laboratory measurement results show that the P-wave anisotropy is strongly influenced by saturated fluid, and the P-wave anisotropy parameter, e, has a much larger value in air saturation than in water and oil saturations. The S-wave anisotropy decreases when the samples are saturated with oil, which can be caused by high fluid viscosity. In the direction perpendicular to the fractures (the 0° direction), shear-wave splitting is negligible, and is similar to the blank sample without fractures, as expected. In the direction parallel to the fractures (the 90° direction) shear-wave splitting is significant. The fractured rock samples show significant P- and S-wave anisotropy caused by the fractures and controlled by the saturated fluids.

Abnormal transmission attenuation and its impact on seismic-fracture prediction — A physical modeling study

Fracture-induced anisotropy can lead to observable azimuthal variations of seismic attributes that then can be used for characterizing a fracture system. Unfortunately, abnormal transmission losses along raypaths also can result in similar azimuthal variations leading to uncertainty in such fracture determination. Using a physical model containing gas-filled fractures, we investigate the impact of abnormal transmission loss on fracture detection from ultrasonic data in a laboratory setting. Recorded reflection amplitudes and traveltimes are used to study ultrasonic responses to the presence of the gas-filled fractures and to understand observed azimuthal attribute anomalies. Experimental results from this study highlight the pitfalls in using azimuthal attribute variations as indicators of the presence of fractures when abnormal transmission attenuation is significant.

Measurements of Seismic Anisotropy in Synthetic Rocks with Controlled Crack Geometry and Different Crack Densities

Seismic anisotropy can help to extract azimuthal information for predicting crack alignment, but the accurate evaluation of cracked reservoir requires knowledge of degree of crack development, which is achieved through determining the crack density from seismic or VSP data. In this research we study the dependence of seismic anisotropy on crack density, using synthetic rocks with controlled crack geometries. A set of four synthetic rocks containing different crack densities is used in laboratory measurements. The crack thickness is 0.06xa0mm and the crack diameter is 3xa0mm in all the cracked rocks, while the crack densities are 0.00, 0.0243, 0.0486, and 0.0729. P and S wave velocities are measured by an ultrasonic investigation system at 0.5xa0MHz while the rocks are saturated with water. The measurements show the impact of crack density on the P and S wave velocities. Our results are compared to the theoretical prediction of Chapman (J App Geophys 54:191–202, 2003) and Hudson (Geophys J R Astron Soc 64:133–150, 1981). The comparison shows that measured velocities and theoretical results are in good quantitative agreement in all three cracked rocks, although Chapman’s model fits the experimental results better. The measured anisotropy of the P and S wave in the four synthetic rocks shows that seismic anisotropy is directly proportional to increasing crack density, as predicted by several theoretical models. The laboratory measurements indicate that it would be effective to use seismic anisotropy to determine the crack density and estimate the intensity of crack density in seismology and seismic exploration.

Creation of synthetic samples for physical modelling of natural shale

Natural shale samples, particularly well-preserved, drilled core samples, are extremely difficult to obtain for laboratory research. Multiple tests must be carried out on one sample, and some samples are disposed after destructive tests. Therefore, rarity and non-reusability of samples strongly restrict shale studies. In this study, based on statistical data from the world’s major shale block, a new type of synthetic shale was physically constructed via a process of interfusion, stuffing, and compaction using quartz, clay, carbonate, and kerogen as the primary materials, according to statistical data from the world’s major shale blocks. Further evaluation of the synthetic shale involved the use of scanning electron microscopy imagery and analysis of its anisotropic characteristics in comparison with natural shale. The synthetic shale had a laminated microstructure similar to natural shale, and its velocity anisotropy corresponded to Thomsen’s anisotropy of a transversely isotropic medium. The results of tests for homogeneity and repeatability indicated that the construction process was stable and that several identical synthetic samples, which were satisfactorily similar to natural shale, could be produced for both iterative and destructive tests. The composition of each mineral, as well as the density, porosity, permeability, and anisotropy of the samples, were all variable. Therefore, a series of synthetic samples could be obtained with properties set to meet the requirements of petrophysical experimentation. Moreover, gas or oil saturation was also considered in the construction of the synthetic shale, meaning that the characteristics of gas or oil saturation (or the complete range nof data from dry to saturated samples) could be tested using the synthetic shale.

Feasibility of theoretical formulas on the anisotropy of shale based on laboratory measurement and error analysis

This paper designs a total angle ultrasonic test method to measure the P-wave velocities (vp), vertically and horizontally polarized shear wave velocities (vsv and vsh) of all angles to the bedding plane on different kinds of strong anisotropic shale. Analysis has been made of the comparisons among the observations and corresponding calculated theoretical curves based on the varied vertical transversely isotropic (TI) medium theories, for which discussing the real similarity with the characterizations of the TI medium on the scope of dynamic behaviors, and further conclude a more accurate and precise theory from the varied theoretical formulas as well as its suitable range to characterize the strong anisotropy of shale. At a low phase angle (theta 0.25, the Berrryman curve will be the best fit for the vp, vsv on shale.

Experimental and theoretical enhancement of the inversion accuracy of the Thomsen parameter δ in organic-rich shale

Experimental physical inversion of rock from the diagonal group velocities is an effective method for the determination of Thomsens δ anisotropy parameter in organic-rich shale. We further enhance the inversion accuracy of δ through conducting more reliable experimental measurements and through theoretical expression. First, we assembled two sets of group velocity acquisition methods, a rotational ultrasonic transducer system and a laser ultrasonic system, and then we assessed which of them was more applicable and accurate by comparing the waveforms and observations on the same cylindrical organicrich shale. Second, we combined the δ-based phase velocity approximation and stricter physical constraints of δ, which are deduced on a standard VTI medium, to improve the theoretical part of the inversion. As a result of better observations by the optimal test methods and the proposed δ inversion methods, the least errors between the best fitted curve to the observations are 3.24% for the traditional method and 2.1% for the proposed method, which verifies the superiority of the proposed method. Based on experimental tests on two cylindrical shale specimens, we find that rotational ultrasonic transducer measurement is more applicable for quick velocity anisotropy measurements, while for observations obtained by the laser technique, system relative error and the necessary scattering effect processing should be conducted. The procedure of the inversion is more robust and accurate when conducted on the proposed δ-based inversion.

Seismic prediction method of multiscale fractured reservoir

Common prestack fracture prediction methods cannot clearly distinguish multiplescale fractures. In this study, we propose a prediction method for macro- and mesoscale fractures based on fracture density distribution in reservoirs. First, we detect the macroscale fractures (larger than 1/4 wavelength) using the multidirectional coherence technique that is based on the curvelet transform and the mesoscale fractures (1/4–1/100 wavelength) using the seismic azimuthal anisotropy technique and prestack attenuation attributes, e.g., frequency attenuation gradient. Then, we combine the obtained fracture density distributions into a map and evaluate the variably scaled fractures. Application of the method to a seismic physical model of a fractured reservoir shows that the method overcomes the problem of discontinuous fracture density distribution generated by the prestack seismic azimuthal anisotropy method, distinguishes the fracture scales, and identifies the fractured zones accurately.

Effects of offset-depth ratio on fracture detection: a physical modeling study

Summary A physical modeling experiment has been conducted to study the effects of different offset-depth ratio on imaging and the capability for fracture detection. The model consists three horizontal layers, where the top and bottom layers are isotropy and the middle layer is HTI medium. There are a dome and a fault block in the fractured layer. Two modeling data are acquired with the same geometry parameters except for offset-depth ratio. Two sets of data are used by the similar analysis methods to detect the fracture from stacking velocity and AVO gradient. The results show that the larger offset-depth benefits the detection of fracture orientation.

Fracture Detection Using 2-D P-wave Seismic Data: A Seismic Physical Modelling Study

We used the seismic physical modelling approach to investigate the effects aligned fractures might have on seismic wave propagation at a larger scale in real Earth imaging. Our primary objective was to examine the effects of aligned fractures on seismic wave amplitude (through the estimation of fracture–induced attenuation) and traveltimes and relating these effects to the fracture orientations. The physical model has two fracture models constructed from a mixture of epoxy resin and silicon rubber and designed to simulate two sets of intersecting fractures. 2-D data were acquired using the pulse-transmission method in three principal directions with the physical model submerged in a water tank. The QVO method, an extension of the classical spectral ratio method for determining attenuation, was used to estimate the quality factor from the pre-processed CMP gathers. The results of our study reveal azimuthal variations in both attenuation expressed through the quality factor Q and the bottom travel-time to the fractured layer. Azimuthal variations in the fractureinduced attenuation and the bottom travel-times were both elliptical to a good approximation, enabling fracture orientations to be obtained quite accurately from the axes of the ellipse. We concluded that attenuation and traveltime anisotropy are potential exploration tools which may be used in fracture detection to complement the use of velocity, amplitude and AVO gradient attributes.

Physical modeling experiment and analysis of seismic response of a reservoir model filled with different fluids

Summary A reservoir model with 7 thin inhomogeneous sand and 8 shale layers of 5 m-thick respectively is made for seismic physical modeling experiment. Seismic physical modeling data are acquired for the model filled with gas, water and oil respectively. The analysis of the data shows that the reflection characteristics of the gas filled model evidently differ from those of the water and oil filled models. The reflection from the water filled reservoir differs subtly from the oil filled model. However the internal multiples are quite different. The differences of internal multiples appear below the reservoir. As a result, attention should be paid to this phenomenon in the interpretation of time lapse seismic data.