Double Shell As a Directly Solvable Model of aMicro-inhomogeneous Poroelastic Medium

We consider pore pressure and fluid compressibility effects on rock properties in the case of inhomogeneous solid matrix. For this purpose we propose a model amenable to exact analytical. It consists of similar double-layered spherical shells with a fluid inside. We show how could be realized different values of effective stress coefficient for porosity, give clear interpretation of it. In addition we validate Brown-Korringa approach. Its predictions exactly coincide with the exact solution. At the same time some new theories of poroelasticity gives predictions deviating from the double-shell model results. These theories claim that the porosity perturbation must be incorporated explicitly in the theory of porous media deformation. We give some possible explanation why this conception is not correct

E1: Seismic survey design technology based on 3D finite-difference wave field modeling

Seismic prediction of complex hydrocarbon reservoirs is a complicated task. Present approaches to this problem often give wrong results. Usage of 3D full waveform seismic computer modelling allows us to improve all stages of seismic prospecting in such regions. It is presented a case study of computer modeling application to Eastern Siberia reservoirs. Aspects of seismo-geological model construction are discussed. It is shown that analysis of synthetic seismograms enables choosing of appropriate data acquisition system, processing graph and interpretation techniques

Effect of crack distribution on elastic properties of rocks revisited

Most models of pressure dependency of rock properties relate such a dependency to the distribution of aspect ratios of pores and cracks. This approach might not always be accurate because moderate roughness of crack boundaries has little effect on effective properties of the medium but asperities heights determine the pressure level at which partial contact between crack faces takes place. We explore how the pressure dependency could be related to distributions of crack radii and their thicknesses (or aspect ratios and thicknesses). For an idealized case of uniform distribution of aspect ratios, the bulk modulus is proportional to an expression of the form √1αp+1 (until pressure p reaches a value pm where all the cracks are closed). The model can be used to derive the distribution of crack properties from pressure dependency of elastic moduli, and to take into account the effect of crack roughness

Effect of asperities on stress dependency of elastic properties of cracked rocks

© 2015 Elsevier Ltd. Rocks are complex heterogeneous materials consisting of solid minerals and fluid-filled pore space. Sedimentary rocks often undergo significant continuous stiffening under stress. The main mechanism of this stiffening is an increase of the number of contacts between adjacent grains and asperities on opposite surfaces of cracks. We propose an analytical model for effective compliance of finite cracks with contacting surfaces. To this end we utilize known exact solutions for an annular crack and a contact of the welded-area type. A set of numerical tests verifies applicability of the model to realistic rock microstructures. A micromechanical theory of changes of the elastic moduli with external loading is based on the crack stiffening by contacting asperities. The proposed theory is then used to analyze results of ultrasonic experiments on a sandstone sample. Parameters of the rock sample recovered from the pressure dependencies of seismic waves are consistent with each other and physically reasonable. Demonstrated applications of the model are based on several simplified assumptions but the theory may incorporate further generalizations, namely: interaction of contacts, inclusion size distribution and more complicated modeling of the microstructure evolution with confining stress

Transforming microseismic clouds into near real-time visualization of the growing hydraulic fracture

Microseismic observations during unconventional reservoir stimulation are typically seen as a proxy for clusters of hydraulic fractures and the extent of the stimulated reservoir. Such straightforward interpretation is often misleading and fails to provide a physically reasonable image of the fracturing process. This paper demonstrates the application of a physics-based machine learning algorithm which enables a rapid and accurate fracture mapping from the microseismic data. Our training and validation data set relies on a history-matched geomechanical modelling workflow implemented in GEOS software for the Hydraulic Fracturing Test Site 1 (HFTS-1) project. For this study we augmented the simulated fracture growth through geostatistical modelling of induced seismicity, so that the synthetic microseismic catalogue matches the main statistical properties of the field observations. We formulated the problem of mapping the actual fracture in the clutter of events to parallel common video segmentation workflows: several past video frames (microseismic density snapshots) are passed through a deep convolutional network to classify whether a given voxel is associated with a fracture or intact rock. We found that for accurate fracture mapping, the network's input and architecture must be augmented to incorporate the fluid injection parameters (pressure, rate, concentration of proppant, and location of the perforation within the cluster). The error rate for the network reached as little as 10 per cent of the fracture area, while a conventional microseismic interpretation approach yielded ∼300 per cent. Our approach also yields must faster predictions than conventional methods (minutes instead of weeks), and could enable engineers to make rapid decisions regarding engineering parameters (pumping rate, viscosity) in real time during stimulation

Use of 3D computer seismic full waveform simulation for validation of porous-fractured reservoirs predictions

Present work concerns with potential of multicomponent seismology to detect porous-fractured reservoirs and to estimate its parameters. This problem is examined in assistance with 3D seismic full waveform computer modeling. It is formulated approaches to construction of digital model of the geological media, optimum processing technology based on layered velocity model building for compressional and converted waves. Seismic records atributes are analysed. It is suggested the set of atributes provided porous-fractured reservoirs localization and estimation of its effective characteristics including symmetry axis direction

CO2 Storage Site Characterisation using Combined Regional and Detailed Seismic Data: Harvey, Western Australia

Some 115 km 2 of regional 3D seismic data were acquired in the first quarter of 2014 near Harvey, Western Australia, for the needs of the South West CO2 Hub project. The survey proved to be of great importance for a regional characterisation of the reservoir, identification of the large structures and key geological interfaces. However, small to medium size structures of interest for the development of the static and dynamic models were poorly imaged in this survey as the recording geometry was adjusted for the greater depths, which was between 2 km and 3 km. To improve the imaging of the shallow structures, a high-resolution (nested) 3D survey centered at Harvey 4 well was undertaken in 2015 (Urosevic et al., 2015). This survey utilised a single geophone and single, 24 s long, broadband (6-150 Hz) sweep combined with high data density to improve signal to noise ratio that was initially lowered by not employing high-power sources and geophone arrays. The results of this high-resolution 3D survey demonstrate that high-density surveys are important even at the characterisation stage and are crucial for development of a detailed static model. For that purpose, both post and pre-stack inversions of these data were utilised to model distribution of paleosols, lenses of high clay content, which are assumed to serve as baffles for CO 2 upward migration. A good correlation was established between very low impedance values and increased percentage of paleosols and on the other end of the scale very high impedance values and low porosity sandstones. A pre-stack migrated high-resolution cube and the attribute derived from it, such as coherency and impedance, enabled improved structural and stratigraphic analysis around Harvey 4 well. The results shown were of a crucial importance for the containment studies, development of the dynamic model and establishment of the injection intervals