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Fractured Reservoirs
Anisotropy & Inhomogeneity
Anisotropy is the seismic wavelength scale expression of sub-seismic inhomogeneity. Anisotropy is almost always to some degree present in rocks, be-it vertical due to thin layering or particle shape (shale) or horizontal due to vertical micro-cracks.
A combination of both results in orthorhombic anisotropy. In this course we will only consider Polar Anisotropy (preferred above the term Vertical Transverse Isotropy as it emphasizes the anisotropic part of the rock) with either a vertical (thin
layering) axis (VTI) or horizontal (vertical micro-cracks) axis (HTI) of symmetry. We will use Thomsen (ε, δ, γ) weak anisotropy parameters. These parameters are defined by combinations of VP and VSH velocities at directions
0°, 45° and 90° relative to the vertical
ε is always positive, δ and γ are small but can be positive as well as negative η’= ε- δ is called the anellipticity, i.e., the deviation of the wave front from an elliptic shape.
A system of fractures can be characterised by two parameters δn and δt which are defined relative to the fracture orientation: index n refers to a direction normal to the fracture plane whereas t refers to a direction tangential to the fracture plane. These parameters are also called normal and tangential weaknesses with values between 0 and 1. They specify the proportion of the compliance that is due to fractures. Note that compliance is the inverse of the elastic modulus or stiffness. The tangential compliance is only a function of the crack density, whereas the normal compliance is a function of crack density, crack aperture and the pore-fluid in the cracks. Obviously, if the crack aperture and pore-fluid (and its compressibility) are known than the anisotropy is only a function of the crack density and the fracture/crack orientation.
Another way of defining the anisotropy due to fractures uses “Thomsen” type parameters: εv, δv, γv. In the case of weak anisotropy they have a simple expression in terms of the usual Thomson parameters: e.g. εv goes to -ε, which means that for VTI: v90=v0(1+ ε)>v0 but for HTI: v90=v0(1+ ε v)<v0 for ε>0, as expected (namely the phase velocity parallel cracks>phase velocity normal to cracks)
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In the exercises we only calculate the AVA response for parallel and normal to the main crack strike direction, but a simple extension allows calculation for any azimuth. There are two ways to determine the crack strike direction, one is by determining the direction of maximum stacking (NMO) velocity if we have multiple azimuth surveys available, the other is by determining the direction of the fastest shear velocity (shear wave splitting or birefringence) in case of a shear wave survey.
In the case of weak anisotropy and small velocity and density contrasts at the interface the difference between the P-wave AVA gradients in the two symmetry planes (normal and strike direction) can be used to derive the crack density.
Knowledge of the strike and density of micro-cracks is important for deriving the maximum permeability magnitude and direction. This knowledge is important in relation to natural fractures in conventional plays as well as induced fractures in unconventional plays.
Note that only the changes in δn and δt across interfaces can be derived from reflection amplitudes. Hence to obtain absolute values they need to be “anchored” at a known un-fractured layer. This is like obtaining absolute acoustic impedance by calibration at a well.
Geophysics provides technology with which we can "look" into the subsurface. It is a key enabler of many activities in the search for hydrocarbons, minerals, fresh water, and geothermal energy. Of the many existing geophysical methods, three are important for monitoring CO2. These are Seismic and Electromagnetic and Gravity methods. CO2 injection can be done for sequestration as well as enhanced oil recovery
In the first method, high resolution seismic up to 250 Hz can be acquired with short offsets. For deeper layers, long offset seismic is collected. Long offsets are needed for Refraction Static corrections and in case Full Waveform Inversion is applied for obtaining the diving waves. For CO2 sequestration, the presence of fractures and their orientation, being natural or induced is a significant hazard that can be determined from seismic.
The second important geophysical method is electrical and electro-magnetic methods. Electrical or Direct Current surveys use grounded electrodes for source and receivers. They measure the potential difference using increasing receiver electrode spacing. Changes in measured potentials contain information on the resistivities of the subsurface, which can be related to changing pore fluids, like CO2 replacing brine. Electro-Magnetic can use either grounded or inductive sources (aerial surveys), but also natural sources as used in Magneto-Telluric surveys. These Electro-Magnetic sources can be a harmonic source (using a single frequency) and the measurement of the magnitude and phase delay, or real and imaginary responses are used. The other Electro-Magnetic source option is a step-off function. The subsurface information is then contained in the amplitude decay after shut-off.
The third approach is using Gravity measurements. These could be changes in the vertical gravity component Gz or the Full-Tensor-Gravity (FTG), which measures gradients in the gravity field. With the increasing accuracy of modern instruments, changes in the CO2-brine interface have been measured in oil (Prudhoe Bay, Schrader-Bluff and Troll field) and gas-storage (Izaute) fields
Geophysics provides technology with which we can "look" into the subsurface. It is a key enabler of many activities in the search for hydrocarbons, minerals, fresh water, and geothermal energy. Of the many existing geophysical methods, two are important for exploring and producing geothermal energy. These are Seismic and Electromagnetic methods.
In the first method, high resolution seismic up to 250 Hz can be acquired with short offsets. For deeper geothermal sources, long offset seismic is collected. Long offsets are needed for Refraction Static corrections and in case Full Waveform Inversion is applied for obtaining the diving waves. For geothermal application, the presence of fractures and their orientation, being natural or induced is an important property that can be derived from seismic.
Velocities have many uses. They are used in processing as well as interpretation. In processing they allow, for example, Normal Moveout (NMO) corrections to offset arrivals which facilitates stacking of events. This increases the signal-to-noise ratio significantly and makes interpretation easier. In processing the correct velocity-depth model allows “true-to-nature” imaging of the subsurface. But especially in interpretation there are huge benefits in obtaining accurate velocities for the different “geo-bodies”, as they can be used for lithology and pore fill determination. In some processing methods, take Full Waveform Inversion (FWI), they can be the main aim of its application. It is fair to say that for an accurate image of the subsurface, an accurate velocity-depth model is paramount in obtaining a “true” image of the subsurface, in terms of structure/geometry as well as amplitudes of reflections needed for quantitative interpretation.
Geophysics provides technology with which we can "look" into the subsurface. It is a key enabler of many activities in the search for hydrocarbons, minerals, and fresh water. It is also extensively used in the domain of monitoring pollution and rejuvenation of polluted sites. The course provides the fundamentals of seismic refraction & reflection methods, the use of gravity, magnetic, electrical, and electromagnetic methods. Modern geophysical acquisition and processing techniques will be taught not only based on a textbook but by applying the theory in mainly Excel based exercises
This course is available as a F2F course as well as an E-Learning course (see the course program).
Of all technologies available
in geophysics, electromagnetic methods have the widest range of applications, a result of the large spectrum of frequencies that can be used.
This course is available as a Blended learning course (see the course program) and as face
to face course (see the course program).
This course will explain how to design optimum seismic
acquisition and processing to obtain the maximum resolution at the targets. This might involve complementary non-seismic data in joint inversion schemes.
Is this the right course for you? Check this with Quiz 0.
- Teacher: J.C. Mondt
Based on Open Source software we offer a Blended learning course (see the course program) and as face to face course (see the course program).
The course consists of PowerPoint presentations with references to publications together with computer-based exercises and Machine Learning related videos and active participant interaction.
Artificial Intelligence is no more and no less than a powerful extension of the human intelligence. Its power is in its capability to master data structures in multi-dimensional space (classification, clustering, regression, prediction) and map the data onto a better-understandable, lower-dimensional space while maintaining the inter-data relationships.
Based on Open Source software, an 8hrs E-learning course (see the course program) is
available, optionally spread over two days, in addition to a one-day F2F
course.
The course consists of presentations, based on publications together
with computer-based exercises and Machine Learning related videos.
Do the Quiz 0 to check whether this course is suitable for you.
We all have seen displays of seismic data in the form of sections or cubes of data. But what do they show and how are they acquired? In this course you will learn to understand that seismic data represents the movement of the surface, resulting from waves generated by a source, dynamite or vibrator, which are reflected by changes in the subsurface rocks. Hence, what we record is related to the properties of the rocks, not only rocks, but also its pore fluids. All information on the subsurface is contained in these records, but almost impossible to extract and understand. Therefor the records need to be processed to make it possible to interpret structure and content of the pore space. In this course, the basic principles of acquisition and processing will be discussed. But also, insights in advanced methods will be provided. These methods allow a much more accurate interpretation of seismic data. The aim is not to fully understand these methods, but to understand its importance in certain cases, to enable interpreters, reservoir engineers to formulate requests for these methods.
