NS34A-01 INVITED
Seismic Reservoir Characterization
Seismic inversion results in elastic parameter estimates that need to be mapped to reservoir properties such as porosity and permeability. This mapping is not trivial. In addition the seismic inversion results in non-unique estimates of elastic parameter attributes. Judicial choice of prior based on well logs is essential to address both the issues. Care must be taken to not introduce any bias. It is easy to match the inversion results to well logs simply by putting hard constraints to prior. However this may result in erroneous mapping away from wells. We applied several methods of seismic inversion to a single dataset to study the effects of data domain, forward modelers, scales and prior on the resulting models. Stable mapping of seismic attributes to flow attributes is the key to effective reservoir model building (Young 2007). Although determinsiting mapping is a common practice, a prbabilistic mapping offers several advantages. Although the current methods assume locally 1D model, their extension to laterally varying media is currently being investigated. Young, R., 2007, Porosity estimation from 3D seismic data, unpublished MS thesis, The University of Texas at Austin
NS34A-02
An unsplit Convolutional perfectly matched layer technique improved at grazing incidence for the differential anisotropic elastic wave equation: application to 3D heterogeneous near surface slices.
In geophysical exploration, high computational cost of full waveform inverse problem can be drastically reduced
by implementing efficient boundary conditions. In many regions of interest for the oil industry or geophysical
exploration, nearly tabular geological structures can be handled and analyzed by setting receivers in wells or/and
at large offset. Then, the numerical modelling of waves travelling in thin slices along wells and near surface
structures can provide very fast responses if highly accurate absorbing conditions around the slice are introduced
in the wave propagation modelling. Here we propose then a Convolutional version of the well known Perfectly
Matched layer technique. This optimized version allows the generation of seismic waves travelling close to the
boundary layer at almost grazing incidence, which allows the treatment of thin 3D slices.
The Perfectly Matched Layer (PML) technique, introduced in 1994 by Bérenger for Maxwell's equations, has
become classical in the context of numerical simulations in electromagnetics, in particular for 3D finite difference
in the time domain (FDTD) calculations. One of the most attractive properties of a PML model is that no reflection
occurs at the interface between the physical domain and the absorbing layer before truncation to a finite-size layer
and discretization by a numerical scheme. Therefore, the absorbing layer does not send spurious energy back
into the medium. This property holds for any frequency and angle of incidence. However, the layer must be
truncated in order to be able to perform numerical simulations, and such truncation creates a reflected wave
whose amplitude is amplified by the discretization process. In 2001, Collino and Tsogka introduced a PML model for the elastodynamics equation written as a
first-order system in velocity and stress with split unknowns, and discretized it based on the standard 2D
staggered-grid finite-difference scheme of Virieux (1986). Then in 2001 and 2004, Zheng applied this technique to
Biot poroelastic systems. Unfortunately, this standard PML suffers from two drawbacks: the fact that the
unknowns are split adds to the memory cost of the simulations because additional arrays must be used to store
all the split components . After numerical discretization, the numerical reflection coefficient between the physical
domain and the PML region becomes large at grazing incidence and therefore the efficiency of the absorbing
layer is poor. In 2000, Roden and Gedney introduced an implementation of the PML for Maxwell's equations
based on the original (unsplit) components of the wave field and optimized for grazing incidence using an
analytical integration of the convolution term. This formulation, which is commonly known as the Convolution-
Perfectly Matched Layer (C-PML), overcomes the two main drawbacks of the classical PML formulation mentioned
above. The PMLs are tested here for
near surface complex structures involving highly dispersive weathered zones and salt domes which can be dealt
with such 3D PMLs.
http:migp.univ-pau.fr
NS34A-03
Subsalt Imaging in the Gulf of Mexico by PSDM and 3D High Resolution Gravity Inversion
In this work we applied a three-dimensional high-resolution gravity inversion method based on simulated annealing to the imaging of salt structures in the Gulf of Mexico. The initial input solution was constrained by using a previously interpreted seismic velocity model and all the available geologic information. Conversion of seismic velocities into densities led to an initial determination of salt distribution at depth. Pre-Stack Depth Migration (PSDM) provided an accurate estimation of top of salt bodies. Estimation of base of salt (and geometry distribution of salt bodies) most of the times is not an easy task for PSDM alone. PSDM was combined with high resolution gravity modeling in three dimensions in order to constrain estimation of base of salt bodies and velocity structure bellow to them. A significant improvement of resolution of PSDM images was achieved by the conversion of densities from gravity inversion into refined velocity models supplied to the PSDM iterative process.
NS34A-04
Modeling of seismic amplitude to discriminate the effects of lithology and fluids in AVO anomalies in deep water of the Gulf of Mexico Basin.
Recently the interest in deep water hydrocarbon exploration of the Gulf of Mexico Basin has increased. Hydrocarbon detection with AVO method in deep water presents greater ambiguity than at shallow depth. For this reason it is necessary to establish an strategy that reduces the risk of perforation associated with the interpretation of AVO anomalies. For the case of sandstone and shale, their petrophysical properties vary widely in the Gulf of Mexico Basin. For example, sandstones located at depths greater than 2-3 km can loose porosity due to cementation and hence this affect rock properties as impedance and Poisson coefficient, producing changes in seismic amplitudes. Reservoir sandstones can produce positive, negative o null reflections in the seismic sections. Throughout the Tertiary basins in the Gulf of México there are areas where acoustic impedance values of shales and gas sandstones are approximately equal. This means that those zones with hydrocarbon can appear as dim spot and are difficult to detect with conventional 3D seismic data. Hydrocarbon saturation affected the velocity of soft rocks at shallow depths, where sandstones is not cemented, but at greater depth when cementation increases susceptibility of rocks to fluid diminishes. For that reason it is important before AVO to make the elastic modeling to study the influence of different factors like mineralogy (obtained from analysis of well core and well logs), pore pressure, attenuation, illumination, anisotropy, etc. The studied elastic models show that it is often difficult to isolate the lithologic and fluid contribution to seismic response. To discriminate lithology from hydrocarbon it is necessary to make anelastic modeling (including attenuation), based on the fact that sands with gas produce attenuation anomaly whereas mineralogical changes do not. A detailed study of attenuation will allow to evaluate if the volume of gas in reservoir is commercial or not.
NS34A-05
Advances in Directional Multicomponent Induction Borehole Measurements
New multicomponent EM wireline and logging-while-drilling (LWD) instruments are changing the way dip is determined and the accuracy of geosteering wells. A new wireline triaxial induction logging tool allows accurate dip and azimuth to be determined along with standard induction logs and the vertical and horizontal resistivities. The new directional LWD resistivity tool allows accurate geosteering within a reasonable formation thickness and indicated whether to steer up or down in real time. Both these instruments are dependent on 3D numerical EM modeling codes for tool design and data interpretation.
NS34A-06
Joint Stochastic Inversion of Pre-Stack 3D Seismic Data and Well Logs for High Resolution Hydrocarbon Reservoir Characterization
This paper describes the successful implementation of a new 3D AVA stochastic inversion algorithm to quantitatively integrate pre-stack seismic amplitude data and well logs. The stochastic inversion algorithm is used to characterize flow units of a deepwater reservoir located in the central Gulf of Mexico. Conventional fluid/lithology sensitivity analysis indicates that the shale/sand interface represented by the top of the hydrocarbon-bearing turbidite deposits generates typical Class III AVA responses. On the other hand, layer- dependent Biot-Gassmann analysis shows significant sensitivity of the P-wave velocity and density to fluid substitution. Accordingly, AVA stochastic inversion, which combines the advantages of AVA analysis with those of geostatistical inversion, provided quantitative information about the lateral continuity of the turbidite reservoirs based on the interpretation of inverted acoustic properties (P-velocity, S-velocity, density), and lithotype (sand- shale) distributions. The quantitative use of rock/fluid information through AVA seismic amplitude data, coupled with the implementation of co-simulation via lithotype-dependent multidimensional joint probability distributions of acoustic/petrophysical properties, yields accurate 3D models of petrophysical properties such as porosity and permeability. Finally, by fully integrating pre-stack seismic amplitude data and well logs, the vertical resolution of inverted products is higher than that of deterministic inversions methods.
NS34A-07
The VLF EM Method Used for Verification of Fracture/ Shear Zone Aquifers in the Hyper-arid Eastern Desert, Egypt
An integrated program using Landsat remote sensing and ground follow-up with the Very Low Frequency (VLF) geophysical method was applied to the basement rocks of the Red Sea Hills (Eastern Desert) to locate fracture and shear-system aquifers. This part of the Nubian Shield was formed by accretion of a complex of ensimatic and ensialic island arcs and interleaving oceanic basins that were later accreted against the old African continent. Hence, melange and ophiolite sequences are common. This basement complex was intensely fractured (630- 530 Mybp) by the Najd transcurrent shear system (NSS) along a NW-SE trend that is up to 350 km wide, and finally the ocean-arc complex was intruded (~550 Mybp) by anorogenic K-granites. A false-color composite image was created, from Landsat thematic mapper band ratio images that are sensitive to the Fe-bearing aluminosilicate, hydroxyl, and opaque phase content of rocks. On these images mafic rocks (e.g., gabbro and mafic volcanics) rich in Fe-bearing aluminosilicates appear in shades of blue, ultramafics (e.g., serpentinites) rich in hydroxyl-bearing phases and opaque phases appear in shades of red, and granitoid rock units poor in the above phases show as green areas. Using this base map (effectively a pseudo geologic map) and a co-registered DEM, locations of potential shallow water occurrence were plotted based on the following criteria: 1) intersection of the NSS system with transverse faults defining wadis, 2) intersection of two or more fault zones, 3) within highly deformed melange units, especially their internal lithologic contacts and their crossings of wadis, 4) relatively unfractured younger dikes and their intersections with wadis. The VLF instrument was first used to make profiles at a number of existing water wells located at the structural intersections described above, to verify that sub-vertical sheet-like electrical conductors (water-filled fissures) could be successfully located with this instrument. Then, other sites that had been marked as having high potential for water using one or more of the above criteria were visited and profiled with the VLF. Many (but not all) of these sites gave anomalies characteristic of conductive fissures. Finally, several sites not chosen by the GIS interpreters were selected at random and profiled with the VLF. One of these (albeit adjacent to a melange area) did show a good conductor. Field navigation was directly on the scrolling false-color image on a laptop computer, linked to an active GPS receiver. VLF transmitters used were mainly those in Europe. Fourteen sites were visited in five days and nineteen total profiles were surveyed, ranging from 160 to 1200 meters in length. In summary, this methodology, beginning with satellite imagery and GIS, and ending with transects on foot with a VLF, proved to be a useful technique in such desert landscapes where fractured bedrock aquifers occur and the discovery of even a low-yield local aquifer is very important.