Åsmund Drottning

Simulated Prestack Local Imaging: a robust and efficient interpretation tool to control illumination, resolution, and time-lapse properties of reservoirs.

Summary The “Simulated Prestack Local Imaging” (SimPLI 1 ) concept is a new method helping interpreters of migrated seismic data to control and constrain their interpretation of local structures such as oil/gas reservoirs. There is no need in this approach to first generate synthetic data, and then migrate to calculate the migrated seismic response of reservoirs models. Simulated prestack migrated sections are quickly obtained as functions of the survey, emitted pulse, wave-modes, and local reservoir structure. An interpreter can therefore check interactively various reservoir scenarios in terms of illumination (survey-planning, reservoir characterization), resolution, and time-lapse evolution related to rock parameter changes during the production. Input can be interpreted horizons with attributes, property grids from reservoir models and inversion results, or hypothetic models. The results are prestack migrated sections, in depth or time. The SimPLI method is a convolution technique, which goes far beyond the classic 1D convolution as being able to predict 2D/3D effects of illumination, resolution and prestack acquisition, but without requiring experts in modeling and migration.

Improved applicability of ray tracing in seismic acquisition, imaging, and interpretation

Ray-based seismic modeling methods can be applied at various stages of the exploration and production process. The standard ray method has several advantages, e.g., computational efficiency and the possibility of simulating propagation of elementary waves. As a high-frequency approximation, the method also has a number of limitations, particularly with respect to a lack of amplitude reliability in the presence of rapid changes of the model functions representing elastic parameters and interfaces. Given the objective of improving the applicability of the standard ray method, we present a strategy that does not require specific extension to finite frequencies. Instead, we define each ray-based process as an element of a system that, as a composite process, is able to obtain better results than the ray-based process applied alone. Other elements of the composite process can be finitedifference modeling or numerical solutions for surface and volume integrals, which are basic constituents of Kirchhoff modeling and imaging. We also include among the process elements recently developed techniques for simulating the migration amplitude on a target reflector and in a local volume, e.g., a reservoir zone. The model is decomposed according to its complexity into volume elements, surface elements, or a combination. The composite process consists of a specified interaction between process elements and model elements, which fits well with the philosophy of modern software design. Model elements that will be exposed to ray-tracing algorithms may need appropriate preparation, e.g., smoothing and resampling. We demonstrate specifically, in a tutorial example, that simulating the seismic response from a reflector by ray-based composite processes can yield better results than applying standard ray tracing alone.

Use of coordinate-free numerics in elastic wave simulation

The modeling of elastic waves in solids and fluids is widely applied in physics and geophysics and often requires large computer codes for its realization. Despite the fact that different wave propagation problems have many similarities from the point of view of the abstract mathematical formulation, such codes are usually hard to adapt to new problems and changing requirements. In this paper we discuss a so-called coordinate-free approach for the general computer implementation of tensorfield equations. We show how it applies to the modeling of seismic waves in isotropic and anisotropic structures, sonic waves in boreholes and ultrasonic waves in poro-elastic fluid-filled materials. The results clearly indicate that the coordinate-free approach is very flexible for the implementation of various wave propagation problems.  2001 IMACS. Published by Elsevier Science B.V. All rights reserved.

Combining Saturation Changes and 4D Seismic for Updating Reservoir Characterizations

This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Information Content in Forward 4D-Seismic Modelling and Elastic Inversion

This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract Elastic seismic inversion is a tool frequently used in analysis of seismic data. Elastic inversion relies on a simplified seismic model, and generally produces 3D cubes for Vp, Vs and density. By applying rock physics theory such volumes may be interpreted in terms of lithology and fluid properties. Understanding the robustness of forward and inverse techniques is important when deciding how much information seismic data really carry. This paper discusses the observed deviation between a reference and simulated reservoir, and its dependency on the seismic parameters and the reservoir characterization parameters. The ability to utilize the results from a 4D seismic survey in reservoir characterization will depend on several aspects. To investigate this, a loop that performs independent forward seismic modeling and elastic inversion at two time stages has been established. The multidisciplinary workflow has several independent steps: 1. Generation of a synthetic reference reservoir, by realistic geostatistical modeling. 2. Flow simulation of the reference reservoir to predict reservoir conditions at survey acquisition times. 3. Establishing a relationship between petrophysical and fluid properties and seismic parameters by a rock physics model. 4. Generation of seismic AVA responses corresponding to reservoir conditions at base and monitor survey times. 5. Elastic seismic inversion of both AVA response sets. 6. Simulation of lithology and fluid parameters conditioned on seismic inversion. 7. Comparison of static reservoir parameters of reference and simulated realization. 8. Comparison of seismic responses at initial and monitor survey times. By working on a realistic synthetic reservoir, full knowledge of the reservoir characteristics is achieved. This …

Seismic Reservoir Characterization: Including Sensitivity to the Overburden, Survey, and Reservoir Properties when Simulating the 3D Seismic Response to the Reservoir

Abstract The simulation of seismic data from 3D Earth models is an integral part of the decision process within the Exploration and Production cycle. Although major advances have been made in the representation of the Earth and the dynamic processes present within the reservoir, the accurate simulation of seismic data from these models still remains a challenge. Conventionally, 1D convolution methods are used to simulate seismic data from the properties within the Earth model. However, this process does not generally consider the effect that the survey and overburden have on the seismic signal. We look at why this is limiting the effectiveness of the 3D Earth model and consider why there is a need to incorporate the 3D illumination and resolution effects from the overburden and survey into the simulation process. We present a new methodology, which integrates rock physics modelling with a new seismic simulation technology to provide a workflow, which enables the explorationist to rapidly simulate 3D PSDM data that incorporates the illumination and resolution effects of the overburden and survey. Using data from an existing field off the Norwegian coast, we demonstrate how the perturbation of rock physics properties followed by the simulation of seismic data, which incorporated illumination and resolution effects, can be used to improve the accuracy of the 3D Earth model and our understanding of the reservoir.

CO2 storage in the high Arctic: efficient modelling of pre-stack depth-migrated seismic sections for survey planning

ABSTRACT The sequestration of CO2 in subsurface reservoirs constitutes an immediate counter‐measure to reduce anthropogenic emissions of CO2, now recognized by international scientific panels to be the single most critical factor driving the observed global climatic warming. To ensure and verify the safe geological containment of CO2 underground, monitoring of the CO2 site is critical. In the high Arctic, environmental considerations are paramount and human impact through, for instance, active seismic surveys, has to be minimized. Efficient seismic modelling is a powerful tool to test the detectability and imaging capability prior to acquisition and thus improve the characterization of CO2 storage sites, taking both geological setting and seismic acquisition set‐up into account. The unique method presented here avoids the costly generation of large synthetic data sets by employing point spread functions to directly generate pre‐stack depth‐migrated seismic images. We test both a local‐target approach using an analytical filter assuming an average velocity and a full‐field approach accounting for the spatial variability of point spread functions. We assume a hypothetical CO2 plume emplaced in a sloping aquifer inspired by the conditions found at the University of Svalbard CO2 lab close to Longyearbyen, Svalbard, Norway, constituting an unconventional reservoir–cap rock system. Using the local‐target approach, we find that even the low‐to‐moderate values of porosity (5%–18%) measured in the reservoir should be sufficient to induce significant change in seismic response when CO2 is injected. The sensitivity of the seismic response to changes in CO2 saturation, however, is limited once a relatively low saturation threshold of 5% is exceeded. Depending on the illumination angle provided by the seismic survey, the quality of the images of five hypothetical CO2 plumes of varying volume differs depending on the steepness of their flanks. When comparing the resolution of two orthogonal 2D surveys to a 3D survey, we discover that the images of the 2D surveys contain significant artefacts, the CO2‐brine contact is misplaced and an additional reflector is introduced due to the projection of the point spread function of the unresolvable plane onto the imaging plane. All of these could easily lead to a misinterpretation of the behaviour of the injected CO2. Our workflow allows for testing the influence of geological heterogeneities in the target aquifer (igneous intrusions, faults, pervasive fracture networks) by utilizing increasingly complex and more realistic geological models as input as more information on the subsurface becomes available.

Rock.XML - Towards a library of rock physics models

Rock physics modelling provides tools for correlating physical properties of rocks and their constituents to the geophysical observations we measure on a larger scale. Many different theoretical and empirical models exist, to cover the range of different types of rocks. However, upon reviewing these, we see that they are all built around a few main concepts. Based on this observation, we propose a format for digitally storing the specifications for rock physics models which we have named Rock.XML. It does not only contain data about the various constituents, but also the theories and how they are used to combine these building blocks to make a representative model for a particular rock. The format is based on the Extensible Markup Language XML, making it flexible enough to handle complex models as well as scalable towards extending it with new theories and models. This technology has great advantages as far as documenting and exchanging models in an unambiguous way between people and between software. Rock.XML can become a platform for creating a library of rock physics models; making them more accessible to everyone. We propose a format for digitally storing rock physics model specifications.The format is based on the Extensible Markup Language XML.It includes details about the constituents and applied theories.It ensures unambiguous documentation and can be integrated into software.It can become a platform for creating a library of rock physics models.