Exploration and Production Technologies
Offshore Oil Field Characterization with EM Methods 

P-301 

Program

This project was funded through DOE's Natural Gas and Oil Technology Partnership Program. The Partnership Program establishes alliances that combine the resources and experience of the nation's petroleum industry with the capabilities of the national laboratories to expedite research, development, and demonstration of advanced technologies for improved natural gas and oil recovery.

Project Goal
The goal is to develop a high-temperature acoustic telemetry tool for the geothermal industry, and construct a low-temperature version of the geothermal tool capable of operating at raw baud rates up to 100.

Performer
Sandia National Laboratories 
Albuquerque, NM

Project Results
Building on SNL's 3D electromagnetic (EM) modeling software, a matrix-free finite difference solution is now available for the marine magnetotelluric (MT) problem. A series of model studies was conducted that computed the synthetic MT response of the Gemini Salt Structure, whose spatial extent was inferred from 3D seismic data provided by Texaco.

The key issue is to assess the ability of the MT method in characterizing the base of salt. Testing of the finite element (FE) code showed favorable agreement between FE solutions and those derived from analytic formulae for simple problems. Preliminary results emphasize the continued need for close collaboration between instrumentation development, data analysis, and numerical modeling efforts. To address the issue of bathymetric effects on seafloor MT response, a 2D finite element code was developed, also utilizing the matrix-free paradigm for enhanced computational efficiency.

Benefits
This new 3D forward engine with its unstructured mesh and mathematically rigorous foundation can immediately address questions of target resolution, the role of bathymetry, and optimal experimental design. In the short term, the code will be put to use in the analysis of data collected using the Scripps instrumentation (e.g. the Gemini data). The long term industrial requirement of a seafloor electromagnetic method is the development of a 3D conductivity model to assist in hydrocarbon evaluation. The algorithm can be readily integrated into a future subsurface 3D conductivity-imaging package to provide this product.

Background
Recent years have seen a dramatic increase by petroleum companies in marine electromagnetic methods. This was motivated by the need to explore areas where seismic methods perform poorly, such as sub-salt, sub-basalt, carbonate terrains, and by the potential to use controlled source EM methods (CSEM) to map gas hydrate reserves. 

While there are many options for 2D MT modeling and inversion, there are serious limitations on access to and design of the very few 3D codes in existence. Furthermore, we note that CSEM and MT data have complementary resolutions to geological structure and may also be efficiently collected together as part of the field operation. Thus, there is a clear need for a joint CSEM/MT modeling and inversion capability. This task is greatly facilitated by using the same underlying algorithm design. 

Project Summary
Project researchers propose to develop a modeling code based on a fully 3D finite element analysis for marine geophysical electromagnetic exploration. The project has many components of the proposed package already in place, such as an existing unstructured tetrahedral mesh generator with local refinement capabilities for detailed representation of bathymetry and sub-seafloor heterogeneities. However, several new features are required for this application.

A major task of this research is thus to implement a new hybrid edge-element approach. Specifically, the formulation involves modification of the hybrid A-ª method developed by electrical engineers at Graz University, Austria. A common framework for planewave (MT) and dipole (CSEM) excitations will be adopted and an efficient matrix-free QMR solver will be used. While these design features enable the code to operate efficiently on a single desktop workstation, the entire algorithm can be readily ported to inexpensive distributed-memory parallel computing clusters now commonly found within the exploration groups of petroleum companies.

Current Status (August 2005)
Project is complete.

Project Start: March 27, 2002
Project End: March 26, 2004

Anticipated DOE Contribution: $210,000
Performer Contribution: $340,000 (62 % of Total)

Contact Information
NETL - Rhonda Jacobs (rhonda.jacobs@netl.doe.gov or 918- 699-2037)
SNL - David Borns (djborns@sandia.gov or 505- 844-7333)


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