Combined Borehole Seismic and Electromagnetic Inversion For High-Resolution Petrophysical Assessment Of Hydocarbon Reservoirs

This report summarizes the work performed between January 2005 and December 2007, under DOE research contract DE-FC26-04NT15507. The project is was performed by the Center for Petroleum and Geosystems Engineering of The University of Texas at Austin and Lawrence Berkeley National Laboratory under the auspices of the National Energy Technology Office (NETL) and the Strategic Center for Natural Gas and Oil (SCNGO). During the three-year project, we developed new methods to combine borehole sonic and electromagnetic (EM) measurements for the improved assessment of elastic and petrophysical properties of rock formations penetrated by a well. Sonic measurements consisted of full waveform acoustic amplitudes acquired with monopole and dipole sources, whereas EM measurements consisted of frequency-domain voltages acquired with multi-coil induction systems. The combination of sonic and EM measurements permitted the joint estimation of elastic and petrophysical properties in the presence of mud-filtrate invasion. It was conclusively shown that the combined interpretation of sonic and EM measurements reduced non-uniqueness in the estimation of elastic and petrophysical properties and improved the spatial resolution of the estimations compared to estimations yielded separately from the two types of measurements. Moreover, this approach enabled the assessment of dynamic petrophysical properties such as permeability, as it incorporated the physics of mud-filtrate invasion in the interpretation of the measurements. The first part of the project considered the development of fast and reliable numerical algorithms to simulate borehole sonic waveforms in 2D, 3D, and radial 1D media. Such algorithms were subsequently used in the quantitative estimation of elastic properties jointly from borehole sonic and EM measurements. In the second part of the project we developed a new algorithm to estimate water saturation, porosity, and dry-rock elastic moduli jointly from borehole sonic and EM measurements. This algorithm assumed radial 1D variations of fluid saturation due to mud-filtrate invasion. Subsequently, we adapted the estimation method to interpret borehole field measurements acquired in both a shaly-sand sedimentary sequence and a tight-gas sandstone formation. In the two cases, we simulated the process of mud-filtrate invasion and concomitantly honored sonic and EM measurements. We produced reliable estimates of permeability and dry-rock moduli that were successfully validated with rock-core measurements. Finally, we introduced a new stochastic inversion procedure to estimate elastic, electrical, and petrophysical properties of layered media jointly from waveform sonic and frequency-domain EM measurements. The procedure was based on Bayesian statistical inversion and delivered estimates of uncertainty under various forms of a-priori information about the unknown properties. Tests on realistic synthetic models confirmed the reliability of this procedure to estimate elastic and petrophysical properties jointly from sonic and EM measurements. Several extended abstracts and conference presentations stemmed from this project, including 2 SEG extended abstracts, 1 SPE extended abstract, and 2 SPWLA extended abstracts. Some of these extended abstracts have been submitted for publication in peer-reviewed journals.

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