Elemental Yields and Complex Lithology Analysis From the Pulsed Spectral
Gamma Log
L. A. Jacobson and D. F. Wyatt Halliburton Energy Services, Inc., Houston,
Texas
Abstract: Induced gamma ray spectrometry, both inelastic and capture, has been established as a viable method for performing elemental analysis in a wellbore. The focus of this article is the extraction and use of elemental yields from capture spectrometry. A key step in using elemental analysis in formation evaluation is the accurate determination of concentrations of elements that can be correlated to minerals of interest. Relative elemental yields for H, Si, Ca, Cl, Fe, K, S, and Ti are obtained by weighted-least-squares (WLS) fitting of pure-element spectra to measured spectra. Pure-element spectra were obtained by stripping laboratory measurements in simple lithologies. Analyst input or multiple passes can be used to exclude elements that are not present and to improve the yield quality.
Flexible interpretation software allows log analysts to use their knowledge of the local geology to reduce interpretation uncertainties. A deterministic cased-hole analysis program is described where volume fractions of various mineralogies (sandstone, limestone, etc.) are obtained from selected elemental yields through known sensitivities and a normalization procedure. Lithologic interpretation of cased-hole Pulsed Spectral Gamma1 (PSG) logs from Alaska, South America, west Texas, and Indonesia compares satisfactorily to available openhole and core data interpretation.
Characterization of Water Saturation AlgorithmsThrough Dual-Salinity
Desaturations
Paul F. Worthington Gaffney, Cline & Associates, Alton, Hants., UK
Abstract: The scope of conventional resistivity and water-saturation measurements on reservoir core plugs is extended to encompass electrolytes other than simulated formation water. Initially, a reservoir rock is classified as Archie or non-Archie in terms of its electrical properties. Non-Archie reservoirs are those for which the non-Archie or "shale" term(s) can be highly significant relative to the Archie or "sand" term in the expression for total rock conductivity. If the reservoir is non-Archie in electrical character, a limited number of representative core plugs are subjected to desaturation and resistivity index determinations at two different salinities. At this stage, two new parameters are introduced; these are model-specific dual-salinity functions of the partially electrolyte-saturated rock conductivities. These parameters allow the determination through regression of the unknown saturation exponents in the Archie and non-Archie terms of the conductivity equation. Therefore, they characterize this equation for the reservoir in question using limited data.
Application of the method to non-Archie reservoirs has revealed that, because of the discrete nature of conventional laboratory data and within the constraints of a particular interpretative algorithm, the determined saturation exponents can show some variation with water saturation. This variation can be incorporated into the algorithm where it is significant. By its very nature, therefore, the form of the characterized algorithm is model specific.
The principal benefit of the approach is to furnish clear guidance on the most appropriate algorithm for the evaluation of water saturation from wireline logs, at an early stage in the life of a field and without materially increasing costs. In this respect, the method is proposed to have a widespread application. Its adoption should enhance the prospects of improved quality assurance in petrophysics, specifically in the form of agreement be-tween log-derived water saturations and those determined directly by extraction of fluids from core.
The Effects of Rock Anisotropy on MWD ElectromagneticWave Resistivity
Sensors
Michael S. Bittar and Paul F. Rodney Sperry-Sun Drilling Services, Houston,
Texas
Abstract: Theoretical modeling and field examples have shown that rock anisotropy affects the phase shift- and attenuation-derived resistivities of an MWD electromagnetic wave resistivity sensor differently in horizontal and deviated wells. These effects depend on the relative dip angle and the ratio between the vertical and the horizontal resistivities (coefficient of anisotropy) of the rock. When a propagating wave resistivity tool is perpendicular to the bedding plane (relative dip angle = 0ø), both the phase shift and attenuation measurements respond only to the horizontal resistivity of the rock. For a small relative dip angle (less than 45ø) the difference between the phase shift- and attenuation-derived resistivities is very small. This difference becomes significant for relative dip angles greater than 50ø, and it is very large in horizontal wells (relative dip angle = 90ø)
In the absence of invasion, rock anisotropy can be identified by the difference between the phase shift- and attenuation-derived resistivities for the single-spacing MWD resistivity sensor or from the difference between the phase shift-derived resistivities from different spacings for multiple-spacing MWD resistivity sensors. Also, rock anisotropy can be identified by the phase shift-derived resistivity from a single-spacing MWD resistivity sensor operating at different frequencies. Values of the horizontal and vertical resistivities of the formation can be determined from the phase shift- and attenuation-derived resistivities for a single-spacing tool. In addition, the dip of the formation can be determined from a minimum of three resistivity measurements of a multiple-spacing or a multiple-frequency tool.