Applications and Derivation of a New Cased-hole Density Porosity in Shaly Sands
Authors: Odom R.C. , Hogan G.P. , Crosby B.W. , Archer M.P. 
Published: December 2007

Applications and Derivation of a New Cased-hole Density Porosity in Shaly Sands R.C. Odom, SPE, Computalog Research, Inc.; G.P. Hogan II, SPE, Computalog Wireline Services; B.W. Crosby, SPE, Computalog Wireline Services; M.P. Archer, Chevron U.S.A., Inc. Copyright 1997, Society of Petroleum Engineers, Inc.

Recently, the physical foundations and derivation of a cased-hole, density-based porosity have been developed for the Computalog PND-S pulsed neutron system. The utility of this measurement is demonstrated in applications to reservoir analysis problems in sand-shale sequences of the Gulf Coast and offshore Gulf of Mexico. This cased-hole density porosity is based on the attenuation of gamma rays produced by inelastic scattering of fast neutrons. These fast-neutron reactions create a dispersed gamma-ray source in close proximity to the accelerator, and the subsequent transport of these gamma rays is strongly affected by the density of the formation. The higher energies and larger geometric scales of this technique give it sufficient penetration to measure a cased-hole density porosity that compares favorably to the open-hole, gamma-gamma density.

Applications (in combination with the cased-hole neutron porosity) include providing porosities where hole conditions make open-hole logging unviable, and in old wells where no modern porosity logs exist or the data is of questionable quality. This measurement can be utilized in cased-hole prospecting by differentiating low porosity from gas-filled porosity. Reservoir monitoring applications include: hydrocarbon typing, estimating pressure changes in gas reservoirs, and monitoring fluid level and hydrocarbon type changes.

Following discussions of the derivation and possible applications, several log examples from the Gulf Coast and offshore Gulf of Mexico demonstrate the uses of the system and the correlation with the conventional open-hole density porosity.

Introduction

The density-neutron crossplot porosity has long been the workhorse of the log analyst. The strength of this union lies in the fact that these two porosities have roughly equal and opposite errors with respect to shale content and gas content. When these measurements are used together, a more accurate estimate of the porosity is obtained. The overlay of these porosities on the well log can be used to visually estimate the shale and fluid content of the logged formations.

Traditionally, in cased-reservoir analyses, the main source of porosity is the hydrogen-based neutron porosity. The neutron radiation is capable of penetrating the well casing and cement and affecting measurable reactions in the formation. The spatial distribution of thermal neutrons (near-to-far ratio) is related to the hydrogen content of the formation. Given the assumptions that the rock matrix does not contain hydrogen and the pore fluid is water, the hydrogen content is mapped into a porosity measure. Of course, these assumptions create errors in formations with shale in the matrix and/or gas in the pore fluid.

In order to have sensitivity to gas-filled porosity versus low porosity (gas vs. tight), current versions of the competitive pulsed neutron systems in the Gulf Coast market have incorporated detector counting bins during the source pulse to measure gamma rays created by inelastic scattering. The count rate during the source pulse is placed in simple ratio algorithms, and qualitative empirical relations are developed. In previous publications, it has been demonstrated that the formation density can be isolated and measured using gamma rays created from inelastic scattering of fast neutrons. This paper describes the continuous process of making this a more accurate and quantitative measurement. In the following section, we will briefly describe the unfolding processes required to measure the effects of the formation density on the received inelastic signal. Currently, the extracted density parameter undergoes a scalar mapping to normalize the well-bore effects. The normalization is facilitated by calibration checks to core data, offset wells or local sections with open-hole data. The end result is a density porosity (PN density) similar to the open-hole, gamma-gamma density porosity (OH density). In the Characterization of Measurements section that follows is a discussion of the development of a prototypical borehole compensated (BHC) algorithm. In this BHC algorithm the measurements of borehole parameters are enfolded, so that external calibration data is not required.

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