The Effect of Formation Absorption on the Thermal Neutron Porosity Measurement

Abstract

Compensated thermal neutron porosity devices provide reliable estimates of formation porosity in standard borehole conditions. Departure from these conditions perturb the measurement and it is necessary to apply corrections to determine the true porosity. One common perturbation is due to the salinity of the formation water.As the salinity of the formation water increases, its hydrogen density (or index) decreases. By itself, this would cause the apparent porosity reading to decrease, reflecting a lower hydrogen content. The additional chlorine, however, causes an increase in the thermal neutron capture cross section (E) that depresses the thermal neutron flux in the formation. This, in turn, tends to increase the apparent porosity. Empirical correction charts have been established for these two competing trends. They are sufficient for correcting porosity readings for salinity effects. However, the presence of additional thermal neutron absorbers associated with either the rock matrix or the pore fluid can result in a residual error on the porosity estimate even after salinity correction. This arises because the correlation between E and porosity, used to derive the salinity correction, is no longer valid.An analysis of the response of a compensated thermal neutron device has been made in terms of the slowing-down length, and diffusion length. It provides a simple method of describing salinity, absorption and lithological effects, and results in a predictive model of the tool response that separates neutron absorption effects from changes in hydrogen index. The model is shown to conform to established salinity corrections as well as to measurements made in laboratory formations with large values of E. It provides a means of correcting apparent neutron porosity values for variations in formation E not associated with salinity. 639

Monitoring of saturation changes and salt precipitation during CO2 injection using pulsed neutron-gamma logging at the Ketzin pilot site

“…The  CO2 value for the Ketzin reservoir conditions results in 0.014 cu. Ellis et al (1987) based on  br . Since CO 2 and NaCl contain no hydrogen, the HI for CO 2 and NaCl is zero.…”

Investigation of Neutron-Porosity Log Uncertainties: Ocean Drilling Program Hole 642E

“…Schlumberger has recognized that the CNL porosity tool, which for our purposes is identical to the CNT-G tool, exhibits such a lithology effect. To account for this effect, correction curves have been published (Ellis et al, 1987) for limestone, sandstone, and dolomite. These curves are based on a nonlinear fit to experimental and computer-generated data.…”

“…The data obtained from the CNT-G Tool do not yield immediately the true porosity, and environment corrections must be made (Gilchrist et al, 1986;Ellis et al, 1987). Other logs run in Hole 642E supplied data pertinent to the environmental correction and they provide a measure of the quality of the data obtained from the CNT-G Tool.…”

New Analytical Method To Calculate Matrix- and Fluid-Corrected Total Porosity in Organic Shale

“…Assuming water saturation in the calculation of porosity from neutron and density measurements can lead to errors of approximately 10% in the case of oil-bearing, and as high as 40% in the case of gas-bearing rocks (Alger et al 1963). An important breakthrough concerning the correction of neutron-porosity logs for presence of hydrocarbon in a clean matrix was proposed by Gaymard and Poupon (1968). The correction invoked density and HI of hydrocarbons into the mixing laws for bulk density and bulk HI.…”

“…However, Gaymard and Poupon's method assumes that thermal-neutron response can be quantified solely as a function of HI. In reality, there are other elements such as oxygen and carbon that account for nonnegligible neutronenergy losses and, therefore, have a significant impact on neutron measurements (Ellis et al 1987). Gaymard and Poupon also proposed the so-called RMS to average neutron and density porosities to approximate total rock porosity; namely,…”