Improved Pulsed Neutron Capture Logging With Slim Carbon-Oxygen Tools: Methodology

Abstract

Recently, two new slim through-tubing carbon-oxygen tools were introduced and described in the literature. In addition to carbon-oxygen water-saturation capability, these new instruments can provide, during the same trip, pulsed neutron capture (PNC) measurements (formation sigma, porosity, borehole fluid salinity, etc.). The answers have improved accuracy and precision compared to prior logging instruments. This paper describes the Dual-Burst* PNC measurement scheme for the RST* Reservoir Saturation Tool, as well as the algorithm methodology for determining corrected sigma, porosity, borehole fluid salinity, log quality and other associated outputs. Also documented are database measurements on which the algorithms are based, accuracy benchmarks in industry-standard calibration facilities, and precision (repeatability) comparisons. Log examples are presented. Diffusion, borehole and lithology effects must be considered when transforming observed quantities such as decay times or near-to-far ratios to actual physical quantities. However, these effects are difficult to account for in direct analytical approaches over the entire range of oilfield conditions. Therefore, a multidimensional dynamic parameterization technique, based on an extensive set of laboratory measurements, has been developed and refined. This technique keeps the order of parameters low, resulting in a well-behaved response both inside and outside the range spanned by the database. The supporting database for each tool includes over 1000 measured points augmented with approximately 400 modeled points spanning different lithologies, porosities, borehole sizes, casing sizes and weights, formation fluid salinities, and borehole fluid salinities typically encountered in the oilfield. Also reported are the sigma and porosity accuracy benchmark measurements made in the industry-standard calibration facilities (EUROPA facility, Aberdeen, Scotland and API test pits, University of Houston, Houston, Texas, USA). Precision (repeatability) is also compared to prior PNC logging instruments, demonstrating logging speeds typically several times faster for comparable precision. Introduction Two new through-tubing carbon-oxygen (C/O) tools, the 1 11/16-in. RST tool ("RST-A") and the 2 1/2-in. RST tool ("RST-B"), were recently introduced. In addition to C/O capability, these new tools can provide, during the same trip in the hole, pulsed neutron capture (PNC) "sigma" logging with substantially improved accuracy, precision, and borehole salinity compensation compared to dedicated PNC tools presently used in the industry. PNC logging determines formation oil saturation indirectly by measuring the "absence of salinity" much in the same way an electrical log does in open hole. PNC logs measure the macroscopic thermal neutron capture cross section () of the formation, a quantity which is highly influenced by the salinity of the formation water. When the formation water is fairly saline an anomalously low sigma provides the hydrocarbon signature much like a low conductivity reading on an electrical log. Unfortunately, the ability of PNC logs to unambiguously detect liquid hydrocarbons breaks down in fresh or low-salinity environments since fresh water and hydrocarbons have nearly the same capture cross section and thus "fresh water looks like oil." Nevertheless, in most situations PNC tools can be logged at acceptable speeds with precision sufficient to detect changes in the reservoir, when run in time-lapse mode. P. 729

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

“…RST data processing involves transfer of apparent into true or intrinsic quantities using environmental corrections, for which borehole and completion data are required as input parameters (Plasek et al, 1995). In the following the main processing parameters are described.…”

“…The RST processing algorithm nevertheless assumes cemented annuli, where no fluid exchange can occur (Plasek et al, 1995). Thus, the borehole status "air" accounts for the borehole effect from the CO 2 within the well, but not for the borehole effect resulting from the CO 2 in uncemented annuli.…”

“…The CO 2 in the uncemented annuli mainly affects TPHI, which is derived from near and far detectors, whereas formation  is derived from the far detector only (Plasek et al, 1995). Nevertheless, TPHI is an input parameter for computation of the  values (Eqs.…”

“…Nevertheless, TPHI is an input parameter for computation of the  values (Eqs. (4) and (5) in Plasek et al, 1995). To account for these short-comings, the total porosity derived from the open hole logging data was additionally used as external knowledge for processing of the repeat  curves, which is equivalent to the approach applied by Müller et al (2007) for processing of a similar time-lapse RST data set from the Frio project.…”

Sensitivity study of pulsed neutron-gamma saturation monitoring at the Altmark site in the context of CO2 storage

“…The difference between the baseline and scenario 1 is in the range of typical PNG tool accuracy (0.21 c.u. ;Plasek et. al.…”