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Collaborating Authors
An LWD Open-Hole Sigma for Time-Lapse Reservoir Monitoring - Case Study from South East Asia
Khairul Azhar, A B (PETRONAS Carigali Sdn Bhd) | Hassan, Nur Atiqah (PETRONAS Carigali Sdn Bhd) | Thanapala, S M (PETRONAS Carigali Sdn Bhd) | Han, Mei (Schlumberger) | Lee, Samie (Schlumberger) | Roberts, Iwan (Schlumberger)
Abstract Sigma log has long been crucial in cased-hole evaluation as it is one of a few measurements that can be made through complicated well completions. In fresh water environments, sigma log was used to discriminate gas from liquid; in some cases can differentiate water from oil, hence used subsequently for time-lapse reservoir monitoring to indicate changes in fluid type over production. A problem arises when no baseline sigma is available for time-lapse comparison. Traditionally a synthetic baseline sigma log may be artificially derived but suffers from uncertainty due to compounding of any errors in original volumetric interpretation as well as errors in the endpoint estimations. The measurement of formation sigma under open hole condition has been enabled by a downhole pulsed neutron generator (PNG), installed in a multi-function logging while drilling (LWD) tool. The relatively large LWD collar reduces significantly the influence of borehole salinity which results in accurate sigma measurement while drilling. Thus a direct sigma baseline can be obtained in open-hole without errors brought from volumetric interpretation and sigma endpoint estimation. Verification of mud filtrate invasion can be done through Multi Depth of Investigation (MDOI) sigma measurements. In order for an LWD baseline sigma measurement to be suitable for time-lapse analysis, the instrument response should be accurate, precise and repeatable by the subsequent cased-hole logging instrument. Sigma logs from both open-hole LWD and cased-hole wireline conveyed tools, over intervals of similar depth, were compared in offset wells within the same license block. Some 25 datasets from 5 producing fields in the region were examined and statistical analysis affirmed the response characteristics of both logs were emanating compatible. It confirms the data compatibility measured by both tools and supports the potential application of open-hole sigma log to serve as a baseline log for future time-lapse application.
Pulsed-neutron capture (PNC) logs are commonly used to determine formation water saturation in cased-hole environments, often for time-lapse monitoring purposes. This paper describes a new diffusion-corrected sigma algorithm developed for a pulse neutron logging tool. In southeast Kuwait, diffusion-corrected sigma log data was recorded in three wells using an array of four optimally spaced gamma ray detectors above a neutron generator. To calculate a diffusion-corrected sigma, an algorithm based on a dual exponential fit was applied to the time-decay spectrum of the near and far detectors. This calculation separates the formation and borehole decays. This approach provides an apparent formation sigma for the near and far detectors. The algorithm uses the near detector for final sigma, and a diffusion correction to the near sigma is determined by a function of a near-far sigma difference. The diffusion-corrected sigma matched the expected results and provided a good statistical qualityโeven at high sigma valuesโbecause it is based on the near detector with its higher count rate, as demonstrated in the examples presented. Also, the formation sigma was independent of different borehole conditions in which the data was recorded. The final formation sigma results were compared to volumetric results from open-hole data (volume of shale, effective porosity and water saturation) and sigma calculated from open-hole volumetric using material balance. The PNC data recorded in the three wells allowed determination of the most recent oil-water contact (OWC) and update of water encroachment maps from the time-lapse monitoring. Comparing with previous sigma data recorded in these wells, it was concluded a normalization transform is not needed because R value of the linear regression is close to 1. The diffusion-corrected sigma algorithm using dual exponential fit showed that this technique was able to extract independent values for borehole sigma and formation sigma for each detector and to perform an accurate diffusion correction. This algorithm will provide reliable sigma values regardless of the borehole conditions in which the data was recorded.
- Asia > Middle East > Kuwait > Ahmadi Governorate > Arabian Basin > Widyan Basin > Wara Formation (0.99)
- Asia > Middle East > Kuwait > Ahmadi Governorate > Arabian Basin > Widyan Basin > Greater Burgan Field > Wara Formation (0.99)
- Asia > Middle East > Kuwait > Ahmadi Governorate > Arabian Basin > Widyan Basin > Greater Burgan Field > Ratawi Formation (0.99)
- (16 more...)
Abstract Pulsed neutron capture (PNC) tools are considered a primary evaluation means in reservoir monitoring and surveillance.Systematic time-lapse PNC logging gives a detailed and dynamic picture of the reservoir saturation response to production and injection operations.The neutron capture cross section measurements offer the essential means to monitor hydrocarbon saturation, water breakthrough, and fluid contact changes in a timely manner. The recent introduction of a new generation of pulsed neutron capture logging tools has lead to a reappraisal of the design of reservoir monitoring logging programs.Efficient reservoir management requires that the latest PNC tool technology be comparable with the older measurement results to provide continuity, precision, and accuracy in the monitoring program.A new sigma processing technique was developed for the dual-burst pulsed neutron capture measurements to accomplish this objective.This technique maintains the resolution and repeatability of the new-generation tools but provides results needed for reconciliation with the older generation tools used for more than 20 years in reservoir monitoring. The sigma processing technique is described relative to the old and new generation tools, which utilize radically different approaches.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.General considerations of repeatability and accuracy are discussed which apply to PNC logs and their use in both stand-alone and time-lapse techniques. More than 30 cases have been evaluated with the new processing technique.Examples are presented that show reservoir monitoring results obtained in a wide variety of formation lithologies, formation fluids, and borehole environments to illustrate the robustness of the technique.These cases include detailed analytic studies between the PNC tools.The results indicate that the new sigma processing can provide direct time-lapse reservoir monitoring continuity and yield accurate reservoir saturation analysis. Introduction The PNC log records the thermal neutron capture cross section of the formation by measuring the thermal neutron's rate of decay.Because chlorine is the strongest neutron absorber of the common earth elements, the response of these tools is determined primarily by the chlorine present (as sodium chloride) in the formation.Since the effects of water salinity, porosity, and shaliness on the thermal neutron decay rate are similar to those of resistivity logs, the PNC log resembles the usual openhole resistivity logs and is easily correlated with them. Pulsed neutron capture tools first became available to the petroleum industry in 1968 with the introduction of the new cased-hole technology by two service companies.These tool systems indirectly measured the rate of decay of thermal neutrons in the formation from the accompanying decay in gamma rays produced as these neutrons are absorbed by the formation.The tools inject pulses of 14 MeV neutrons into the formation that, through scattering, are rapidly reduced to thermal energy levels.As these neutrons spread through the formation, they are captured by various atomic nuclei making up the formation at a rate proportional to their population.Capture events produce gamma rays, some of which are detected by the tool.As the neutron population decays, so does the number of gamma rays detected.The decay time value was originally presented on the log presentation as โTauโ. With initial work at qualitative interpretation of this data, the term โMacroscopic Capture Cross Sectionโ was devised and became what is now termed โSigmaโ of the formation.The relationship between Tau and Sigma is a fixed relationship: Sigma = 4545/Tau, where Tau is in microseconds (of decay time).In a sense, this was the first attempt at qualitative interpretation as the PNC technology was born.
Conventional resistivity-based water saturation estimates using Archie's equation depend on petrophysical parameters strongly related to reservoir rock texture, pore structure, wettability, and formation salinity. These parameters are often not readily available and can vary significantly within and across depositional sequences. At the same time, for high-angle wells, the different bed boundary effects mask the true formation resistivity only to add to the grim of saturation evaluation. This makes formation sigma particularly interesting because it offers saturation estimation independent of resistivity measurement and empirical Archie exponents. However, the method has not been extensively used as the unknown invasion effect in single-depth sigma measurement can lead to erroneous interpretation. With the availability of multiple-depth-of-investigation (MDOI) formation sigma, the latest logging-while-drilling (LWD) service provided means to invert for undisturbed formation sigma (Mauborgne et al. 2013). An invasion-free sigma and the resistivity log can be used together to simultaneously solve for the Sw and formation water salinity with a reasonable knowledge of the porosity and Archie parameters (m and n). The method solves for a salinity value that is consistent with the Sw calculated from resistivity and from sigma. This technique was used for the first time to evaluate Sw in the Gamba-sand and Dentale, a reservoir with very low resistivity. Following the success, the method was used in another field (Robin) to evaluate similar reservoirs. The technique provided accurate oil saturations in the oil leg and proved its applicability in these fields. This paper will illustrate the methodologies of deriving true formation sigma and the advanced steps for combining sigma and resistivity to derive a precise answer. The availability of LWD sigma opened new frontiers of applications beyond the conventional baseline monitoring logs. The capability of deriving true formation sigma along with the novel technique of integrating two different measurements, sigma and resistivity, to derive one single answer was the key to the success.
- Africa > Gabon > South Atlantic Ocean (0.48)
- Asia > Middle East > Saudi Arabia (0.29)
- North America > United States > Wyoming > Sweetwater County (0.24)
- Asia > Middle East > UAE > Abu Dhabi Emirate > Abu Dhabi (0.15)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.71)
- Geology > Sedimentary Geology > Depositional Environment > Continental Environment (0.47)
- Asia > Middle East > Saudi Arabia > Eastern Province > Al-Ahsa Governorate > Arabian Basin > Widyan Basin > Ghawar Field > Lower Fadhili Formation (0.99)
- Asia > Middle East > Saudi Arabia > Eastern Province > Al-Ahsa Governorate > Arabian Basin > Widyan Basin > Ghawar Field > Khuff D Formation (0.99)
- Asia > Middle East > Saudi Arabia > Eastern Province > Al-Ahsa Governorate > Arabian Basin > Widyan Basin > Ghawar Field > Khuff C Formation (0.99)
- (8 more...)
Abstract The capability of current pulsed neutron capture logging technology for determining oil and gas saturations has been investigated. The currently advancing technology provides more accurate saturation determination, leading to better reservoir management and hence more economic production than hitherto. Experiments have been carried out at our laboratory test facility with various commercially available logging tools. This has increased our understanding of the physical processes in pulsed neutron capture, such as the diffusion of thermal neutrons. The derived log value depends strongly on the way these effects are taken into account in the processing. Examples are shown of the results of processing. Examples are shown of the results of various processing techniques. The comparison between different tools has led to recommendations for field applications. In addition to the experimental work, the basic tool response (the pulsed neutron decay) has been numerically simulated. With a Monte Carlo simulator, the detector count rates were modelled for several combinations of formation and borehole contents. In addition, the influence of porosity on the tool response has been investigated. In this way, the conclusions from the experimental results were extended. The details of a number of pulsed neutron capture tools were studied. As a result of these studies, a reprocessing technique was developed for one of these tools to simulate the log that would have been obtained by its predecessor. This enables time-lapse comparisons of the logs from the new generation of tools with previously obtained logs. Introduction The advancing maturity of many oil and gas fields makes a reliable reserves estimation necessary. This includes an accurate assessment of the proportion of hydrocarbons in the reservoir fluids, i.e. the hydrocarbon saturation. One of the techniques for determining the hydrocarbon saturation around the wellbore is pulsed neutron capture (PNC) logging. PNC logging tools such as Schlumberger's TDT series are widely used in the industry. Often, these tools have been used qualitatively to distinguish between gas, oil and water or quantitatively to derive oil and gas saturations. With the progress in technology, the accuracy of the hydrocarbon saturations derived becomes gradually higher, enabling the monitoring of saturations during flooding and measurement of residual oil and gas saturations. To enable employment of the present generation of PNC tools to its full potential and to quantify their capabilities and limitations, we have studied the details of a number of present and previous tools. Experiments were carried out in the laboratory under well-defined conditions. These investigations were mainly aimed at the accuracy of the tested tools to derive hydrocarbon saturations. We have studied the effects of the borehole and of neutron diffusion, which cause the measured quantity to deviate from its true value. In addition, numerical simulations of the tool response were performed using a Monte Carlo model. The simulations performed using a Monte Carlo model. The simulations provided support to the experiments and improved our provided support to the experiments and improved our understanding of the physical processes governing pulsed neutron capture logging. pulsed neutron capture logging. P. 437