Pulsed Neutron Logging Research Articles

Machine learning and deep learning for mineralogy interpretation and CO2 saturation estimation in geological carbon Storage: A case study in the Illinois Basin

Carbon capture and storage (CCS) is a promising approach to simultaneously maintaining energy security and reducing carbon dioxide (CO2) emissions under the current energy portfolio that is dominated by fossil fuel energy. Pre-injection formation characterization and post-injection CO2 monitoring are two critical tasks to guarantee storage efficiency in CCS. The CCS projects in the Illinois Basin, the first large-scale CO2 injection into saline aquifers in the United States, employed conventional and the latest pulsed neutron logging (PNL) tools for mineralogy interpretation and CO2 saturation estimation, which provide valuable references for future CCS projects. Because of the inherent fuzziness of petrophysical measurements and complex subsurface heterogeneity, interpreting well-logging data is time-consuming, and its accuracy can be user-biased. In recent years, data-driven methods have been widely used to capture the non-linear patterns between input features and interpretation results. This work applied and evaluated four commonly used machine learning (ML) models, including ridge regression (RR), random forest (RF), gradient boosting regression (GBR), support vector regression (SVR), and one deep learning (DL) model, the artificial neural network (ANN). We optimized the hyperparameters of the four ML models and the DL model using the simulated annealing algorithm and the grid search strategy, respectively. The input features of the mineralogy interpretation models were eleven conventional well-logging parameters, and the label data (i.e., ground truth) were the porosity and volumetric fractions of six minerals, including quartz, feldspar, dolomite, calcite, clay, and iron minerals. The results demonstrated that the GBR and RF models were superior in predicting volumetric fractions of minerals and porosity; label data with low coefficient of variation (CV) values tended to yield better performance. For CO2 saturation estimation, the RF was the best-performing model, followed by SVR, ANN, GBR, and RR. Furthermore, we conducted feature importance ranking using the permutation importance algorithm and found that the formation sigma and well pressure were the most important features in this study. The study of CCS projects in the Illinois Basin bridges the gap between the limited knowledge and understanding of geological carbon storage and the increasing demand for reliable, cost-effective, and sustainable energy solutions.

Time-Lapse Pulsed-Neutron Logs for Carbon Capture and Sequestration: Practical Learnings and Key Insights

Pulsed-neutron logs are a staple of time-lapse monitoring programs for saline aquifer carbon capture and sequestration (CCS) projects and are unsurprisingly the most frequently run wireline logs in both injection and monitoring wells. While the emphasis imposed by government regulators and the focus of operators to date has been on the verification of CO2 containment, it is envisioned that a savvy interpretation of the multiple independent measurements should be able to unlock much greater value for the project than merely detecting the location of stored CO2. Recently introduced capabilities for novel measurements and improved environmental compensation should further increase the repeatability, interpretability, and value of these logs. We reviewed more than 30 time-lapse runs of pulsed-neutron logs acquired over a period of 15 years on three mature CCS projects using both previous- and new-generation pulsed-neutron tools, including measurements of formation sigma, hydrogen index, and fast neutron cross section. Special attention in processing is required when changes occur to the wellbore environment between runs, although this is mitigated by the improved environmental compensation scheme of the newer tool. We performed both standalone estimates of CO2 saturation from single-physics time-lapse measurements and simultaneous interpretation of multiple independent time-lapse measurements and studied the results side-by-side with openhole log interpretation, core analysis, and well test results from the evaluation phase. The apparent changes in saturation were framed within the context of the injection history and important events in the life of the wells. A first finding is that differences in apparent CO2 saturation between the various independent measurement physics of the pulsed-neutron tool are often reconcilable and may carry additional information about the state of the well or reservoir. With respect to verification of containment, depending on the well configuration, it may be possible to differentiate between CO2 in the formation and CO2 in the annulus. The interpreted CO2 saturation itself can have different significance depending on the timing of acquisition and the type of well. Measured at the right time, it is a direct in-situ measurement of formation CO2 storage efficiency. In other cases, the interpretation reveals formation dryout in the near-wellbore region of injection wells, a condition that may presage loss of injectivity. We now understand that it is important for operators to plan the timing and frequency of pulsed-neutron runs according to what they want to measure and not based solely on regulatory obligations. In a CCS project, time-lapse pulsed-neutron logs should be thought of as much more than simple indicators of the presence and migration of CO2. They give important information about migration pathways. They can also help to quantify essential uncertainties on reservoir performance that are difficult to ascertain during evaluation. For example, storage efficiency is difficult to quantify with openhole logs since the formation is initially at zero CO2 saturation. Yet it is one of the keys to determining the ultimate storage capacity of any reservoir. Time-lapse pulsed-neutron logs provide an abundance of information that, when properly history matched, can greatly improve our models of CCS reservoirs to better navigate both the economic and operational risks associated with these projects.

A new neutron-gamma porosity measurement method for pulsed neutron logging tools

As an alternative method for the compensated neutron porosity measurement method, the neutron-gamma porosity measurement method is widely used in cased holes. However, the neutron-gamma porosity measurement method has some problems, such as the lack of detailed theoretical analysis and low sensitivity in high-porosity formations. The purpose of this paper is to clarify the theory of neutron-gamma porosity measurement, improve the measurement sensitivity, and eliminate the effects of environmental factors. Based on the neutron diffusion theory and the gamma transport theory, the distribution of capture gamma rays is described. A new neutron-gamma porosity measurement method is developed based on inelastic and capture gamma-ray distribution. The applicability of the new method in different environments is analyzed using the Monte Carlo method, and the effectiveness of this method is verified using the synthetic model. The results indicate that the capture gamma-ray ratio is only related to the source-detector distance, gamma-ray attenuation length, and neutron migration length. By combining density, different neutron cross sections, inelastic gamma-ray count ratio, and capture gamma-ray count ratio, the parameter secondary gamma-ray hydrogen index (SGH) related to hydrogen index can be extracted. The dynamic range of SGH is much higher than thermal neutron count ratio and the capture gamma-ray count ratio. The errors in porosity calculation by this method under different borehole and lithologic conditions are generally less than 2 pu. The calculation error of this method is close to that of the compensated neutron porosity measurement method. In addition, the applicability of the new method is analyzed without density data, and the results indicate that the calculation results are basically consistent with the results of calculation with density data. The new method has great potential for extensive applications in cased holes or open holes as an alternative for the compensated neutron porosity logging measurement method.

A study on characteristics of background gamma spectrum from LaCl3 detector in pulsed neutron logging

The scintillator detectors such as LaCl3(Ce) play an important role in some fields of scientific research, environment, safeguards, medicine, security and industry due to its superior energy resolution and exceptional luminescence properties, etc. However, Cl element in a LaCl3 crystal produces uncertainty of determining oil saturation in pulsed neutron logging because of the background spectrum caused by secondary gamma ray from the reaction of Cl nuclei with the neutron. In this paper, we employed Monte Carlo method to simulate secondary gamma ray generated LaCl3 crystal induced by thermal neutron with different borehole and formation conditions and establish a reference spectrum of Cl element. The relations between elemental window or peak areas counts and borehole and formation conditions were also investigated. The background was obtained by combining the reaction rate derived from thermal neutron capture cross section for Cl element and neutron flux with the reference spectrum. The results indicate that the contribution of secondary gamma ray to measuring spectrum decreases with formation porosity, limestone content, borehole diameter, and water salinity increasing. Nevertheless, the relative peak areas of Cl at different energies remain constant, indicating that the logging conditions have less of an effect on the background spectrum shape. As evidenced by the measured spectra in the sandstone and limestone calibration wells processed, the peaks of Si and Ca elements are enhanced while the peaks of Cl element are weakened. After subtracting detector background, the computations of oil saturation based on calibration wells are 38% more accurate than the original method.

Study on Monitoring Reservoir Gas Density by Pulsed Neutron Logging

In mature multilayer gas reservoirs, it is a common problem to evaluate whether a potential reservoir is still productive after casing. Monitoring of unproduced or developing gas formations can guide production action plans. Intervention in unperforated zones with high gas density can increase gas production, whereas intervention in zones with low gas density is uneconomical. An RPM instrument stratum model was established by using the Monte Carlo method in this study. The model was used to simulate the response characteristics of pulsed neutron logging in a sandstone reservoir with 100% gas saturation to different reservoir gas densities, formation porosity, and shale content. The data obtained from the simulation were interpolated to construct a capture gamma ratio plate and a nonbounce gamma ratio plate. Using the constructed chart, the gas density of the reservoir can be quantitatively calculated, and then, the recovery value of the potential gas layer can be evaluated.

КОМПЛЕМЕНТАРНОСТЬ МЕТОДОВ ИМПУЛЬСНОГО НЕЙТРОННОГО КАРОТАЖА И ЭЛЕКТРИЧЕСКОГО ДИВЕРГЕНТНОГО КАРОТАЖА В ОБСАЖЕННЫХ СКВАЖИНАХ

In production wells, nuclear-physical and electrical logging methods are used to study the backwater space. More well-known and widespread is pulsed neutron logging (PNL) [16] and its modifications. Electric divergent logging (EDC) is considered less common [2] in cased wells. Both methods solve the same problem, but they are still not ideal and have their drawbacks. This article presents the advantages and disadvantages of the PNL and EDC methods in comparison with each other. A significant criterion when choosing the preferred method for determining the current oil and gas saturation is the minimum error of the output results of the study by the method. The operating conditions of a particular deposit must be taken into account. When choosing the optimal method for determining the current oil and gas saturation from the point of view of the minimum error, the operating conditions of each specific field should be taken into account. The best results, however, will obviously be achieved by combining the above methods, which radically differ significantly both in their physical characteristics and in the principles of measuring nuclear physical and electrical parameter.

РЕЗУЛЬТАТИ ГЕОФІЗИЧНОГО МОНІТОРИНГУ РОЗРОБЛЕННЯ ГОРИЗОНТУ ПК-2 ПІДКИРМАКІНСЬКОЇ СВІТИ РОДОВИЩА НАФТИ ДАШЛАРИ

The article is devoted to the study of the current state of development of the second horizon of the Podkirmakinsky suite (PK-2) of the Productive stratum (PS) of the Neft Dashlary field, which has been in operation for more than 60 years. The results of the study by the method of complex areal interpretation of geophysical survey data of wells and geological and field information are presented. Three-dimensional geo models, built on the basis of petrophysical parameters obtained after the interpretation of geophysical and geological data using the DV Seys Geo program are given in the article. The results of the analysis of the state of water impact are presented, the values of the oil saturation factor during well drilling are summarized, the values of the current oil saturation factor are calculated taking into account the extraction of oil, the results of studying the nature of saturation of reservoirs in the context of production wells by the method of pulsed neutron logging (PNL), as well as a comparative analysis of these values along the horizon PK-2 of the PS of the Neft Dashlary field.

An Integrated Petrophysical Characterization of a Siliciclastic Tight Gas Reservoir in Neuquén Basin, Western Argentina

The Río Neuquén Field is located between Neuquén and Río Negro provinces, Argentina. Historically, it has been a conventional oil producer, but it was converted to a tight gas producer from deeper reservoirs. The targeted geological formations are Lajas, which is already a known tight gas producer, and the less-known overlaying Punta Rosada Formation, which is the main objective of the current work. Punta Rosada presents a diverse lithology, including shaly intervals separating multiple stacked reservoirs that grade from fine-grained sandstones to conglomerates. The reservoir pressure can change from the normal hydrostatic gradient to up to 50% of overpressure. There is little evidence of movable water. The key well in this study has a comprehensive set of openhole logs, including pulsed-neutron spectroscopy data, and is supported by a full core study over 597 ft. Additionally, data from several offset wells were used, containing sidewall cores and complete sets of electrical logs. This allowed the development of rock-calibrated mineral models, adjusting the clay volume with X-ray diffraction data, porosity, and permeability with core measurements, and linking the log interpretation to dominant pore-throat radius models from MICP Purcell tests. Several water saturation models were tested, attempting to adjust the irreducible water saturation with NMR and Purcell tests at reservoir conditions. As a result, three hydraulic units were defined and characterized, identifying a strong correlation with lithofacies observed in cores and image logs. A cluster analysis model allowed the propagation of the facies to the rest of the wells (50). Finally, lithofacies were distributed in a full-field 3D model, guided by an elastic seismic inversion. In the main key well, in addition to the openhole logs and core data, a casedhole pulsed-neutron log (PNL) was also acquired, which was used to develop algorithms to generate synthetic pseudo-openhole logs such as bulk density and resistivity, integrated with the spectroscopy mineralogical information and other PNL data, to perform the petrophysical evaluation. This enables the option to evaluate wells in contingency situations where openhole logs are not possible or too risky, and also in planned situations to replace the openhole data in infill wells, saving considerable drilling rig time during this field development phase. Additionally, the calibrated casedhole model can be used in old wells. This paper explores the integration of different core and log measurements and explains the development of rock-calibrated petrophysical and rock type models addressing the characterization challenges found in tight gas sand reservoirs. The results of this study will be crucial to optimize the field development.

A method of monitoring gas saturation in carbon dioxide injection heavy oil reservoirs by pulsed neutron logging technology

A method is proposed to characterize the fast neutron scattering cross-section (σf) quantitatively by the combination of inelastic gamma rays and captured gamma rays, so as to realize the gas saturation evaluation of CO2-injected heavy oil reservoirs based on the three-detector pulsed neutron logging technology. Factors influencing of the evaluation effect of this method are analyzed and the effectiveness of this method is verified by a simulation example. By using the Monte Carlo simulation method and the physical model of bulk-volume rock, the relationship between σf and CO2 saturation is studied, and the saturation interpretation model is established. The influences of formation temperature and pressure, heavy oil density, borehole fluid and reservoir methane content on the evaluation results of CO2 saturation are analyzed. The results show that the characterization of σf by the combination of secondary gamma information can eliminate the influence of formation lithology, borehole fluid and methane content are the main factors affecting the quantitative monitoring of CO2 saturation, and the effects of formation temperature and pressure and heavy oil density are negligible. The simulation example verified the feasibility of the method for evaluating the CO2 saturation of CO2-injected heavy oil reservoirs.

Research on recognition of gas saturation in sandstone reservoir based on capture mode

Pulsed neutron technology has been widely used to monitor the remaining oil saturation of the formation. In order to solve the problem of the remaining oil saturation and porosity of the reservoir after casing, and to study the condition of the reservoir after water flooding, the pulsed neutron logging tool with the same technology has been recommended and used in China. Most of these instruments have many measurement modes such as inelastic, capture, carbon–oxygen ratio, and gas-view, and these can measure a large amount of stratum information. By studying the influencing factors of the capture mode in sandstone reservoirs, it can be used to research the methods of gas reservoir identification and quantitatively evaluate the gas saturation. This paper studies the detection principles, modes, advantages, and disadvantages of pulsed neutron logging tools. The principle of gas recognition is analyzed in the capture mode, and the formation model is established based on the Monte Carlo method. The common influencing factors are studied, and the sensitivity of detection is also analyzed. It realizes the identification of gas layers by the combined detector, as well as the calculation of gas saturation in comprehensive logging interpretation. The results show that the source distance of the detector affects the detection sensitivity, and the sensitivity of the near-ultra-far detector combination is better. As the porosity increases, the sensitivity of the detector increases, and the capture count ratio also increases significantly. Based on the same pore conditions, the higher the gas saturation, the lower is the capture gamma ratio. The shale content is less sensitive to the capture gamma count ratio, and it can still be concluded that the higher the mud content, the lower the capture count ratio. The conclusions can be used in the research method of gas formation identification and can also be used to accurately explain and quantitatively analyze the gas saturation of the formation.

Current Status and Development Trend Analysis of Neutron Logging in Uranium Mines in China

Neutron logging is a method of measuring uranium fission neutrons by bombarding uranium in ore beds with neutrons emitted by neutron sources or neutron generators. The pulsed neutron logging technology of uranium ore has been popularized and applied in the United States, Russia, Australia and other countries. The key technology is almost monopolized by these countries, but the technology is prohibited in China. In the field of uranium logging, natural gamma logging has always been used in China for uranium quantification, which is characterized by large error, low efficiency, high cost and technical and economic bottlenecks. Due to the fact that the research and application of pulsed neutron log in the field of petroleum exploration and production is relatively mature in China, it is urgent to develop relevant logging instruments and uranium quantitative interpretation methods as independent intellectual property rights. This will lead to a change in the backward appearance of uranium logging. This paper analyzes the research status and development trend of neutron logging in uranium at home and abroad.

Water-Shutoff Technique Extends Productive Life Cycle of Cretaceous Sandstone

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 199070, “Water-Shutoff Technique Extends the Productive Life Cycle of Cretaceous U Sandstone: The Iro Field Case in Ecuador,” by Luis Roberto Bailón, SPE, Ney Holger Orellana, SPE, and Santiago Villegas, Repsol, et al., prepared for the 2020 SPE Latin American and Caribbean Petroleum Engineering Conference, originally scheduled to be held in Bogota, Colombia, 17-19 March. The paper has not been peer reviewed. The water-shutoff technique is used in some wells of the U reservoir in the Iro field of the Oriente Basin in Ecuador as a remediation plan to restore production after an early water breakthrough. The production historical data, workovers, and sand-body correlation of wells are compared to understand reservoir behavior, shale-baffle-sealing continuity, the existence of different sand units, and the effect on production. Introduction The Iro field is in the south of Block 16. Production began in March of 1996. Iro is considered a mature field that produces heavy crude oil. The U sand-stone reservoir at Iro field is constituted by quartz grains subtransparent with fine grain sizes to medium, moderately classified, occasionally clay-like matrix. A thin limestone layer subdivides the U sandstone reservoir into two main stratigraphic units, Upper U and Lower U sandstone. Logging acquisition during the drilling campaign revealed heterogeneous sand-body deposition throughout the field. Depositional features of fluvial channels are developed from the base of the reservoirs, which are overlaid by sand bars. In addition, interbedded shale layers and baffles are present in the U reservoir, in some cases locally. However, the main shale layers are effective seals when they subdivide the Upper and Lower U sandstone units into two or more subunits. A good example is the shale layer that separates channels and bars in the Lower U sandstone unit. This identification was possible after the development of the well-drilling campaign, well correlation, and years of production behavior. Two subunits of the Lower U reservoir, Ui1 and Ui2, were classified as a result of the acquired data. Cased-Hole Logs (Pulsed Neutron) Given the maturity of the fields, during the last 2 years, a logging campaign of pulsed-neutron cased-hole logs has been performed. In the case of the Iro field, pulsed-neutron logs were run in six wells; three of these reached the Lower U reservoir. These three wells have a good correlation between the analog density and neutron curves of the cased-hole and the original openhole curves, providing certainty in the reading of the tool. The logging program obtained data of chemical-element spectra in capture and inelastic modes. This information was processed and analyzed to derive hydrocarbon saturation. In this way, by-passed oil can be identified as well as reservoir zones already drained by production of the same wells or by neigh-boring wells. Data in the Lower U reservoir show fluid movement.

A novel borehole/annulus holdup calculation method based on pulsed neutron logging

Pulsed neutron logging tool (PNL), particularly small-diameter multifunction PNL, is an essential instrument used in casing well for evaluating residual oil saturation. In a borehole with a multiple casing string structure, PNL tools log in the oil tube and even the space between the casing and oil tube. Borehole/annulus holdup have a significant influence on the evaluation of oil saturation. Hence, the requirement of the estimation of borehole/annulus liquid holdup is highlighted. This study proposes a new method to determine the annulus oil holdup in a casing well with a complex string structure. For this method, the annulus holdup are derived from ΔS, which is defined as the difference between the relative variations of carbon/oxygen and carbon/hydrogen ratios. To demonstrate the feasibility of the proposed method, we first simulate the responses of ΔS under various borehole and formation conditions using the Monte Carlo method. The outcome shows that ΔS covaries with annulus oil holdup, confirming the feasibility of the proposed method in theory. The proposed method is applied to real data measured in an offshore production well. The result confirms that the formation water saturation calculated by the proposed method agrees well with the actual well production status, further confirming that the proposed method has a promising application in residual oil dynamic monitoring.

New Generation of Pulsed-Neutron Multidetector Comparison in a Challenging Multistack Clastic Reservoir: A Case Study in Brown Field, Malaysia

Running pulsed-neutron logs in Malaysia has previously been plagued by results with high uncertainties, especially in brown fields with complex multistacked clastic reservoirs. Together with a wide range of porosities and permeabilities, the acquired logs quite often tended to yield inconclusive results. In addition, the relatively fresh aquifer water (where salinity varies from 5,000 to 40,000 ppm) makes reservoir fluid typing and distinguishing between oil and water even more challenging. As a result, the inconsistencies and uncertainties of the results tend to leave more questions than answers. Confidence in using pulsed-neutron logging, especially to validate fluid contacts for updating static and dynamic reservoir models, deteriorated within the various study teams. Due to this fact, the petrophysics team took the initiative to conduct a three-tool log off in one of their wells with the objective of making a detailed comparison of three pulsed-neutron tools in Malaysia’s market today. The main criteria selected for comparisons were the consistency of the data, repeatability, and statistical variations. With recent advancements in pulsed-neutron (multidetector) tool technology, newer tools are being equipped with more efficient scintillation crystals, improving the repeatability of the measurements as well as the number of gamma ray (GR) count rates associated with the neutron interactions. In addition, the newer tools now have up to five detectors per tool, with the farthest detector supposedly being able to “see” deeper into the formation, albeit at a lower resolution. With these new features in mind, the log off was conducted in a single well with a relatively simple completion string (single tubing, single casing), logged during shut-in conditions only, and the logs were acquired directly one after the other (back to back) to avoid bias to any particular tool. Both sigma and spectroscopy measurements were acquired to compare the capabilities of each tool. Due to the relatively freshwater salinity, the carbon-oxygen (C/O) ratio from the spectroscopy measurements is used to identify the remaining oil located in the reservoirs, while the sigma measurements determine the gas-oil or gas-water contact, if present. This paper will illustrate the steps taken by Petronas Carigali Sdn Bhd (PCSB) to compare the raw data and interpreted results from the three pulsed-neutron tools. Consequently, a comparison from all the tools was made to the current understanding of the reservoir assessed. The points from these comparisons will then show which tools are favored over the rest.

Multidetector Pulsed-Neutron Tool Application in Low-Porosity Reservoir–A Case Study in Mutiara Field, Indonesia

In mature fields, pulsed-neutron logging is commonly used to solve for the remaining saturation behind the casing. For years, sigma-based saturation has been used to calculate gas saturation behind casing; however, the high dependency of sigma-to-water salinity of the formation, especially the low-dynamic range at porosity near 12 p.u., has proven to be challenging in low-porosity gas rock. A new measurement from the third detector from a multidetector pulsed-neutron tool (MDPNT) is proposed to provide a better estimation of the gas saturation in a low-porosity reservoir. Two sets of independently measured sigma and the third detector were taken in a casedhole well, with a dual-tubing system of a long string and short string. For the third-detector measurement, the measurement was based on the ratio of the slow capture gate and inelastic gate component from the decay curve created by the long detector. This ratio can be used to detect gas in a tight reservoir with a minimum salinity and lithology effect. This data will then be used to calculate the gas saturation from the third detector, and the result is compared to sigma-based gas saturation. At an interval where the porosity is above 12 p.u., the sigma-based gas saturation and MDPNT-based gas saturation are very much in agreement. However, in a low-porosity reservoir near 12 p.u. or below, the sigma-based measurement starts to show its limitation. Meanwhile, the MDPNT-based gas saturation clearly shows the remaining gas saturation where sigma-based measurements failed to detect it. The subsequent decision was made based on the log analysis result, and perforation was done at a potential interval based on the MDPNT result. The results from the production test confirm the MDPNT-based gas saturation with 700-Mscf/d gas production added. This study showcases a new technology to solve a low-porosity gas reservoir issue where a sigma-based measurement underestimates the remaining gas saturation. Using two different measurements in the same well, the results from the MDPNT measurement demonstrated a better result compared to the sigma-based measurement in low-porosity rock

Maximizing the Value of Pulsed-Neutron Logs: A Complex Case Study of Gas Pressure Assessment Through Casing

Casedhole logging for formation evaluation and input to determine the redevelopment potential of an oil producer with a challenging production history was conducted. This included an intelligent assessment of formation gas pressure through casing, which was later confirmed by perforating. The target reservoirs are Triassic sandstones drilled as a gas exploration prospect. Based on openhole log data, the prospect appeared to be oil bearing. The well has been producing oil for several years and is now a candidate for a gas cap blowdown. The presence of heterogeneous layers with varied rock quality and producibility indexes coupled to complexity in fluids distribution and zonal isolation issues complicates the development process and ability to optimize recovery from any contributing level. Recently, a new-generation casedhole formation evaluation tool that provides multiple independent formation property measurements was deployed to enhance knowledge of the formation parameters while describing the current gas and oil volumes. Sigma, neutron porosity, fast-neutron cross section (FNXS), and elemental concentrations, including total organic carbon from inelastic and capture spectroscopy, were simultaneously recorded. Because the well is highly deviated in the zones of interest, the tool was efficiently conveyed on wireline using tractor technology. The evaluation techniques used to study this rich set of data reveal several pieces of information that are essential to the petrophysicists and geologists, and to the reservoir and production engineers. A multimineral solver analysis guided by the prior knowledge of the rocks using cores from offset wells was conducted to quantify the porosity and gas-oil contact levels while giving access to detailed knowledge of matrix and rock composition for refining the reservoir models. Additionally, a novel method to determine gas pressure at the current time from the casedhole log measurements was applied to support reservoir management. The highly sensitive sigma, neutron porosity, and FNXS gas properties can be parameterized as a function of pressure and temperature if the formation and fluid properties are known. This is a well-established principle that can finally be applied independently and directly to multiple measurements. The computations are done independently and checked against each other for consistency and to support optimal parameter setting, in an iterative manner. This is particularly important in this scenario where the complexity of the wellbore environment and history of the well could have complicated the ability to achieve enough precision on the estimated pressures when coming from a single method or if modeling is required. The log results, with validation, and implications for the well redevelopment are presented together with a general discussion on the methodology and applicability based on this well experience. The importance of meticulous job preparation, prejob modeling, and data quality control is also highlighted. The information is key to defining the future well development management strategy and clarify the role that casedhole logging can play as part of the complete well evaluation process.

How well can time-lapse seismic characterize a small CO2 leakage into a saline aquifer: CO2CRC Otway 2C experiment (Victoria, Australia)

Injection of CO2 into brine-saturated reservoir rocks reduces their elastic moduli and thus changes the seismic response. However, estimation of the stiffness reduction and 3D plume morphology have large uncertainty for regular Signal-to-Noise Ratio (SNR) and amount of prior geological information. This paper examines achievable accuracy of the time-lapse seismic inversion based on Stage 2C of the CO2CRC Otway Project, which was specifically designed to test the sensitivity of seismic monitoring. Thanks to rich geological dataset, we could build a set of adequate subsurface models to optimise the inversion workflow and assess its capability. Firstly, 1D stochastic simulations and analytical models are used to estimate the effects of imperfect repeatability and limited bandwidth of the seismic (SNR ≈ 4). Then, we test a prototype inversion workflow on a virtual seismic survey - full-scale time-lapse synthetic seismic produced by 3D simulations in a detailed full-earth model. This test shows that the errors associated with approximate nature of the seismic inversion algorithm reduces SNR to 2.1. Furthermore, detectable thickness of the plume reduces to 10 m when the plume is laterally finite. The key findings of the synthetic study help design the inversion workflow for the Stage 2C field data. Inverted parameters of the CO2 plume agree well with independent measurements, including repeat pulsed-neutron logging, in- and above-zone pressure monitoring and borehole seismic. To extract the plume body from the noisy inversion output, we use a Neyman-Pearson detector augmented by a spatial connectivity constraint. The proposed extraction criterion is then employed to compare history-matched reservoir with the monitoring data. For the simulated CO2 distribution, the proposed workflow would detect 70% of the plume areal footprint. The missed samples correspond to thin parts of the plume with low saturation, and hence their effect on the estimated total mass of CO2 in the reservoir is minimal.

Illuminating the geology: Post-injection reservoir characterisation of the CO2CRC Otway site

Proper site characterisation is vital in the planning stages of a CO2 storage project; but we can also learn a good deal about the reservoir once the injection is underway or has been completed. During CO2CRC Otway Project Stage 2C, sources of valuable information about storage performance have been generated as a consequence of the staged injection of 15,000 t of CO2 rich gas, as well as observations from time-lapse seismic surveys and well monitoring data. Now that injection has ceased for Stage 2C, the geological model is compared against field observations for the period spanning injection and 23 months after injection ended. The post-injection reservoir characterisation has proven important to refine the static and dynamic models for future field development and added assurance about the long-term stabilisation of the CO2 plume. The south-eastern progress of plume development, as seen on the time-lapse seismic data, has led to a review of the structural interpretation and horizon-fault geometry represented in the models. The developing plume has illuminated the extent of splay faults previously unresolved on the baseline seismic data. Saturation profiles interpreted from pulsed-neutron logs at the injection and observation wells show a preference for higher saturations occurring in high permeability distributary channels penetrated by each of the wells. This has reduced the uncertainty in predicting connectivity of this facies between the wells. The pressure data from numerous injection events has been used to refine the characterisation of the average horizontal permeability of the reservoir zone, and the vertical permeability of the intra-formational seal. It has also been used to infer near-field bounding conditions of the interior splay fault, which in turn improves our understanding of containment at the site.

Interpreting CO2 Saturation Changes from Pulsed Neutron Logs at the Aquistore Site

The use of repeated pulsed neutron (PN) logging has proved effective in numerous CO2 storage monitoring portfolios around the world. It is regarded as the go-to technique for interpreting saturation changes at the well-bore and has been routinely used for monitoring saturation changes in hydrocarbon reservoirs since tools appeared in the 1960s. The output from this type of tool that is almost always used for saturation monitoring is the so-called ‘Sigma’ curve, which can be used to distinguish formation fluids on the basis of how fast thermal neutrons are captured. Through the AQUISTORE Project in Saskatchewan, Canada, the Petroleum Technology Research Centre (PTRC) is demonstrating time-lapse pulsed neutron logging is an appropriate method to monitor near well CO2 saturation changes in a highly saline reservoir due to a large Sigma contrast between the saline formation waters and the injected CO2. PTRC has conducted time‐lapsed PN logging at the injection well to observe the saturation in the perforated interval and monitor any above zone migration. In addition, a total of 20 PN logging runs were also undertaken to detect the arrival of CO2 at the project's observation well, which is located approximately 150 m from the injection well. Here we present an interpretation of a secondary measurement produced by the tools, which is a measure of the hydrogen index obtained from the total count rates of secondary gamma-rays. We use the Ratio output to determine the suitability of this curve for monitoring CO2 movement and to see any additional benefits compared to the ‘Sigma’ curve. We find that the Ratio shows slightly better repeatability than the Sigma curve and could certainly be used in its place to monitor saturation changes. Furthermore, at the Aquistore site the wells were drilled with oil based mud. The Ratio measurement is almost unaffected by the presence of oil, and responds strongly to carbon dioxide thus is more reliable for interpreting saturation.

Research on Dual Mode Pulsed Neuron Spectral Logging Method and Its Application

Pulsed neutron logging is one of the key techniques for monitoring residual oil saturation in the late stages of oilfield exploration. Dual mode pulsed neutron spectral logging is discussed, which can be used to obtain time spectra and energy spectra of inelastic scattering of gamma radiation during a borehole measurement. With optimized pulse radiation and measurement timing, the tool simultaneously measures Σ and C/O to calculate the residual oil saturation. In this paper, based on Monte Carlo numerical simulation, a mathematical model is proposed of the dependence of temporal and energy spectra of gamma radiation on reservoir parameters, and the characteristics of C/O and capture cross-section are calculated. An interpretation model is proposed that can be used to calculate oil saturation from the C/O-Σ cross-plots. Comparative analysis and field tests showed that dual mode PSL spectra make it possible to determinate residual oil saturation in an unknown porosity formation with increased accuracy, and have technological advantages over the single C/O energy spectrum and neutron lifetime logging.