Nuclear Magnetic Resonance Logging Research Articles

Determination of pore size distribution in tight gas sandstones based on Bayesian regularization neural network with MICP, NMR and petrophysical logs

Mercury injection capillary pressure (MICP) is one of the most important means for quantitative study of pore structure, however, its application is often limited due to its expensive, destructive and dangerous characteristics. Nuclear magnetic resonance (NMR) then becomes a common alternative by log-linear conversion of transverse relaxation time (T2) spectrum into pseudo-MICP or pore size distribution (PSD) though the accuracy and generality is unconvincing. Petrophysical logs have rich geological-petrophysical information but are seldom used in pore structure research. In this paper, PSD determination of tight gas sandstones based on Bayesian regularization neural network (BRNN) with MICP, NMR and petrophysical logs is studied. A numerical workflow is developed to preprocess MICP and NMR experimental data and determine PSD. Subsequently, two BRNN models are constructed respectively with joint laboratory T2 spectrum and petrophysical logs as main inputs and pore radius and mercury saturation from MICP measurement as training target. It is revealed that, accurate PSD can be determined from prediction results of BRNN models, wherein the NMR-based BRNN can be employed to obtain PSD from more easily accessible NMR measurement and the log-based one is expected to be generalized in the field. Conversion of PSD by fitting pore radius with T2 through their piecewise linear or quintic polynomial relation in log-log coordinate is equivalent to underfitting or overfitting in accuracy of machine learning and is confined to laboratory NMR with joint MICP. Detailed discussion on BRNN modeling, pore radius fitting and the corresponding numerical workflow proposed here may provide valuable insight into petrophysical-log analysis and quantitative evaluation on pore structure of complex reservoirs.

Study on the logging response characteristics and the quantitative identification method of solid bitumen at different thermal evolution stages

Solid bitumen is the intermediate products during the oil and gas generation, and widely exists in petroliferous basins all over the world. Besides damaging the reservoir physical properties, solid bitumen affects the logging parameters and is hard to be identified by the conventional logging method. In this study, two different thermal evolution stages of solid bitumen, separately from the Devonian outcrop profile and the Well ST3 in the northwestern Sichuan Basin, are taken as an example. Through a series of analyses on the porosity, permeability, resistivity, S-wave time difference, P-wave time difference and nuclear magnetic resonance (NMR) before and after bitumen dissolution, the logging response characteristics of solid bitumen at different thermal evolution stages are clarified, and the logging quantitative identification method of solid bitumen is established. The results are as below: (1) The solubility of solid bitumen is linearly correlated with the increment of porosity, permeability, P-wave time difference and S-wave time difference, and exponentially correlated with the decrement of resistivity. (2) Under the equal solid bitumen content and with the increase of thermal evolution, the influence of solid bitumen on porosity and permeability gradually decreases, while the influence on acoustic time difference and resistivity gradually increases. (3) Two kinds of NMR logging interpretation models for bitumen reservoir are established based on the NMR response characteristics of solid bitumen at different thermal evolution stages. The solid bitumen from low thermal evolution stage has a strong response in NMR, while that from high thermal evolution stage is otherwise. The Peak 1 (0.1 ms – 10 ms) of the T2 spectrum can respond to most of the low thermal evolution stage solid bitumen, but just a few of the high thermal evolution stage solid bitumen. (4) The NMR logging quantitative calculation methods on different thermal evolution stage solid bitumen are established respectively to solve the difficult issues of quantitative identification of solid bitumen. All the researches above have important guiding significance for the logging quantitative calculation of solid bitumen and the selection of effective oil/gas test layers.

Assessment of natural gas hydrate reservoirs at Site GMGS3-W19 in the Shenhu area, South China Sea based on various well logs

To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19, extensive geophysical logging data and cores were analyzed to assess the reservoir properties. Sediment porosities were estimated from density, neutron, and nuclear magnetic resonance (NMR) logs. Both the resistivity and NMR logs were used to calculate gas hydrate saturations, the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data. The density porosity was closely in agreement with the core-derived porosity, and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates. The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity. For the gas hydrate reservoirs at GMGS3-W19, the porosity, gas hydrate saturation and free gas saturation was 52.7%, 42.7% and 10%, on average, respectively. The various logs provide different methods for the comprehensive evaluation of hydrate reservoir, which supports the selection of candidate site for gas hydrate production testing.©2022 China Geology Editorial Office.

Influencing mechanics and correction method of nuclear magnetic resonance measurement in igneous rocks reservoir

Nuclear magnetic resonance (NMR) measurement provides lithology-independent porosity and pore structure for sedimentary reservoirs. Recently, oil and gas were discovered in igneous rock formations, however, NMR logging porosity is substantially lower than the true porosity, and the T2 distributions does not characterize the pore-size distributions. These observations limit the use of NMR logging in igneous rock formations and new research is needed to be developed. To address this gap in the literature, using numerical simulation of NMR behavior, we conduct a theoretical study to examine the impact of magnetic susceptibility, echo spacing, and fluid type on the T2 distributions in igneous rock. The simulation results and laboratory experiment show that, the paramagnetic minerals affect the magnetization of fluids in pores and rate of the echo decay, and NMR porosity is thus significantly reduced. To effectively correct the NMR measurement, a new approach is proposed to improve the underestimation of NMR porosity and T2 distribution shifting to shorter times. For a pore model, the corrected porosity and T2 distribution are both in good agreement with the model. In a case study from Chepaizi area of Junggar Basin, China, the corrected NMR porosity matches well with core measurement, and the corrected T2 distribution shifts to longer times more than before. Moreover, bound fluid volume and movable fluid volume are both corrected, and the NMR permeability also agrees well with core laboratory measurement, which proves that the correction method effective. Therefore, this technology enables NMR log for evaluating the igneous rock reservoir quality.

NMR Logging Application for Complicated Reservoir Formation Evaluation of M Oil Field in Middle East

There are many carbonate oil fields laying-out in and around Middle-East region, which are the hot productive spot with the huge crude oil reserves. However, these reservoirs also face the challenges of complex lithology, diverse pore configuration, and strong formation heterogeneity to affect the field operation performance. Take Iraq oil companies as an example, before joint operations with overseas oil companies from 2010, only the conventional logging technologies were mainly applied on site, but which can not provide enough support for the development of oil field. After 2010, the advanced logging technologies, such as Nuclear Magnetic Resonance (NMR) logging which are superior to conventional logging in some aspects, are introduced in order to support oil fields development effectively, and which actually makes good performance to direct the identification of non-pay zones, hidden effective zones and micro-fracture distributions. Technical analysis is based on M Oil Field in Iraq to share the experiences of NMR logging application from the view of reservoir engineering in the complicated carbonate reservoir.

Spatial assessment of gross vertical reservoir heterogeneity using geostatistics and GIS-based machine-learning classifiers: A case study from the Zubair Formation, Rumaila oil field, southern Iraq

The study of oil-field reservoir heterogeneity is an important task in the oil industry as it affects waterflooding, developing injection production systems, and optimizing hydrocarbon production. In this study, vertical reservoir heterogeneity was quantified using the Lorenz statistical index, empirical Bayesian kriging, and seven machine-learning classifiers (Classification and Regression Trees, Boosted Regression Trees, Random Forest, Naïve Bayes, Logistic Regression, K-Nearest Neighbors, and Support Vector Machine with three different kernels (linear, radial, and polynomial) under the geographic information system platform. The main pay zone of the Zubair Formation in the Rumaila oil field from southern Iraq was used as a case study. The degree of heterogeneity was first quantified using the Lorenz index, and a borehole-heterogeneity inventory location map was prepared according to the determined Lorenz index. Information about five factors influencing the heterogeneity, namely, porosity, permeability, volume of shale, reservoir-unit thickness, and depth to the top of reservoir unit, was collected based on available cores, nuclear magnetic resonance log, gamma-ray logs, and drilling-information logs. Factors from these sources were interpolated to show their spatial distribution using the empirical Bayesian kriging technique. The relationship between the borehole inventory map of vertical heterogeneity and the five factors was examined using the seven machine-learning classifiers. Two statistical-error measures, namely, accuracy and Cohen's kappa, were used to verify the performance of the classifiers in both training and testing stages. Results proved that Random Forest, Support Vector Machine with radial kernel function, and Logistic Regression were the best models. The probabilities of the best performance models were then interpolated and classified into five heterogeneity zones: Very low, low, moderate, high, and very high. The high-very high classes for each of these models approximately occupy 60% of the oil field and are mainly distributed in the middle and north of the field, whereas the other classes encompass about 40% of the field and mostly occur in the south. This distribution of classes is most likely related to the distribution and complexity of former depositional environments. • The spatial vertical heterogeneity of a clastic reservoir unit is quantified in this study. • Five factors including porsoity and permeability are used as influencing factors for modeling heterogeneity. • The high zone of heterogeneity encompasses 60% of the reservoir unit.

Characterization of porous media by nuclear magnetic resonance beyond fast diffusion limit with numerical simulation

Nuclear magnetic resonance logging is an important formation detection technique in the petroleum industry, which is always used to detect formation pore structure and fluid identification. However, conventional nuclear magnetic resonance characterization methods assume that the formation pore medium are in fast diffusion regime. In complex formations, non-fast diffusion regimes (intermediate diffusion and slow diffusion) are also present. In the previous paper [13], a new method for characterizing porous media with non-fast diffusion regime and uniform pore size distribution by T2-T2 pulse sequence is proposed. In this research, the T2-T2 signals of periodic stacked pore models beyond fast diffusion regime are numerically simulated by the random walk algorithm, and the pore size - surface relaxivity maps are respectively inverted, the relationship between pore structure and correlation maps are discussed. The results provide theoretical foundation for characterizing pore structure beyond fast diffusion limit.

Insights into the pore structure and oil mobility in fine-grained sedimentary rocks: The Lucaogou Formation in Jimusar Sag, Junggar Basin, China

Unconventional shale/tight oil resources in fine-grained sedimentary rocks play essential roles in global energy structure. Fine-grained sedimentary rocks are characterized by complex pore assemblage and connectivity. Insights into the links between pore structure and movable oil are important for exploration and production of tight/shale oil or gas in fine-grained sedimentary rocks. A series of analyses including thin sections, mercury injection capillary pressure (MICP), and nuclear magnetic resonance (NMR) were carried out to unravel the linkage between pore structure and oil movability. Various centrifugation speeds were used in the NMR tests to remove the movable oil to obtain the movable oil saturation corresponding to various centrifugation speed. Fractal dimension is calculated from NMR T2 spectrum. Then four types of pore structures were divided according to the parameters derived from the MICP and NMR results, and the characteristics of various pore structure are summarized. From type Ⅰ to type Ⅳ, the threshold pressure, irreducible water saturation (Swi), and the fractal dimension increased while the SHgmax, rapex, T2gm, porosity, permeability, and movable oil saturation decreased. Then the key factors controlling the movable oil saturation were investigated, including permeability, porosity, T2gm, rapex, and mineral composition of the rocks. The movable oil saturation is dominated by porosity, T2gm, fractal dimension, and rapex. Then a quantitative evaluation model of the movable oil saturation was established. In addition, the NMR logs and the derived parameters were utilized to evaluate the reservoir quality and pore structures in a single well. At last the T2 value of 25 ms (the ratio of the T2 components >25 ms to the total T2 components) was used to calculate the movable oil saturation in the upper member of the Lucaogou Formation, and the T2 value of 28 ms was used for the lower member of the Lucaogou Formation in the Jimusar Sag. The results are testified by the high-speed (10000 r/min) centrifugation NMR measurements. The results above help improve understanding of tight/shale oil enrichment and provide a new model for calculating the movable oil saturation in fine-grained sedimentary rocks.

A novel method for NMR data denoising based on discrete cosine transform and variable length windows

Raw nuclear magnetic resonance (NMR) data detected exhibits a low signal-to-noise ratio (SNR). The inversion results suffer from the low SNR of echo data. It is therefore essential to denoise the data before NMR data inversion, to improve the accuracy of inverted NMR spectrum. In this paper, a denoising method was firstly proposed based on discrete cosine transform (DCT), whereby four window schemes were designed into one window, multiple non-overlapping windows of equal length, multiple overlapping windows of equal length, and variable length windows. The effectiveness of four schemes was verified and compared by numerical simulations, and it indicates that Denoising Fig. 5 with variable length windows is the optimum. Core experimental data and NMR logging data were used to verify the practicality of Denoising Fig. 5. The results indicated that Denoising Fig. 5 can be effectively used to reduce the noise of actual NMR data, and the processed NMR spectrum with Denoising Fig. 5 is more reliable. • Discrete Cosine Transform was first applied to the noise reduction of NMR echo data. • Variable Length Windows scheme can achieve the best denoising effect. • The proposed method can effectively improve the application effect of NMR logging.

Lithofacies Architecture and Distribution Patterns of Lacustrine Mixed Fine-Grained Rocks —a Case Study of Permian Lucaogou Formation in Jimsar Sag, NW China

As the controlling effect of complex lithofacies of lacustrine mixed fine-grained rocks on the shale oil sweet spot remains unclear, core, outcrop, general logging, nuclear magnetic resonance (NMR) logging, testing, and production data were used to study the types, combination pattern, and genesis of lithofacies architectures of lacustrine mixed fine-grained rocks in the study area by lithofacies hierarchy analysis, X-ray fluorescence (XRF) logging, UAV, and 3D geological modeling. The research shows that: 1) According to lithology and sedimentary structure, the mixed fine-grained rocks can be divided into 13 lithofacies types of different origins in 5 sub-categories and 2 categories. 2) UAV photography was combined with a traditional field survey to characterize the 3D spatial distribution of lithofacies architecture of the Lucaogou Formation on the outcrop, and it is found that the lithofacies architecture patterns of mixed fine-grained rocks include three types: gradual change type, abrupt change type, and special type. The gradual change type with higher sand development degree and symmetrical lithofacies architecture has a high quality reservoir with dissolution pores, and is mixed beach-bar sand in the mixed zone. It is high in development degree and often appears as several similar cycles stacking over each other. The abrupt change type can be subdivided into two sub-types, asymmetric and smaller in reservoir thickness. It is very high in development degree and often comes in several similar cycles. The special type belongs to thick clastic rock relatively independent in the mixed fine-grained rocks with a high development degree of sand. The sand is a higher quality reservoir with properties of tight reservoir. It often appears as stacking of single cycle sand. 3) The different lithofacies architectures in the mixed fine-grained rocks have significant differences in distribution. The gradual change type is mainly composed of mudstone, dolomitic siltstone, and sandy dolomite, dolomitic siltstone, and mudstone, and appears in lenticular shape overlapping with each other on the plane. The abrupt change type is made up of felsic siltstone, dolomitic siltstone, sandy dolomite, and mudstone, and appears as isolated thin layers on the plane. The special type is mainly composed of mudstone and felsic siltstone, and mudstone, and turns up as lenses of different sizes on the plane.

Optimization of shaped pulses for radio frequency excitation in NMR logging.

The radio frequency (RF) excitation pulse of the nuclear magnetic resonance (NMR) logging tool can realize slice measurement by designing shaped pulses. In the case of a certain main magnetic field, the accuracy of the shaped pulse designhas a very important impact on the signal-to-noise ratio (SNR) of the NMR signal and the measurement of the short relaxation signal. Hard pulse excitation will produce an undesirable infinite number of side lobes that may perturb the spins in unwanted regions. Soft pulse can achieve selective excitation and has a better slice profile and shorter energy release time while it is not conducive to the measurement of short relaxation signals. This article focuses on the designof shaped pulses in extreme downhole environments and analyzes the characteristics of the three shaped pulses in the two cases of equivalent bandwidth and equivalent pulse duration. At the same time, a kind of RF-shaped pulse transmitting circuit with phase difference control is realized. According to the pulse type optimization strategy, the appropriate shaped pulse is selected. When echo spacing (TE) >0.6ms, the SNR can be increased to more than 12%. When TE is small, it will automatically switch to the hard pulse mode, which is good for short relaxation measurement.

Estimation of nuclear magnetic resonance (NMR) log permeability by integrating artificial neural network and imperialist competition optimization algorithm (ICA) in one of the oil fields of southwestern Iran

Estimation of nuclear magnetic resonance (NMR) log permeability by integrating artificial neural network and imperialist competition optimization algorithm (ICA) in one of the oil fields of southwestern Iran

High depth resolution design of small diameter nuclear magnetic resonance logging probe based on compensating magnets for soil moisture measurement in situ

The soil moisture in the surface layer (1 or 2 m) interacts with the atmosphere through evapotranspiration and influences land–atmosphere interactions, impacts rainfall-runoff processes, regulates net ecosystem exchange, and constrains food security. Thus, the accurate measurement with high depth resolution of surface soil moisture is of great significance to hydrology, meteorology, and agriculture. The nuclear magnetic resonance (NMR) logging technology has the ability to study hydrogen-containing fluid dynamics in porous media and is expected to achieve in situ detection for soil moisture. In this paper, we propose a new design of a small diameter NMR logging probe based on compensating magnets, which increases the signal-to-noise ratio (SNR) and have high depth resolution (1 cm). The uniform design method is adopted to obtain the optimal compensation magnet parameters. Compared with the probe without compensation magnets, the SNR of the new probe can be increased by 5.2 times in theory. Then a soil moisture experiment of three group samples is conducted to demonstrate the effectiveness of the new probe. The experiment results show that the new probe can obtain accurate results of soil moisture, and can also give the proportion of free water and bound water, which has been verified with the results of the Lab-NMR experiments. Finally, the factors that have an important influence on the measurement effect, such as the uniformity of the excitation magnetic field, the radial depth of investigation, and the echo time have been fully discussed.

Study on Nuclear Magnetic Resonance Logging T2 Spectrum Shape Correction of Sandstone Reservoirs in Oil-Based Mud Wells.

The oil-based mud filtrate will invade the formation under the overbalanced pressure during drilling operations. As a result, alterations will occur to the nuclear magnetic resonance (NMR) response characteristics of the original formation, causing the relaxation time of the NMR T2 spectrum of the free fluid part to move towards a slower relaxation time. Consequently, the subsequent interpretation and petrophysical evaluation will be heavily impacted. Therefore, the actual measured T2 spectrum needs to be corrected for invasion. For this reason, considering the low-porosity and low-permeability of sandstone gas formations in the East China Sea as the research object, a new method to correct the incorrect shape of the NMR logging T2 spectrum was proposed in three main steps. First, the differences in the morphology of the NMR logging T2 spectrum between oil-based mud wells and water-based mud wells in adjacent wells were analyzed based on the NMR relaxation mechanism. Second, rocks were divided into four categories according to the pore structure, and the NMR logging T2 spectrum was extracted using the multidimensional matrix method to establish the T2 spectrum of water-based mud wells and oil-based mud wells. Finally, the correctness of the method was verified by two T2 spectrum correction examples of oil-based mud wells in the study area. The results show that the corrected NMR T2 spectrum eliminates the influence of oil-based mud filtrate and improves the accuracy of NMR logging for calculating permeability.

Improving the hydrogeologic conceptualization of a remote semiarid palaeovalley groundwater system using airborne electromagnetics, seismic refraction and reflection, and downhole nuclear magnetic resonance

A hydrogeologic conceptualization is critical to understand, manage, protect, and sustain groundwater resources, particularly in regions where data are sparse and accessibility is difficult. We used airborne electromagnetics (AEM), shallow seismic reflection and refraction, and downhole nuclear magnetic resonance (NMR) logs to improve our understanding of an arid groundwater system influenced by palaeovalleys. We found that there is a limited connection between the palaeovalley and fractured bedrock aquifers because they are separated by a spatially variable layer of saprolite, which is the layer of chemically altered rock on top of the fractured bedrock. The AEM data provided an estimate of the top of the saprolite but failed to effectively image the bottom. In contrast, the seismic data provided an estimate of the bottom of the saprolite but failed to image the top. This geophysical combination of electrical and seismic data allowed us to map saprolite thickness in detail along a 1.7 km long transect that runs perpendicular the main trunk of a well-defined palaeovalley. These data indicate that the palaeovalley is lined with a heterogeneous layer of saprolite (approximately 3–120 m thick) that is thickest near its edges. Despite the observed variability, only a small percentage of the fractured bedrock aquifer (8%–17%) appears to be in contact with the palaeovalley aquifer. Furthermore, the lack of an elastic boundary at the top of saprolite suggests that the porosity of the saprolite is similar to the palaeovalley sediments — an observation that is supported by the downhole NMR-derived water contents. The electrical change at the top of saprolite is caused by a combination of a decrease in total dissolved solids of the groundwater in the saprolite and a change in pore structure associated weathering in situ versus transported weathered materials. The presence of saprolite, which commonly behaves as an aquitard, may limit the groundwater exchange between the palaeovalley and bedrock aquifers, with implications for the regional groundwater resource potential.

Logging evaluation of lamina structure and reservoir quality in shale oil reservoir of Fengcheng Formation in Mahu Sag, China

The Fengcheng Formation in the Mahu Sag of the Junggar Basin is composed of a set of fine-grained sedimentary rocks located under a layer of alkaline lacustrine rocks. There are alternating dolomite lamina, siltstone and organic matter laminae that show strong heterogeneity. The lamina structure determines reservoir quality and oil-bearing potential in shales. Therefore, the identification of lamina structure is a crucial basis for identifying reserves of shale oil. Identified as massive, layered and laminated, three different types of lamina structures are divided according to core observations in terms of the density of the laminae and the overlapping relationships. High-resolution processing is also performed on dynamic image log data to form slab images for recognizing the various types of lamina structures. Compared to core slab photos, it is believed that slab images offer an excellent thin-layer resolution of up to a millimeter in scale and can effectively identify the lamina structure of the shale reservoir. There are no evident beddings on the core of the massive pattern, and the slab image shows a bright or dark block pattern. There are centimeter-level felsic bands of fine silt sand on the thin-layered sedimentary fabric core, and the slice image shows a band-like pattern. A large number of millimeter-level dolomitic laminae are enriched on the core of the laminated type, and the slab image shows millimeter-level light and dark lamina. Slab images were used for the division and identification of lamina structures in a single well. Then, the relationships between the three types of lamina structures and the two-dimensional nuclear magnetic resonance logs and oil test data were recorded. The results show that massive and laminated pores are dominated by intergranular pores with low porosity and movable oil content. Therefore, only low oil flow can be obtained during an oil test. However, the layered type due to centimeter-level silty bands, T1 and T2, is bimodal with high porosity, movable oil content, and high levels of hydrocarbon production. Therefore, the optimal areas of shale reservoirs are closely related to the layered type. The more significant the proportion of layered types in shale reservoirs, the higher the quality of the reservoir. The present study is intended to identify lamina structure-controlled reservoir quality and oil accumulation and to provide insights into the exploration and development of shale oil.

Key Development Fields and Construction of Technical System for Logging of Continental Shale Oil

The research on continental shale oil has just started in China. The key development fields of logging, the link of geology-engineering integration, are not yet clear. There is no systematic establishment of a corresponding collection and evaluation technical system, which restricts the development and full exploitation of shale oil logging to a certain extent. After a systematic analysis of current status of shale oil logging and the requirements of the geology-engineering integration of shale oil, the contents, difficulties, and shortcomings of logging evaluation were analyzed in detail from the four aspects, including a quantitative analysis of mineral components and identification of favorable lithofacies while drilling, reservoir and oil-bearing property evaluation, mobility evaluation, and fracrability evaluation. It is proposed that the development of diffuse reflection infrared Fourier transform spectroscopy (DRIFTS), online nuclear magnetic resonance (NMR) logging for the oil-bearing property of drilling fluids, T1-T2 two-dimensional NMR logging of rock samples, and logging rock mechanics should be emphasized. On this basis, according to the principles of pertinence, effectiveness and economy, continental shale oil can be divided into two types: medium-low maturity and medium-high maturity, and the technical system for logging based on the evaluation of reservoir property, oil-bearing property, mobility, fracrability, and drillability was developed, in a way that can guide the development and production practice in shale oil logging.

Static Measurements Enhance Saturation and Permeability Interpretation

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202683, “Marrying the Static and Dynamic Worlds: Enhancing Saturation and Permeability Interpretation Using a Combination of Multifrequency Dielectric, Nuclear Magnetic Resonance, and Wireline Formation Testers,” by Hassan Mostafa, Ghassan Al-Jefri, SPE, and Tania Felix Menchaca, SPE, ADNOC, et al., prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually 9–12 November. The paper has not been peer reviewed. Accurate water saturation evaluation and permeability profiling are crucial factors in determining volumetrics and productivity of multiple, stacked carbonate reservoirs offshore Abu Dhabi and derisking reservoir management. The case study presented in the complete paper illustrates how the integration of static measurements, such as dielectric dispersion and nuclear magnetic resonance (NMR) with dynamic measurements improves understanding of reservoir properties and supports more-accurate reservoir evaluation. Sampling and downhole fluid analysis (DFA) performed by wireline formation tester (WFT) identifies the fluid and rock properties in various flow units. Field Background and Challenges Optimal field development requires accurate estimations of water saturation and permeability. In this greenfield, the hydrocarbon is generally oil (medium to light) with very low asphaltene content. Overall, the reservoir quality is controlled by a combination of depositional environment, sequence stratigraphy, and diagenesis. Some reservoirs have good porosity, but reconciliation of log-based water saturation results with well-test results has been an issue. The objective in this case study was to drill a pilot hole for data gathering in a poorly characterized field location. Phase I included drilling a hole with a 55° deviation to cover all reservoirs for data gathering only, with the openhole reservoir section then being plugged and abandoned. Phase II of the plan was to sidetrack and complete the well as dual water-injector boreholes. In the reservoir section of the pilot borehole, a variety of logs was acquired for evaluation, including both logging-while-drilling and wireline measurements. While drilling, triple- combination data were acquired, consisting of gamma ray, resistivity, and nuclear logs (density neutron) along with resistivity images. The wireline-logging program was carried out in two stages to avoid differential sticking. In the first stage, the WFT was used to acquire 10 pressure points, seven points in the first reservoir and three points in the second. Two DFA stations were also recorded in Zone 1 to confirm whether the oil/water contact was deeper than expected. Logging was conducted using a high-tension wireline cable, which facilitates quicker accessibility to the openhole sections. In the second stage, multiple wireline runs were performed for the formation evaluation of the complete section, followed by the WFT pressure and fluid-sampling run on the drillpipe conveyance. Another critical challenge was to obtain accurate water saturations in the heterogeneous, minor, thin reservoirs, which are bounded by dense layers above and below and cause shoulder-bed effects. The third challenge in this well was to obtain an accurate, continuous, and representative permeability profile for the multiple reservoirs. WFT mini-drillstem test (DST) stations along with NMR logs were used to address this important requirement.

Electron spin resonance (ESR) in petrophysical characterization of pure carbonatic rocks and containing oil

Petrophysical evaluation from laboratory and well logging data, such as gamma ray, Nuclear Magnetic Resonance (NMR) and resistivity has been used to identify and evaluate oil zones in reservoirs. Gamma ray log is able to predict lithologies along the well, and X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) techniques also provide important lithological laboratory information. The NMR log directly estimates the porosity of the rock. Resistivity log identifies the water region, but cannot provide information about oil saturation. However, the application of unconventional techniques, such as Electron Spin Resonance (ESR), can be of great relevance in the well logging, adding information about lithologies, oil quality and saturation. The ESR technique, which can directly estimate the oil saturation inside the rock, can also provide important information about the origin and formation of rocks. This technique could be an advance for the oil industry, helping to understand the geothermal processes suffered by the rocks and identifying important regions of the reservoir. Thus, in this work, we studied carbonate rock (limestone and dolomite) samples with and without oil, by ESR technique. Firstly, the effectiveness of ESR in the differentiation of carbonates (calcitic and dolomitic) without oil was tested, through the analysis of manganese ion impurity, which promotes the replacement of calcium and magnesium ions in calcites and dolomites. The samples had a typical manganese ESR signature. The hyperfine parameter, A, was homogeneous for both limestone and dolomites. The zero-field split parameter, D, showed variability, as reported in the literature. The studies were carried out at X-band frequency. The analysis by XRF and XRD provided information on the chemical and mineralogical composition of the carbonates, which were added to the ESR data for application in a correlation matrix, by which it was identified that the components CaO and MgO were directly related to the intensity of the ESR signal and the concentration of CaO was inversely related to the concentration of Al2O3 and SiO2. In one limestone rock sample was incorporated oil, with 25° API and moderate biodegradation level, to identify by ESR, the oil presence inside this rock. The technique was able to identify the oil and its saturation in the rock through the detection of free radical signal, g = 1.9951. The innovative application of the ESR technique to identify oil in varying saturation, in carbonate rock samples from the Illinois Basin (USA), was successful. These results demonstrated that this technique in addition to being accurate, non-destructive and relatively low cost, could be used in mud logging operations providing results for lithological characterization, oil identification and its saturation inside the reservoirs.

An experimental study on the effect of magnetic field strength and internal gradient on NMR-Derived petrophysical properties of sandstones

Nuclear Magnetic Resonance (NMR) is being widely deployed as a powerful tool for estimating petrophysical properties. Compared to conventional carbonate reservoirs, sandstone reservoirs contain a high amount of paramagnetic ions (such as iron, nickel, or manganese) usually found in clays. The interpretation of NMR-derived petrophysical properties for sandstone formation can be complicated due to the paramagnetic ions which cause inhomogeneity in the magnetic field called internal gradient. Earlier studies focused on investigating the impact of clays and field strength on internal gradient, but implications for estimating NMR-derived petrophysical properties are not well understood yet. This study aims to investigate the impact of the internal gradient and magnetic field strength on the T2 relaxation times and the NMR-derived porosity, T2 logarithmic mean value (T2,LM), and permeability. NMR T2 measurements at several echo times were performed to evaluate the internal gradient for six sandstone samples characterized by variable porosity, permeability, and clay contents/mineralogy. The analysis was executed at two different operating Larmor frequencies (2 and 12 MHz), which are the most common frequencies for rock core analysis. The results show that the internal gradient magnitude increases with the increase of the paramagnetic-rich clays (Chlorite and Illite in the studied samples) while no correlation is observed with Kaolinite content. Furthermore, the internal gradient's magnitude and effect are consistently more pronounced at the 12 MHz measurements than those performed at 2 MHz. Porosity estimates from NMR are found to be independent of the internal gradient and the magnetic field strength as long as sufficiently low echo time (0.2 ms) is used. Unlike porosity, noticeable discrepancies in T2,LM were reported between 2 MHz and 12 MHz measurements, with the later showing consistently lower T2,LM values. The discrepancies in T2,LM values result in significant differences (up to 300%) when calculating NMR-derived permeability using the same formulations for both 2 and 12 MHz data. That is, calibration done using the T2 data from 12 MHz equipment cannot be applied for permeability estimation from lower field NMR data (like logging NMR). The discrepancies between 2 and 12 MHz in terms of T2,LM and derived-permeability increase exponentially as the internal gradient increases but such discrepancies diminish for tight rocks (permeability < 1 mD). This behavior of the tight sandstone samples is attributed to the relaxation regime, which is impacted by pore sizes. If pores are small enough, such as the case occasionally for shale or tight sandstone, relaxations move from a short time regime to the motionally averaged. In such a regime, the internal gradient is averaged by diffusion over the length scale, meaning that relaxation time has no dependence on echo time.