NMR Log Data Research Articles

Petrophysical Evaluation of the Zubair Formation’s Main Pay Zone Using Conventional and NMR Log Data: Insights from the North Rumaila Field, Southern Iraq

This study investigates the calculation of petrophysical properties using both conventional logging tools and Nuclear Magnetic Resonance logs for the main pay zone of the Zubair Formation, Recognized as one of the most significant hydrocarbon-producing formations in the North Rumaila Field, located in southern Iraq, the main pay zone is categorized into three permeable units (AB, DJ, and LN) and two impermeable isolated zones (C and K) based on log responses. The petrophysical analysis of these zones indicates that the shale volume increases toward the northern part of the field due to the depositional environment. Effective porosity values across these zones range from fair to good; however, some wells in the AB layer exhibit excellent porosity, notably wells X-7, X-62, X-83, and X-110. Additionally, hydrocarbon saturation levels remain consistently high in the AB and DJ layers, whereas certain wells in the LN layer display lower saturation values, particularly wells X-85, X-78, X-63, X-110, X-84, X-19, and X-115. Furthermore, permeability values were calculated using the KSDR equation based on Nuclear Magnetic Resonance logs that span from good to very good, except well X-115, demonstrating excellent permeability in the AB layer.

Estimation of gas hydrate and free gas saturation using the rock physics model based on constrained least squares and trust region method in the Shenhu Area, South China Sea

Gas hydrate saturation is a critical parameter for quantitative assessment of hydrate reservoirs. To accurately determine gas hydrate saturation, it is essential to consider various morphological occurrences of hydrates and the coexistence of hydrates with free gas. In this study, we devised a method for estimating the saturations of gas hydrate and free gas as well as the hydrate morphologies by employing the generalized effective medium model based on the constrained least squares and trust region method. Four hydrate occurrence morphologies, namely, contact-cementing, grain-coating, load-bearing, and pore-filling, were integrated into the rock physics model, with the coexistence of hydrates and free gas also being considered. We formulated the inversion as a multi-parameter constrained least squares problem and then used the trust region algorithm to find the optimal least-squares solution. The saturations of gas hydrate and free gas, along with the percentages of hydrate morphologies, were computed using the sonic logs at sites GMGS3-W11 and SH-W01-2017 in the Shenhu Area. The predicted hydrate saturation exhibited comparability to the saturation calculated from resistivity log, NMR log, and chloride data. The gas hydrate saturations predicted for these sites using the rock physics model were found to reach maximum values of 54% and 72%, respectively, while the saturation of free gas could attain up to 22%. The quantitative calculation results of hydrate morphologies reveal that in the Shenhu Area, load-bearing hydrates and pore-filling hydrates predominantly coexist, whereas contact-cementing and grain-coating hydrates are relatively fewer. Additionally, the application results indicate that the trust region algorithm demonstrates favorable global convergence.

Study on Quantitative Characterization Method of Micro-Pore Structure of Carbonate Reservoir Based on Nuclear Magnetic Resonance Logging

Carbonate reservoirs have various types of reservoir spaces and complex pore structures, so the evaluation of microscopic pore structures is of great significance to favorable reservoir identification. In order to accurately characterize the micro-pore structure of carbonate reservoir, this paper uses the NMR experiment, high-pressure mercury injection, and NMR logging data to establish a conversion model between the NMR T2 spectrum and the capillary pressure curve by piecewise power function method. The nuclear magnetic T2 spectrum of the reservoir is mainly bimodal, with small pore T2 ranging from 0.1 to 6 ms, the peak value being about 2 ms, and the large pore throat T2 ranging from 100 to 6000 ms. The throat radius of small pores is 0.04–0.1 μm, the peak value is 0.08 μm, and the throat of large pores is 0.1–10 μm. The Bash layer has the smallest pore throat radius and median radius, higher median pressure, and poorer pore structure. By converting the T2 spectrum of nuclear magnetic logging into a pseudo-capillary pressure curve, the continuous and quantitative characterization of reservoir pore structure parameters was achieved vertically. The secondary method has important reference significance for the quantitative characterization of pore structure in reservoirs of the same type.

A Method to Identify Pore Fluids in Heterogeneous Conglomerate Reservoirs Using a Nuclear Magnetic Resonance Log with Oil-Based Mud Invasion

Summary Nuclear magnetic resonance (NMR) logging has been extensively utilized for identifying pore fluids in recent years. However, after oil-based mud (OBM) invasion, OBM filtrate partially takes the place of the original fluid in the reservoir and the morphology of the NMR T2 (transverse relaxation time) spectrum changes. OBM invasion makes it difficult to identify the formation fluid using routine NMR fluid identification methods. In this study, we took heterogeneous conglomerate formation in the northwestern Junggar Basin as a case study and carried out core experiments under four conditions (viz., water-saturated, OBM displacing water, oil-saturated, and OBM displacing oil) and simulated the states of OBM invasion into water and hydrocarbon-bearing formation. Comparative analysis finds that when the OBM invades the water layer, the movable peak of the T2 spectrum primarily reflects the bulk relaxation characteristics of OBM filtrate, whereas when the OBM invades into the oil layer, the T2 spectrum may exhibit a three-peak distribution, where the first peak mainly indicates irreducible water, the second peak reflects OBM filtrate, and the third peak primarily reflects nondisplaced oil. To facilitate fluid identification, two T2 cutoffs are adopted to divide the T2 spectrum into three segments, and combined with the T2 geometric average, a fluid identification factor (ifluid) is proposed. Finally, identification criteria for reservoir type are established on the NMR logging and drillstem test data. The field application verifies the reliability of the proposed methods. These methods realize the identification of oil layers under OBM drilling and guide the subsequent production and development of oil reservoirs.

Prediction of lithology in lacustrine carbonates using well logs: The Cretaceous Barra Velha Formation in Santos Basin, offshore Brazil

Abundant hydrocarbons have been produced from the pre‐salt lacustrine carbonates of the Cretaceous Barra Velha Formation in the Santos Basin of offshore Brazil. The highly heterogeneous lithology and pore systems make it difficult to predict the reservoir quality via petrophysical well logs. Core description, computed tomography (CT) scanning, thin sections and wireline well logs are utilized to characterize the lithology, pore spaces are reservoir quality of the Cretaceous Barra Velha Formation pre‐salt lacustrine carbonates in the Santos Basin. The results show that the lithology mainly includes shrubstone, grainstone, laminated micrite and massive micrite. The pore systems are dominated by secondary pores, such as intergranular pores, dissolution pores and intercrystalline pores as well as modified primary pores. Geological well logs (conventional well logs, electrical and sonic image logs and nuclear magnetic resonance [NMR] logs) are integrated to build the predictable models of the carbonate reservoirs. The shrubstone, which is dominated by intergranular pores, has the highest reservoir quality, which can be derived from NMR logs. Grainstone contains both primary intergranular pores and abundant secondary dissolution pores. The laminated and massive micrite contain only micropores and therefore have the lowest reservoir quality. Lastly, conventional well logs integrated with electrical and sonic image logs are utilized to predict the distribution of lithology and reservoir quality in single wells, and the results are testified by the NMR log data. The shrubstone and grainstone are future exploration targets of the Cretaceous Barra Velha Formation in the Santos Basin. The results are hoped to give insights into the petrophysical behaviours of various lithologies, and provide theoretical and technical guidance for reservoir evaluation and prediction.

Reconstruction of LWD-NMR T2 water spectrum and fluid recognition based on microscopic pore structure constraints

Reconstruction of LWD-NMR T2 water spectrum and fluid recognition based on microscopic pore structure constraints

A Technique to Determine the Breakthrough Pressure of Shale Gas Reservoir by Low-Field Nuclear Magnetic Resonance

The porous and low-permeability characteristics of a shale gas reservoir determine its high gas storage efficiency, which is manifested in the extremely high breakthrough pressure of shale. Therefore, the accurate calculation of breakthrough pressure is of great significance to the study of shale gas preservation conditions. Based on a systematic analysis of a low-field NMR experiment on marine shales of the Longmaxi Formation in the Sichuan Basin, a shale gas breakthrough pressure determination technique different from conventional methods is proposed. The conventional methods have low calculation accuracy and are a tedious and time-consuming process, while low-field NMR technique is less time-consuming and of high accuracy. Firstly, the NMR T2 spectrum of shale core sample in different states is measured through low-field NMR experiment. The NMR T2 spectra of sample in water-saturated state and dry state are combined to model the mathematical relationship between shale gas breakthrough pressure and NMR T2 spectrum. It is found that the gas breakthrough pressure is power-exponentially related to the geometric mean of NMR T2 spectrum and positively related to the proportion of micropores. Accordingly, the shale gas breakthrough pressure is quickly and accurately calculated using continuous NMR logging data and then the sealing capacity of the shale caprocks is evaluated, providing basic parameters for analyzing unconventional hydrocarbon accumulation, preservation and migration. This technique has been successfully applied with actual data to evaluate the sealing capacity of shale caprock in a shale gas well in the Sichuan Basin. It can provide a good basis for the evaluation and characterization of shale oil and gas reservoirs.

A machine learning framework for low-field NMR data processing

A machine learning framework for low-field NMR data processing

A study on water saturation predictions in igneous reservoirs based on the relationship between the transverse relaxation time and the resistivity index

A study on water saturation predictions in igneous reservoirs based on the relationship between the transverse relaxation time and the resistivity index

A data mining approach for automatic classification of rock permeability

A data mining approach for automatic classification of rock permeability

Study on discriminant method of rock type for porous carbonate reservoirs based on Bayesian theory

Rock typing is an extremely critical step in the estimation of carbonate reservoir quality and reserves in the Middle East. In order to recognize the rock types of carbonate reservoirs in the Mishrif Formation better, classify the reservoirs accurately, and establish the permeability model in line with the study area precisely, it is necessary to study the recognition method conforming to the actual situation of the study area. The practice shows that the current recognition methods based on capillary pressure curve, flow unit and NMR logging data can effectively distinguish rock types, but a large number of accurate experimental data are required, which can only be applied in a few cored well, however, cannot be applied in the whole oil field. In this study, based on core, thin section, logging data, the sedimentary characteristics of carbonate reservoir, logging response of four rock types as well as porosity and permeability characteristics of Mishrif Formation in W are comprehensively studied. Based on Bayesian stepwise discriminant theory in multivariate statistics, the Bayesian discrimination model based on conventional logging data is established. The examining results showed that, compared with the description of logging and coring, the accuracy of Bayesian discriminant model and cross confirmation rate have achieved more than 80% for the original sample. Reliability verification showed that the matching degree of the rock type recognized in the non-cored well with the core and mud logging was as high as 90%, which matched the depositional environment of the entire region. The study results confirm the validity and generalizability of the Bayesian method to identify and predict rock types, which can be applied to the entire Middle East region to solve the problem of the lack of core data to accurately evaluate the quality of non-cored wells and accurately predict production, meeting the needs of actual reservoir evaluation and production development in the Middle East.

Study on pore structure evaluation of shale gas reservoir-Taking the Lower Cambrian Niutitang Formation in the Cenggong Block, Guizhou Province as an example

The storage and seepage space of shale is mainly composed of pores and fractures, while the microscopic pore structure and fracture distribution are very complicated. The accuracy of calculation of pore structure parameters is related to whether the reservoir evaluation is correct and effective. Taking the Niutitang Formation in the Cengong area of Guizhou as the research object. Firstly, based on the Archie formula, the process of the wellbore mud intrusion is approximated as the process of the laboratory high pressure mercury intrusion, combined with conventional and nuclear magnetic resonance logging data. The formula deduces a new model for the T2 spectrum conversion pseudo-capillary pressure curve. Then the model is calibrated by the high pressure mercury intrusion experimental data, and the pore structure parameters such as reservoir pore size distribution curve and maximum pore throat radius are calculated. The results show that the maximum pore throat radius and total porosity data calculated by NMR logging are relatively reliable, the median radius error is general, and the displacement pressure and median pressure error are relatively large. The pore volume percentage of 1-10 μm is up to 60%, and the micro-cracks are relatively developed, which is beneficial to the fracturing of the reservoir. Therefore, the use of NMR logging data combined with conventional logging can better reflect the pore structure characteristics of reservoirs, which provides a strong support for complex reservoir identification and qualitative prediction of productivity, and has a good application prospect.

Implementing Direct and Indirect Wireline Methods in Determination of Total Organic Carbon: A Case Study from a West African Hydrocarbon Field

Since the development bloom in unconventional reservoirs in North America, total organic carbon (TOC) has become a more essential parameter, as the indicator of the efficiency of these reservoirs. In this paper, by using conventional well logs and NMR log data, the TOC content of an unconventional reservoir in West Africa is estimated. Passy’s, Issler’s, and Schmoker’s methods were used as indirect wireline methods to estimate TOC content, along the well paths. Afterward, NMR log data, as a direct method, was used to provide more precise calculations of TOC. Both methods showed almost similar trends, with the NMR method indicating lower values for the TOC. Then, an adjusted Schmoker equation was proposed, which showed the best fit between NMR and conventional well logs results. By using the equation, the TOC content was calculated in three other wells, where NMR data were unavailable. The results were then used to prepare a 3D model of the TOC distribution, within the reservoir.

Reservoir rock properties estimation based on conventional and NMR log data using ANN-Cuckoo: A case study in one of super fields in Iran southwest

Reservoir rock properties estimation based on conventional and NMR log data using ANN-Cuckoo: A case study in one of super fields in Iran southwest

Introducing a method for calculating water saturation in a carbonate gas reservoir

Introducing a method for calculating water saturation in a carbonate gas reservoir

Porosity and Permeability Calculation Using NMR Logging in an Iraqi Oil Field

This study aims of analysing the NMR data and extracting the petrophysical properties of a well in an Iraqi oil field, T2 analysis forms the basis of more complex analysis for the interpretation of NMR, including wettability analysis, facies discrimination and the identification of reservoir units from NMR data. T2 analysis can also be used to calculate porosity, calculate the geometric mean of the distribution, track the position of the modes (peaks of the distribution), and estimate fluid viscosity. NMR logging data has been used for an Iraqi oil field and interpreted to calculate PHIT, PHIE, BVI, CBW, FFI and K. In this current research the petrophysical properties have been calculated and the moveable fluid zone has been detected, where the porosity and permeability values are significant with a good value of FFI which represents the reflection of high value of T2.

Pore structure evaluation of tight reservoirs in the mixed siliciclastic-carbonate sediments using fractal analysis of NMR experiments and logs

Pore structure evaluation of tight reservoirs in the mixed siliciclastic-carbonate sediments using fractal analysis of NMR experiments and logs

K-Mean Cluster Analysis for Better Determining the Sweet Spot Intervals of the Unconventional Organic-Rich Shale: A Case Study

Abstract The petrophysical analysis is the crucial task for evaluating the quality of unconventional organic-rich shale and tight gas reservoirs. The presence of organic matter and the ultra-tight with over complex pore system have remained a lack of understanding of how to evaluate the extensive parameters of porosity considering organic content, gas saturation, organic richness, brittleness index, and sweet spot interval by only using conventional log. Therefore, this study offers effectively applied techniques and better analysis for interpreting these parameters by maximizing and integrating geological, geochemical, rock mechanical and engineering data. In general, the field data used in this study are from the first dedicated well for source rock exploration in the North Sumatra Basin, Indonesia. The developed method was derived by using conventional log. All interpretation results were validated by laboratory data measurements of routine and special core analysis, petrography, total organic carbon (TOC) and organic maturation, and brittleness index (BI) calculation. Moreover, the high quality of NMR log data was used as well to ensure our developed techniques present good estimations. Briefly about the methods, we started to determine the total and effective porosity based on the density log by including the presence of organic matter and multi-mineral analysis in these estimations. Then, we used the revised water saturation-TOC of water saturation while the TOC was predicted in advance by averaging three results from the correlation of TOC-Density, modified CARBOLOG and Passey’s ΔlogR methods. Equally important, in order to obtain the reliable gas saturation prediction, we used saturation exponent (n), cementation factor (m), and the tortuosity factor (a) parameters which obtained from laboratory measurement of formation resistivity factor and resistivity index (FFRI). In addition, the brittleness index was predicted based on sonic log data. Finally, all parameters needed for determining gas shale sweet spot have been made. Then, we developed a way to evaluate the sweet spot interval by using K-mean clustering. In conclusion, this clustering result properly follows the shale quality index parameters which consist of organic richness and maturation, brittleness index, the storage capacity of porosity and gas saturation. This study shows that these petrophysical applied techniques leads us to interpret the best position of shale interval to be developed with a simple, fast, and accurate prediction way. Furthermore, as a novelty, this method can be used as rock typing method and obviously can reduce uncertainty and risks in organic-rich shale exploration.

Pore structure classification and logging evaluation method for carbonate reservoirs: A case study from an oilfield in the Middle East

ABSTRACTBased on the data of 121 thin sections, 24 mercury injection and physical properties of the carbonate reservoir in A oilfield of the Middle East, the reservoir in the study area is divided into four pore-throat systems by analyzing the pore-throat volume verses permeability-contribution curve of core mercury injection and corresponding depth NMR logging data. Taking into account the contribution of each pore-throat system to the rock, a new pore structure parameter P based on NMR logging data is proposed. On this basis, the P and flow porosity calculated from NMR logging data are used as variables, and the pore structure of carbonate reservoirs is divided into four types by using the K-means clustering method in combination with the characteristics of capillary pressure curves and thin sections. With the input of NMR logging data and conventional logging data, the classification model of pore structure is established by Rotation Forest algorithm. The accuracy of the classification model based on NMR logging is 98.56%, and the accuracy of the classification model based on conventional logging is 89.9%. Compared with the Random Forest algorithm and the Fisher discriminant method, the Rotation Forest algorithm has high prediction accuracy and strong stability. The research shows that the pore structure classification method proposed in this paper is in good agreement with the interpretation results, which can provide some reference value for finding effective reservoirs in the future.

NMR logging activation sets selection and fluid relaxation characteristics analysis of tight gas reservoirs: A case study from the Sichuan Basin

NMR logging activation sets selection and fluid relaxation characteristics analysis of tight gas reservoirs: A case study from the Sichuan Basin