Petrophysical Research Articles

Hybrid parameters for fluid identification using an enhanced quantum neural network in a tight reservoir

This paper proposes a fluid classifier for a tight reservoir using a quantum neural network (QNN). It is difficult to identify the fluid in tight reservoirs, and the manual interpretation of logging data, which is an important means to identify the fluid properties, has the disadvantages of a low recognition rate and non-intelligence, and an intelligent algorithm can better identify the fluid. For tight reservoirs, the logging response characteristics of different fluid properties and the sensitivity and relevance of well log parameter and rock physics parameters to fluid identification are analyzed, and different sets of input parameters for fluid identification are constructed. On the basis of quantum neural networks, a new method for combining sample quantum state descriptions, sensitivity analysis of input parameters, and wavelet activation functions for optimization is proposed. The results of identifying the dry layer, gas layer, and gas–water co-layer in the tight reservoir in the Sichuan Basin of China show that different input parameters and activation functions affect recognition performance. The proposed quantum neural network based on hybrid parameters and a wavelet activation function has higher fluid identification accuracy than the original quantum neural network model, indicating that this method is effective and warrants promotion and application.

Experimental Study of Seismic Wave Attenuation in Carbonate Rocks

Summary Seismic wave attenuation has a great potential for studying saturated and fractured media, due to its high sensitivity to the physical properties of geological media. However, accurately estimating this parameter can be challenging due to its sensitivity to signal noise, particularly in heterogeneous media such as carbonate rocks. This explains the paucity of attenuation studies carried out in carbonate rocks compared with sandstones, and the ambiguity around its mechanisms and its relationship with petrophysical properties. To investigate further, we conducted an experimental study of ultrasonic waveform signals (0.5–3 MHz) reordered under dry and fully saturation conditions in 13 samples covering a wide range of petrophysical values and subjected them to differential pressure reaching reservoir pressure. The resulting increase in attenuation magnitudes and their variation with pressure due to brine saturation were more pronounced than in velocity magnitudes, confirming the higher sensitivity of attenuation to fluid content. However, understanding the relationship between attenuation and petrophysical properties required a careful examination of the results and more elucidation about attenuation mechanisms. We suggested that multiple attenuation mechanisms coexist, including scattering, cracks slipping, solid frictional relative motion, and global and squirt flow. This explains the frequency dependence of attenuation, with higher magnitudes at sonic frequencies, where the squirt flow mechanism may be dominant. In contrast to sandstone, the magnitude of compressional to shear attenuation ratio (QP−1/QS−1) was found to be greater than unity in both dry and brine fully saturated carbonate samples at ultrasonic frequencies. This result may be due to the complex porosity structure of carbonate rocks, which makes it not appropriate to the sandstone rock physics models.

Acoustical-electrical models of tight rocks based on digital rock physics and double-porosity theory

The acoustical-electrical (AE) properties of reservoir rocks are affected by their microstructure (pores, microfractures, cracks and their geometry) and saturating fluids. To aid the interpretation, digital rock physics (DRP) is a useful technique to characterize this microstructure, as well as equivalent petrophysical models (EPM) to obtain joint AE properties. We perform thin-section analysis on rocks from tight-oil reservoirs, along with X-ray diffraction (XRD) and computed tomography (CT) to study rock lithology and minerals. We also perform porosity, permeability, ultrasound, and electrical conductivity experiments as a function of confining pressure to analyze pore structure. First, the 3D digital multicomponent cores are created based on a geology-driven multiphase segmentation workflow and images of the samples, and verified based on the porosity and minerals. Then, the AE properties and permeability are calculated by using numerical simulations (finite difference and finite volume methods). Next, the rigid and microcracked (soft) porosities are determined by using the Shapiro model to create the rock skeleton. We develop a joint EPM based on the effective medium AE and the Biot-Rayleigh equations with double porosity to describe the rock properties. The ultrasonic, sonic and seismic multiple data are used to compare and analyze the two approaches. The results show that the DRP techniques based on real cores are effective in characterizing the microstructure and the proposed EPM can describe the AE properties of real rocks, indicating a potential for quantitative characterization of reservoirs.

Time-lapse seismic inversion for CO2 saturation with SeisCO2Net: An application to Frio-II site

Seismic monitoring of geological CO2 storage (GCS) involves highly nonlinear seismic inversion and petrophysical inversion, making it challenging to estimate CO2 volume efficiently and detect possible early CO2 leakages. Deep learning (DL) using convolutional neural networks (CNNs) has shown promise in solving highly nonlinear seismic inversion problems. However, direct estimation of CO2 plume extent/saturation from time-lapse seismic gathers using DL is still underexplored, with no reported field applications to date. The investigation of field data is primarily hindered by scarcity of field data for neural network training. Other obstacles include highly nonlinear seismic-petrophysics inverse relationship, and presence of noise in field seismic data. We introduce SeisCO2Net, a deep CNN that predicts CO2 saturation maps directly from time-lapse full waveform shot gathers. For training, we use site-specific geological information, fluid flow physics, rock physics, and seismic modeling to generate synthetic datasets that closely resemble the CO2 storage site. Synthetic tests show promising results, inspiring us to apply SeisCO2Net's trained weights on field data collected at Frio-II GCS site by leveraging transfer learning principles. As reference, we compare SeisCO2Net's predicted CO2 saturation maps with results obtained from physics-based inversion. Our analyses show both methods display similar CO2 plume shapes, reasonable CO2 plume characteristics, and comparable saturation values. Our results suggest pre-training CNNs on physics-informed synthetic datasets and then applying the learned weights to field data is a viable approach to estimating field CO2 saturation. This method effectively addresses the scarcity of field training data, thus encouraging the feasibility of long-term GCS monitoring.

Using rock physics analysis driven feature engineering in ML-based shear slowness prediction using logs of wells from different geological setup

Using rock physics analysis driven feature engineering in ML-based shear slowness prediction using logs of wells from different geological setup

Introducing a simplified rock physics model to estimate shear velocity to consider the geometry of pore spaces and minerals

Introducing a simplified rock physics model to estimate shear velocity to consider the geometry of pore spaces and minerals

Synergy in the Subsurface: Revolutionizing Reservoir Characterization through Integrated Geophysics and Rock Physics

Synergy in the Subsurface: Revolutionizing Reservoir Characterization through Integrated Geophysics and Rock Physics

Multi-Wave Stochastic Inversion Physical Parameters Prediction Method Driven by Linearized Rock Physics

Multi-Wave Stochastic Inversion Physical Parameters Prediction Method Driven by Linearized Rock Physics

Joint Inversion Method of Rock Physics Based on Hunger Games Search Correction and Bidirectional Long-Short-Term Memory Network

Joint Inversion Method of Rock Physics Based on Hunger Games Search Correction and Bidirectional Long-Short-Term Memory Network

Integration of Rock Properties Crossplot and Petrophysics for Enhancement of Litho-Fluid Discrimination and Compartmentalizationto Reduce Uncertainty and Mitigate Hydrocarbon Exploration Risk. ‘A Case of ‘NICK' Field, Onshore Niger Delta’.

The main components of reservoir rocks are hydrocarbons and immiscible water in varying ratios. It is essential to precisely identify, characterize, and divide the fluids in these reservoirs into distinct groups according to the characteristics of their rock properties to conduct a successful hydrocarbon exploration. For this reason, petrophysics and rock physics analysis were combined on the "NICK" field in the onshore Niger Delta. Through precise litho-fluid discrimination in the field, this study seeks to improve field hydrocarbon production, lower uncertainty, and mitigate risks related to hydrocarbon exploration. The suites of well logs (Gamma-ray neutron, bulk density, sonic, and resistivity logs) from three wells—NICK-1, NICK-3, and NICK-6—make up the data used. Gamma-ray log signatures were used to identify and correlate lithologies throughout the field. Potential reservoirs and fluid content were identified and delineated by high resistivity and adequate neutron-porosity log signatures. Hydrocarbon-bearing sands were recorded at low values of elastic attributes (acoustic impedance, rigidity, incompressibility, and others), which were computed to aid in the characterizations. Two potentialreservoirs’ Sands A and B delineated, constituted the correlated pay zones observed in three wells across the field at depths ranging from 1986.24 to 2599.82m. Petrophysics results generally revealed fair to good porosities of (11-25%) for easy accumulation of hydrocarbon. Permeability ranged from 210- 809mD for Sand A and 27 – 887mD for Sand B, showing that there are suitable permeabilitiesfor fluid movement/migration within the reservoirs. Cross plot of Lambdarho versus Murho, Lambdarho versus Velocity Ratio, and Velocity Ratio versus Acoustic Impedance gave four distinct clusters for litho-fluid zones identification given as gas-sand, oil-sand, brine-sand, and shale. This study has assisted in better characterization and distinguishing of the litho-fluid details for enhancement of hydrocarbon production in the field.

Seismic facies characterization using Rock Physics Templates and brittleness indices: Stybarrow field

This paper proposes a method for identifying the lithological properties of the medium based on the joint analysis of the Lamé parameters, Young's Modulus, and Poisson's Ratio. Additionally, an analysis of brittleness was proposed to identify brittle/ductile intervals and areas of potential reservoirs. The petroelastic properties were analyzed at well and seismic scales using ternary rock physics templates. The ternary templates were built from a self-consistent micromechanical model. In addition, the analysis allows for preserving the conditions of the environment subsurface in the seismic and log information. A workflow for petroelastic lithology interpretation was coupled with a workflow of brittleness modeling. The results correlate well with conventional qualitative methodologies applied in previous studies. A brittleness analysis methodology was developed and tested to identify reservoirs associated with the Lower Cretaceous in the Stybarrow field in Australia; the results highlight the high brittleness zones attenuated by hydrocarbons ( ). The proposed seismic-based methodology is an improvement to conventional analysis trends for identifying lithologies and prospective hydrocarbon zones.

Mechanical behaviour and microstructure of methane hydrate-bearing sandy sediment observed at various spatial scales

Methane hydrates (MHs) are considered an alternative energy resource but also a potential source of geo-hazards and climate change. The physical/mechanical properties of gas hydrate-bearing sandy sediments are strongly dependent on the distribution of hydrates within the pore space. The purpose of this study is to investigate morphologies and pore-habits of MHs formed in sandy sediments by means of experiments that probe a wide range of scales, from the pore scale – using Synchrotron X-Ray Computed Tomography (SXRCT) and optical microscopy – to the core scale, through mechanical property measurements. The same synthetic sands are used, in which MHs are generated successively under excess gas and excess water conditions. At the macroscopic (core) scale, MH pore habits are inferred by comparing the measured sonic wave velocities to velocities calculated from rock physics models and further assessed via triaxial compression tests. Furthermore, Magnetic Resonance Imaging is used to investigate the kinetics of MH formation and distribution along the core height. The pore habits and MH morphologies are directly visualized at the pore (grain) scale by SXRCT and, with still better spatial and temporal resolution, by transmission optical microscopy, revealing some more complex morphologies than in the hydrate pore habits commonly admitted.

Homogenisation method based on energy conservation and independent of boundary conditions

The foundation of homogenisation methods rests on the postulate of Hill–Mandel, describing energy consistency throughout the transition of scales. The consideration of this principle is therefore crucial in the discipline of Digital Rock Physics which focuses on the upscaling of rock properties. For this reason, numerous studies have developed numerical schemes for porous media to enforce the Hill–Mandel condition to be respected. The most common method is to impose specific boundary conditions, such as periodic ones. However, these boundary conditions influence both the effective property and the size of the REV. The recent study of Thovert and Mourzenko (2020) has shown that most boundary conditions still result in the same intrinsic effective physical property if the averaging is applied outside the range of the boundary layer. From this discovery, it becomes logical to question the status of Hill–Mandel postulate in porous media when homogenising away from the boundary. In this contribution, we simulate Stokes flow through random packings of spheres and a range of rock microstructures. For each, we plot the evolution of the ratio micro- vs macro-scale of the energy of the fluid transport outside the boundary layer, for a growing subsample size of porous media. Here, we prove that we naturally find energy consistency across scales when reaching the size of the Representative Elementary Volume (REV), which is a known condition for rigorous upscaling. Furthermore, we show that this index for the energy consistency is a more accurate indicator of REV convergence since the mean value is already known to be unitary.

Research on the innovation system of physical climbing in college physical education under big data technology

Abstract Rock climbing, as one of the outdoor sports gradually sought after by young people nowadays, plays an important role in innovating the traditional physical education system. This study integrates the training related to physical rock climbing with the conventional college physical education system and utilizes a synergistic teaching method to innovate and improve the college physical education system. Through the implementation of rock-climbing teaching experiments for students in school A, the data related to the experiments were analyzed using the literature method, questionnaire survey method, and mathematical statistics method to explore the effectiveness of the practical application of physical rock-climbing in college physical education. In terms of physical function, the heart rate indexes of students in the experimental group decreased by 8.96 and 9.08, respectively, and the lung capacity and anaerobic threshold were significantly improved, but the male students did not show any significant improvement in the index of maximal oxygen uptake. Except for the lower limb stirrup strength, the p-value of the experimental group of students in seated forward bending, dorsal extension, left-handed grip strength, right-handed grip strength, back strength, and longitudinal jump were all less than 0.05. The capacity of motor memory of the experimental group was higher than that of the control group by 2.91. In terms of social functioning, compared to the control group of students, all the experimental groups showed stronger characteristics.

Universal apparatus module of radioactivity logging for the investigation of oil-and-gas reservoirs while drilling

Logging-while-drilling (LWD) is the most advanced technology in geophysical well logging. It is widely used for determining the petrophysical and other parameters of oil-and-gas reservoirs in vertical, deviated, and horizontal boreholes.
 The LWD is particularly important for horizontal boreholes, obtained by directional drilling from vertical boreholes into predetermined hydrocarbon reservoirs. Horizontal boreholes allow multiplying the yield of hydrocarbons, increasing overall production, and using effectively both new vertical boreholes and exploited ones, as well as old boreholes. The opportunity of horizontal drilling to local (isolated) deposits, which are at a certain distance from vertical boreholes, is of also essential importance. All these factors are favourable for increased hydrocarbon production, which is of great significance for Ukraine.
 This paper concentrates on the apparatus developments aimed at creating a new module of radioactivity logging for LWD; the module is universal in the sense that it can be used in drill collars, which are employed while drilling all types of boreholes. The tool implements neutron-neutron logging, neutron-gamma logging, gamma-gamma logging, and gamma-ray logging. It allows a direct and combined determination of a broad set of petrophysical reservoir parameters.
 The advantages and disadvantages of the known nuclear logging modules for the LWD have been analyzed, and the features and merits of the proposed one are shown. During the laboratory and borehole operations with a pilot sample of the module, trends for its improvement were outlined.
 As an example of using the new module for LWD, the results of determining the petrophysical parameters of oil reservoirs of varying lithology in three horizontal boreholes are given. The process of LWD measurements demonstrated high technologicness and efficiency of the module. Comparison of the obtained parameters of the reservoirs with the data of independent measurements carried out in the open hole of the same boreholes using the PEX tool (Schlumberger) showed a qualitative consistency and a quantitative convergence of the results.

Rock-physics model of a gas hydrate reservoir with mixed occurrence states

Seismic interpretation of gas hydrates requires the assistance of rock physics. Changes in gas hydrate saturation can alter the elastic properties of formations, and this relationship can be considerably influenced by the occurrence state of gas hydrates. Pore-filling, load-bearing, and cementing types are three single gas hydrate occurrence states commonly considered in rock-physics investigations. However, many gas hydrate-bearing formations are observed to have mixed occurrence states, and their rock-physics properties do not fully conform to models of single occurrence states. We develop a generalized rock-physics model for gas hydrate-bearing formations with three mixed occurrence states observed in the field or laboratory experiments: coexisting pore-filling-type and matrix-forming-type gas hydrates (case 1); pore-filling type when [Formula: see text] (gas hydrate saturation) < [Formula: see text] (critical saturation) and pore-filling + matrix-forming type when [Formula: see text] (case 2); and matrix-forming type when [Formula: see text] and matrix-forming + pore-filling type when [Formula: see text] (case 3). Instead of initial porosity, the apparent porosity (the volume fraction of an effective pore filler) [Formula: see text] represents the influence of occurrence states on the pore space. These three mixed occurrence states can be modeled using a unified workflow, in which the volume fractions of various gas hydrate types are expressed in general forms in terms of the apparent porosity. In addition, the model considers the effect of a pore filler on shear modulus. The developed model is validated through calibration with real well-log data and published experimental data corresponding to five gas hydrate-bearing formations. The model effectively interprets the influences of gas hydrate saturation and occurrence state on these formations. Thus, the generalized model provides a theoretical basis for the analysis of sensitive elastic parameters and quantitative interpretation for gas hydrate reservoirs.

Rock physics and basin modeling nexus for predicting pore pressure

Improving accuracy in pre-drill pore pressure predictions is a fundamental starting point for well design including setting boundaries for casing points and mud weights. Although rock physics modeling is used routinely in pore pressure estimates, existing rock physics modeling approaches have been largely empirical and face challenges to predict pore pressure accurately, especially for deeper drilling targets where rock matrix properties deviate from normal compaction trends, due to chemical alterations of the rocks. Moreover, existing rock physics models require petrophysical inversion for porosity and volume of shale. It is well-known that petrophysical inversion has non-uniqueness issues and creates additional uncertainty and inaccuracy in pore pressure prediction. Therefore, we developed an integrated modeling approach that involves rock physics modeling and physics-based basin modeling. Through fundamental rock physics modeling and in-depth understanding of the process of clay diagenesis, we recognize that the acoustic impedance from seismic inversions and level of organic metamorphism (LOM, as a proxy for thermal stress) estimated from basin modeling are critical for pore pressure prediction in mudrocks. Based on extensive post-drill data validation, we demonstrate that our modeling approach improves accuracy in pore pressure prediction for depth zones undergoing clay-diagenesis. In addition, we demonstrate that the integrated rock physics modeling greatly reduces the number of calibration parameters as needed in existing models. Blind tests of our method on well data in two different basins with drastically different burial history demonstrates the validity and generalizability of our approach.

Theoretical Simulation of the Resistivity and Fractured–Cavernous Structures of Carbonate Reservoirs

Recently, theoretical modeling based on rock physics has emerged as a pivotal approach to studying the resistivity of complex fractured–cavernous microstructures. In this work, to study the effects of fractured–cavernous structures on carbonate reservoir resistivity, electrical conductivity models were developed based on the effective medium theory and Ohm’s Law, and theoretical simulations were performed to examine how the porosity and resistivity of the rock matrix, the formation water resistivity, and the parameters of the fractured–cavernous microstructure affect the resistivity of rocks saturated with petroleum or water. Furthermore, the modeling results revealed the specific relationships between these factors in petroleum-saturated and water-saturated rocks. For vuggy reservoirs, a significant negative correlation between throat diameter and resistivity was revealed when variations in the rock matrix and formation water resistivity were negligible. Furthermore, the pore shape—especially the extension of pores in the direction of the current—severely reduced the resistivity of petroleum-saturated rocks. For fractured reservoirs, the porosity and resistivity of the rock matrix were the primary factors affecting resistivity, with the fracture inclination angle and width also exhibiting pronounced effects on the resistivity of water-saturated rocks. The rock cementation exponent was much smaller when the matrix pores were interconnected through fractures than when they were interconnected through throats. The findings reveal that the effects of the structural parameters of fractured–cavernous carbonate reservoirs on reservoir resistivity differ between petroleum-saturated and water-saturated rocks. The conventional Archie’s equation is insufficient for evaluating fluid saturation in carbonate reservoirs. A saturation evaluation model with a variable rock cementation exponent tailored to the specific reservoir type should thus be developed.

Elastic rock properties analysis of diagenetic facies identification of deep tight sandstone in the Xihu Sag, China

In order to investigate the physical relationship between elastic properties and reservoir parameters, and determine sensitive seismic elastic parameters of diagenetic facies of deep tight sandstone in the sparse well network area, 16 samples of typical diagenetic facies were selected for rock physics experiment. Diagenetic characteristics and diagenetic facies of tight sandstone were analyzed using petrographic data, and mineral associations and diagenetic pathways of diagenetic facies were investigated. Then, based on rock physics experiment data and petrographic data, sensitive seismic elastic parameters were determined, by comprehensively using rock physics analysis, correlation analysis, and relationship between diagenetic characteristics, mineral composition and seismic elastic parameters. Sensitive seismic elastic parameters can identify four diagenetic facies of tight sandstone. The results show that shear modulus, p-wave to s-wave velocity ratio, p-wave impedance and porosity data volume are sensitive to diagenetic facies, and the correlation between four seismic elastic parameters is weak. The determination of sensitive seismic elastic parameters in this study makes seismic diagenetic facies research targeted to distinguish four diagenetic facies. Understanding the basic physical significance of four elastic parameters is very important for their application to diagenetic facies prediction.

Third‐order elasticity of transversely isotropic field shales

AbstractThe formations above a producing reservoir can exhibit large mechanical changes, creating a risk of significant subsidence and loss of rock integrity. These changes can be monitored by time‐lapse seismic acquisition, which measures the corresponding velocity changes via time‐shifts. Third‐order elastic theory can be used to connect subsurface strains and stress changes to these seismic attribute changes. Existing models assume isotropic strain dependence of the dynamic stiffness in shales. It is important to re‐evaluate this isotropic assumption considering the inherent anisotropy of shales and their abundance in the overburden. Thus, we instead propose a third‐order elastic model with a transversely isotropic strain dependence of the dynamic stiffness. When calibrated, this new model satisfactorily predicted P‐wave velocity changes determined in undrained laboratory experiments conducted on overburden field shales, covering a wide range of propagation directions and stress variations. The shales exhibit anisotropic dynamic strain sensitivity, resulting in a significantly higher strain sensitivity predicted for Thomsen's anisotropy parameters epsilon and delta subjected to a uniaxial strain parallel to the horizontal bedding plane compared to the vertical direction. Geomechanical modelling, considering a depleting disk‐shaped reservoir surrounded by shales, was employed to predict the dynamic stiffness changes of the overburden using the laboratory‐calibrated third‐order elastic model. The overburden time‐shifts increased with offset angle, peaking at about 45°, suggesting a strong influence of shear strains on the time‐shifts. In contrast, a corresponding model with an isotropic third‐order elastic tensor, calibrated to the same data, exhibited a significantly lower sensitivity to the shear strains. These results underscore the importance of considering the anisotropic strain dependence of the dynamic stiffness when studying shales. Interpreting offset‐dependent trends in pre‐stack time‐lapse seismic data, along with geomechanical modelling and an appropriate strain‐dependent rock physics model, can assist in quantifying subsurface strains and stress changes.