Search

A large number of high-quality fractured-vuggy reservoirs (FVRs) are buried in ultra-deep (6500–9000 m) carbonate layers in the Tarim Basin, represented by the strong "beaded" reflection (SBR) on seismic profiles. This characteristic corresponds to the vast majority of high yield oil and gas wells, but there have also been a number of instances where the reservoir was filled with solid, leading to exploration failure. To avoid drilling failure, it is crucial to accurately identify fake reservoirs using geophysical methods. This paper proposes a novel method to eliminate the seismic characteristics of solid-filled in FVRs. Based on the waveform component decomposition (WCD) data, combined with drilling and logging data, an adaptive component reconstruction (ACR) can be completed to distinguish whether the reservoir is filled with solid or not, achieving the goal of eliminating the seismic characteristics of filling. Firstly, supervised WCD is performed on the target seismic data. Then, the waveform characteristics, which indicate the difference in the FVRs filled with solids, are combined accordingly with the sensitive attribute characteristics (instantaneous amplitude, heterogeneous anomalies, arc length, etc.) of each component data, and the feature templates of filled and non-filled FVRs are defined. Finally, the information entropy method is applied to optimise the component set that can distinguish the solid filled and non-filled FVRs and complete the component reconstruction. Forward modelling shows that this method can effectively distinguish the characteristics of FVRs that have been filled by solids from those that not. This method has achieved favourable results in the M block in the Tarim Basin. Successfully removed some or all the characteristic information of solid filled reservoirs confirmed by drilling from seismic data. The effectiveness of the method in identifying solid filled reservoirs has been confirmed by drilling, and the prediction rate of FVRs non-filled with solids has increased from 78% to 95%. This method can be used for reference in blocks with similar geological characteristics. [ABSTRACT FROM AUTHOR]

The fractured-vuggy carbonate oil resources in the western basin of China are extremely rich. The connectivity of carbonate reservoirs is complex, and there is still a lack of clear understanding of the development and topological structure of the pore space in fractured-vuggy reservoirs. Thus, effective prediction of fractured-vuggy reservoirs is difficult. In view of this, this work employs adaptive point cloud technology to reproduce the shape and capture the characteristics of a fractured-vuggy reservoir. To identify the complex connectivity among pores, fractures, and vugs, a simplified one-dimensional connectivity model is established by using the meshless connection element method (CEM). Considering that different types of connection units have different flow characteristics, a sequential coupling calculation method that can efficiently calculate reservoir pressure and saturation is developed. By automatic history matching, the dynamic production data is fitted in real-time, and the characteristic parameters of the connection unit are inverted. Simulation results show that the three-dimensional connectivity model of the fractured-vuggy reservoir built in this work is as close as 90% of the fine grid model, while the dynamic simulation efficiency is much higher with good accuracy. [ABSTRACT FROM AUTHOR]

Fractured-vuggy reservoirs are crucial for increasing unconventional oil storage and production, but the controlling mechanism of this dominant flow channel remains vague, and the jamming mechanism of modulator particles is unclear. This study explores the filling and jamming processes of particles in the fractures by conducting a computational fluid dynamics−discrete element method (CFD−DEM) coupled simulation, considering the variation of fracture width, fluid velocity, particle size, and concentration. Results suggest that four sealing modes are proposed: normal filling, local jamming, complete sealing, and sealing in the main fracture. The ratio of particle size to the main fracture width exerts the primary role, with the ratio having a range of 0.625 < D/W ≤ 0.77 revealing complete jamming. Furthermore, an optimal particle size for achieving stable sealing is observed when the particle size varies from 2 to 2.5 mm. A higher concentration of particles yields better results in the fracture-shrinking model. Conversely, a greater velocity worsens the sealing effect on fractures. This research can offer technical support for the large-scale dissemination of flow regulation technology. [ABSTRACT FROM AUTHOR]

Fractured-vuggy carbonate reservoirs are tectonically complex; their reservoirs are dominated by holes and fractures, which are extremely nonhomogeneous and are difficultly exploited. Conventional water injection can lead to water flooding, and the recovery effect is poor. This paper takes the injection of foam and solid particles to control bottom water as the research direction. Firstly, the rheological properties of foam were studied under different foam qualities and the presence of particles. The ability of foam to carry particles was tested. By designing a microcosmic model of a fractured-vuggy reservoir, we investigated the remaining oil types and the distribution caused by bottom water. Additionally, we analyzed the mechanisms of remaining oil mobilization and bottom water plugging during foam flooding and foam–particle co-injection. The experimental results showed that foam was a typical power-law fluid. Foam with a quality of 80% had good stability and apparent viscosity. During foam flooding, foam floated at the top of the dissolution cavities, effectively driving attic oil. Additionally, the gas cap is released when the foam collapses, which can provide pressure energy to supplement the energy of the reservoir. Collaborative injection of foam and solid particles into the reservoir possessed several advantages. On one hand, it inherited the benefits of foam flooding. On the other hand, the foam transported particles deep into the reservoir. Under the influence of gravity, particles settled and accumulated in the fractures or cavities, forming bridge plugs at the connection points, effectively controlling bottom water channeling. The co-injection of foam and solid particles holds significant potential for applications. [ABSTRACT FROM AUTHOR]

The fractured-vuggy carbonate reservoir comprises various types of storage and seepage spaces, and is composed of multi-scale dissolution pores and fractures. The frequent changes to working systems make the characteristics of water breakthrough complex, and the production data nonlinear and non-stationary, resulting in great difficulty in real-time prediction. Traditional production forecasting methods only consider temporal correlations, neglecting the spatial correlations between production wells and local geological features. In this paper, adopts a modular design approach that comprehensively considers the spatiotemporal characteristics by abstracting each production well in the unit as a directed graph network node. We establish a graph attention network module based on the connectivity between wells to simulate fluid motion patterns and extract spatial features. To address the autocorrelation characteristics of the production sequences, we use a self-attention mechanism module to capture the temporal dependency relationships between production sequences. Finally, considering the fusion of spatiotemporal features, a gating mechanism is designed to adaptively aggregate spatiotemporal characteristics produced by the previous two modules, enabling dynamic production forecasting. We validate our proposed model using real-world production data from the Tarim Basin in China. Our experimental results demonstrate the superiority of the new model over existing production prediction models in fractured-vuggy carbonate reservoirs.

The complexity and strong heterogeneity of carbonate reservoirs with fracture-vuggy structures present significant challenges in reservoir characterization. To address these challenges, we propose a novel multi-element information fusion modeling approach. This approach is designed to integrate multiple methods and incorporate multi-probability fusion at various facies and scales, thereby bridging the gap between geological information and reservoir modeling. Our methodology involves four key steps. First, the statistics between frequency of karst and geological information are acquired, and we quantify the statistics to regression equations. Second, these regression equations are transferred to probability bodies. The probability bodies can be applied in modeling as a soft control. But just one single body can be input in modeling process. Third, multiple probability bodies are fused into a fusion probability body by a probability fusion algorithm, which can keep the potential information of probability bodies. Finally, we apply the probability body in modeling workflow. By this way, the fusion method bridges the gap between geological information and modeling. The model established through our proposed method showed a significant level of consistency with reservoir re-evaluation, achieving an impressive 90% degree of alignment. Furthermore, the history match analysis revealed a high correlation, indicating the model's reliability. The method effectively integrates various scales and types of geological information, offering an accurate approach to complex carbonate reservoir modeling. [ABSTRACT FROM AUTHOR]

The exploration of fractured-vuggy carbonate reservoirs usually involves hydraulic fracturing to maximize recovery. At present, effectively communicating natural discontinuities is a technical challenge. In this article, we investigated the origin and propagation of cracks in fractured-vuggy reservoirs using discrete element hydraulic fracturing simulations that included poromechanical effects. A particular focus on the microscopic force-displacement symmetry of adjacent pore pressures is introduced. Our results demonstrate that the poromechanical effect significantly increases the strength of overpressurized reservoir formations. Moreover, the effect of injected fluid viscosity on the hydraulic fracturing effectiveness was studied through two simulation tests. The outcomes highlight the critical influence of fluid viscosity on the propagation of micro-cracks in overpressure fractured-vuggy reservoirs. [ABSTRACT FROM AUTHOR]

Fractured-vuggy reservoirs are known for containing substantial amounts of oil in high positions of reservoir, even after natural energy development and water injection development. However, due to their poor physical properties and fracture distribution, gas channeling becomes a common occurrence when injecting large amounts of gas, which hinders the formation of an effective gas cap, resulting in reduced oil displacement efficiency. This phenomenon results in a lengthy period of effective gas cap formation and reduces the oil displacement efficiency of an artificial gas cap. In this paper, according to the actual geological characteristics, logging data, and production data, the mechanism model and the numerical model of Oilfield A are established. The variation law of flow potential difference before and after gas injection channeling is studied by simulation, and the control method of artificial gas cap gas channeling in fractured-vuggy reservoir is put forward. The results show that the production gas–oil ratio method is the most convenient and practical in the oil field, and the flow potential difference can effectively predict the occurrence of gas channeling. It likely occurs when the ratio of flow potential difference between injection and production wells is less than 0.972. Gas channeling can be controlled effectively by altering the energy of position and pressure, as well as body measures including injection–production well pattern adjustment, injection–production parameter optimization. This technology provides a new approach for controlling gas channeling through gas cap drive in fractured-vuggy reservoirs. After the implementation of this technology, the effect is obvious, and can effectively improve the efficiency of gas top oil displacement and save costs. This gas channeling control technology is of great significance for the development of fractured-vuggy reservoirs. [ABSTRACT FROM AUTHOR]

There are abundant marine carbonate rock resources in China, which are dominated by fractured-vuggy carbonate oil and gas reservoirs accounting for over two thirds of proved reserves. The fractured-vuggy carbonate condensate gas reservoir in the Tarim Basin is at a burial depth of 4500–7000 m with the characteristics of extremely strong reservoir heterogeneity, complicated and diverse reservoir seepage media, complex fluid properties and quite difficult static characterization, which brings many challenges to reserve evaluation, development plan design and performance analysis. In order to improve the recovery factor of this type of oil and gas reservoir, this paper takes the dynamic characterization to supplement the static characterization and combines each other to improve the accuracy. In addition, based on many years' of dynamic and static research and development practice, the key enhanced gas recovery (EGR) technologies for fractured-vuggy carbonate condensate gas reservoirs are innovatively developed, such as the high-accuracy dynamic characterization technology with dynamic and static iteration for fractured-vuggy body in the strongly attenuated area of desert by taking seismic inversion information as the basis and 3D numerical well test as the core, the multi-target 3D development technology in one well to improve reserve production of fractured-vuggy condensate gas reservoirs, and the gas-lift depressurization EGR technology for fractured-vuggy condensate gas reservoirs. In conclusion, this technological system better solves the key difficulties in the fine characterization of fractured-vuggy body, reserve production improvement and abandonment pressure reduction of well. What's more, it realizes the fine reservoir characterization of fractured zones and feather-shaped fractured zones and the accurate prediction of key development indexes, and increases the deployment success rate of effective wells and efficient wells by 26% compared with that in the initial stage of large-scale productivity construction. To sum up, these technologies provide powerful technical support and reference experience for the scientific and efficient development of fractured-vuggy carbonate condensate gas reservoirs.

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Fractured-vuggy reservoirs are a new target in carbonate oil and gas exploration and development. Because of well-developed reservoir bodies, including fractures and caverns, bottom water invasion can be observed in oilfield development, with low utilization efficiency of crude oil in the reservoir. Accordingly, this study focused on the intersection region of the S99 unit of the Tahe fractured-vuggy reservoirs. Based on seismic data, the reservoir bodies can be divided into three types—caverns, fractures, and broken solution pores. Using the same location condition assignment algorithm, four single-type models are fused into a multi-scale discrete three-dimensional geological model of fractured and cavernous reservoirs, and the corresponding fractured-vuggy reservoir model was established for numerical simulation. The single-well historical fitting precision exceeded 85%. Furthermore, the development can be divided into four stages—initial stage of production, peak production stage, liquid control and oil stabilization stage, and scale gas injection stable. Streamlining sweep analysis determined the utilization and distribution characteristics of the remaining oil in the reservoir. It can be concluded that structure, caverns, and fractures were the main controlling factors affecting the remaining oil distribution in the fractured-vuggy reservoir. The fluid exchange among single-well reserve zones was calculated using streamline-based quantitative sweep analysis and interwell flow quantitative analysis method. Through source-sink quantitative analysis, interwell flow relations were derived, and three water breakthrough modes were further concluded: violent flooding, slow ascending of water cut, and low cut or intermittent water production. [ABSTRACT FROM AUTHOR]

To get a deeper understanding on the formation mechanisms and distribution laws of remaining oil during water flooding, and enhanced oil recovery (EOR) mechanisms by reversing water injection after water flooding, 3D visualization models of fractured-vuggy reservoir were constructed based on the elements and configuration of fractures and vugs, and typical fracture-vug structures by using advanced CT scanning and 3D printing technologies. Then, water flooding and reversing water injection experiments were conducted. The formation mechanisms of remaining oil during water flooding include inadequate injection-production well control, gravity difference between oil and water, interference between different flow channels, isolation by low connectivity channel, weak hydrodynamic force at the far end. Under the above effects, 7 kinds of remaining oil may come about, imperfect well-control oil, blind side oil, attic oil at the reservoir top, by-pass residual oil under gravity, by-pass residual oil in secondary channel, isolated oil in low connectivity channel, and remaining oil at far and weakly connected end. Some remaining oil can be recovered by reversing water injection after water flooding, but its EOR is related to the remaining oil type, fracture-cavity structure and reversing injection-production structure. Five of the above seven kinds of remaining oil can be produced by six EOR mechanisms of reversing water injection: gravity displacement, opening new flow channel, rising the outflow point, hydrodynamic force enhancement, vertically equilibrium displacement, and synergistic effect of hydrodynamic force and gravity.

Fractured-vuggy carbonate reservoirs are mainly composed of holes, caves, and fractures, with large disparity in scale, irregular shape, and discontinuous distribution. Quantitatively depicting the distribution of holes, caves, and fractures in three-dimensional space has always been a technical problem in the development of this type of reservoir, and there is currently no effective geological modeling method. Taking the Yijianfang Formation in the Halahatang area of the Tarim Basin as an example, this paper proposes to model the cavities and fractures separately. The hole model is generated by seismic resampling, and the attribute model uses the inverted porosity as the data volume combined with dynamic and static data constraints to simulate and generate. There are two main methods for fracture modeling. One is deterministic fracture modeling, which is mainly for large-scale fractures; the other is random fracture modeling, which is mainly for small-scale fractures. The deterministic fracture modeling method is based on the seismic data, by extracting the fracture slices of the seismic ant body data, combining the fracture characterization results, screening the fractures, and finally generating a large-scale fracture network; finally integrating hole, the hole model and fracture attributes model to establish a geological model of fracture-cavity reservoirs. Model the cavities and fractures separately was used to drill well Pro1 in yijianfang Formation, revealing that the target zone was 28 meters, and the high yield oil flow was obtained by sidetracking the high point target, which proved that the simulation results were credible.

Copyright of Petroleum Geology & Oilfield Development in Daqing is the property of Editorial Department of Petroleum Geology & Oilfield Development in Daqing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The exploration of fractured-vuggy carbonate reservoirs usually involves hydraulic fracturing to maximize recovery. At present, effectively communicating natural discontinuities is a technical challenge. In this article, we investigated the origin and propagation of cracks in fractured-vuggy reservoirs using discrete element hydraulic fracturing simulations that included poromechanical effects. A particular focus on the microscopic force-displacement symmetry of adjacent pore pressures is introduced. Our results demonstrate that the poromechanical effect significantly increases the strength of overpressurized reservoir formations. Moreover, the effect of injected fluid viscosity on the hydraulic fracturing effectiveness was studied through two simulation tests. The outcomes highlight the critical influence of fluid viscosity on the propagation of micro-cracks in overpressure fractured-vuggy reservoirs.

A modified instrument was designed to evaluate foam properties under high temperature and pressure. The type and molar ratio of betaine surfactants were screened to develop the heat‐resistant and salt‐tolerant foam for Tahe oilfield (130°C, 220 g/L), and the effects of temperature and pressure on foam properties were also investigated. The synergism between surfactants and the enhanced oil recovery (EOR) mechanism of foam flooding in fractured‐vuggy reservoirs were studied. Experimental results showed the developed foam had excellent foaming ability and foam stability when the lauramidopropyl hydroxyl sulfobetaine (LHSB): erucic amide propyl betaine (EAB) molar ratio ranged from 1:1 to 1:2 (initial foam volume was 392 ml when the molar ratio was 1:1, drainage half‐time was 5.75 min and foam half‐time was 72 min when the molar ratio was 1:2 at 130°C and 2 MPa). The synergistic effect was found to reach its maximum when the LHSB:EAB molar ratio ranged from 1:1 to 1:2 according to interaction parameters, which agreed with the results of foam properties. Foam stability was found to considerably increase with increasing pressure, but decrease with increasing temperature. However, temperature and pressure were found to have consistent effects on foaming ability, that is, the foaming ability increased with increasing temperature and pressure. The flooding test showed foam flooding exhibited better sweep efficiency and higher recovery ratio in the fractured‐vuggy model than gas flooding and water flooding. This could be because injected foam did not channel through the top (or bottom) path due to its high viscosity and moderate density. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Permeability analysis is an essential process in reservoir characterization. As one of the most development potential kind of reservoirs, the coexistence of fractures and vugs brings great challenges to permeability analysis of fractured-vuggy carbonate reservoirs due to its highly heterogeneity on multi scales. In this work, a novel permeability analysis model is developed for fractured-vuggy carbonate reservoirs based on fractal theory, which can predict the permeability of fractured-vuggy porous media within a reliable range. Besides, a Monte Carlo simulation has also been implemented to obtain feasible values of permeability based on experimental sample parameters. The validity of the proposed fractal model is verified by both the experimental data and numerical simulation results. Each parameter in the model has clear physical meaning. The results show that the permeability of a fractured-vuggy carbonate reservoir is related to the fracture fractal dimension, fracture fractal tortuosity dimension, the maximum vug diameter and the maximum fracture aperture. [ABSTRACT FROM AUTHOR]

Due to the complex connectivity of fractured-vuggy reservoirs, there is always a large amount of remaining oil after water flooding development. The blind cave remaining oil, one of the primary remaining oil occurrence forms, is the most difficult to recover. Based on the distribution pattern of these types of remaining oil, the paper designed and manufactured a variety of tube-cave models. Through physical experiment, numerical experiment and theoretical calculation, the ability for water to displace blind cave remaining oil in different conditions, oil-water convective displacement characteristics and the critical parameters for remaining oil development were studied, respectively. The results show that the interfacial tension, fracture width and oil-water gravity difference are the most important parameters to determine whether the remaining oil can be exploited; the wall wettability and oil viscosity mainly affect the oil-water displacement rate due to the loss of non-wetting internal friction. In addition, the interfacial tension should be reduced to 1 mN/m or lower to accelerate the convective flow in the fractures, so that most of the blind cave remaining oil can be developed. This paper provides the influencing factors and exploitation measures of the blind cave remaining oil in fractured-vuggy reservoirs. [ABSTRACT FROM AUTHOR]

The fracture-vuggy carbonate reservoir possess strong heterogeneity, and the reservoir space of it is mainly composed of fractures and vugs. After water injection and gas injection development, there is a large amount of remaining oil in the reservoir space. A two-dimensional visual physical model which can characterize the structural characteristics of different fractures and vugs was designed and made according to the geological characteristics of fracture-vuggy carbonate reservoirs in Tahe oilfield. The physical experiments of foam flooding were carried out based on these visualization model. The foam flooding effect and the production mechanism of different fracture-vuggy structures were revealed, and the foam production effect on remaining oil in different displacement stages was clarified. The study shows that foam has excellent mobility control ability and high permeability channel plugging ability. It has a good displacement effect for horizontal, vertical and high-angle fractures, and can effectively improve the oil recovery of fracture-vuggy carbonate reservoirs at different development stages. The existence of filling medium is beneficial to enlarge the foam sweep volume and improve the microscopic oil displacement efficiency. Enhancing the strength and stability of the foam can enhance the ability of the foam to change fluid flow direction. Reduce the density of foam can reduce the interfacial tension and improve the foam flooding recovery in fracture-cave reservoir. The results of laboratory research can provide a theoretical basis for the field application of foam flooding in fracture-vuggy carbonate reservoirs. [ABSTRACT FROM AUTHOR]

The fractured-vuggy reservoirs (FVRs) wrapped in strike-slip fault-damaged zones (SSFDZs) are the main drilling targets for oil-gas exploration in the Tarim Basin. The seismic characteristics of the FVRs are strong wave crest, strong wave trough, chaotic waveform and the random combination of these three types of waveforms, which are distributed in strips. Constrained sparse pulse inversion (CSSI) results based on the traditional relatively homogeneous low frequency impedance model (LFIM) cannot characterize heterogeneous FVRs. To solve this bottleneck, this paper proposes the viewpoint that the SSFDZ-transformed reservoirs are geologically isochronous. A multi-iterative inversion method (MIIM) is proposed, including multi-iterative CSSI and poststack geostatistics inversion (PSGI). This method extracts the information implicit in seismic data by AI-assisted local heterogeneity and establishes an SSFDZ framework to form a heterogeneous LFIM (HLFIM). The process performs multiple iterative constrained inversion based on the HLFIM. Finally, by combining the relative impedance with the HLFIM, absolute impedance data consistent with drilling and geological understanding are obtained. The result of MIIM improves the ability to distinguish different scale FVRs features controlled by SSFDZs. The recovery rate of reservoirs with different scales in the SSFDZs of the forward geological model is 98.15%. In the FuMan area, the success rate of commercial oil and gas flow by drilling has increased from 75 to 95% in two years and has been maintained for three years. This method can be popularized and applied, and can be used as a reference for the extraction of heterogeneous information in remote sensing data. [ABSTRACT FROM AUTHOR]

The existence of large fractures and caves and frequent working system adjustments have resulted in the diverse characteristics of water cut in the fractured-vuggy reservoir, which makes it difficult to early warn the water flooding.Aiming at the problem of time delay of traditional early warning methods, this paper uses K-line theory to describe the change trend of water cut production indicators, and summarizes the pre-flooding abnormal patterns such as abundant type, breakthrough type and reversal type.Since the recurrent neural network can memorize the long-term correlation between production data, LSTM is used to automatically identify the features of abnormal pattern to realize early warning of water flooding.The experimental results show that by transforming the data scale, the proposed abnormal pattern recognition model based on LSTM can successfully extract the overall trend of data before water flooding.The recognition accuracy of the proposed model is significantly higher than that of support vector machine and naïve Bayes and other models.Various kinds of abnormal patterns described by K-line can effectively solve the traditional problem of prediction delay.The proposed model realizes early warning of water flooding one to three weeks in advance, and provides new ideas for early warning of water flooding.

Due to the strong heterogeneity of fractured-vuggy reservoirs in Tahe Oilfield, the results of reserves calculated using the Volumetric Method are not ideal. To improve the calculation accuracy of reserves, this study selected 524 evaluation Wells of the fractured-vuggy reservoir in Tahe Oilfield, and adopt the probability method (or Monte Carlo method) to establish the characteristics and distribution model of reserves per unit coefficient (RPUC) based on the reservoir geological conditions, crude oil density, and development process. From the mathematical test, the Beta function is more applicable to represent the probability model of the RPUC parameters. The RPUC of reservoirs controlling by weathering crust is found to be the greatest, and proved to be the most favorable reservoir in this area. The calculation results obtained using the probability method are more reliable, compared to the other three methods, which are in line with the estimated reserves during oil and gas exploration. The RPUC values of reservoirs reveal that Aidingbei, and Yuqi porous reservoirs were relatively developed with better resource prospects. The above research can provide a reference for the calculation of the reserves for the fractured-vuggy carbonate reservoir of the new area in the Tarim Basin. [ABSTRACT FROM AUTHOR]

Various storage and seepage spaces exist in fractured-vuggy carbonate reservoirs composed of multi-scale dissolution pores and fractures. The frequent regulation of the working system causes nonlinear and unstable production data along with complex water breakthrough characteristics, which lead to difficulty in real-time prediction. Traditional methods based on the water drive curve and autoregressive machine learning ignore the spatial correlation among production wells and local geological characteristics. To address these problems, we propose a model based on improved attention, the spatiotemporal multi-graph convolutional network (STMCN), for production prediction. The unit production wells are abstracted as directed graph network nodes to establish adjacency, connectivity and correlation graphs to extract the spatial and semantic features from different perspectives through graph convolutional networks. To depict the law of fluid movement, we realize the fusion of spatial and semantic information through dynamic routing. Aiming at the autocorrelation characteristics of production sequences, the model uses the self-attention mechanism to capture the dependencies in production sequences. The gating mechanism is designed to achieve dynamic production prediction by adaptively aggregating the spatiotemporal characteristics. This paper evaluates the predictive performance of our model by two real-world datasets of the Tahe Oilfield. The results show that the mean absolute error (MAE), root mean squared error (RMSE), and mean absolute percentage error (MAPE) of our proposed model for the prediction of well A2 were 3.19, 4.66 and 0.07, respectively, which were better than the relatively new baseline model. [ABSTRACT FROM AUTHOR]

To define the channel flow control (CFC) mechanism of the equidensity particle agent, the effect of CFC treatment in fractured-vuggy carbonate reservoirs was systematically analyzed by visual simulation experiments and field tests. The typical visual experimental model of the fractured-vuggy reservoir was designed based on the fractured-vuggy reservoir outcrop characteristics. It overcomes the shortcomings of the conventional model, whose experimental results are greatly affected by subjective factors. The water breakthrough in the dominant flow channel caused substantial shielded remaining oil to be left in the reservoir, resulting in violent flooding of the oil well. The shielded remaining oil accounted for approximately 43.8% of reserves, which is the potential point of the CFC treatment. The CFC treatment can effectively enhance the shielded remaining oil recovery by 22.2% in laboratory experiments. Based on its own efficient suspension and migration ability, the equidensity particle agent can be transferred to the whole dominant flow channel in the fractured-vuggy carbonate reservoirs. By using the multistage distributed shrinkage joint at the outlet of each vuggy and inside of each fracture to control the flow capacity, the equidensity particle agent can equably control the dominant flow channel along the path to realize a wide range of water flooding in the fractured-vuggy carbonate reservoirs. [ABSTRACT FROM AUTHOR]

The fractured-vuggy reservoir is one important carbonate reservoir in the world. Compared with the sandstone reservoir, the carbonate fractured-vuggy reservoir has some significant characteristics such as the strong reservoir dispersion and the single interwell connectivity. Currently, the evaluation indexes, the evaluation standards and evaluation methods for water-flooding development effect in the fractured-vuggy reservoir are not uniform. To solve the issues include large uncertainty in evaluation and a big difference in evaluation results caused by the different indexes in the fractured-vuggy reservoir, we choice five typical evaluation indexes to estimate waterflooding development effect in fractured-vuggy reservoir. The indexes include increase in recovery rate, energy keeping rate, annual comprehensive oil production natural decline rate, water/oil exchange rate and water content rise rate. Meanwhile, a new evaluation method of evaluating fractured-vuggy reservoir water-flooding development effect is established based on the uncertain measure. The field application shows that the newly established waterflooding development effect evaluation method has a good match with reservoir dynamic analysis results, and our method can be used for the waterflooding development effect evaluation in fractured reservoir. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A reservoir-scale numerical conceptual model was established according to the actual geological characteristics of a carbonate fractured-vuggy reservoir. Considering the difference in density and viscosity of fluids under reservoir conditions, CFD (computational fluid dynamic) porous medium model was applied to simulate the process of nitrogen displacement in a fractured-vuggy reservoir after water flooding. The effects of gas injection rate, injection mode, and injector–producer location relation were studied. The results show that nitrogen flooding can yield additional oil recovery of 7–15% after water flooding. Low-speed nitrogen injection is beneficial in obtaining higher oil recovery. High speed injection can expand the sweep area, but gas channeling occurs more easily. In gas–water mixed injection mode, there is fluid disturbance in the reservoir. The gas channeling is faster in low injector–high producer mode, while the high injector–low producer mode is beneficial for increasing the gas sweep range. Nevertheless, the increment of recovery is closely related to well pattern. After nitrogen flooding, there are still a lot of remaining oil distributed in the trap area of gas cap and bottom water in the reservoir that water and gas injection can’t sweep. The establishment of the numerical conceptual model compensates for the deficiency of physical simulation research, stating that only limited parameters can be simulated during experiments, and provides theoretical bases for nitrogen flooding in fractured-vuggy reservoir.

Large cracks are important seepage channels inside fractured-vuggy reservoirs. Therefore, in this thesis, the calculation method of fully coupled modeling of the fractured saturated porous medium based on the extended finite element method (XFEM) is established to study the expanding regularity of cracks in fractured-vuggy reservoirs. Fully coupled governing equations are developed for hydro-mechanical analysis of deforming porous medium with fractures based on the stress balance equation, the seepage continuity equation and the effective stress principle. The final nonlinear fully coupled equations reflect not only the coupling effect of the physical quantity within the porous medium but also the coupling between the medium and the fracture. During the spatial dispersion of coupled equations based on XFEM, two kinds of additional displacement functions are introduced in the displacement model of the fracture area to reflect the strong discontinuity of the fracture surface. The pore pressure enhancement function is also applied to represent the weak discontinuous features of the normal pore pressure. The validity and efficiency of this model and calculation are verified through three calculating examples. The following crack propagation laws are obtained: (1) The larger the water flow rate is, the longer the crack propagation length is, and the larger the propagation width is (2) The greater the crack angle and the crack length, the easier it is to expand the crack. Besides, compared with dip angle, the crack length has a more sensitive influence to the crack propagation. (3) When multiple cracks exist, the larger the fracture spacing is, the easier the crack will expand. [ABSTRACT FROM AUTHOR]

Visual models of fractured-vuggy reservoirs were designed and manufactured to conduct experiments of nitrogen assisted gravity drainage (NAGD). The impacts of flooding pattern, gas injection rate, well type, and displacement direction (vertical or horizontal) on development performances and remaining oil distribution were studied. The results show that during NAGD, the sweep scope is decided by the connections between producer and reservoir, and the local sweep efficiency is decided by fracture-vuggy configuration. The homogenous fractured reservoir has higher oil recovery, and the bigger the aperture of fracture is, the higher the recovery. The main regions of remaining oil due to poor connectivity and gas-oil gravity difference include blind fractures and vugs below the connected fractures, the bottom of vugs, and the narrow and low-angle fractures. The accumulation of remaining oil in the bottom of reservoir is easily formed and controlled by the connections between producers and reservoir. The higher the gas injection rate and the stronger the fracture heterogeneity, the earlier the gas channeling and the lower the oil recovery of the producer will be. Horizontal wells have the best development effect, so horizontal well can be applied in fractured-vuggy reservoirs without bottom water. Producers should be preferentially drilled at low structural position. Gas channeling firstly occurs in the producer at high structural position, and it should be shut in timely to improve the utilization of injected gas. [ABSTRACT FROM AUTHOR]

Abstract Universal existence of the filled vug or cave in carbonate reservoir complicates the fluid flow as the coexistence of free flow and porous flow. The fills in vug increase the flow resistance and affect the water invasion for a natural bottom-water reservoir. This influence was explored through experimental tests on a conceptual discrete fractured-vuggy model. This model was abstracted from Tahe Oilfield in China, in the form of isolated vugs connected by fractures network. A new scaling method was proposed for this discrete media according to the similarity of the displacement front of water drive. It was found that vug filling displays a negative role during the natural bottom-water drive. This impact depends on the geological feature of fractured-vuggy reservoir. A stronger sensitivity occurred to the fracture well-developed stratum that the vug filling made the pre-existing or new preferential corridors more significant. As a result, the systems with multiple flow paths became more "heterogeneous". In this condition, a lower water intensity and oil viscosity can ease these adverse effects. Highlights • Establish a new fractured-vuggy conceptual model in the form of isolated vugs connected by fractures network. • Establish a new scaling method for the discontinuous fracture-vug media based on the similarity of displacement front advance. • Conduct natural bottom-water drive in discrete fractured-vuggy model. • Show the effects of vug filling, bottom-water intensity, and fluid viscosity on oil displacement efficiency in this media. [ABSTRACT FROM AUTHOR]

Abstract To effectively solve the problem of lost circulation and well kick frequently occurring during the drilling of abnormally high temperature and pressure fractured-vuggy reservoirs in the Tazhong block, a rigid particle material, GZD, with high temperature tolerance, high rigidity (> 8 MPa) and low abrasiveness has been selected based on geological characteristics of the theft zones in the reservoirs. Through static pressure sealing experiments, its dosage when used alone and when used in combination with lignin fiber, elastic material SQD-98 and calcium carbonate were optimized, and the formula of a new type (SXM-I) of compound lost circulation material with high temperature tolerance and high strength was formed. Its performance was evaluated by compatibility test, static sealing experiment and sand bed plugging experiment. The test results show that it has good compatibility with drilling fluid used commonly and is able to plug fractures and vugs, the sealed fractures are able to withstand the static pressure of more than 9 MPa and the cumulative leakage is 13.4 mL. The mud filtrate invasion depth is only 2.5 cm in 30 min when the sand bed is made of particles with sizes between 10 mesh and 20 mesh. Overall, with good sealing property and high temperature and high pressure tolerance, the lost circulation material provides strong technical support for the safety drilling in the block. [ABSTRACT FROM AUTHOR]

Copyright of Journal of China University of Petroleum is the property of China University of Petroleum and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Universal existence of the filled vug and fracture in carbonate fractured-vuggy reservoir upgrades the complexity of fluid flow as the coexistence of free flow and porous flow. The highly discrete distribution of fracture and vug makes it possible to apply Dykstra-Parsons method in this kind of medium. A modification of Dykstra-Parsons method is proposed to depict the displacement in fracture-vug system, which consists of multiple non-communicating fracture-vug units with each containing fracture, fills, and vug. The displacement front advance equation is derived piecewise corresponding to the position of the front in fracture, fills, and unfilled vug of one unit. The solution to this equation for every fracture-vug unit is used to estimate the production dynamics of the fracture-vug system, including the fractional oil recovery, the water cut, the injected pore volume, and the injectivity ratio. A calculating example with log-normal distribution for fracture aperture is involved to make the sensitivity analysis of gravity number, viscosity ratio, fills properties, and variation of the fracture aperture. In terms of increased oil recovery, decreased water cut, and delayed water breakthrough, it was found that a better performance reached in the condition of weak heterogeneity of fracture, large gravity number, and lower oil//water viscosity ratio; the fills with low permeability and high filling degree is favorable when all vugs are filled identically. [ABSTRACT FROM AUTHOR]

Ying Mai-2 Block of Tarim Oil Field is a karst fractured-vuggy reservoir. Along with the research on well location demonstration and recovery ratio improvement in the developing progress, the importance of fracture-vug unit division is gradually revealed. This paper presents an integrated method for establishing a fractured element/flow unit in the oil field and discusses its importance and its influence on future well location optimization. With more dynamic data from the oil field, this workflow can be reused to divide the unit during the whole life of the well. This paper begins with the understanding and characterization of involved reservoirs, then establishes a geological concept model and makes clear reservoir type, geometric boundary, reserves and aquifer energy of the fracture-vug unit. Dynamic and static data are used by multidisciplinary means to describe the fracture-vug unit, and reasonable recommendations are proposed on well testing for these reservoirs. A comprehensive evaluation on the fracture-vug combination model and dominant reservoir type is made based upon the flow experiments of a large-scale physical model in combination with production performance and well testing data. Based on this, the fracture-vug units in an individual well are divided using dynamic production characterization. Finally, the fracture-vug units for Ying Mai-2 Block are divided by seismic prediction and gridding judgment taking the units with well data as the standard, and thus a powerful guidance for the reservoir development at present stage is provided. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Fractured-vuggy carbonate reservoirs have great potential for development with diverse reservoir space types and very complicated fluid flow dynamics. The height of high-angle fractures and cavities in fractured-vuggy carbonate reservoirs can reach several meters and fluids are extremely prone to transport and mass exchange along the vertical direction. The vertical equilibrium phenomenon can significantly affect the oil-water spatial distribution in this type of reservoir. However, the existing numerical simulation methods do not consider the effect of fluid vertical equilibrium mechanism, and the predicted results often have large deviations from the actual situation. In this paper, considering the connectivity structure of typical fractured-vuggy unit, several visualized fractured-vuggy physical models are firstly prepared by 3D printing technology, and the vertical equilibrium time of oil and water under different fractured-vuggy combination modes are determined using the vertical equilibrium experiments. 'Dynamic reconstruction', a multi-scale vertical equilibrium method, is employed for the decoupling calculation of fluid saturation and pressure profiles in coarse-scale and fine-scale grids of fractured-vuggy reservoirs. Compared with indoor experiments, the simulated results when considering the fluid vertical equilibrium are much closer to the experimental phenomena than that calculated by the traditional method, proving the proposed method's reliability. Finally, the vertical equilibrium model is applied to study two-phase fluid flow in a typical fractured-vuggy unit of Tarim oilfield. The proposed method has been proven to reveal the flow characteristics of oil-water two-phase and remaining oil distribution in fractured-vuggy carbonate reservoir more accurately. • Diagnostic condition of vertical equilibrium time in fractured-vuggy carbonate reservoirs is obtained. • A multi-scale 'dynamic reconstruction' method is used to decouple calculation of fluid saturation and pressure profiles. • A numerical simulation method by considering vertical equilibrium is developed. • The calculated results are more reliable than those of the traditionalmodel. [ABSTRACT FROM AUTHOR]