Large-scale Reservoir Research Articles

Research and Application of Reservoir Flood Control Optimal Operation Based on Improved Genetic Algorithm

This paper took the Foziling Reservoir in the Pi River Basin as an example, used an improved genetic algorithm to optimize the flood control dispatching during the flood process, and compared the results with the traditional genetic algorithm and the dispatching plan in the 2020 large-scale reservoir flood control operation plan. The results showed that, compared with the traditional genetic algorithm, the improved genetic algorithm saved the time for the model to determine the penalty coefficients and made the model application more convenient. At the same time, the design of the original scheduling scheme also has certain limitations. The scheduling results obtained by improving the genetic algorithm could occupy a small flood control capacity as much as possible under the premise of ensuring the safety of the reservoir itself and the downstream area.

Coupling Relationship Between Densification Characteristics and Tight Oil Accumulation of the FGSR Reservoir: A Case Study of the Chang 7 Member of the Triassic Yanchang Formation in the Zhenyuan Area of the Southwest Ordos Basin, Central China

Fine-grained sedimentary rock (FGSR) reservoirs in a deep-water sedimentary environment from the Late Triassic Yanchang Formation in the Ordos Basin in central China have huge potential for tight oil production. According to the comprehensive research of fluid inclusion experiment, scanning electron microscopy, cathodoluminescence, thin section identification, and whole rock analysis, the petrographic characteristics, fluorescence spectrum parameters, and the homogenization temperature of oil inclusions and associated brine inclusions in the FGSR reservoir were analyzed to explore the coupling relationship between densification characteristics and tight oil accumulation of the FGSR reservoir. The results show the FGSR reservoir diagenesis has experienced compaction, cementation, and multiple stages of dissolution. A comprehensive analysis of burial history shows that the Chang 7 Member FGSR reservoir in the study area has three phases of oil charging, corresponding to the geologic time range of 132–124, 120–102, and 98–97 Ma. The organic acid produced by the first-stage oil charging accelerated the dissolution of feldspar and cuttings, and the dissolution pores increased significantly. Then, the organic acid in the second-stage oil charging accelerated the dissolution, and the abnormally high hydrocarbon expulsion pressure inhibited the damage of compaction on the pores of FGSR reservoir. Finally, the thermal evolution degree of source rocks reached the highest in the third-stage oil charging. The constructive effect of large-scale organic acids on pores and the protective effect of the chlorite membrane on pores inhibited the densification process of the FGSR reservoir. In short, the FGSR reservoir has experienced a cyclical reservoir densification process of “reducing porosity–increasing porosity–preserving porosity.” The coupling of pore evolution and oil charging of the FGSR reservoir is the key to the extensive accumulation and large-scale reservoir formation of tight oil in the Chang 7 Member of the Yanchang Formation in the Zhenyuan area, southwest Ordos Basin. The significance of the study may help to understand the process of the tight oil accumulation and predict the tight oil of the FGSR reservoir distribution.

Multi-objective water-sediment optimal operation of cascade reservoirs in the Yellow River Basin

Sediment deposition in sediment-laden rivers seriously restricts the normal operation of reservoirs and the socioeconomic development of the basin. Studying the water–sediment joint operation of cascade reservoirs is important for extending the life of reservoirs. The objective of this paper is to provide a sound scientific approach to optimize the water–sediment operation of cascade reservoirs considering multiple objectives. An innovative time-dependent objective function was proposed, in which the sediment release efficiency equations suitable for different sediment inflow scenarios were introduced for sediment flushing calculation. The Sanmenxia (SMX) and Xiaolangdi (XLD) Reservoirs on the Yellow River were selected as case studies to test the proposed method. First, two typical data sets, including high and low flow magnitudes, were taken as model input scenarios. Second, the multiple-objective joint optimal operation model was established. Discrete differential dynamic programming (DDDP) was used to solve the operation model. Finally, the operation results based on different weights were discussed. Results indicate that, for sediment-laden rivers, such as the Yellow River, the proposed model can achieve the maximization of the hydropower generation and sediment flushing benefits for the high flow and low flow series. And the proposed model can guarantee the flood control, ice flood control, water supply, and ecological objectives. For the SMX-XLD cascade reservoirs, when the water resources are abundant, the hydropower generation reservoirs increase significantly. And in flood season, the weight for sediment flushing can be increased appropriately to improve the comprehensive benefits of the reservoir in sediment-laden rivers. In addition, the time-dependent objective function can provide a reference for the multi-objective optimal operation of cascade reservoirs. The optimal operation model provides a model basis for the water–sediment-energy joint operation of large-scale reservoir group in the sediment-laden basins.

Numerical Simulation of Hydrate Formation in the LArge-Scale Reservoir Simulator (LARS)

The LArge-scale Reservoir Simulator (LARS) has been previously developed to study hydrate dissociation in hydrate-bearing systems under in-situ conditions. In the present study, a numerical framework of equations of state describing hydrate formation at equilibrium conditions has been elaborated and integrated with a numerical flow and transport simulator to investigate a multi-stage hydrate formation experiment undertaken in LARS. A verification of the implemented modeling framework has been carried out by benchmarking it against another established numerical code. Three-dimensional (3D) model calibration has been performed based on laboratory data available from temperature sensors, fluid sampling, and electrical resistivity tomography. The simulation results demonstrate that temperature profiles, spatial hydrate distribution, and bulk hydrate saturation are consistent with the observations. Furthermore, our numerical framework can be applied to calibrate geophysical measurements, optimize post-processing workflows for monitoring data, improve the design of hydrate formation experiments, and investigate the temporal evolution of sub-permafrost methane hydrate reservoirs.

The sharp ALMA view of infall and outflow in the massive protocluster G31.41+0.31

Context.To better understand the formation of high-mass stars, it is fundamental to investigate how matter accretes onto young massive stars, how it is ejected, and how all this differs from the low-mass case. The massive protocluster G31.41+0.31 is the ideal target to study all these processes because observations at millimeter and centimeter wavelengths have resolved the emission of the Main core into at least four massive dust continuum sources, named A, B, C, and D, within 1″ or 0.018 pc, and have identified signatures of infall and several outflows associated with the core.Aims.We study the interplay between infall and outflow in G31.41+0.31, at a spatial resolution of a few 100 au, by investigating their properties and their possible impact on the core.Methods.We carried out molecular line observations of typical high-density tracers, such as CH3CN or H2CO, and shock and outflow tracers, such as SiO, with ALMA at 1.4 mm that achieved an angular resolution of ~0.′′09 (~340 au).Results.The observations reveal inverse P Cygni profiles in CH3CN and H2CO toward the four sources embedded in the Main core, suggesting that all of them are undergoing collapse. The infall rates estimated from the redshifted absorption are on the order of ~10−2M⊙yr−1. The individual infall rates imply that the accretion timescale of the Main core is an order of magnitude smaller than its rotation timescale. This confirms that rotating toroids such as the G31 Main core are non-equilibrium, transient collapsing structures that need to be constantly replenished with fresh material from a large-scale reservoir. For sources B, C, and D the infall could be accelerating inside the sources, while for source A the presence of a second emission component complicates the interpretation. The SiO observations have revealed the presence of at least six outflows in the G31.41+0.31 star-forming region, and suggest that each of the four sources embedded in the Main core drives a molecular outflow. The outflow rates are on the order of ~10−5–10−4M⊙yr−1, depending on the SiO abundance. The feedback of the outflows appears sufficient to sustain the turbulence in the core and to eventually disrupt the core or prevent its further collapse. The mass accretion rates onto the individual sources, estimated from the highest values of the outflow mass loss rates, are on the order of 10−4M⊙yr−1. The difference of about two orders of magnitude between the accretion rates and the infall rates suggests that the central stars and the jets and outflows might not be efficient in removing disk material, which could lead to gravitational instabilities, fragmentation of the disk, and episodic accretion events.Conclusions.Infall and outflows are simultaneously present in all four sources embedded in the high-mass star-forming core G31.41+0.31. This indicates that these sources are still actively accreting and have not reached their final mass yet, which confirms the youth of this massive core.

Advanced workflow for multi-well tracer test analysis in a geothermal reservoir

Interpretation of tracer tests in geothermal reservoirs is carried out by fitting the measured data either with simplified two-dimensional (2-D) analytical solutions or with complex numerical models. Available analytical solutions commonly only describe isotropic conditions in 1-D or 2-D, which is generally unsatisfactory to construct realistic reservoir models. Moreover, due to the large spatial and temporal scale of a tracer test in deep reservoirs, the concentration levels measured in the field are relatively low due to dispersion that complicates the assessment of breakthrough curve tailing and residence time. Fitting tracer data with fully resolving 3-D numerical models thus may be more appropriate, but even with a rich set of field data, reliable calibration is often compromised by the high computational effort and data hunger of such models. In this study, an advanced workflow is presented for evaluating tracer test data. Firstly, a 3-D analytical model in which solute transport is considered in anisotropic porous media is developed. Green's function method is applied to obtain a moving line source solution of the convection-dispersion-diffusion equation for solute transport in a 3-D porous medium. In addition, Green's function equation is analytically convoluted with a rectangular pulse function, which represents tracer injection. Secondly, the analytical model results are fitted to the tracer test data by Monte-Carlo simulations to obtain feasible ranges of flow velocities, as well as longitudinal and transversal dispersivities. Finally, the derived parameter values are implemented in a 3-D numerical model to evaluate the solute residence time in a large-scale reservoir. The results applied to field data from the Kızıldere field in Turkey demonstrate that the workflow provides robust estimates of effective parameters from well-to-well data in complex reservoir systems with anisotropic flow paths. Thus, despite the higher effort of applying convolution and stochastic parameter estimation, the preceding analytical step of the workflow substantially eases numerical model set-up.

Connection element method for reservoir numerical simulation

<p indent=0mm>Abstract: The reservoir numerical simulation presently encounters problems based on the fine grid. These include poor convergence and difficult history matching. In practical application, the coarse grid often leads to the destruction of the flow structure and the significant reduction of the prediction accuracy. This paper proposes a new reservoir numerical simulation method different from the traditional grid-based methods. The proposed method discretizes the reservoir calculation domain into a series of connection elements from the flow perspective. According to the physical properties of nodes, the support domain and the moving least squares are used to estimate the Laplace operator of pressure and provide the calculation method of the attribute parameters reflecting the flow ability (connection transmissibility) and mass basis (connection volume) of the connection element. Taking the connection element as the object, the flow flux in the element is obtained by solving the node pressure equation. The transport equation about saturation is also solved by using a semianalytical method along the connection element to obtain the production dynamics. The path tracking algorithm is introduced to quantitatively represent the flow interaction and connectivity between the source and sink nodes. Finally, a reservoir numerical simulation method based on the connection element system, which is referred to herein as the connection element method (CEM), is introduced. Practical applications show that the CEM can realize the fast prediction and history matching of the reservoir production performance and identification of the injection-production connectivity. The conceptual example also verifies that in the case of the same node (grid) distribution, the CEM shows better calculation accuracy and efficiency compared with the traditional finite difference method. At the same time, the node allocation in the CEM is more flexible and can ensure the flow path integrity under fewer nodes to improve the prediction accuracy. It also supports the parallel computing of transport equations on connection elements. In conclusion, the CEM can better meet the requirements of the rapid prediction of actual large-scale reservoirs and provide a new idea for reservoir numerical simulation with complex geometric characteristics, such as fracture, fracture-cavity type, and complex boundary.

Dynamic coordination and control technology of the operating water level during flood season and its application in Xiluodu-Xiangjiaba - Three Gorges cascade reservoirs

To satisfy the increasing multi-objective scheduling requirements in flood season of the Xiluodu-Xiangjiaba - Three Gorges multi-function cascade reservoir system located in the lower reaches of Jinsha River and the upper reaches of Yangze River, and simultaneously solve the key technical problems of the resource utilization of floods by joint optimal scheduling of cascade reservoirs. This paper explorers and presents a scientific solution of joint dynamic control of the reservoirs’ operating water level during flood season that suitable for the flood resources comprehensive utilization of the large-scale cascade reservoirs in the Yangtze River Basin. Specifically, the solution is composed of a dynamic control mode for the operating water level of the cascade reservoir during the main flood season (ie, forecast and pre-impounding, grade regulating, and staged discharge control), a phased release theory of the flood control capacity at the end of flood season and a risk analysis model for the decision of the cascade reservoir scheduling. These new technical solutions were successfully applied to the 2016-2019 scheduling practices of the Xiluodu- Xiangjiaba - Three Gorges cascade reservoir system, the world’s largest multi-function multi-reservoir system that operation and management by the China Three Gorges Corporation (CTG). The comprehensive benefits of the cascade reservoir in flood control, power generation, shipping, ecology, and the water supply during reservoir impounding and falling periods have been significantly improved, including greatly reducing downstream flood pressure, increasing power generation and water supply, improving navigation condition, promoting fish reproduction. Meanwhile, it also verifies that these new technologies are scientific, practical, universal and have a broad application prospect.

Dynamic effective permeability of a laminated structure with cross flow in the transient flow process and its application to reservoir simulation

The effective permeability of a laminated structure is an important parameter related to the practical necessity of coarsening of grid blocks in large scale reservoir simulations. Traditional effective permeability derived under steady state flow conditions can cause great errors in the transient flow process due to the great heterogeneity and anisotropy of laminated formations. To handle this error, the unsteady oil flow in the matrix between hydraulic fractures of a laminated system during an oil depletion process is analyzed in this study. Based on material balance, we present an analytical method for calculating the dynamic effective permeability of the laminated structure considering the cross flow between laminae in the transient flow process. The proposed dynamic effective permeability can be used to conduct coarsening of grid blocks and can be incorporated into numerical models for accurate modeling of transient flow in a laminated structure in the reservoir simulation. It is shown that the simulation results using the proposed effective permeability model match well with those from two-dimensional (2D) simulations. Further, the effects of the horizontal permeability, vertical permeability, thickness of the laminae, and the length of the matrix between fractures are studied.

The first power generation test of hot dry rock resources exploration and production demonstration project in the Gonghe Basin, Qinghai Province, China

Hot dry rock (HDR) is a kind of clean energy with significant potential. Since the 1970s, the United States, Japan, France, Australia, and other countries have attempted to conduct several HDR development research projects to extract thermal energy by breaking through key technologies. However, up to now, the development of HDR is still in the research, development, and demonstration stage. An HDR exploration borehole (with 236 °C at a depth of 3705 m) was drilled into Triassic granite in the Gonghe Basin in northwest China in 2017. Subsequently, China Geological Survey (CGS) launched the HDR resources exploration and production demonstration project in 2019. After three years of efforts, a sequence of significant technological breakthroughs have been made, including the genetic model of deep heat sources, directional drilling and well completion in high-temperature hard rock, large-scale reservoir stimulation, reservoir characterization, and productivity evaluation, reservoir connectivity and flow circulation, efficient thermoelectric conversion, monitoring, and geological risk assessment, etc. Then the whole-process technological system for HDR exploration and production has been preliminarily established accordingly. The first power generation test was completed in November 2021. The results of this project will provide scientific support for HDR development and utilization in the future.©2022 China Geology Editorial Office.

Identifying key drivers of harmful algal blooms in a tributary of the Three Gorges Reservoir between different seasons: Causality based on data-driven methods

Intense harmful algal blooms (HABs) can occur in the backwaters of tributaries supplying large-scale reservoirs. Due to the characteristics of process-based models and difficulties in modelling complex nonlinear processes, traditional models have difficulties disentangling the driving factors of HABs. In this study, we used data-driven methods (i.e., correlation analysis and machine-learning models) to identify the most important drivers of HABs in the Xiangxi River, a tributary of the Three Gorges Reservoir, China (2017–2018), for the dry season (from October to mid-April) and wet season (from April to September). We utilized the maximal information coefficient (MIC) combined with a time lag strategy and prior knowledge to quantitatively identify the driving variables of HABs. An extra trees regression (ETR) model was developed to assess the relative importance of causal variables driving algal blooms for the different periods. The results showed that water temperature was the most important driver for the duration of the study, followed by total nitrogen. Nitrogen had a stronger effect on algal blooms than phosphorus during both the wet and dry seasons. HABs were mainly affected by ammonia nitrogen in the wet season and by other forms of nitrogen in the dry season. In contrast, rather than the water temperature and nutrients, the operation of the Three Gorges Dam (difference between inflow and outflow discharge rate) was the most significant factor for algal blooms during the dry season, but its influence sharply declined during the wet season. This study showed that the key drivers of HABs can differ between seasons and suggests that HAB management should take seasonality into account.

Tectonic-lithofacies paleogeography, large-scale source-reservoir distribution and exploration zones of Cambrian subsalt formation, Tarim Basin, NW China

To predict the large-scale source rock and reservoir distribution in Cambrian subsalt formations (Upper Sinian to Mid-Cambrian) in the Tarim Basin, more than 50 2D seismic lines and 3D data volume acquired latest, 22 wells and some outcrops around this area were used to study the tectonic-lithofacies paleogeography, define the distribution of large scale source rocks and reservoirs, and sort out favorable zones. (1) The basin experienced evolution from ramp to deep-water mud-rich ramp, low-gradient slope, weak rimmed to strong rimmed platform, and the differentiation in the platform was controlled by pre-Cambrian palaeo-rifts. (2) The Luonan-Yubei ancient rift controlled the distribution of source rocks in the Lower Cambrian Yuertusi Formation, and this rift together with the northern depression are the main source rock area. (3) There are three sets of large-scale reservoirs, the Upper Sinian Qigebulake Formation, the Lower Cambrian Xiaorbulake Formation and the Wusongger Formation, and shoal-mounds, early dolomitization and multi-levels of unconformities controlled reservoir development. (4) Four favorable zones, the north slope of Tazhong, upper slope of Maigaiti area, Keping-Wensu periphery, and south slope of Tabei, were evaluated. Of them, the north slope of Tazhong is most likely to have exploration breakthrough; the south slope of Tabei is the best area for exploring the Upper Sinian large scale weathering crust dolomite; Maigaiti slope and Keping-Wensu periphery area improved in exploration potentials significantly, and are worth prospecting faster.

Real-time optimization of a large-scale reservoir operation in Thailand using adaptive inflow prediction with medium-range ensemble precipitation forecasts

Study regionThe Sirikit Dam in the Nan River Basin is located on a main tributary of the Chao Phraya River in Thailand. Study focusThis study investigates forecasting river flows and real-time optimization of dam release using a distributed hydrological model with ensemble weather forecasting for reservoir operations which provide hydropower and irrigation facilities in Thailand during a case study of the 2019 drought event. Medium-range ensemble precipitation forecasts were employed using a hydrological model to predict the real-time reservoir inflow. Real-time optimization of the water release strategy determined a week in advance with an effective initial condition for hydropower generation and irrigation was conducted with different scenarios using dynamic programming considering inflow predictions. New hydrological insights for the regionThe medium-range ensemble precipitation forecast conducted by the European Centre for Medium Range Weather Forecasts was used to quantify precipitation for the study basin. The ensemble precipitation forecast with the hydrological model was employed for inflow prediction of the study basin which was located in a tropical climate with a distinct wet and dry season. The initial conditions of the hydrological model largely influenced the real-time inflow forecast. To determine the initial conditions of the model, the empirical data assimilation considering a drainage area factor was utilized and observed precipitation data were used for model input forcing data during the initial analysis period. This method improved the reservoir inflow prediction and real-time reservoir optimization using dynamic programming with considering ensemble forecasts provided more efficient operating decisions than employing historical data. The resulting information will be useful for water resource management, which may be adapted to other basins in the study region.

Study on the Evolution Law of Sandstone Porosity under Soaking-Drying Cycles

In view of the problem whereby the rock masses in water-level fluctuation zones of reservoir banks are susceptible to deterioration due to soaking-drying cycles, the sandstone of a typical landslide in the Three Gorges Reservoir area is selected as the test object to determine the ion concentration of an immersion solution and rock sample via periodic water-rock microscopic images, quantitative back analysis of the porosity changes caused by the dissolution of rock minerals under soaking-drying cycles, and evaluation of the sandstone porosity evolution. The results show that, 1) in the first two soaking-drying cycles, the generation rate of each ion is relatively high, and after 6 soaking-drying cycles, the ion generation rate tends to remain stable. 2) The volume reduction in calcite is the largest, followed by that in albite and potash feldspar. 3) Small porosity changes result in a considerable decrease in the compressive strength. 4) Sandstone porosity increase leads to an increase in rock pores, which is manifested as a deterioration of the macroscopic rock mechanical properties. Relevant research results could provide a reliable data basis for the study of large-scale reservoir bank slope protection and geological disaster evaluation and prediction.

Synchronous Cycle of Available Phosphorus, Iron, and Sulfur in the Sediment of Lancang River Reservoirs

Large-scale deep reservoirs associated with hydropower cascade development are known to influence the cycle of phosphorus (P). However, there is scarce information on the fractions and availability of P in sediments of large-scale deep reservoirs constructed due to hydropower cascade development. In this study, we researched the fractions and release mechanism of P in the sediments of large-scale deep reservoirs by analyzing the fractions and availability of P in the sediments of the Xiaowan (XW) and Nuozhadu (NZD) reservoirs in the middle and lower reaches of the Lancang River (China). According to the results, there is a significant difference in the P fractions in the sediments of the XW and NZD reservoirs, but not for the available P in the sediments. Compared to the NZD reservoir, there was less solid bioavailable phosphorus (BAP) in the sediments of the XW reservoir, but the replenishment degree of active solid phase P into pore water was higher in the XW. There was a significant positive correlation between the available P and the BAP; the Fe/P ratio measured by the diffusive gradients in thin films reflects the control of active iron (oxyhydr) oxides over labile P in the sediments. In addition to the reductive dissolution of iron-bound P, the release of P into the large deep reservoirs may be related to factors such as the sulfate reduction and the degradation of organic materials. The P cycling in deep reservoir sediments is mainly controlled by the Fe, and there is a clear spatial distribution of this mechanism in deep reservoirs.

Shale gas enrichment model and exploration implications in the mountainous complex structural area along the southwestern margin of the Sichuan Basin: A new shale gas area

Marine shale gas in the southern Sichuan Basin is the most successful area of shale gas exploration and development in China. In order to open up new shale gas fields and search for new shale gas reserves and production replacement blocks, it is necessary to continuously establish and complete the standards on shale gas reservoir evaluation and area selection under different structural settings. The early exploration practice shows that shale in the mountainous complex structural area along the southwestern margin of the Sichuan Basin varies greatly in the gas bearing property, so systematical analysis and study on the shale gas enrichment mechanisms in this area is of great significance to searching for new shale gas exploration areas. Based on drilling data of 5 wells in the mountainous complex structural area along the southwestern margin of the Sichuan Basin, the main factors controlling the shale gas bearing property and the shale gas enrichment model were discussed based on the systematical experiments and analysis of the outcrops, cores and cuttings of Longmaxi Formation shale from the aspects of organic geochemistry, physical property, gas content, hydrocarbon generation history and pore evolution history, so as to lay a solid geological basis for the birth of a new shale gas area. And the following research results were obtained. First, the shale of the first submember of first Member of Longmaxi Formation (S1l11) in the mountainous complex structural area along the southwestern margin of the Sichuan Basin has an average TOC of 3.02%–4.97% and Ro of 2.38%–3.37%, and the average total gas content in local enrichment zones is up to 4.62 m3/t, so it is classified as quality shale. Second, the detailed studies on hydrocarbon generation history indicate that the shale has the characteristics of “low thermal evolution rate and low maturity”. It is characterized by late hydrocarbon generation, low thermal evolution rate, low current maturity and short late diffusion time, which are favorable for shale gas enrichment. Third, the pore evolution history reveals that shale pore evolution can be divided into six stages, i.e., sharp pore reduction, pore reduction, organic pore formation, pore preservation, organic pore dissipation and karstification, and organic pore and TOC are the most direct control factors of shale gas content. Fourth, favorable shelf facies belt is conducive to the formation of large-scale reservoir space and effective pores, and diversities of preservation conditions under different structural styles and at different structural positions control different pore evolution stages. The shale gas in the mountainous complex structural area is characterized by horizontal zoning and differential enrichment. “Low thermal evolution rate and low maturity” and “slow structural uplifting” are beneficial to the long-term enrichment and preservation of shale gas. In conclusion, the mountainous complex structural area along the southwestern margin of the Sichuan Basin has a shale gas enrichment model of “sedimentation controlling source rocks, diagenesis controlling reservoirs and structure controlling preservation”. This research result provides idea and reference for searching for new shale gas areas and fields.

Lithofacies paleogeography and characteristics of large-scale reservoirs of the Middle Triassic Leikoupo Formation in Sichuan Basin, China

Material exploration discoveries have been achieved in the Middle Triassic Leikoupo Formation in the Sichuan Basin in recent years, and a large-scale reservoir is one of the key factors controlling reservoir formation. In this paper, the lithofacies paleogeographic map of the Leikoupo Formation is prepared based on field outcrops, exploratory wells, well logs, and seismic data. The characteristics of large-scale reservoirs have been analyzed, which resulted in three new summarized understandings: (1) At the Leikoupo stage, the Sichuan Basin was relatively closed in the south, east and north, and the seawater entered through the west side, so it was a relatively closed carbonate platform; during the sedimentary stage of Lei I and Lei II members, it was a weakly fringed carbonate platform. In the sedimentary period of Lei III (T2l3) and Lei IV (T2l4) members, it was a rimmed carbonate platform. (2) The sedimentary facies including the platform margin, open platform, restricted platform, and evaporative platform were developed in the Leikoupo Formation; these were mainly the restricted platform facies covering the subfacies of the lagoon, intra-platform shoal, and tidal flat. Two types of hydrocarbon reservoirs, namely the platform-margin shoal and intra-platform shoal reservoirs, were developed in this period. (3) Two sets of large-scale reef beach facies reservoirs were developed in the Leikoupo Formation, one is the platform margin reef beach facies of Lei III (T2l3) and Lei IV (T2l4) members in northwestern Sichuan with a large thickness (generally larger than 50 m); the other one is the intra-platform shoal facies in the lower part of Lei I Member (T2l1) in Central Sichuan with a small thickness, stable distribution, and a large area (larger than 20,000 km2) forming large-scale reservoirs. The two types of large-scale reservoirs experienced multi-stage karstification transformation to become reservoirs with a good quality, laying the material basis for large-scale oil & gas reservoir formation. The research results have important theoretical and practical significance to improve the understanding on the sedimentation and reservoirs of the Leikoupo Formation in the Sichuan Basin, and to provide guidance for the oil and gas exploration in this region.

Sandy riverbed shoal under anthropogenic activities: The sandy reach of the Yangtze River, China

Large-scale reservoirs change downstream water and sediment conditions, causing adaptive adjustments in erosion and deposition along the riverbed, ultimately impacting sandbar morphology. This study examined the sandy reaches downstream of the China Three Gorges Project along the Yangtze River for river erosion and siltation, riverbed morphology, beach evolution, and other physical processes over the past 40 years, to explain the evolution of river geomorphology under human activities, such as reservoirs, improvement projects, and sand mining. The Shashi reach is approximately 52 km in length; it is the first sandy riverbed downstream of the China Three Gorges Dam. Analysis of topographic data from 1975 to 2018 showed that decreased sediment from the basin resulted in increased riverbed erosion (93.1%, in low-water channel), with sand mining activities destroying the integrity of the central bar. Simultaneously, total sandbar area decreased, thalwegs deepened, river morphological coefficients decreased, and the riverbed became narrower and deeper. The erosion and deposition patterns of the Shashi reach beach and channel have changed from “low shoal erosion-high shoal deposition” to “both low and high shoal erosion”. Additionally, waterway engineering has changed sandbar boundaries, and impacted the evolution of interactions among beach groups. Under quasi-natural conditions, strong interactions occur in sandbar evolution, such as the development of the Taipingkou Channel Bar (TPKCB), which led to Lalinzhou Point Bar (LLZPB) erosion, tail section widening, as well as Sanbatan (SBT) shrinkage and recession of its eastward edge. Owing to the implementation of waterway engineering, the linkage between the TPKCB and the upper section of the LLZPB weakened. The widening of the lower section of the latter has caused the SBT to shrink further and shift westward. Changes in the morphologies and dynamic axes of upstream beach groups have resulted in a sharp reduction of incoming sediment causing the downstream Jinchengzhou Point Bar (JCZPB) to gradually evolve into a shrunken channel bar.

Classification of slope area types and hydrocarbon accumulation rules in Wuerxun-Beier Depression

After more than 50 years of exploration and development, Wuerxun-Beier depression has entered a high exploration stage. It is more and more difficult to find large-scale structural reservoirs. Therefore, structural-lithologic reservoirs in slope areas have gradually become the focus of exploration. Based on the overall study and dissection of the depression, the geological differentiation characteristics of the large-scale slope area are discussed. According to the occurrence of the slope, the intensity of tectonic activity, the intensity of source supply and the pattern of synsedimentary fault combination, the slope zone in the study area can be divided into six types, namely, the steep slope with strong source along the fault step type, the compound fault step type of medium slope with strong source, the reverse fault step type of the gentle slope with strong source, and the weak supply type. Different types of slope have different sand control mechanisms and reservoir forming characteristics, such as the source gentle slope forward fault terrace type, the weak source gentle slope reverse fault terrace type and the weak source middle slope forward fault terrace type. Source supply intensity and slope dip control the development type of sedimentary system, the combination of synsedimentary faults and valleys controls the distribution characteristics of sedimentary system. Through exploration practice, significant discoveries have been made on the southwestern, southeastern and northern slopes of Beixi, Wuerxun-Beier depression, which confirms that the slope area is the main breakthrough direction for oil and gas exploration and large-scale reservoir increase in continental fault-depression lake basins.

Deep Sequential Surrogate Model Enables New Work Flows

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 201775, “An End-to-End Deep Sequential Surrogate Model for High-Performance Reservoir Modeling: Enabling New Work Flows,” by Jiri Navratil, IBM; Giorgio De Paola, SPE, Repsol; and Georgos Kollias, IBM, et al., prepared for the 2020 SPE Annual Technical Conference and Exhibition, originally scheduled to be held in Denver, 5–7 October. The paper has not been peer reviewed. Despite considerable progress in the development of rapid evaluation methods for physics-based reservoir simulators, a significant gap still exists in the acceleration and accuracy needed to enable complex optimization methods. The complete paper describes an end-to-end deep surrogate model capable of modeling field and individual-well production rates given arbitrary sequences of actions (schedules), including varying well locations, controls, and completions. Results provide concrete measures of the efficacy of the deep surrogate model as an enabling technology for the development of optimization techniques previously out of reach because of computational complexity. Introduction Recent advances in machine learning, particularly in deep learning, have enabled breakthroughs in technically challenging areas such as computer vision, natural language processing, and speech processing. The authors expand on these powerful algorithms to develop a deep neural network (DNN) surrogate model for reservoir dynamics. The objective of the DNN surrogate model is to predict field rates and individual well rates. The field rates typically include aggregate oil and water production and water-injection rates reported at equidistant timesteps over a desired production horizon. Economic metrics such as net present value are computed using the field rates. The individual well rates are optional, and, typically, the oil production per well is the desired output. The contribution of this paper is twofold: It expands on the sequence-to-sequence approach by capturing additional varying inputs, namely well controls and completions, and it presents a thorough experimental analysis and validation on a publicly available synthetic reservoir model (SPE9) as well as a real large-scale reservoir model. Methods and Procedures Reservoir Models. SPE9 is a standard benchmark to validate and compare simulations of the black-oil model. The reservoir is described by a 24×25×15 grid, having a 10° dipping angle in the x direction and a heterogeneous geostatistically based permeability field. The grid has a uniform size of 300 ft in the x and y directions and is nonuniform along the z direction. The porosity values for each layer are constant and vary across layers. In this work, the original SPE9 benchmark has been modified to be suitable for field development plan (FDP) optimization. Different well-completion data and ranges have been used for the number of producer and injector wells. The areal well locations, the number of wells, and the well-drilling sequence are variables specified by a suitable FDP. A fixed well control has been used in all SPE9 simulations. In the original SPE9 benchmark, the field operating conditions led to free-gas formation and most of the wells switched from rate control to pressure control. The conditions that trigger the switch in well controls are provided as inputs to a reservoir model and are determined on the basis of operational logistics and geomechanical stability criteria. Neglecting such switches in well controls is disastrous in the real world. However, this is benign from the point of view of a numerical benchmark to test FDP optimization.