Visualization and characterization of experimental hydraulic fractures interacting with karst fracture-cavity distributions

Karst fracture-cavity carbonate reservoirs,in which natural cavities are connected by natural fractures to form cavity clusters in many circumstances,have become significant fields of oil and gas exploration and exploitation.Proppant fracturing is considered as the best method for exploiting carbonate reservoirs;however,previous studies primarily focused on the effects of individual types of geological formations,such as natural fractures or cavities,on fracture propagation.In this study,true-triaxial physical simulation experiments were systematically performed under four types of stress difference conditions after the accurate prefabrication of four types of different fracture-cavity distributions in artificial samples.Subsequently,the interaction mechanism between the hydraulic fractures and fracture-cavity structures was systematically analyzed in combination with the stress distribution,cross-sectional morphology of the main propagation path,and three-dimensional visualization of the overall fracture network.It was found that the propagation of hydraulic fractures near the cavity was inhibited by the stress concentration surrounding the cavity.In contrast,a natural fracture with a smaller approach angle(0°and 30°)around the cavity can alleviate the stress concentration and significantly facilitate the connection with the cavity.In addition,the hydraulic fracture crossed the natural fracture at the 45°approach angle and bypassed the cavity under higher stress difference conditions.A new stimulation effectiveness evaluation index was established based on the stimulated reservoir area(SRA),tortuosity of the hydraulic fractures(T),and connectivity index(CI)of the cavities.These findings provide new insights into the fracturing design of carbonate reservoirs.

A novel fracture-cavity reservoir outcrop geological knowledge base construction method considering parameter collection and processing,mutual transformation of data-knowledge,application and update

This study endeavors to formulate a comprehensive methodology for establishing a Geological Knowledge Base(GKB)tailored to fracture-cavity reservoir outcrops within the North Tarim Basin.The acquisition of quantitative geological parameters was accomplished through diverse means such as outcrop observations,thin section studies,unmanned aerial vehicle scanning,and high-resolution cameras.Subsequently,a three-dimensional digital outcrop model was generated,and the parameters were standardized.An assessment of traditional geological knowledge was conducted to delineate the knowledge framework,content,and system of the GKB.The basic parameter knowledge was extracted using multiscale fine characterization techniques,including core statistics,field observations,and microscopic thin section analysis.Key mechanism knowledge was identified by integrating trace elements from filling,isotope geochemical tests,and water-rock simulation experiments.Significant representational knowledge was then extracted by employing various methods such as multiple linear regression,neural network technology,and discriminant classification.Subsequently,an analogy study was performed on the karst fracture-cavity system(KFCS)in both outcrop and underground reservoir settings.The results underscored several key findings:(1)Utilization of a diverse range of techniques,including outcrop observations,core statistics,unmanned aerial vehicle scanning,high-resolution cameras,thin section analysis,and electron scanning imaging,enabled the acquisition and standardization of data.This facilitated effective management and integration of geological parameter data from multiple sources and scales.(2)The GKB for fracture-cavity reservoir outcrops,encompassing basic parameter knowledge,key mechanism knowledge,and significant representational knowledge,provides robust data support and systematic geological insights for the intricate and in-depth examination of the genetic mechanisms of fracture-cavity reservoirs.(3)The developmental characteristics of fracturecavities in karst outcrops offer effective,efficient,and accurate guidance for fracture-cavity research in underground karst reservoirs.The outlined construction method of the outcrop geological knowledge base is applicable to various fracture-cavity reservoirs in different layers and regions worldwide.

Construction of carbonate reservoir knowledge base and its application in fracture-cavity reservoir geological modeling

To improve the efficiency and accuracy of carbonate reservoir research,a unified reservoir knowledge base linking geological knowledge management with reservoir research is proposed.The reservoir knowledge base serves high-quality analysis,evaluation,description and geological modeling of reservoirs.The knowledge framework is divided into three categories:technical service standard,technical research method and professional knowledge and cases related to geological objects.In order to build a knowledge base,first of all,it is necessary to form a knowledge classification system and knowledge description standards;secondly,to sort out theoretical understandings and various technical methods for different geologic objects and work out a technical service standard package according to the technical standard;thirdly,to collect typical outcrop and reservoir cases,constantly expand the content of the knowledge base through systematic extraction,sorting and saving,and construct professional knowledge about geological objects.Through the use of encyclopedia based collaborative editing architecture,knowledge construction and sharing can be realized.Geological objects and related attribute parameters can be automatically extracted by using natural language processing(NLP)technology,and outcrop data can be collected by using modern fine measurement technology,to enhance the efficiency of knowledge acquisition,extraction and sorting.In this paper,the geological modeling of fracture-cavity reservoir in the Tarim Basin is taken as an example to illustrate the construction of knowledge base of carbonate reservoir and its application in geological modeling of fracture-cavity carbonate reservoir.

Air pressure law of a reservoir constructed in karst sinkholes

Karst sinkholes with natural negative landform provide favorable conditions for the pumped storage reservoir construction for less excavation work.However,the construction of the reservoir would plug the natural karst channels for water and air,which would cause remarkable air pressure in karst channels when the groundwater level fluctuates.A large laboratory simulation test was carried out to study the air pressure variation of a reservoir built on the karst sinkhole.The air pressure in the karst channel and inside the model was monitored during the groundwater rising and falling process.Result showed that the variation of air pressure in the karst channel and the surrounding rock exhibited a high degree of similarity.The air pressure increased rapidly at the initial stage of water level rising,followed by a slight decrease,then the air pressure increased sharply when the water level approached the top of the karst cave.The initial peak of air pressure and the final peak of air pressure were defined,and both air pressure peaks were linearly increasing with the water level rising rate.The negative air pressure was also analyzed during the drainage process,which was linearly correlated with the water level falling rate.The causes of air pressure variation in karst channels of a pumped storage reservoir built on the karst sinkhole were discussed.The initial rapid increase,then slight decrease and final sudden increase of air pressure were controlled by the combined effects of air compression in karst channel and air seepage into the surrounding rock.For the drainage process,the instant negative air pressure and gradual recovering of air pressure were controlled by the combined effects of negative air pressure induced by water level falling and air supply from surrounding rock.This work could provide valuable reference for the reservoir construction in karst area.

A Numerical Study on the Propagation Mechanisms of Hydraulic Fractures in Fracture-Cavity Carbonate Reservoirs

Field data suggests that carbonate reservoirs contain abundant natural fractures and cavities.The propagation mechanisms of hydraulic fractures in fracture-cavity reservoirs are different from conventional reservoirs on account of the stress concentration surrounding cavities.In this paper,we develop a fully coupled numerical model using the extended finite element method(XFEM)to investigate the behaviors and propagation mechanisms of hydraulic fractures in fracture-cavity reservoirs.Simulation results show that a higher lateral stress coefficient can enhance the influence of the natural cavity,causing a more curved fracture path.However,lower confining stress or smaller in-situ stress difference can reduce this influence,and thus contributes to the penetration of the hydraulic fracture towards the cavity.Higher fluid viscosity and high fluid pumping rate are both able to attenuate the effect of the cavity.The frictional natural fracture connected to the cavity can significantly change the stress distribution around the cavity,thus dramatically deviates the hydraulic fracture from its original propagation direction.It is also found that the natural cavity existing between two adjacent fracturing stages will significantly influence the stress distribution between fractures and is more likely to result in irregular propagation paths compared to the case without a cavity.

Application of Spectral Decomposition to Detection of Fracture-Cavity Carbonate Reservoir Beds in the Tahe OUfield,Tarim Basin,NW China

Ordovician fracture-cavity carbonate reservoir beds are the major type of producing formations in the Tahe oilfield, Tarim Basin. The seismic responses of these beds clearly changes depending on the different distance of the fracture-cavity reservoir bed from the top of the section. The seismic reflection becomes weak or is absent when the fracture-cavity reservoir beds are less than 20 ms below the top Ordovician. The effect on top Ordovician reflection became weaker with deeper burial of fracture-cavity reservoir beds but the developed deep fracture-cavity reservoir beds caused stronger reflection in the interior of the Ordovician. This interior reflection can be divided into strong long-axis, irregular and bead string reflections, and was present 80 ms below the top Ordovician. Aimed at understanding reflection characteristics, the spectral decomposition technique, which uses frequency to ＂tune-in＂ bed thickness, was used to predict Ordovician fracture-cavity carbonate formations in the Tahe oilfield. Through finely adjusting the processing parameters of spectral decomposition, it was found that the slice at 30 Hz of the tuned data cube can best represent reservoir bed development. Two large N-S-trending strong reflection belts in the mid-western part of the study area along wells TK440- TK427-TK417B and in the eastern part along wells TK404-TK409 were observed distinctly on the 30 Hz slice and 4-D time-frequency data cube carving. A small N-S trending reflection belt in the southern part along wells T403-TK446B was also clearly identified. The predicted reservoir bed development area coincides with the fracture-cavities connection area confirmed by drilling pressure testing results. Deep karst cavities occur basically in three reservoir bed-development belts identified by the Ordovician interior strong reflection. Spectral decomposition proved to be a useful technique in identifying fracture-cavity reservoir beds.

Grading evaluation and prediction of fracture-cavity reservoirs in Cambrian Longwangmiao Formation of Moxi area,Sichuan Basin,SW China

By using core, thin section, well logging, seismic, well testing and other data, the reservoir grading evaluation parameters were selected, the classification criterion considering multiple factors for carbonate reservoirs in this area were established, and the main factors affecting the development of high quality reservoir were determined. By employing Formation MicroScanner Image(FMI) logging fracture-cavity recognition technology and reservoir seismic waveform classification technology, the spatial distribution of reservoirs of all grades were predicted. On the basis of identifying four types of reservoir space developed in the study area by mercury injection experiment, a classification criterion was established using four reservoir grading evaluation parameters, median throat radius, effective porosity and effective permeability of fracture-cavity development zone, relationship between fracture and dissolution pore development and assemblage, and the reservoirs in the study area were classified into grade I high quality reservoir of fracture and cavity type, grade II average reservoir of fracture and porosity type, grade Ⅲ poor reservoir of intergranular pore type. Based on the three main factors controlling the development of high quality reservoir, structural location, sedimentary facies and epigenesis, the distribution of the 3 grades reservoirs in each well area and formation were predicted using geophysical response and percolation characteristics. Follow-up drilling has confirmed that the classification evaluation standard and prediction methods established are effective.

Application of Prediction Techniques in Carbonate Karst Reservoir in Tarim Basin

Carbonate karst reservoir is the emphases of Tarim＇s carbonate exploration. However, it is buried at a large depth, which results in Weak seismic reflection signal and low S/N ratio. In addition, the karst reservoir contains great heterogeneity, so reservoir prediction is very difficult. Through many years of research and exploration, we have established a suite of comprehensive evaluation technology for carbonate karst reservoir using geophysical characteristics and a geological concept model, including a technique for reconstructing the paleogeomorphology of buried hills based on a sequence framework, seismic description of the karst reservoir, and strain variant analysis for fracture estimation. The evaluation technology has been successfully applied in the Tabei and Tazhong areas, and commercial production of oil and gas has been achieved. We show the application of this technology in the Lunguxi area in North Tarim in this paper.

Research on the Seismic Wave Field of Karst Cavern Reservoirs near Deep Carbonate Weathered Crusts

Fracture and cavern hydrocarbon reservoirs in carbonates are an important pool type worldwide. The karst cavern reservoirs are easiest to identify on seismic reflection data. The prediction, exploration, and development of this type of reservoir require theoretical research on seismic wave fields reflected from complex inhomogeneous media. We compute synthetic seismic sections for fluidfilled cavern reservoirs of various heights and widths using random media models and inhomogeneous media elastic wave equations. Results indicate that even caverns significantly smaller than 1/ 4 wavelength are detectible on conventional band-width seismic sections as diffractions migrated into bead-type events. Diffraction amplitude is a function of cavern height and width. We introduce a width-amplitude factor which can be used to calculate the diffraction amplitude of a cavern with a limited width from the diffraction amplitude computed for an infinitely wide cavern.

Eogenetic karst and its control on reservoirs in the Ordovician Majiagou Formation, eastern Sulige gas field, Ordos Basin, NW China

To further ascertain the origin of the Ordovician Majiagou Formation reservoirs in the Ordos Basin,the M54-M51 sub-members of the Ordovician Majiagou Formation in the eastern Sulige gasfield of Ordos Basin were taken as examples to analyze the vertical development characteristics of eogenetic karst and to discover the dissolution mechanism and its control on reservoirs through observation of a large number of cores and thin sections.According to detailed analysis of petrologic characteristics,the reservoir rock types include micritic dolomite,grain dolomite and microbialite which have mainly moldic pore,intergranular(dissolved)pore,and(dissolved)residual framework pore as main reservoir space respectively.The study area developed upward-shallowing sequences,with an exposure surface at the top of a single upward-shallowing sequence.The karst systems under the exposure surface had typical exposure characteristics of early dissolution and filling,indicating these reservoirs were related to the facies-controlled eogenetic karstification.With the increase of karstification intensity,the reservoirs became worse in physical properties.

Architecture characterization of Ordovician fault-controlled paleokarst carbonate reservoirs in Tuoputai,Tahe oilfield,Tarim Basin,NW China

Based on outcrop,core,logging,seismic and production data,and the formation of fault-controlled karst reservoirs,the types and characterization of Ordovician fault-controlled karst reservoir architectures in the Tuoputai area of the Tahe oilfield are studied.According to the concept of genetic geologic body,the fault-controlled karst reservoir is divided into architecture elements of four levels,the strike-slip fault impact zone is the level-1 architecture element,the fault-controlled karst reservoir the level-2 architecture element,the fracture-cave zone(which can be further subdivided into dissolution cave,dissolution pore and vug,and fracture zones)inside the fault-controlled karst reservoir the level-3 architecture element,and fillings inside caves is the level-4 architecture element(which can be further divided based on the filling degree and lithologic types of the fillings).Specific characterization techniques have been optimized according to the characteristics of various architecture elements.The zone impacted by strike-slip fault is characterized by seismic coherence and artificial interpretation.Under the constraint of zone impacted by strike-slip fault,fault likelihood(FL)property is used to characterize the outline of fault-controlled karst reservoir.Under the constraint of fault-controlled karst reservoir outline,the internal structures are divided based on seismic texture attribute.Finally,the cavern filling pattern is interpreted based on drilling and logging data.The fault-controlled karst reservoirs can be interpreted in 3-dimensional space by architecture element levels,and the characterization technology combining log and seismic data for fault-controlled karst reservoir has been worked out,which has complemented the development theory and technologies for this kind of reservoirs in the Tahe oilfield.

Characteristics and development model of karst reservoirs in the fourth member of Sinian Dengying Formation in central Sichuan Basin,SW China

The reservoir space,types and distribution characteristics of karst carbonate gas reservoirs in the fourth member of Sinian Dengying Formation(Deng 4 Member)in central Sichuan Basin are analyzed based on the drilling,logging and seismic data.A development model of karst reservoirs is constructed to support the high-efficiency development of gas pools.The research shows that the reservoirs in Deng 4 Member have mainly small-scale karst vugs and fractures as storage space,and can be divided into three types,fracture-vug,pore-vug and pore types.The development patterns of the karst reservoirs are determined.On the plane,the karst layers increase from 65 m to 170 m in thickness from the karst platform to the karst slope,and the high-quality reservoirs increase from 25.0 m to 42.2 m in thickness;vertically,the reservoirs at the top of Deng 4 Member appear in multiple layers,and show along-bedding and along fracture dissolution characteristics.The reservoirs at the bottom are characterized by the dissolution parallel to the water level during the karstification period,and have 3-5 large-scale fracture-cave systems.Based on the reservoir development characteristics and the genetic mechanism,three types of reservoir development models of karst reservoir are established,i.e.,bed-dissolved body,fracture-dissolved body and paleohorizon-dissolved body.The construction of karst reservoir development models and seismic response characteristics of the three types of reservoirs can provide parameter for well placement and trajectory design,and substantially improve productivity and development indices of individual wells and gas reservoirs.The designed production capacity of the gas reservoir has enhanced from the initial 3.6 billion to 6 billion cubic meters,making the profit of the reservoir development increase noticeably.

Type and distribution of Mid-Permian Maokou Formation karst reservoirs in southern Sichuan Basin,SW China

Based on the analysis of the responses of conventional logs such as natural gamma(GR), density(DEN), acoustic interval transit time(AC), compensated neutron(CNL), dual lateral resistivity(Rlld, Rlls), and caliper log(CAL), combined with drilling data,cores, thin section and productivity of 65 wells, the reservoirs in the Mid-Permian Maokou Formation of southern Sichuan Basin were divided into four types, fractured-vuggy, pore-vuggy, fractured and fractured-cavity. The main reservoirs in high productivity wells are fractured-vuggy and pore-vuggy. The reservoirs of Maokou Formation are generally thin, and can be divided into the upper reservoir segment(layer a of the second member to the third member of Maokou Formation, P_2 m^2 a-P_2 m^3) and the lower segment(layer b of the second member of Maokou Formation, P_2 m^2 b). The two reservoir segments are mainly controlled by two grain beaches during the sedimentation of P_2 m^2 a-P_2 m^3 and P_2 m^2 b, the vertical zonation of karst, and the fractures. The upper reservoir segment is generally better than the lower one in development degree and single well productivity, and is much thicker than the lower one. It is thicker in the Yibin-Zigong-Weiyuan-Dazu area, the southwestern area of Chongqing and the southeastern area of Luzhou, while the lower segment is thicker in the Neijiang-Zigong-Luzhou area and the Dazu-Luzhou area. The areas with big reservoir thickness at tectonic slope or syncline parts are the favorable exploration areas.

Numerical simulation of groundwater under complex karst conditions and the prediction of roadway gushing in a coal mine:a case study in the Guang'an Longtan Reservoir in Sichuan Province, China

Numerical simulation of groundwater in karst areas has long been restricted by the difficulty of generalizing the hydrogeological conditions of reservoirs and of determining the relevant parameters due to the anisotropy and discontinuity of the karst water-bearing media in these areas. In this study, we used the Guang'an Longtan Coal mine in Sichuan as an example, and generalized the complex hydrogeological conditions in the reservoir area. A finite element numerical flow model was used to simulate current and future scenarios of roadway gushing at the bottom of the coal mine at pile number 1 + 700 m. The results show that the roadway section corresponding to valleys has a gushing quantity of 4323.8–4551.25 m^3/d before impoundment. Modeled water inflow after impoundment increased to 1.6 times the water inflow before impoundment, which threatens the impoundment as well as the roadway's normal operation. Therefore, roadway processing measures are needed to guarantee the safety of the impoundment and of the mining operation.

Strike-Slip Faults and Their Control on Differential Hydrocarbon Enrichment in Carbonate Karst Reservoirs: A Case Study of Yingshan Formation on Northern Slope of Tazhong Uplift, Tarim Basin

Objective Oil and gas are abundant in the Ordovician Yingshan Formation carbonate karst reservoirs on the northern slope of Tazhong uplift in the Tarim Basin, and have extremely complicated oil-gas-water distribution, however. The difference in burial depth of the reservoirs between east and west sides is up to 1000 m. Water-bearing formations exist between oil- and gas-bearing formations vertically and water-producing wells are drilled between oil- and gas-producing wells. Macroscopically, oil and gas occur at low positions, while water occurs at high positiona on the northern slope of Tazhong uplift. The mechanism of differential hydrocarbon enrichment in heterogeneous reservoirs is by far not clarified, which has affected the efficient exploration and development of oil and gas fields in this area.

Differential structure of Ordovician karst zone and hydrocarbon enrichment in paleogeomorphic units in Tahe area,Tarim Basin,NW China

Based on a large number of drilling,logging,seismic and production data,the differential structures of karst zone and hydrocarbon distribution in different paleogeomorphic units of the Tahe area,Tarim Basin,are discussed by analyzing the karst drainages and flowing channels.The karst paleogeomorphy of Ordovician in Tahe area is composed of watershed,karst valley and karst basin.The watershed has epikarst zone of 57.8 m thick on average and vadose karst zone of 115.2 m thick on average with dense faults,fractures and medium-small fracture-caves,and 76.5%of wells in this area have cumulative production of more than 5×10^(4) t per well.The karst valleys have epikarst zone,vadose karst zone and runoff karst zone,with an average thickness of 14.6,26.4 and 132.6 m respectively.In the runoff karst zone,the caves of subsurface river are mostly filled by fine sediment,with a filling rate up to 86.8%,and 84.9%of wells in this area have cumulative production of less than 2×10^(4) t per well.The karst basin has no karst zone,but only fault-karst reservoirs in local fault zones,which are up to 600 m thick and closely developed within 1 km around faults.Different karst landforms have different water flowing pattern,forming different karst zone structures and resulting in differential distribution of oil and gas.The watershed has been on the direction of oil and gas migration,so medium-small sized connected fracture-caves in this area have high filling degree of oil and gas,and most wells in this area have high production.Most caves in subsurface river are filled due to strong sedimentation and transportation of the river,so the subsurface river sediment has low hydrocarbon abundance and more low production oil wells.The faults linking source rock are not only the water channels but also the oil-gas migration pathways,where the karst fractures and caves provide huge reservoir space for oil and gas accumulation.

Reservoir characteristics and genetic differences between the second and fourth members of Sinian Dengying Formation in northern Sichuan Basin and its surrounding areas

This study investigated the characteristics and genesis of reservoirs in the 2^(nd) and 4^(th) members of Sinian Dengying Formation in northern Sichuan and its surrounding areas, on the basis of outcrop, drilling cores and thin section observation and geochemical analysis. The reservoirs of 2^(nd) member are distributed in the middle part of the stratum. The reservoir quality is controlled by supergene karst and the distribution of mound-shoal complex. The bedded elongated isolated algal framework solution-cave and the residual "grape-lace" cave, which are partially filled with multi-stage dolosparite, constituted the main reservoir space of the 2^(nd) member. There is no asphalt distribution in the pores. The pore connectivity is poor, and the porosity and permeability of the reservoir is relatively low. The reservoirs of 4^(th) member were distributed in the upper and top part of the stratum. The reservoir quality is controlled by burial dissolution and the distribution of mound-shoal complex. The bedded algal framework solution-pores or caves, intercrystalline pores and intercrystalline dissolved pores constituted the main reservoir space of the 4^(th) member. It's partially filled with asphalt and quartz, without any dolosparite fillings in the pores and caves. The pore connectivity is good. Most of the 4^(th) member reservoirs had medium-low porosity and low permeability, and, locally, medium-high porosity and medium permeability. Affected by the development of mound-shoal complex and heterogeneous dissolution, the platform margin along Ningqiang, Guangyuan, Jiange and Langzhong is the most favorable region for reservoir development. Deep buried Dengying Formation in the guangyuan and langzhong areas should be the most important hydrocarbon target for the future exploration.

Controlling factors of remaining oil distribution after water flooding and enhanced oil recovery methods for fracturecavity carbonate reservoirs in Tahe Oilfield

Based on comprehensive analysis of core, well logging, seismic and production data, the multi-scale reservoir space, reservoir types, spatial shape and distribution of fractures and caves, and the configuration relationship with production wells in fracture-cavity carbonate reservoirs were studied systematically, the influence of them on the distribution of residual oil was analyzed, and the main controlling factors mode of residual oil distribution after water flooding was established. Enhanced oil recovery methods were studied considering the development practice of Tahe oilfield. Research shows that the main controlling factors of residual oil distribution after water flooding in fracture-cavity carbonate reservoirs can be classified into four categories: local high point, insufficient well control, flow channel shielding and weak hydrodynamic. It is a systematic project to improve oil recovery in fracture-cavity carbonate reservoirs. In the stage of natural depletion, production should be well regulated to prevent bottom water channeling. In the early stage of waterflooding, injection-production relationship should be constructed according to reservoir type, connectivity and spatial location to enhance control and producing degree of waterflooding and minimize remaining oil. In the middle and late stage, according to the main controlling factors and distribution characteristics of remaining oil after water flooding, remaining oil should be tapped precisely by making use of gravity differentiation and capillary force imbibition, enhancing well control, disturbing the flow field and so on. Meanwhile, backup technologies of reservoir stimulation, new injection media, intelligent optimization etc. should be developed, smooth shift from water injection to gas injection should be ensured to maximize oil recovery.

Karst地区明渠伏流交替河道水动力学模型及应用

基于圣维南方程组,根据测压管水头在明渠、承压水流中具有相同的表达形式,实现了自由水面水流、承压水流的统一描述。提出采用三角形子断面法描述伏流非规则封闭断面,进而建立了适用于真实Karst地区明渠伏流交替河道的统一型混合流模型。采用半隐方法离散压力梯度项,使用欧拉-拉格朗日法求解对流项,并使用预测-校正法实现了枝状、环状河网各支汊的耦合求解。以夹岩水库工程为背景,采用2000年水文过程对模型进行了测试,结果表明流量计算误差在1.6%以内。进一步研究了明渠伏流交替河道纵向沿程的流速变化规律以及伏流河段前布置人工隧洞的分流规律。在入流洪峰流量、坝前正常蓄水位的水流条件下,明渠段流速一般在0.25 m/s以下,相比之下,伏流段流速可达1.0~3.0 m/s,有利于伏流过流通道的保持。通过1971—1990年水文过程的模拟,得出大中天桥伏流前的人工隧洞的分流比为40.3%~42.8%,小天桥伏流前的人工隧洞的分流比为76.8%~78.6%。研究成果可为工程建设提供设计依据和技术支撑,具有重要的工程应用价值。

DISCUSSION ON THE ORIGIN OF GAS-BEARING KARST GEODES IN MAOKOU LIMESTONE IN CHONGQING REGION, SICHUAN

Many karst geodes have been found in the pure Iimestone on the top Maokou formation (Permian) in the axial part of anticlines and lower part hydrological networks in Chongqing region. It is sometimes a reservoir natural gas. Calcite megacryst decorates the wall of the geodes which believed to he formed in Tertiary.