Progress and prospects of EOR technology in deep,massive sandstone reservoirs with a strong bottom-water drive

The Triassic massive sandstone reservoir in the Tahe oilfield has a strong bottom-water drive and is characterized by great burial depth,high temperature and salinity,a thin pay zone,and strong heterogeneity.At present,the water-cut is high in each block within the reservoir;some wells are at an ultrahigh water-cut stage.A lack of effective measures to control water-cut rise and stabilize oil production have necessitated the application of enhanced oil recovery(EOR)technology.This paper investigates the development and technological advances for oil reservoirs with strong edge/bottom-water drive globally,and compares their application to reservoirs with characteristics similar to the Tahe oilfield.Among the technological advances,gas injection from the top and along the direction of structural dip has been used to optimize the flow field in a typical bottom-water drive reservoir.Bottom-water coning is restrained by gas injection-assisted water control.In addition,increasing the lateral driving pressure differential improves the plane sweep efficiency which enhances oil recovery in turn.Gas injection technology in combination with technological measures like channeling prevention and blocking,and water plugging and profile control,can achieve better results in reservoir development.Gas flooding tests in the Tahe oilfield are of great significance to identifying which EOR technology is the most effective and has the potential of large-scale application for improving development of deep reservoirs with a strong bottomwater drive.

Gas-Water Production of a Continental Tight-Sandstone Gas Reservoir under Different Fracturing Conditions

A numerical model of hydraulic fracture propagation is introduced for a representative reservoir(Yuanba continental tight sandstone gas reservoir in Northeast Sichuan).Different parameters are considered,i.e.,the interlayer stress difference,the fracturing discharge rate and the fracturing fluid viscosity.The results show that these factors affect the gas and water production by influencing the fracture size.The interlayer stress difference can effectively control the fracture height.The greater the stress difference,the smaller the dimensionless reconstruction volume of the reservoir,while the flowback rate and gas production are lower.A large displacement fracturing construction increases the fracture-forming efficiency and expands the fracture size.The larger the displacement of fracturing construction,the larger the dimensionless reconstruction volume of the reservoir,and the higher the fracture-forming efficiency of fracturing fluid,the flowback rate,and the gas production.Low viscosity fracturing fluid is suitable for long fractures,while high viscosity fracturing fluid is suitable for wide fractures.With an increase in the fracturing fluid viscosity,the dimensionless reconstruction volume and flowback rate of the reservoir display a non-monotonic behavior,however,their changes are relatively small.

Rate transient analysis methods for water-producing gas wells in tight reservoirs with mobile water

Tight gas reservoirs with mobile water exhibit multi-phase flow and high stress sensitivity.Accurately analyzing the reservoir and well parameters using conventional single-phase rate transient analysis methods proves challenging.This study introduces novel rate transient analysis methods incorporating evaluation processes based on the conventional flowing material balance method and the Blasingame type-curve method to examine fractured gas wells producing water.By positing a gas-water two-phase equivalent homogenous phase that considers characteristics of mobile water,gas,and high stress sensitivity,the conventional single-phase rate transient analysis methods can be applied by integrating the phase's characteristics and defining the phase's normalized parameters and material balance pseudotime.The rate transient analysis methods based on the equivalent homogenous phase can be used to quantitatively assess the parameters of wells and gas reservoirs,such as original gas-in-place,fracture half-length,reservoir permeability,and well drainage radius.This facilitates the analysis of production dynamics of fractured wells and well-controlled areas,subsequently aiding in locating residual gas and guiding the configuration of well patterns.The specific evaluation processes are detailed.Additionally,a numerical simulation mechanism model was constructed to verify the reliability of the developed methods.The methods introduced have been successfully implemented in field water-producing gas wells within tight gas reservoirs containing mobile water.

Effect of CO_(2)flooding in an oil reservoir with strong bottom-water drive in the Tahe Oilfield,Tarim Basin,Northwest China

The dissolution and diffusion of CO_(2)in oil and water and its displacement mechanism were investigated by laboratory experiment and numerical simulation for Block 9 in the Tahe oilfield,a sandstone oil reservoir with strong bottom-water drive in Tarim Basin,Northwest China.Such parameters were analyzed as solubility ratio of CO_(2)in oil,gas and water,interfacial tension,in-situ oil viscosity distribution,remaining oil saturation distribution,and oil compositions.The results show that CO_(2)flooding could control water coning and increase oil production.In the early stage of the injection process,CO_(2)expanded vertically due to gravity differentiation,and extended laterally under the action of strong bottom water in the intermediate and late stages.The CO_(2)got enriched and extended at the oil-water interface,forming a high interfacial tension zone,which inhibited the coning of bottom water to some extent.A miscible region with low interfacial tension formed at the gas injection front,which reduced the in-situ oil viscosity by about 50%.The numerical simulation results show that enhanced oil recovery(EOR)is estimated at 5.72%and the oil exchange ratio of CO_(2)is 0.17 t/t.

Deformation and failure mechanism of Yanjiao rock slope influenced by rainfall and water level fluctuation of the Xiluodu hydropower station reservoir

With the construction of the Xiluodu hydropower station on the Jinsha River,the reservoir impoundment began in 2013 and the water level fluctuates annually between 540 m and 600 m above sea level.The Yanjiao rock slope which is located on the left bank of the Jinsha River 75 km upstream of the Xiluodu dam site,began to deform in 2014.The potential failure of the slope not only threatens Yanjiao town but also affects the safe operation of the Xiluodu reservoir.This paper is to find the factors influencing the Yanjiao slope deformation through field investigation,geotechnical reconnaissance,and monitoring.Results show that the Yanjiao slope can be divided into a bank collapse area(BCA)and a strong deformation area(SDA)based on the crack distribution characteristics of the slope.The rear area of the slope has been experiencing persistent deformation with a maximum cumulative displacement(GPS monitoring point G4)of 505 mm and 399 mm in the horizontal and vertical directions,respectively.The potential failure surface of the slope is formed 36 m below the surface based on the borehole inclinometer.The bank collapses of the Yanjiao slope are directly caused by the reservoir impoundment while the deformation area of the slope is affected by the combination of the rainfall and reservoir water level fluctuation.Based on mechanism of the Yanjiao slope,prestressed anchor combined with the surface drainage and slope unloading are recommended to prevent potential deformation.

Dissolution mechanism of a deep-buried sandstone reservoir in a deep water area:A case study from Baiyun Sag,Zhujiang River(Pearl River)Mouth Basin

Dissolution mechanism and favorable reservoir distribution prediction are the key problems restricting oil and gas exploration in deep-buried layers.In this paper,the Enping Formation and Zhuhai Formation in Baiyun Sag of South China Sea was taken as a target.Based on the thin section,scanning electron microscopy,X-ray diffraction,porosity/permeability measurement,and mercury injection,influencing factors of dissolution were examined,and a dissolution model was established.Further,high-quality reservoirs were predicted temporally and spatially.The results show that dissolved pores constituted the main space of the Paleogene sandstone reservoir.Dissolution primarily occurred in the coarse-and medium-grained sandstones in the subaerial and subaqueous distributary channels,while dissolution was limited in fine-grained sandstones and inequigranular sandstones.The main dissolved minerals were feldspar,tuffaceous matrix,and diagenetic cement.Kaolinization of feldspar and illitization of kaolinite are the main dissolution pathways,but they occur at various depths and temperatures with different geothermal gradients.Dissolution is controlled by four factors,in terms of depositional facies,source rock evolution,overpressure,and fault activities,which co-acted at the period of 23.8–13.8 Ma,and resulted into strong dissolution.Additionally,based on these factors,high-quality reservoirs of the Enping and Zhuhai formations are predicted in the northern slope,southwestern step zone,and Liuhua uplift in the Baiyun Sag.

Evaluation of reservoir environment by chemical properties of reservoir water‒A case study of Chang 6 reservoir in Ansai oilfield,Ordos Basin,China

The Ordos Basin is the largest continental multi-energy mineral basin in China,which is rich in coal,oil and gas,and uranium resources.The exploitation of mineral resources is closely related to reservoir water.The chemical properties of reservoir water are very important for reservoir evaluation and are significant indicators of the sealing of reservoir oil and gas resources.Therefore,the caprock of the Chang 6 reservoir in the Yanchang Formation was evaluated.The authors tested and analyzed the chemical characteristics of water samples selected from 30 wells in the Chang 6 reservoir of Ansai Oilfield in the Ordos Basin.The results show that the Chang 6 reservoir water in Ansai Oilfield is dominated by calcium-chloride water type with a sodium chloride coefficient of generally less than 0.5.The chloride magnesium coefficients are between 33.7 and 925.5,most of which are greater than 200.The desulfurization coefficients range from 0.21 to 13.4,with an average of 2.227.The carbonate balance coefficients are mainly concentrated below 0.01,with an average of 0.008.The calcium and magnesium coefficients are between 0.08 and 0.003,with an average of 0.01.Combined with the characteristics of the four-corner layout of the reservoir water,the above results show that the graphics are basically consistent.The study indicates that the Chang 6 reservoir in Ansai Oilfield in the Ordos Basin is a favorable block for oil and gas storage with good sealing properties,great preservation conditions of oil and gas,and high pore connectivity.

Temporal variations in geochemistry of hydraulic fracturing fluid and flowback water in a tight oil reservoir

Hydraulic fracturing facilitates the development and exploitation of unconventional reservoirs.In this study,the injected hydraulic fracturing fluid(HFF)and flowback and produced water(FPW)in tight oil reservoirs of the Lucaogou Formation in the Junggar Basin are temporally sampled from day 1 to day 64.Freshwater is used for fracturing,and HFF is obtained.The chemical and isotopic parameters(including the water type,total salinity,total dissolved solids(TDS),pH,concentrations of Na^(+),Cl^(-),Ba^(+),K^(+),Fe^(2+)+Fe^(3+),and CO_(3)^(2-),dD,and δ^(18)O)are experimentally obtained,and their variations with time are systematically analyzed based on the flowback water.The results show that the water type,Na/Cl ratio,total salinity,and TDS of the FPW change periodically primarily due to the HFF mixing with formation water,thus causing δD and δ^(18)O to deviate from the meteoric water line of Xinjiang.Because of watererock interaction(WRI),the concentrations of Fe^(2+)+Fe^(3+)and CO_(3)^(2-)of the FPW increase over time,with the solution pH becoming more alkaline.Furthermore,based on the significant changes observed in the geochemistry of the FPW,three separate time intervals of flowback time are identified:Stage Ⅰ(<10 days),where the FPW is dominated by the HFF and the changes in ions and isotopes are mainly caused by the WRI;Stage Ⅱ(10-37 days),where the FPW is dominated by the addition of formation water to the HFF and the WRI is weakened;and finally,Stage Ⅲ(>37 days),where the FPW is dominated by the chemistry of the formation water.The methodology implemented in this study can provide critical support for the source identification of formation water.

Seismic Attribute Gradient Analysis and Reservoir Configuration Study of Shallow Water Delta Reservoir in Huanghekou Sag

The geological conditions of shallow offshore delta oil reservoirs are complex. Under the condition of less well data and larger well spacing, the traditional reservoir configuration method is difficult to solve the detailed study of such reservoirs in offshore oil fields. Based on the comprehensive analysis of the seismic phase, data of well log. The paper identifies criteria of the quaternary configuration boundary in shallow water delta of different types with distributary sand dam is established. At the same time, this paper used sensitive factor to construct the edge detection operator based on the amplitude attribute, characterizing the boundary of sand body thickness mutation or physical property mutation quantitatively, realizing the quantitative characterization of four-stage configuration boundary in the region with no wells or few wells, guiding the efficient development of offshore shallow water delta oilfield, and realizing the increase of storage and production of Bohai oilfield.

Spatial and Temporal Variation and Trend Analysis of Water Quality in Large Reservoirs in Ji’an City

Based on the water quality monitoring data of 5 large reservoirs in Ji’an City,Jiangxi Province from 2015-2021,the temporal and spatial variation characteristics and trends of water quality of the reservoirs were analyzed by single-factor evaluation method and seasonal Kendall test to evaluate the trophic status of the reservoirs and explore the influencing factors of characteristic pollutants.The results showed that:①the water quality of the reservoirs was good and could meet the water needs of various functions;②the water quality of the reservoirs had generally changed from bad to good in recent years,indicating that the implementation of“river chief system”has achieved certain results;③Kendall test analysis showed that,except for individual projects which showed an upward trend in water quality,other projects showed no obvious change trend or downward trend,indicating that the water quality of the reservoirs is indeed improving;④the causes of water pollution in reservoir area were further analyzed by exploring the natural and human factors of the characteristic pollutant total phosphorus.It is recommended to strengthen supervision in the later stage to control point and non-point source pollution.

Adaptability of Development Methods for Offshore Gas Cap Edge Water Reservoirs under Different Permeability Levels

The BZ 34-1 oilfield is a typical gas cap edge water reservoir in the Bohai oilfield. The main characteristics of the oilfield were multi-phase sand body stacking and the sand body was composed of three parts: gas cap, oil reservoir, and edge water. The actual production site results show that the permeability difference of multi-layer sand bodies has a serious impact on the development effect. This article establishes a typical reservoir model numerical model based on the total recovery degree of the reservoir and the recovery degree of each layer, and analyzes the impact of permeability gradient. As the permeability gradient increases, the total recovery degree of all four well patterns decreases, and the total recovery degree gradually decreases. The recovery degree of low permeability layers gradually decreases, and the recovery degree of high permeability layers gradually increases. As the permeability gradient increases, the degree of recovery gradually decreases under different water contents. As the permeability gradient increases, the reduction rate of remaining oil saturation in low permeability layers is slower, while the reduction rate of remaining oil saturation in high permeability layers was faster. By analyzing the impact of permeability gradient on the development effect of oil fields, we could further deepen our understanding of gas cap edge water reservoirs and guide the development of this type of oil field.

Paradigm for Determining the Optimal Ultradeep and Superthick Saline Aquifer for High-TDS Mine Water Geological Storage

Saline aquifers are the most popular waste and CO_(2)injection and storage reservoirs worldwide.This project proposes that several optimal injection positions should be investigated as hydraulic pressure-focused positions,in order to relieve the high demands of pump performance.The comprehensive indices(F_(i))representing the injectivity of different burial depths were obtained by using information entropy,based on the mercury injection experimental data of 13 rock samples.The results demonstrated that the burial depths of No.4,No.1 and No.2 in the Liujiagou Formation were the most suitable positions for hydraulic focused injection,which means the upper 30 m thickness could be regarded as the hydraulic focused range in the saline aquifer with an average thickness of 400 m.In addition,some laboratory experiments and in situ tests were carried out for the purpose of certifying and analyzing results,including SEM,XRD,brittleness index and logging.The results suggested that the rock samples at the No.4,No.1 and No.2 burial depth ranges have loose microstructure,weak cementation,as well as dual pores and fractures.The lithology is mainly quartz and feldspar,but the clay mineral content is high(10%-25%),which is positive for dissolution.The lithology is suitable for hydraulic fracturing to form extended cracks and micro-fissures during high-TDS(total dissolved solids)mine water injection,because of the high brittleness index.Finally,a theoretical and technical framework for high-TDS mine water injection was established,based on operating pilot engineering.Some theoretical defects and drawbacks learned from the field practices were summarized and solutions proposed.The research in this study could provide guidance and a paradigm for the inexpensive treatment of high-TDS mine water by injection and storage.

Study on the Fine Configuration of Reservoir in River Facies Oilfield in Bohai Sea Area

The geological conditions of offshore shallow water delta oil reservoirs were complex, with limited well data and large well spacing. Taking A Oilfield in the Bohai Sea Area, China as an example, the target sand body was formed in a shallow water delta sedimentary environment, with well-developed underwater distributary channels and frequent branching and diversion. The reservoir was strong non-uniformity and uneven plane water cut pressure. To this end, based on the existing work of predecessors, combined with seismic, logging, and production dynamics data, and based on the genesis mechanism of shallow water delta reservoirs, the boundary of composite river channels was identified through seismic facies, and logging facies were used to subdivide them into single river levels within the composite river channels. Then, seismic waveform characteristics were applied to track and characterize the plane distribution of single river channels, guiding the efficient development of offshore shallow water delta oil fields and achieving increased storage and production in Bohai Oilfield, China.

Climate Characteristics in Three Gorges Reservoir Area after Water Storage and Its Impact on the Production Potential

Based on the meteorological data from 33 stations of Three Gorges Reservoir from 1960 to 2008,climate yield of rice,corn and winter wheat and the changes of climatic potential productivity after water storage in Three Gorges Reservoir were calculated by the dynamic statistic model of crop growth.The results showed that the temperature in Three Gorges Reservoir was fluctuant decreased before late 1980s,and warmed rapidly after the late 1980s.The precipitation had little change before the late 1990s and had a slight decrease after the late 1990s.Sunshine hours were more in 1960s and 1970s,and then it changed little after 1980s.After water storage,the temperature increased in Three Gorges Reservoir as a whole.The precipitation decreased in the south of Three Gorges Reservoir,while it increased in the northwest of Three Gorges Reservoir.The sunshine hours were reduced except that in the vicinity of Dianjiang.After water storage,climatic potential productivity of rice decreased in the northwest and the northeast,while it increased in the south of Three Gorges Reservoir.The climatic potential productivity of corn decreased in the northeast and the southwest,but increased in the rest of Three Gorges Reservoir.The climatic potential productivity of winter wheat increased almost in total.

A Study on Water Pollution and Bioremediation in Hedi Reservoir

Statistical work and analysis were made based on data of monitored wa- ter quality data in Hedi Reservoir during 2000-2010 and the results show that the content of nutritive salt was higher and water was moderate or light eutrophication. Based on status quo and practical survey research of the Reservoir, the causes for eutrophication deterioration were analyzed and countermeasures were proposed, providing scientific references for sustainable development of Hedi Reservoir.

Evaluation, prediction, and protection of water quality in Danjiangkou Reservoir, China

The water quality in the Danjiangkou Reservoir has attracted considerable attention from the Chinese public and government since the announcement of the Middle Route of the South to North Water Diversion Project （SNWDP）, which commenced transferring water in 2014. Integrated research on the evaluation, prediction, and protection of water quality in the Danjiangkou Reservoir was carried out in this study in order to improve environmental management. Based on 120 water samples, wherein 17 water quality indices were measured at 20 monitoring sites, a single factor evaluation method was used to evaluate the current status of water quality. The results show that the main indices influencing the water quality in the Danjiangkou Reservoir are total phosphorus （TP）, permanganate index （CODM,）, dissolved oxygen （DO）, and five-day biochemical oxygen demand （BODs）, and the concentrations of TP, BODs, ammonia nitrogen （NH3--N）, CODM,, DO, and anionic surfactant （Surfa） do not reach the specified standard levels in the tributaries. Seasonal Mann--Kendall tests indicated that the CODMn concentration shows a highly significant increasing trend, and the TP concentration shows a significant increasing trend in the Danjiangkou Reservoir. The distribution of the main water quality indices in the Danjiangkou Reservoir was predicted using a two-dimensional water quality numerical model, and showed that the sphere of influence from the tributaries can spread across half of the Han Reservoir if the pollutants are not controlled. Cluster analysis （CA） results suggest that the Shending River is heavily polluted, that the Jianghe, Sihe, and Jianhe rivers are moderately polluted, and that they should be the focus of environmental remediation.

Changjiang River sediment delivering into the sea in response to water storage of Sanxia Reservoir in 2003

The Sanxia Reservoir on the Changjiang River stored water from 1 to 10 June and from 25 October to 5 November in 2003, elevating the water level to 135 and 139 m above mean sea level at the dam, respectively. A monthly dataset of water discharge, suspended sediment concentration （SSC） and sediment load of the Changjiang River from 1953 to 2003 measured at the Datong Hydrological Gauging Station of the downstreammost Changjiang River was mainly used to examine the Changjiang River sediment delivering into the sea in 2003 in response to the Sanxia Reservoir water storages in the same year. The results show that （1） compared with those in 2002, 2001, and the multi-yearly （1953-2000） average, both annual SSC and sediment load at Datong in 2003 were markedly reduced, and they were even smaller than the multi-yearly （1953-2000） minimum, although the annual runoff in 2003 did not change largely; and （2） compared with those in the corresponding months in 2002, 2001 and the multi-monthly average from 1953 to 2000, monthly SSC and sediment load at Datong both in June and November of 2003 were also markedly reduced, and those in June 2003 were even smaller than the multi-monthly minimum from 1953 to 2000. These may indicate that sediment sedimentation in the Sanxia Reservoir resulting from the Sanxia Reservoir water storage should be the main cause of the decreased annual and monthly SSC and sediment load of the Changjiang River into the sea in 2003. Besides, it seems that the Sanxia Reservoir water storage in the early June （flood season） of 2003 had more impacts on the decreased monthly SSC into the sea than that in the late October and early November （approximately non-flood season） of 2003.

Effects of physical parameter range on dimensionless variable sensitivity in water flooding reservoirs

The similarity criterion for water flooding reservoir flows is concerned with in the present paper. When finding out all the dimensionless variables governing this kind of flow, their physical meanings are subsequently elucidated. Then, a numerical approach of sensitivity analysis is adopted to quantify their corresponding dominance degree among the similarity parameters. In this way, we may finally identify major scaling law in different parameter range and demonstrate the respective effects of viscosity, permeability and injection rate.

Spatio-temporal distribution of phytoplankton in the Danjiangkou Reservoir, a water source area for the South-to-North Water Diversion Project (Middle Route), China

One of the water source areas of the South-to-North Water Diversion Project is the Danjiangkou Reservoir （DJKR）. To understand seasonal variation in phytoplankton composition, abundance and distribution in the DJKR area before water diversion, as well as to estimate potential risks of water quality after water diversion, we conducted an investigation on phytoplankton in the DJKR from August 2008 to May 2009. The investigation included 10 sampling sites, each with four depths of 0.5, 5, 10, and 20 m. In this study, 117 taxa belonging to 76 genera were identified, consisting of diatoms （39 taxa）, green algae （47 taxa）, blue-green algae （19 taxa）, and others （12 taxa）. Annual average phytoplankton abundance was 2.01×10^6 ind./L, and the highest value was 14.72 ×10^6 ind/L （at site 3 in August 2008）. Phytoplankton abundance in front of the Danjiangkou Dam （DJKD） was higher than that of the Danjiang Reservoir Basin. Phytoplankton distribution showed a vertical declining trend from 0.5 m to 20 m at most sites in August 2008 （especially at sites of 1, 2, 4 and 10）, but no distinct pattern in other sampling months. In December 2008 and March 2009, Stephanodiseus sp. was the most abundant species, amounting to 55.23% and 72.34%, respectively. We propose that high abundance ofStephanodiscus sp. may have contributed greatly to the frequent occurrence of Stephanodiscus sp. blooms in middle-low reaches of the Hanjiang River during the early spring of 2009. In comparison with previous studies conducted from 1992 to 2006, annual average phytoplankton density, green algae and blue-green algae species, as well as major nutrient concentrations increased, while phytoplankton diversity indices declined. This indicates a gradual decline in water quality. More research should be conducted and countermeasures taken to prevent further deterioration of water quality in the DJKR.

Predicting the height of water-flow fractured zone during coal mining under the Xiaolangdi Reservoir

It is very important to determine the extent of the fractured zone through which water can flow before coal mining under the water bodies.This paper deals with methods to obtain information about overburden rock failure and the development of the fractured zone while coal mining in Xin'an Coal Mine.The risk of water inrush in this mine is great because 40%of the mining area is under the Xiaolangdi reservoir.Numerical simulations combined with geophysical methods were used in this paper to obtain the development law of the fractured zone under different mining conditions.The comprehensive geophysical method described in this paper has been demonstrated to accurately predict the height of the water-flow fractured zone.Results from the new model, which created from the results of numerical simulations and field measurements,were successfully used for making decisions in the Xin'an Coal Mine when mining under the Xiaolangdi Reservoir.Industrial scale experiments at the number 11201,14141 and 14191 working faces were safely carried out.These achievements provide a successful background for the evaluation and application of coal mining under large reservoirs.