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.

Time-dependent model for two-phase flow in ultra-high water-cut reservoirs:Time-varying permeability and relative permeability

For the ultra-high water-cut reservoirs,after long-term water injection exploitation,the physical properties of the reservoir change and the heterogeneity of the reservoir becomes increasingly severe,which further aggravates the spatial difference of the flow field.In this study,the displacement experiments were employed to investigate the variations in core permeability,porosity,and relative permeability after a large amount of water injection.A relative permeability endpoint model was proposed by utilizing the alternating conditional expectation(ACE)transformation to describe the variation in relative permeability based on the experimental data.Based on the time dependent models for permeability and relative permeability,the traditional oil-water two-phase model was improved and discretized using the mimetic finite difference method(MFD).The two cases were launched to confirm the validation of the proposed model.The impact of time-varying physical features on reservoir production performance was studied in a real water flooding reservoir.The experimental results indicate that the overall relative permeability curve shifts to the right as water injection increases.This shift corresponds to a transition towards a more hydrophilic wettability and a decrease in residual oil saturation.The endpoint model demonstrates excellent accuracy and can be applied to time-varying simulations of reservoir physics.The impact of variations in permeability and relative permeability on the reservoir production performance yields two distinct outcomes.The time-varying permeability of the reservoir results in intensified water channeling and poor development effects.On the other hand,the time-varying relative permeability enhances the oil phase seepage capacity,facilitating oil displacement.The comprehensive time-varying behavior is the result of the combined influence of these two parameters,which closely resemble the actual conditions observed in oil field exploitation.The time-varying simulation technique of reservoir physical properties proposed in this paper can continuously and stably characterize the dynamic changes of reservoir physical properties during water drive development.This approach ensures the reliability of the simulation results regarding residual oil distribution.

Study on Optimal Water ControlMethods for Horizontal Wells in Bottom Water Clastic Rock Reservoirs

The segmented water control technology for bottom water reservoirs can effectively delay the entry of bottom water and adjust the production profile.To clarify the impact of different methods on horizontal well production with different reservoir conditions and to provide theoretical support for the scientific selection of methods for bottom water reservoirs,a numerical simulation method is presented in this study,which is able to deal with wellbore reservoir coupling under screen tube,perforation,and ICD(Inflow Control Device)completion.Assuming the geological characteristics of the bottom-water conglomerate reservoir in the Triassic Formation of the Tahe Block 9 as a test case,the three aforementioned completion methods are tested to predict the transient production characteristics.The impact of completion parameters,reservoir permeability,bottom-water energy,and individual well control on the time to encounter water in horizontal wells(during a water-free production period)is discussed.A boundary chart for the selection of completion methods is introduced accordingly.The results show that the optimized ICD completion development effect for heterogeneous reservoirs is the best,followed by optimized perforation completion.Permeability is the main factor affecting the performances of completion methods,while bottom water energy and single well controlled reserves have a scarce impact.The average permeability of the reservoir is less than 500 mD,and ICD has the best water control effect.If the permeability is greater than 500 mD,the water control effect of perforation completion becomes a better option.

Impacts of interactions with low-mineralized water on permeability and pore behavior of carbonate reservoirs

Laboratory filtration experiments are employed to investigate effective well killing while minimizing its impacts on surrounding rocks.The novelty of this experimental study lies in the prolonged exposure of rock samples to the killing fluid for seven days,corresponding to the average duration of well workovers in the oilfields in Perm Krai,Russia.Our findings indicate that critical factors influencing the interactions between rocks and the killing fluid include the chemical composition of the killing fluid,the mineralogical composition of the carbonate rocks,reservoir pressure and temperature,and the contact time.Petrophysical analyses using multi-scale X-ray computed tomography,field emission scanning electron microscopy,and X-ray diffraction were conducted on samples both before and after the well killing simulation.The experiments were performed using real samples of cores,crude oil,and the killing fluid.The results from this study indicate that low-mineralized water(practically fresh water)is a carbonate rock solvent.Such water causes the dissolution of rock components,the formation of new calcite crystals and amoeba-like secretions,and the migration of small particles(clay,quartz,and carbonates).The formation of deep channels was also recorded.The assessment reveals that the change in the pH of the killing fluid indicates that the observed mineral reactions were caused by carbonate dissolution.These combined phenomena led to a decrease in the total number of voids in the core samples,which was 25%on average,predominantly among voids measuring between 45 and 70μm in size.The change in the pore distribution in the bulk of the samples resulted in decreases in porosity of 1.8%and permeability of 67.0%in the studied core samples.The results from this study indicate the unsuitability of low-mineralized water as a well killing fluid in carbonate reservoirs.The composition of the killing fluid should be optimized,for example,in terms of the ionic composition of water,which we intend to investigate in future research.

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.

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.

A novel steady-state productivity equation for horizontal wells in bottom water drive gas reservoirs

It is known that there is a discrepancy between field data and the results predicted from the previous equations derived by simplifying three-dimensional（3-D） flow into two-dimensions（2-D）.This paper presents a new steady-state productivity equation for horizontal wells in bottom water drive gas reservoirs.Firstly,the fundamental solution to the 3-D steady-state Laplace equation is derived with the philosophy of source and the Green function for a horizontal well located at the center of the laterally infinite gas reservoir.Then,using the fundamental solution and the Simpson integral formula,the average pseudo-pressure equation and the steady-state productivity equation are achieved for the horizontal section.Two case-studies are given in the paper,the results calculated from the newly-derived formula are very close to the numerical simulation performed with the Canadian software CMG and the real production data,indicating that the new formula can be used to predict the steady-state productivity of such horizontal gas wells.

A new mathematical model for horizontal wells with variable density perforation completion in bottom water reservoirs

Horizontal wells are commonly used in bottom water reservoirs,which can increase contact area between wellbores and reservoirs.There are many completion methods used to control cresting,among which variable density perforation is an effective one.It is difficult to evaluate well productivity and to analyze inflow profiles of horizontal wells with quantities of unevenly distributed perforations,which are characterized by different parameters.In this paper,fluid flow in each wellbore perforation,as well as the reservoir,was analyzed.A comprehensive model,coupling the fluid flow in the reservoir and the wellbore pressure drawdown,was developed based on potential functions and solved using the numerical discrete method.Then,a bottom water cresting model was established on the basis of the piston-like displacement principle.Finally,bottom water cresting parameters and factors influencing inflow profile were analyzed.A more systematic optimization method was proposed by introducing the concept of cumulative free-water production,which could maintain a balance（or then a balance is achieved）between stabilizing oil production and controlling bottom water cresting.Results show that the inflow profile is affected by the perforation distribution.Wells with denser perforation density at the toe end and thinner density at the heel end may obtain low production,but the water breakthrough time is delayed.Taking cumulative free-water production as a parameter to evaluate perforation strategies is advisable in bottom water reservoirs.

Layer regrouping for water-flooded commingled reservoirs at a high water-cut stage

Layer regrouping is to divide all the layers into several sets of production series according to the physical properties and recovery percent of layers at high water-cut stage, which is an important technique to improve oil recovery for high water-cut multilayered reservoirs. Dif- ferent regroup scenarios may lead to different production performances. Based on unstable oil-water flow theory, a multilayer commingled reservoir simulator is established by modifying the production split method. Taking into account the differences of layer properties, including per- meability, oil viscosity, and remaining oil saturation, the pseudo flow resistance contrast is proposed to serve as a characteristic index of layer regrouping for high water-cut multilayered reservoirs. The production indices of multi- layered reservoirs with different pseudo flow resistances are predicted with the established model in which the data are taken from the Shengtuo Oilfield. Simulation results show that the pseudo flow resistance contrast should be less than 4 when the layer regrouping is implemented. The K-means clustering method, which is based on the objec- tive function, is used to automatically carry out the layer regrouping process according to pseudo flow resistances. The research result is applied to the IV-VI sand groups of the second member of the Shahejie Formation in the Shengtuo Oilfield, a favorable development performance is obtained, and the oil recovery is enhanced by 6.08 %.

Impact of formation water on the generation of H2S in condensate reservoirs： a case study from the deep Ordovician in the Tazhong Uplift of the Tarim Basin, NW China

A number of condensate reservoirs with high concentrations of H2S have been discovered in the deep dolomite reservoirs of the lower Ordovician Yingshan Formation（O1y） in the Tazhong Uplift, where the formation water has a high p H value. In the O1y reservoir, the concentrations of Mg^2＋ and SO4^2-in the formation water are higher than those in the upper Ordovician formation.The concentration of H2 S in the condensate reservoirs and the concentration of Mg^2＋in the formation water correlate well in the O1y reservoirs of the Tazhong Uplift, which indicates a presumed thermochemical sulfate reduction（TSR） origin of H2S according to the oxidation theory of contact ion-pairs（CIPs）. Besides, the p H values of the formation water are positively correlated with the concentration of H2S in the condensate reservoirs, which may indicate that high p H might be another factor to promote and maintain TSR. Oil–source correlation of biomarkers in the sulfuretted condensates indicates the Cambrian source rocks could be the origin of condensates. The formation water in the condensate reservoirs of O1y is similar to that in the Cambrian; therefore, the TSR of sulfate-CIPs likelyoccurred in the Cambrian. High H2S-bearing condensates are mainly located near the No. 1 Fault and NE-SW strikeslip faults, which are the major migration pathway of deep fluids in the Tazhong Uplift. The redox between sulfateCIPs and hydrocarbons is the generation mechanism of H2S in the deep dolomite condensate reservoirs of the Tazhong Uplift. This finding should be helpful to predict the fluid properties of deep dolomite reservoirs.

Coastal Reservoirs Strategy for Water Resource Development—A Review of Future Trend

Water use and access become a more and more important determinant of environmental equity and human development according to the view held by the UN [1]. Water scarcity is one of the major crises which has overarching implications for other world problems especially poverty, hunger, ecosystem degradation, desertification, climate change, threatening world peace and security [2]. In the decades to come, freshwater consumed by human will get to a tipping point. Many projects and concepts have been proposed and implemented for several years to improve the effectiveness of using water. These research activities can be grouped as: desalination plants;water detention (like rainwater tanks for collection and reuse);wastewater reuse;dams and reservoirs. This paper summarized the characters of these water solutions. But these methods can’t provide enough fresh water due to limitations imposed by these methods as well as following industry and population’s development. To overcome these shortcomings, coastal reservoir strategy is proposed in the paper. This new strategy is technically feasible, environmentally sustainable and cost effective by demonstration and comparison.

Theoretical exploration of water injection gravity flooding oil in ultra-deep fault-controlled fractured-cavity carbonate reservoirs

Based on the analysis of geological characteristics of ultra-deep fault-controlled fracture-cavity carbonate reservoirs and division of reservoir units, two physical models were made, and physical simulations of oil displacement by water injection were carried out to find out water flooding mechanism in the fault-controlled fracture-cavity carbonate reservoir under complex flow state. On this basis, a mathematical model of fault-controlled carbonate reservoir with coexisting seepage and free flow has been established. Pilot water injection tests have been carried out to evaluate the effects of enhancing oil recovery by water injection. The results show that: fault-controlled fracture-cavity carbonate reservoir units can be divided into three types:the strong natural energy connected type, the weak natural energy connected type and the weak natural energy isolated type;the fault-fracture activity index of the fault-controlled fractured-cavity body can effectively characterize the connectivity of the reservoir and predict the effective direction of water injection;the mathematical model of fault-controlled carbonate reservoir with coexisting seepage and free flows can quantitatively describe the fluid flow law in the fracture-cavity body;the water injected into the fault-controlled fracture-cavity body is weakly affected by the capillary force of the lithologic body, and the oil-water movement is mainly dominated by gravity. The development modes of single well water injection, unit water injection,and single well high pressure water injection proposed based on the connection structure of fracture-cavity space and well storage space configuration are confirmed effective by pilot tests, with obvious water injection gravity flooding effect.

A novel method for predicting breakthrough time of horizontal wells in bottom water reservoirs

Dimensional analysis and numerical simulations were carried out to research prediction method of breakthrough time of horizontal wells in bottom water reservoir. Four dimensionless independent variables and dimensionless time were derived from 10 influencing factors of the problem by using dimensional analysis. Simulations of horizontal well in reservoir with bottom water were run to find the prediction correlation. A general and concise functional relationship for predicting breakthrough time was established based on simulation results and theoretical analysis. The breakthrough time of one conceptual model predicted by the correlation is very close to the result by Eclipse with less than 2% error. The practical breakthrough time of one well in Helder oilfield is 10 d, and the predicted results by the method is 11.2 d, which is more accurate than the analytical result. Case study indicates that the method could predict breakthrough time of horizontal well under different reservoir conditions accurately. For its university and ease of use, the method is suitable for quick prediction of breakthrough time.

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.

A new model for predicting irreducible water saturation in tight gas reservoirs

The irreducible water saturation(Swir) is a significant parameter for relative permeability prediction and initial hydrocarbon reserves estimation.However,the complex pore structures of the tight rocks and multiple factors of the formation conditions make the parameter difficult to be accurately predicted by the conventional methods in tight gas reservoirs.In this study,a new model was derived to calculate Swir based on the capillary model and the fractal theory.The model incorporated different types of immobile water and considered the stress effect.The dead or stationary water(DSW) was considered in this model,which described the phenomena of water trapped in the dead-end pores due to detour flow and complex pore structures.The water film,stress effect and formation temperature were also considered in the proposed model.The results calculated by the proposed model are in a good agreement with the experimental data.This proves that for tight sandstone gas reservoirs the Swir calculated from the new model is more accurate.The irreducible water saturation calculated from the new model reveals that Swir is controlled by the critical capillary radius,DSW coefficient,effective stress and formation temperature.

Optimum development options and strategies for water injection development of carbonate reservoirs in the Middle East

Through the research on several carbonate reservoirs developed in the Middle East, the basic characteristics of different types of carbonate reservoirs are determined, and a set of high-efficiency water injection development options and strategies are presented. Hidden baffles and barriers exist in carbonate reservoirs in the Middle East, so the reservoirs could be divided into different separated development units based on the baffles and barriers characteristics. Flexible and diverse profile control techniques such as high angle wells and simple and applicative zonal water injection have been introduced to improve the control and development degree of reservoirs. Three principal water injection development methods suitable for different carbonate reservoirs in the Middle East are proposed, including the combination of crestal gas injection and peripheral water injection, bottom interval injection and top interval production(buoyancy underpinning), and "weak point and strong plane" area well pattern. Based on the characteristics of very low shale content, fast and far pressure transmission in the Middle East carbonate reservoirs, a large well-spacing flood pattern is recommended, and reasonable development strategies have been made such as moderate water injection rate and maintaining reasonable production pressure drawdown and voidage replacement ratio, so as to maximize the recovery of reservoirs in the none or low water cut period.

The Use of Coastal Reservoirs and SPP Strategy to Provide Sufficient High Quality Water to Coastal Communities

Water quality-induced water shortage is emerging as one of the main threats for the growth of the world’s population and economic development, especially for coastal cities in developing nations. This paper discusses how to supply enough sufficiently clean water to such cities using the technologies of coastal reservoirs and wetland pre-treatments, as well as employing the SPP strategy. The so-called coastal reservoir is defined as a freshwater reservoir situated in seawater which sources its water from river runoff;to improve its water quality, a wetland is used to pre-treat the runoff that is potentially polluted by domestic, agricultural and industrial contaminants. Generally, the existing lakes in the urban-rural fringe are severely polluted;the Separation, Protection and Prevention (SPP) strategy can quickly restore the lakes’ water quality to a drinkable standard. In this study, we take Shanghai, the largest city in China, as an example to investigate the feasibility of the proposed strategies. This investigation shows that using the technology of coastal reservoirs in Shanghai, namely the Qingcaosha Reservoir, freshwater can be pumped from the sea without requiring the desalination process and if an agricultural wetland on Chongming Island is used to pre-treat the river water prior to its storage, its water quality will be improved to Class II. It is also found that Dianshan Lake, which has been polluted and had a water quality of worse than class 5 in 2015, can have its water quality improved to class II in a short period if the SPP strategy is applied. It is expected that it will be a cost-effective and sustainable water supply method for coastal cities.

Timing of advanced water flooding in low permeability reservoirs

It is very important to design the optimum starting time of water injection for the development of low permeability reservoirs. In this type of reservoir the starting time of water injection will be affected by a reservoir pressure-sensitive effect. In order to optimize the starting time of water injection in low permeability reservoirs, this effect of pressure change on rock permeability of low permeability reservoirs was, at first, studied by physical simulation. It was shown that the rock permeability decreases exponentially with an increase in formation pressure. Secondly, we conducted a reservoir engineering study, from which we obtained analytic relationships between formation pressure, oil production rate, water production rate and water injection rate. After our physical, theoretical and economical analyses, we proposed an approach which takes the pressure-sensitive effect into consideration and designed the optimum starting time of water injection, based on the principle of material balance. Finally, the corresponding software was developed and applied to one block of the Jiangsu Oilfield. It is shown that water injection, in advance of production, can decrease the adverse impact of the pressure-sensitive effect on low permeability reservoir development. A water-flooding project should be preferably initiated in advance of production for no more than one year and the optimum ratio of formation pressure to initial formation pressure should be maintained at a level between 1.05 and 1.2.

Displacement behavior and mechanism of long-term water flooding in sandstone oil reservoirs

Sandstone oil reservoirs with huge bottom water and high permeability are generally developed with high flow rate.After long-term water flooding(LTWF),the water flooding characteristics are quite different from that of original reservoir.In this paper,the effects of the PV number,viscosity,and displacement rate during LTWF are studied through experiments.The mechanism is analyzed based on analysis of changes in oil composition,rock mineral composition and wettability.The oil-water relative permeability curves,oil recovery and wettability were obtained with new experiments methods,which avoids the oil metering error by measuring oil and water separately.The research indicates that when the viscosity increases,the water phase permeability decreases,the residual oil saturation increases,and the water content rate increases earlier.A higher water flooding rate results in a higher ultimate recovery.A higher asphaltene content results in a higher viscosity and more oil-wet reservoir conditions.After LTWF,the wettability tends to water-wet,which is more favorable for heavy oil recovery.Moreover,LTWF reduces the clay content,which creates a more water-wet surface and a larger reservoir pore throat environment.This research provides insightful characteristics of offshore sandstone oil reservoirs,which can be used to enhance oil recovery.

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.