Integrated numerical simulation of hydraulic fracturing and production in shale gas well considering gas-water two-phase flow

Based on the displacement discontinuity method and the discrete fracture unified pipe network model,a sequential iterative numerical method was used to build a fracturing-production integrated numerical model of shale gas well considering the two-phase flow of gas and water.The model accounts for the influence of natural fractures and matrix properties on the fracturing process and directly applies post-fracturing formation pressure and water saturation distribution to subsequent well shut-in and production simulation,allowing for a more accurate fracturing-production integrated simulation.The results show that the reservoir physical properties have great impacts on fracture propagation,and the reasonable prediction of formation pressure and reservoir fluid distribution after the fracturing is critical to accurately predict the gas and fluid production of the shale gas wells.Compared with the conventional method,the proposed model can more accurately simulate the water and gas production by considering the impact of fracturing on both matrix pressure and water saturation.The established model is applied to the integrated fracturing-production simulation of practical horizontal shale gas wells.The simulation results are in good agreement with the practical production data,thus verifying the accuracy of the model.

Simulation of Gas-Water Two-Phase Flow in Tight Gas Reservoirs Considering the Gas Slip Effect

A mathematical model for the gas-water two-phase flow in tight gas reservoirs is elaborated.The model can account for the gas slip effect,stress sensitivity,and high-speed non-Darcy factors.The related equations are solved in the framework of a finite element method.The results are validated against those obtained by using the commercial software CMG(Computer Modeling Group software for advanced recovery process simulation).It is shown that the proposed method is reliable.It can capture the fracture rejection characteristics of tight gas reservoirs better than the CMG.A sensitivity analysis of various control factors(initial water saturation,reservoir parameters,and fracturing parameters)affecting the production in tight gas wells is conducted accordingly.Finally,a series of theoretical arguments are provided for a rational and effective development/exploitation of tight sandstone gas reservoirs.

Knowledge flow analysis of knowledge co-production-based climate change adaptation for lowland rice farmers in Bulukumba Regency,Indonesia

To increase the resilience of farmers’livelihood systems,detailed knowledge of adaptation strategies for dealing with the impacts of climate change is required.Knowledge co-production approach is an adaptation strategy that is considered appropriate in the context of the increasing frequency of disasters caused by climate change.Previous research of knowledge co-production on climate change adaptation in Indonesia is insufficient,particularly at local level,so we examined the flow of climate change adaptation knowledge in the knowledge co-production process through climate field school(CFS)activities in this study.We interviewed 120 people living in Bulukumba Regency,South Sulawesi Province,Indonesia,involving 12 crowds including male and female farmers participated in CFS and not participated in CFS,local government officials,agriculture extension workers,agricultural traders,farmers’family members and neighbors,etc.In brief,the 12 groups of people mainly include two categories of people,i.e.,people involved in CFS activities and outside CFS.We applied descriptive method and Social network analysis(SNA)to determine how knowledge flow in the community network and which groups of actors are important for knowledge flow.The findings of this study reveal that participants in CFS activities convey the knowledge they acquired formally(i.e.,from TV,radio,government,etc.)and informally(i.e.,from market,friends,relatives,etc.)to other actors,especially to their families and neighbors.The results also show that the acquisition and sharing of knowledge facilitate the flow of climate change adaptation knowledge based on knowledge co-operation.In addition,the findings highlight the key role of actors in the knowledge transfer process,and key actors involved in disseminating information about climate change adaptation.To be specific,among all the actors,family member and neighbor of CFS actor are the most common actors in disseminating climate knowledge information and closest to other actors in the network;agricultural trader and family member of CFS actor collaborate most with other actors in the community network;and farmers participated in CFS,including those heads of farmer groups,agricultural extension workers,and local government officials are more willing to contact with other actors in the network.To facilitate the flow of knowledge on climate change adaptation,CFS activities should be conducted regularly and CFS models that fit the situation of farmers’vulnerability to climate change should be developed.

Gas-water stratified flow patterns from electromagnetic tomography

Because of gravitational differentiation of multi-phase fluids, gas-water flow is usually stratified in highly inclined or horizontal gas wells. By using electrode arrays to scan flowing fluids, electromagnetic tomography can identify the flow patterns of mixed fluid from the different electrical properties of gas and water. The responses for different gas-water interface locations were calculated and then physical measurements were undertaken. We compared the results of the numerical simulation with the experimental data, and found that the response characteristics were consistent in the circumstances of uniform physical fields and stratified flows. By analyzing the signal characteristics, it is found that, with the change of the interface location, the response curves showed ＂steps＂ whose position and width were decided by the location of fluid interface. The measurement accuracy of this method depended on the vertical distance between adjacent electrodes. The results showed that computer simulation can simulate the measurement of the electromagnetic tomography accurately, so the physical experiment can be replaced.

Pore-Throat Combination Types and Gas-Water Relative Permeability Responses of Tight Gas Sandstone Reservoirs in the Zizhou Area of East Ordos Basin, China

With the aim of better understanding the tight gas reservoirs in the Zizhou area of east Ordos Basin,a total of 222 samples were collected from 50 wells for a series of experiments.In this study,three pore-throat combination types in sandstones were revealed and confirmed to play a controlling role in the distribution of throat size and the characteristics of gas-water relative permeability.The type-I sandstones are dominated by intercrystalline micropores connected by cluster throats,of which the distribution curves of throat size are narrow and have a strong single peak(peak ratio>30%).The pores in the type-II sandstones dominantly consist of secondary dissolution pores and intercrystalline micropores,and throats mainly occur as slice-shaped throats along cleavages between rigid grain margins and cluster throats in clay cement.The distribution curves of throat size for the type-II sandstones show a bimodal distribution with a substantial low-value region between the peaks(peak ratio<15%).Primary intergranular pores and secondary intergranular pores are mainly found in type-III samples,which are connected by various throats.The throat size distribution curves of type-III sandstones show a nearly normal distribution with low kurtosis(peak ratio<10%),and the micro-scale throat radii(>0.5μm)constitute a large proportion.From type-I to type-III sandstones,the irreducible water saturation(Swo)decreased;furthermore,the slope of the curves of Krw/Krg in two-phase saturation zone decreased and the two-phase saturation zone increased,indicating that the gas relative flow ability increased.Variations of the permeability exist in sandstones with different porethroat combination types,which indicate the type-III sandstones are better reservoirs,followed by type-II sandstones and type-I sandstones.As an important factor affecting the reservoir quality,the pore-throat combination type in sandstones is the cumulative expression of lithology and diagenetic modifications with strong heterogeneity.

Mechanism of gas-water flow at pore-level in aquifer gas storage

By means of the pore-level simulation, the characteristics of gas-water flow and gas-water distribution during the alternative displacement of gas and water were observed directly from etched-glass micromodel. The results show that gas-water distribution styles are divided into continuous phase type and separate phase type. The water lock exists in pore and throat during the process of gas-water displacement, and it reduces the gas flow-rate and has some effects on the recovery efficiency during the operation of gas storage. According to the experimental results of aquifer gas storage in X area, the differences in available extent among reservoirs are significant, and the availability of pore space is 33% 45%.

Pore-scale simulation of gas-water flow in low permeability gas reservoirs

A novel method was developed to establish a realistic three dimensional(3D) network model representing pore space in low permeability sandstone.Digital core of rock sample was established by the combination of micro-CT scanning and image processing,then 3D pore-throat network model was extracted from the digital core through analyzing pore space topology,calculating pore-throat parameters and simplifying the shapes of pores and throats.The good agreements between predicted and measured porosity and absolute permeability verified the validity of this new network model.Gas-water flow mechanism was studied by using pore-scale simulations,and the influence of pore structure parameters,including coordination number,aspect ratio and shape factor,on gas-water flow,was investigated.The present simulation results show that with the increment of coordination number,gas flow ability in network improves and the effect of invading water on blocking gas flow weakens.The smaller the aspect ratio is,the stronger the anisotropy of the network is,resulting in the increase of seepage resistance.It is found that the shape factor mainly affects the end points in relative permeability curves,and for a highly irregular pore or throat with a small shape factor,the irreducible water saturation(Swi) and residual gas saturation(Sgr) are relatively high.

Droplets Turbulence Effect of Gas-Water Separator with Corrugated Plates

Droplet turbulence effect on gas-water separator with corrugated plates is explored using the Eulerian-Lagrangian two-way coupled multiphase approach of FLUENT. It is concluded that the inertial force is dominant in separating large droplets, while droplet turbulence dispersion plays a decisive role in separating fine droplets. Good agreement exists between calculations and air-water experiments. The numerical method developed provides a rea-sonable description of the droplet trajectories and separating efficiency, and it can be applied to predicting the performance of gas-water separator with corrugated plates.

A method for identifying coalbed methane co-production interference based on production characteristic curves: A case study of the Zhijin block, western Guizhou, China

Efficient detection of coalbed methane(CBM) co-production interference is the key to timely adjusting the development plan and improving the co-production efficiency. Based on production data of six typical CBM co-production wells in the Zhijin block of western Guizhou Province, China, the production characteristic curves, including production indication curve, curve of daily water production per unit drawdown of producing fluid level with time, and curve of water production per unit differential pressure with time have been analyzed to explore the response characteristics of co-production interference on the production characteristic curves. Based on the unit water inflow data of pumping test in coal measures, the critical value of in-situ water production of the CBM wells is 2 m^(3)/(d·m). The form and the slope of the initial linear section of the production indication curves have clear responses to the interference, which can be used to discriminate internal water source from external water source based on the critical slope value of 200 m^(3)/MPa in the initial linear section of the production indication curve. The time variation curves of water production per unit differential pressure can be divided into two morphological types: up-concave curve and down-concave curve. The former is represented by producing internal water with average daily gas production greater than 800 m^(3)/d, and the latter produces external water with average daily gas production smaller than 400 m^(3)/d. The method and critical indexes for recognition of CBM co-production interference based on the production characteristic curve are constructed. A template for discriminating interference of CBM co-production was constructed combined with the gas production efficiency analysis, which can provide reference for optimizing co-production engineering design and exploring economic and efficient co-production mode.

How Does the Government Facilitate the Co-Production of Digital Public Safety Services?——Based on Empirical Evidence from Shenzhen

The application of big data technology provides support for the co-production of public safety services.Existing research often focuses on how technology influences co-production,but lacks attention to the key actors that drive co-production and the mechanisms that facilitate it.This study examines the role of government in the digital co-production of public safety services,using the practice of Shenzhen as a case study.Shenzhen has built 125 information systems based on over 100 billion pieces of big data,forming a model of digital safety service co-production.The study reveals three types of digital co-production,characterized by"government-business joint planning,passive participation of businesses and the public,and active cooperation among government,businesses,and the public"in the"design-production-application"stages.The study shows that the government is not only a co-producer but also a proactive actor in activating the willingness of non-governmental entities to participate.Local governments mobilize non-governmental participation through three mechanisms:empowerment,profit enhancement,and value co-creation.The"power-interest-value"paradigm is applicable for analyzing the co-production of public services and helps to explain the transformation mechanisms of co-production behavior.

Simulation of Two-Phase Flowback Phenomena in Shale Gas Wells

The gas-water two-phaseflow occurring as a result of fracturingfluidflowback phenomena is known to impact significantly the productivity of shale gas well.In this work,this two-phaseflow has been simulated in the framework of a hybrid approach partially relying on the embedded discrete fracture model(EDFM).This model assumes the region outside the stimulated reservoir volume(SRV)as a single-medium while the SRV region itself is described using a double-medium strategy which can account for thefluid exchange between the matrix and the micro-fractures.The shale gas adsorption,desorption,diffusion,gas slippage effect,fracture stress sensitivity,and capillary imbibition have been considered.The shale gas production,pore pressure distribution and water saturation distribution in the reservoir have been simulated.The influences of hydraulic fracture geometry and nonorthogonal hydraulic fractures on gas production have been determined and discussed accordingly.The simulation results show that the daily gas production has an upward and downward trend due to the presence of a large amount of fracturingfluid in the reservoir around the hydraulic fracture.The smaller the angle between the hydraulic fracture and the wellbore,the faster the daily production of shale gas wells decreases,and the lower the cumulative production.Nonplanar fractures can increase the control volume of hydraulic fractures and improve the production of shale gas wells.

Microscopic experiment on efficient construction of underground gas storages converted from water-invaded gas reservoirs

Based on the microfluidic technology,a microscopic visualization model was used to simulate the gas injection process in the initial construction stage and the bottom water invasion/gas injection process in the cyclical injection-production stage of the underground gas storage(UGS)rebuilt from water-invaded gas reservoirs.Through analysis of the gas-liquid contact stabilization mechanism,flow and occurrence,the optimal control method for lifecycle efficient operation of UGS was explored.The results show that in the initial construction stage of UGS,the action of gravity should be fully utilized by regulating the gas injection rate,so as to ensure the macroscopically stable migration of the gas-liquid contact,and greatly improve the gas sweeping capacity,providing a large pore space for gas storage in the subsequent cyclical injection-production stage.In the cyclical injection-production stage of UGS,a constant gas storage and production rate leads to a low pore space utilization.Gradually increasing the gas storage and production rate,that is,transitioning from small volume to large volume,can continuously break the hydraulic equilibrium of the remaining fluid in the porous media,which then expands the pore space and flow channels.This is conducive to the expansion of UGS capacity and efficiency for purpose of peak shaving and supply guarantee.

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.

MRI insight on multiphase flow in hydrate-bearing sediment and development mechanism of hydrate seal

Gas and water migration through the hydrate-bearing sediment are characteristic features in marine gas hydrate reservoirs worldwide.However,there are few experimental investigations on the effect of water-gas flow on the gas hydrate reservoir.In this study,gas-water migration in gas hydrate stability zone(GHSZ)was investigated visually employing a high-resolution magnetic resonance imaging(MRI)apparatus,and the formation of hydrate seal was experimentally investigated.Results revealed that normal flow of gas-water at the low flow rate of 1–0.25 mL/min will induce the hydrate reformation.Conversely,higher gas-water flow rates(at 2–0.5 and 4–1 mL/min)need higher reservoir pressure to induce the hydrate reformation.In addition,the hydrate reformation during the gas-water flow process produced the hydrate seal,which can withstand an over 9.0 MPa overpressure.This high overpressure provides the development condition for the underlying gas and/or water reservoir.A composite MRI image of the whole hydrate seal was obtained through the MRI.The pore difference between hydrate zone and coexistence zone produces a capillary sealing effect for hydrate seal.The hydrate saturation of hydrate seal was more than 51.6%,and the water saturation was more than 19.3%.However,the hydrate seal can be broken through when the overpressure exceeded the capillary pressure of the hydrate seal,which induced the sudden drop of reservoir pressure.This study provides a scientific explanation for the existence of high-pressure underlying gas below the hydrate layer and is significant for the safe exploitation of these common typical marine hydrate reservoirs.

P-wave and S-wave response of coal rock containing gas-water with different saturation: an experimental perspective

The acoustic response of gas and/or water saturated coal rock is fundamental for establishing the correspondence between the physical properties of the coal reservoir and the characteristics of the well-logging response,which is the technology essential for the geophysical exploration of coalbed methane(CBM).This acoustic response depends on water(Sw)and gas(Sg)saturation among other factors.In this study,we performed acoustic tests on dry and different gas-water saturated coal samples with different degrees of metamorphism and deformation,collected from several coal mining areas in China.These tests enabled us to analyze the influence of coal type and gas-water saturation on the acoustic response of CBM formations.Our results show that the acoustic velocity of P-wave and S-wave(Vp and Vs,respectively),and the relative anisotropy of and Vs,increased with increasing vitrinite reflectance,density,Vp and Sw.WithSw increasing from 0 to 100%,the growth rate of the acoustic velocity decreased with increasing vitrinite reflectance.The Vp/Vs ratio of tectonic coal was generally higher than that of primary coal.The growth rate of the relative anisotropy in tectonic coal was markedly higher than that in primary coal.

Coalbed methane desorption characteristics controlled by coalification and its implication on gas co-production from multiple coal seams

In this work,CH4 isothermal adsorption measurements were carried out on 64 coal samples collected from western Guizhou Province of China,and the coalbed methane(CBM)desorption processes were quantitatively analyzed.The results show that the Langmuir volume and the Langmuir pressure are controlled by coalification,and tend to increase as the vitrinite reflectance changes from 0.98% to 4.3%.Based on a division method of CBM desorption stages,the CBM desorption process were divided into four stages(inefficient,slow,fast and sensitive desorption stages)by three key pressure nodes(the initial,turning and sensitive pressures).The fast and sensitive desorption stages with high desorption efficiency are the key for achieving high gas production.A theoretical chart of the critical desorption pressure(P_(cd))and its relationship with different pressure nodes was established.The higher-rank coals have the higher initial,turning and sensitive pressures,with larger difference between pressure nodes.Most CBM wells only undergo partial desorption stages due to the differences in P_(cd) caused by the present-gas content.Under the same gas content conditions,the higher the coal rank,the less desorption stages that CBM needs to go through.During coalbed methane co-production from multiple coal seams within vertically superposed pressure systems,the reservoir pressure,the P_(cd),the initial working liquid level(WLL)height,and coal depth are key factors for evaluating whether coal seams can produce CBM simultaneously.It must be ensured that each production layer enters at least the fast desorption stage prior to that the WLL was lower than the depth of each layer.Only on this basis can all layers achieve the maximum gas production.

Sintering microstructure and properties of copper powder prepared by electrolyzation and atomization

The almost completely dense copper was prepared by ultrafine copper powder prepared with both methods of electrolysis and novel water-gas atomization through cold isostatic pressing(CIP)and sintering under atmospheric hydrogen.Fine copper powder possesses the higher sintering driving force,thereby promoting shrinkage and densification during the sintering process.The grain size of sintered samples by electrolytic copper powder is smaller than that prepared by the atomized copper powder,and the twin crystals are particularly prone to forming in the former sintered microstructure due to the raw powder with low oxygen content and high residual stress originating from the CIP process.The relative density of samples by electrolytic and atomized powder at 1000℃ sintering temperature achieves 99.3%and 97.4%,respectively,significantly higher than that of the powder metallurgy copper parts reported in the literature.Correspondingly,the ultimate tensile strength and yield strength of samples by both kinds of copper powder are approximately similar,while the elongation of the sintered sample by the electrolytic powder(60%)is apparently higher than the atomized powder(44%).The superior performance of samples fabricated by electrolytic powder is inferred from the full density and low oxygen level for there is no cuprous oxide in the grain boundaries.

Flow structures of gaseous jets injected into water for underwater propulsion

Gaseous jets injected into water are typically found in underwater propulsion, and the flow is essentially unsteady and turbulent. Additionally, the high water-to-gas density ratio can result in complicated flow structures; hence measuring the flow structures numerically and experimentally remains a challenge. To investigate the performance of the underwater propulsion, this paper uses detailed NavierStokes flow computations to elucidate the gas-water interactions under the framework of the volume of fluid （VOF） model. Furthermore, these computations take the fluid compressibility, viscosity, and energy transfer into consideration. This paper compares the numerical results and experimental data, showing that phenomena including expansion, bulge, necking/breaking, and back-attack are highlighted in the jet process. The resulting analysis indicates that the pressure difference on the rear and front surfaces of the propul- sion system can generate an additional thrust. The strong and oscillatory thrust of the underwater propulsion system is caused by the intermittent pulses of the back pressure and the nozzle exit pressure. As a result, the total thrust in underwater propulsion is not only determined by the nozzle geometry but also by the flow structures and associated pressure distri- butions.

Controlling Effects of Tight Reservoir Micropore Structures on Seepage Ability: A Case Study of the Upper Paleozoic of the Eastern Ordos Basin, China

In this study, the types of micropores in a reservoir are analyzed using casting thin section(CTS) observation and scanning electron microscopy(SEM) experiments. The high-pressure mercury injection(HPMI) and constant-rate mercury injection(CRMI) experiments are performed to study the micropore structure of the reservoir. Nuclear magnetic resonance(NMR), gas-water relative seepage, and gas-water two-phase displacement studies are performed to examine the seepage ability and parameters of the reservoir, and further analyses are done to confirm the controlling effects of reservoir micropore structures on seepage ability. The experimental results show that Benxi, Taiyuan, Shanxi, and Shihezi formations in the study area are typical ultra-low porosity and ultra-low permeability reservoirs. Owing to compaction and later diagenetic transformation, they contain few primary pores. Secondary pores are the main pore types of reservoirs in the study area. Six main types of secondary pores are: intergranular dissolved pores, intragranular dissolved pores, lithic dissolved pores, intercrystalline dissolved pores, micropores, and microfracture. The results show that reservoirs with small pore-throat radius, medium displacement pressure, and large differences in pore-throat structures are present in the study area. The four types of micropore structures observed are: lower displacement pressure and fine pores with medium-fine throats, low displacement pressure and fine micropores with fine throats, medium displacement pressure and micropores with micro-fine throats, and high displacement pressure and micropores with micro throats. The micropore structure is complex, and the reservoir seepage ability is poor in the study areas. The movable fluid saturation, range of the gas-water two-phase seepage zone, and displacement types are the three parameters that well represent the reservoir seepage ability. According to the characteristic parameters of microscopic pore structure and seepage characteristics, the reservoirs in the study area are classified into four types(Ⅰ–Ⅳ), and types Ⅰ, Ⅱ, and Ⅲ are the main types observed. From type Ⅰ to type Ⅳ, the displacement pressure increases, and the movable fluid saturation and gas-water two-phase seepage zone decrease, and the displacement type changes from the reticulation-uniform displacement to dendritic and snake like.

Thermodynamic Consistency Test for Binary Gas ＋ Water Equilibrium Data at Low and High Pressures

Phase equilibrium in binary gas + water mixtures over wide ranges of temperatures and pressures are modeled and tested for thermodynamic consistency. For modeling, the Peng-Robinson equation of state was used and the Wong-Sandler mixing rules were incorporated into the equation of state parameters. In the Wong-Sandler mixing rules the van Laar model for the excess Gibbs energy was applied. In addition, a reasonable and flexible method is applied to test the thermodynamic consistency of pressure-temperature-concentration(P-T-x) data of these binary mixtures.Modeling is found acceptable in all cases, meaning that deviations in correlating the pressure and the gas phase concentration are low. For all cases the thermodynamic consistency method gives a clear conclusion about consistency or inconsistency of a set of experimental P-T-x data.