The Hijiori Hot Dry Rock test site, Japan: Evaluation and optimization of heat extraction from a two-layered reservoir

The Hijiori hot dry rock (HDR) system, Japan, consists of a shallow and a deep reservoir, both in fractured granitic rocks. During a long-term circulation test (LTCT) lasting approximately 18 months and which tested different fluid production scenarios, large changes were observed in output fluid temperatures, pressures, and flow rates. A multi-reservoir, numerical model of the Hijiori HDR system was developed using the finite element heat and mass (FEHM) transfer code and applied to simulations of the LTCT. The model reproduced the pressure, temperature, and flow data observed during the test. Based on the modeling study, it was concluded that most of the produced fluid came from the shallow reservoir, that the permeability of the deep reservoir changed during the initial part of the LTCT, and that the redistribution of injected water between the two reservoirs had little impact on the relative amounts of deep and shallow fluid production. After validating the model on the LTCT, it was used to optimize injection rates in both reservoirs. The model was also used in simulations of reservoir performance where an additional heat transfer surface area has been created in the subsurface through hydraulic fracturing.     

Numerical investigation of production behavior of deep geothermal reservoirs at super-critical conditions

Numerical simulations have been performed to predict pressure transients in deep geothermal reservoirs at sub- and super-critical temperatures. First, pressure drawdown and buildup tests of reservoirs with different initial conditions were simulated. The calculated pressure responses are dominated by non-linear changes of fluid kinematic viscosity and compressibility. The pressure of a super-critical zone is shown to cause complex behavior. Short- and long-term production tests in both unbounded and bounded reservoirs were then simulated. Unbounded and bounded reservoirs exhibit very similar short-term production behavior near or above the critical temperature (375–400°C). Unbounded reservoirs of low transmissivity (kh = 1 darcy-meter) exhibit long-term production behavior that depends on whether the reservoir is sub-critical (300–375°C) or super-critical (400°C); substantial increases in flowing enthalpy and declines in feedpoint pressure occur at early times in the super-critical reservoir.     

Waste water reinjection at tongonan geothermal field: Results and implications

Reinjection of waste water in geothermal reservoirs is primarily intended for the disposal of the separated brine containing toxic minerals. It also increases the recovery of internal energy in the reservoir, aside from sustaining pressure and fluid mass in-place. However, there are also associated detrimental effects in the system once premature breakthrough of reinjected fluid occurs. There will be losses in well deliverability, due to decline in enthalpy and steam fraction, and reduction in permeability if deposition takes place. Induced seismicity also increases in the area, especially along faulted regions. Increases in iniectivity during the early span of reinjection in Tongonan have been experienced, followed by levelling and reduction after a certain period. A drop in well output of the well followed a premature fluid breakthrough. In the production sector, pressure drawdown is minimized due to reinjection returns.     

Fluid circulation and heat extraction from engineered geothermal reservoirs

A large amount of fluid circulation and heat extraction (i.e., thermal power production) research and testing has been conducted on engineered geothermal reservoirs in the past 15 years. In confined reservoirs, which best represent the original Hot Dry Rock concept, the flow distribution at any given time is primarily determined by three parameters: (1) the nature of the interconnected network of pressure-stimulated joints and open fractures within the flow-accessible reservoir region, (2) the mean pressure in the reservoir, and (3) the cumulative amount of fluid circulation—and therefore reservoir cooling—that has occurred. For an initial reservoir rock temperature distribution and mean fluid outlet temperature, the rate of heat extraction (i.e., thermal power) is at first only a function of the production flow rate, since the production temperature can be expected to remain essentially constant for some time (months, or even years). However, as reservoir circulation proceeds, the production temperature will eventually start to decline, as determined by the mean effective joint spacing and the total flow-accessible (i.e., heat-transfer) volume of the reservoir. The rate of heat extraction, which depends on the production flow rate, can also vary with time as a result of continuing changes in the flow distribution arising from reservoir cooling.The thermal power of engineered reservoirs can most readily be increased by increasing the production flow rate, as long as this does not lead to premature cooldown, the development of short-circuit flow paths, or excessive water losses. Generally, an increase in flow rate can be accomplished by increasing the injection pressure within limits. This strategy increases the driving pressure drop across the reservoir and the mean reservoir pressure, which in turn reduces the reservoir flow impedance by increasing the amount of joint dilation. However, the usefulness of this strategy is limited to reservoir operating pressures below the fracture extension pressure, and may lead to excessive water losses, particularly in less-confined reservoirs. Under such conditions, a downhole production-well pump may be employed to increase productivity by recovering more of the injected fluid at lower mean reservoir operating pressures.     

Two-phase heat and mass transfer experiment at well BR21 Broadlands

BR21 is a “two-phase” well in Broadlands geothermal field, producing at excess enthalpy from a liquid-dominated reservoir. Detailed transient measurements were made, measuring the changes in mass flow, enthalpy, chemistry and pressure after the well was opened, and after later changes in flow. These results are interpreted in terms of possible models of heat and mass transfer in the reservoir near the well. It is found that there is a delayed thermal equilibrium which requires a fractured rather than a homogeneous porous medium model, and that variations in discharge chemistry did not distinguish between the possible models. An estimate of the steam saturation in the reservoir was obtained.     

Mass transfer from hot dry rock to water flowing through a circular fracture

Numerical simulation of heat and mass transfer from hot dry rock to flowing water in a circular fracture was conducted to estimate the concentration of the dissolved silica at the production well. The local mass transfer coefficients between the rock and the fluid, obtained by using the electrochemical method during the laboratory experiments, were used in the calculation. The results of the simulation indicate that the concentration of the silica at the production well increases with increasing distance between the injection and the production wells, increasing fracture diameter and increasing pressure. It decreases with increases in flow rate and porosity in the fracture. The concentration of silica at the production well first increases and then decreases with increasing initial rock temperature due to the decrease in the solubility of silica at high temperatures. Finally, the mass transfer by forced convection of flowing water plays an important role in the variation in the concentration of silica in water, and the assumption that the concentration of dissolved silica is equal to that at the fracture surface is not valid for estimation of the concentration at the production well.     

库水位骤降条件下动水压力型滑坡稳定性分析

三峡水库蓄水后库区部分滑坡已显著变形,且随库水位下降过程响应明显,因此分析日降幅条件下库区动水压力型滑坡稳定性就十分重要。为此,采用GEO-studio软件分析库水位骤降条件下滑坡体不同区段的稳定性,发现库水位作用于滑坡体的下滑段时,滑坡体稳定系数随库水位下降而下降,库水位下降速度越大,稳定系数下降越快,但若最终库水位一致,滑坡稳定性系数相差不大;库水位下降作用于滑坡体阻滑段时,滑坡表现出一定的动水压力效应,整体结果为浮托减重,稳定系数先下降后上升;整体而言提高日降幅对动水压力型滑坡稳定性的影响有限。     

地热流体在井筒中的流动及碳酸钙结垢过程模拟

以地热井筒流动和碳酸钙结垢过程为研究对象,建立了地热流体从井底到井口流动过程中的质量、能量以及压降数学模型并进行了求解。通过数学模型计算了质量流量和井口压力对应的关系;模拟分析了温度、压力、干度和CO₂分压沿井筒不同位置的变化情况;总结了井口干度、温度、压力和CO₂分压随热储压力、温度、流量、CO₂含量以及井壁粗糙度的变化规律;分析了结垢位置、结垢厚度以及结垢延续长度对井口压力的影响。结果表明,通过数学模型可以对井筒内的流动进行模拟,确定不同截面上的参数变化情况,并可进一步推导热储的特性变化情况。     

Geochemical evidence of drawdown in the Cerro Prieto geothermal field

Some wells of the Cerro Prieto geothermal field have undergone changes in the chemistry of fluids produced which reflect reservoir processes. Pressure decreases due to production in the southeastern part of the field have produced both drawdown of lower chloride fluids from an overlying aquifer and boiling in the aquifer with excess steam reaching the wells. These reservoir changes are indicated by changes in fluid chloride concentrations, Na/K ratios and measured enthalpies and by comparisons of aquifer fluid temperatures and chloride concentrations calculated from enthalpy and chemical measurements. Fluid temperatures have not been greatly affected by this drawdown because heat contained in the rock was transferred to the fluid. When this heat is exhausted, fluid temperatures may drop rapidly.     

Geothermal reservoir monitoring by continuous self-potential measurements,Mori geothermal field,Japan

A self-potential (SP) monitoring study was conducted at the Mori geothermal field, Japan, to detect fluid flow changes in the reservoir caused by well operations. In 2000, eight observation points were deployed in and around the reservoir region for about 2.5 months, during which the production and reinjection wells were shut in and then re-opened. Since no reliable remote reference was available, the SP results were plotted in two ways: adopting an observation point in the target area as reference, and using the average value of all the observation points as a reference (called the “relative SP” plot). Although changes in SP corresponding to well operations were detected in both plots, more reliable results were obtained with the relative SP plot. Based on numerical simulation of SP using a simplified model of the Mori reservoir, the observed changes are interpreted as being caused by reservoir pressure changes through electrokinetic coupling.     

向心涡轮可调导向叶栅内流动损失机理的分析

通过对向心涡轮可调导向叶栅三维流场数值模拟,分析在不同叶片安装角下,可调叶片表面静压系数和出口总压损失系数的变化规律。导叶安装角从21°增加到44°,通流面积调节范围为50%～116%设计通流面积。结果表明:叶栅开度减小时,叶片的气动负荷增加,总压损失增加。与设计工况相比,导叶关小15°总压损失增加了1倍多。叶栅端部间隙增加了导向叶栅的流动损失,间隙增加2%,损失增加1.5%,端部损失范围从20%叶高增加到40%叶高。叶栅开度减小,端部损失与叶型损失的变化较小,而间隙损失无论是数量还是占总压损失的比重都明显增加,是非设计工况下总压损失增加的主要原因。     

Experimental study of a series-connected two-evaporator refrigerating system with propane (R-290) as the refrigerant

The performance of a refrigerating system with an environment-friendly refrigerant, propane (R-290) as the refrigerant, was experimentally studied. There were two evaporators connected in series within the system under study.The results show that with both lengths of the two capillary tubes fixed, both the mass flow rate of the refrigerant and the suction pressure of the system increase with the condensing pressure. In addition, the cooling capacity of the high-temperature evaporator decreases, but that of the low-temperature evaporator increases. As the condensing pressure is fixed and the length of the capillary tube for the high-temperature evaporator is increased while that for the low-temperature evaporator is fixed, the cooling capacity of the high-temperature evaporator increases while that of the low-temperature evaporator decreases. On the other hand, as the capillary tube for the low-temperature evaporator is lengthened while that for the high-temperature evaporator is fixed, the variations in the cooling capacity of these two evaporators reverse. The enthalpy changes of the refrigerant within the evaporators are strongly affected by the length of the high-temperature capillary tube, while the evaporating pressures are influenced mainly by the length of the low-temperature capillary tube.     

Numerical investigation of the solid particle erosion rate in a steam turbine nozzle

A numerical investigation of solid particle erosion in the nozzle of 300 MW steam turbine is presented. The analysis consists in the application of the discrete phase model, for modelling the solid particles flow, and the Eulerian conservation equations to the continuous phase. The numerical study employs a computational fluid dynamics (CFD) software, based on a finite volume method.The investigation permits us to know the influence of the parameters such as: particle diameter, impact angle, particle velocity and particle distribution on the erosion rate in the surface of the nozzle. These parameters are analyzed to different operational conditions in the turbine.The results show the eroded zone which increases the throat area through the nozzle provoking changes in the operation conditions. When the throat area increases the turbine demands an increase of steam flow to maintain the power supply. On the other hand, it is shown that the solid particles flow cause the most severe the erosion rate whereas the steam mass flow rate is the most sensitive parameter. Finally it is obtained that the erosion rate decreases as the diameter of the particle increases in a nearly linear form.     

基于CFD数值模拟下的蒸汽喷射器性能预测

建立了蒸汽喷射器热力学模型,并验证了该模型性能及结构计算的可靠性。并针对实际情况中,对该模型进行了动态模拟。结果显示：当工作流体的温度升高时,工质流体的质量流量都会增加,引射系数存在峰值,峰值所对应温度为热力学模型设定温度值;当引射流体的温度升高时,其质量流量也会随之增大,而工作流体的质量流量则较稳定,因此引射流体温度与压力的升高可以改善喷射器的性能;当背压升高时,在一定压力范围内,工质流体的质量流量都趋于稳定,而当背压超过热力学模型设定背压值时,引射流体的质量流量便随背压的升高而急剧下降,喷射器性能严重恶化,故认为该压力值为喷射器的临界背压。本文研究结果对喷射器的设计计算具有一定的指导作用。 关键词：蒸汽喷射器;热力学模型;数值模拟;引射系数     

Longevity evaluation for optimum development in a liquid dominated geothermal field: effects of interaction of reservoir pressure and fluid temperature on steam production at operating conditions

The steam production rate of a well at fixed operating conditions in a liquid-dominated geothermal field is reduced at first by a decline in reservoir pressure and then by a decrease in fluid temperature, if reinjected water returns to the production well. In many cases, the fluid temperature decrease reduces the steam production rate more than does the reservoir pressure decline. Those effects should therefore be taken into due account in the evaluation of the longevity of an area, because sufficient longevity and recoverable electric energy are the minimum requirements for planning field development.     

The Effect of a Number of Baffles on the Performance of Shell-and-Tube Heat Exchangers

The number of baffles has an impact on the thermal-hydraulic performance of a shell-and-tube heat exchanger (STHX), thus a model was developed using Engineering Equations Solver software to solve the governing equations. The program uses Kern, Bell-Delaware, and flow-stream analysis (Wills Johnston) methods to predict both the heat-transfer coefficient and pressure drop on the shell side of an STHX. It was found that Bell-Delaware method is the most accurate method when compared with the experimental results. The effect of a number of baffles, mass flow rate, tube layout, fluid properties and baffle cut were investigated. The analysis revealed that an increase in the number of baffles increases both the heat-transfer coefficient and pressure drop on the shell-side. Increasing the mass flow rate, the heat transfer coefficient increases; however, the pressure drop increases at a higher rate. For a large number of baffles, the pressure drop decreases with an increase in the baffle cut. It also shows that the heat transfer coefficient increases at a higher rate with the square tube layout, whereas the rotated square and triangular layouts have approximately the same behavior.     

泌238断块提高水驱开发效果对策研究及应用

针对下二门油田泌238断块地层非均质性强、大孔道发育、平面上注水方向性强、纵向上注入水沿高渗透层段窜流严重、常规注水开发效果差的开发状况,介绍了不稳定注水和多轮次调驱技术原理、施用条件、施用方案制定,并进行了现场应用。应用结果表明：①经过7个月的不稳定注水,非中心区油层动用程度提高,日产能力由18t／d上升至45t／d。综合含水率下降5．9个百分点,自然递减由11．3％下降到0,压力保持水平上升到68．1％;②对试验区3口注水井实施了3轮次调驱,窜流大孔道得到有效封堵,累积增油4996．4t,综合含水率下降4．5个百分点。     

Sustainable development of the Kamojang geothermal field

Geothermal electricity production in Indonesia began with the operation of a 0.25 MW_e pilot project in Kamojang geothermal field, in 1978. Commercial operation started in 1983, with the commissioning of the 30 MW_e Unit-1 power plant. In 1987, an additional capacity of 110 MW_e was provided by the Unit-2 and Unit-3 power plants. The addition of the 60 MWe Unit-4 power plant in 2008 increased the total generating capacity to 200 MW_e. The 27 years of commercial operation have led to a slight decline in reservoir pressure and temperature within the active production sector. The most recent significant change in the field conditions and performance occurred following the 2008 increase in generating capacity from 140 to 200 MWe. The production decline of individual wells has been relatively low, at an average of 3%/yr. However, the increased rate of steam withdrawal might negatively affect long-term sustainability of energy production at Kamojang unless suitable field management strategies are implemented. In order to stabilize the steam flow, it has been necessary to drill about three make-up wells every 2–3 years. The unbalanced mass extraction, where less than 30% of the produced steam mass can be injected, is a serious concern for long-term reservoir management in Kamojang. The field operator (Pertamina) plans to increase the Kamojang generating capacity from 200 to 230 MWe (Unit 5) and optimize the long-term performance of the Kamojang geothermal resource. The response of the reservoir during the previous three decades is being used to guide reservoir development for the planned increase in production capacity.     

Simulation and resistivity modeling of a geothermal reservoir with waters of different salinity

Apparent resistivities measured by means of repetitive dipole-dipole surveys show significant changes within the Cerro Prieto reservoir. The changes are attributed to production and natural recharge. To understand better the observed geophysical phenomena, we performed a simple reservoir simulation study combined with the appropriate DC resistivity calculations to determine the expected magnitude of apparent resistivity change. We consider production from a liquid dominated reservoir with dimensions and parameters of the Cerro Prieto ‘A’ reservoir and assume lateral and vertical recharge of colder and less saline waters. Based on rather schematic one- and two-dimensional reservoir simulations, we calculate changes in formation resistivity which we then transform into changes in apparent resistivity that would be observed at the surface. Simulated changes in apparent resistivities over the production zone show increases of 10 to 20% over a 3 year period at the current rate of fluid extraction. Changes of this magnitude are not only within our ability to discern using proper field techniques, but are consistent in magnitude with some of the observed effects. However, the patterns of apparent resistivity changes in the simulated dipole-dipole pseudosection only partially resemble the observed field data. This is explained by the fact that the actual fluid recharge into the ‘A’ reservoir is more complicated than assumed in our simple, schematic recharge models. DC resistivity monitoring appears capable of providing indirect information on fluid flow processes in a producing geothermal reservoir. Such information is extremely valuable for the development of quantitative predictions of future reservoir performance.     

Boundary layer effects on the critical nozzle of hydrogen sonic jet

When hydrogen flows through a small finite length constant exit area nozzle the viscous effects create a fluid throat which acts as a converging-diverging nozzle and lead to Mach number greater than one at the exit if the jet is under-expanded. This phenomenon influences the mass flow rate and the dispersion cloud size. In this study, the boundary layer effect on the unsteady hydrogen sonic jet flow through a 1 mm diameter pipe from a high pressure reservoir (up to 70 MPa) is studied using computational fluid dynamics with a large eddy simulation turbulence model. This viscous flow simulation is compared with a non-viscous simulation to demonstrate that the velocity is supersonic at the exit of a small exit nozzle and that the mass flow is reduced.