Pressure Air Injection Research Articles

High pressure air injection in ultra-low permeability reservoirs: Effects of physical and chemical reactions on oil recovery

This research aims to investigate the effects of physical and chemical reactions on oil recovery during high-pressure air injection (HPAI) in ultra-low permeability reservoirs. The oxidation reaction characteristics of dead oil were investigated using thermal analysis experiments and oxidation tube experiments. Additionally, pressure-volume-temperature (PVT) analysis was employed to simulate the physical and chemical reactions between air and live oil under formation conditions. The results indicated that the fuel deposition (FD) stage exhibited less prominence in light oil due to its lower content of resins and asphaltenes. Additionally, the presence of cuttings impeded the volatilization of lighter components during low-temperature oxidation (LTO) stage, effectively storing fuel required for subsequent high-temperature oxidation (HTO) stage, consequently facilitating a direct transition from LTO to HTO. Notably, the debris derived from the ultra-low permeability reservoirs possesses an enlarged specific surface area, facilitating enhanced contact between the oil and oxygen. This heightened contact effectively reduces the activation energy required for the LTO stage by an appreciable 22.9 kJ/mol. Furthermore, the PVT experiments reveals a quadratic relationship between pressure and viscosity, while a cubic relationship exists between pressure and the expansion rate of crude oil. Moreover, the thermal effects of oxidation reactions and the dissolution of the gas phase have been found to exert a more substantial influence on reducing oil viscosity, in contrast to the oxygen addition reaction and the extraction of lighter components, which have a minor influence on increasing oil viscosity. Lastly, in the context of developing HPAI techniques for ultra-low permeability reservoirs, optimizing the gas injection rate emerges as a viable strategy to effectively enhance oil recovery, provided that the occurrence of gas channeling is effectively mitigated.

Combustion tube experiments on key factors controlling the combustion process of air injection with light oil reservoir

High pressure air injection (HPAI) has unique advantages due to its rich gas source and unique high-energy displacement effect of the thermal front. Thermal effect of air injection after combustion is the key factor affecting the oil displacement effect. Combustion tube experiments are conducted to study the combustion characteristics of light oil and the influence of different factors on combustion process. We find that light oil (content of asphaltene is only 1.13%) can be ignited and combustion front can advance forward stably, which proves that the implement of HPAI in light oil reservoir can achieve good results. High temperature makes combustion more intense and the advancing of combustion front more stable. The experimental results show that when the air injection rate is low, low temperature oxidation of light oil will dominant, or the combustion front cannot successfully advance. But when the injection rate is too high, the combustion front moves forward too fast and gas channeling will occur. With pressure increasing, the solubility of O2 in oil increases, causing a more violent oxidation reaction. Increasing pressure can reduce the air/oil ratio and improve the utilization rate of O2. Thus, HPAI is more ideal for the reservoirs with high temperature and high pressure. And for a specific reservoir, an optimal injection rate exists. The conclusions of the research can provide references for field applications of HPAI in light oil reservoirs.

New insights on low temperature oxidation characteristics and possibility of auto-ignition in light oil reservoir

High pressure air injection (HPAI) can improve oil recovery by 10%–20% due to its unique oxidation heat effect. The high temperature and pressure thermostatic device is used to study the low temperature oxidation characteristics and oxidation kinetic parameters of light oil. Then a numerical model based on the oxidation kinetic parameters is established to study the possibility of auto-ignition of low temperature oxidation of light oil. Temperature rises due to the heat release of low temperature oxidation. Higher temperature makes low temperature oxidation more intense, so the reaction rate increases. When temperature is relatively low, the oxygen addition reaction dominant, the content of oxygen-containing functional groups increases continuously, and aromatic hydrocarbon is converted into resin. With the progress of oxidation, resin is further oxidized into asphaltene. When temperature rises, carbon bond stripping reaction begins to occur, which makes macromolecular oxygenated hydrocarbons in oil decompose into small molecules of hydrocarbons and CO2. After considering gas (mainly CO2 generated by oxidation and natural gas) dissolution into oil, the viscosity of underground oil after low temperature oxidation is lower than that of original oil, which improves the mobility of oil. The activation energy of oxygen addition reaction is 49.0 kJ/mol and pre-exponential factor is 8.72 × 103s−1. The activation energy of carbon bond stripping reaction is 68.9 kJ/mol and pre-exponential factor 1.6 × 106s−1. The static low temperature oxidation experiments are simulated by CMG-STARS module based on experimental results. The simulation results show that decreasing the heat loss of experiments makes the temperature rise rapidly, which indicates that the light oil with little asphaltene also has high oxidation activity, and auto-ignition can occur in the light oil reservoir for HAPI. The conclusions of the research can provide references for field applications of HPAI in light oil reservoir.

Failure analysis of the collapse of a raised steel wine tank

As structural damage or even catastrophic collapses are sometimes due to improper design, underestimated design loads or unexpectedly extreme loads, progressive collapse and structural robustness has recently received a great deal of attention, while the number of scientific papers published on the topic has grown considerably. This paper describes an analysis of the failure of a 106 m3 steel raised wine tank belonging to a local wine producer that suddenly collapsed, fortunately without personal injury, but causing significant damage to the company's facilities. The collapse generated the progressive failure of other tanks next to the first one that failed. After several visits to inspect the accident on site, a diagnosis was made to determine the causes. Possible reasons were examined by visual inspections and experimental tests on steel specimens recovered from the collapsed structures, which allowed to determine the mechanical properties and study the fracture surfaces by scanning electron microscopy and micrographs. On the other hand, the load-carrying capacities of the structure were studied by a linear-static finite-element and a nonlinear-static push-over analysis. The collapse mechanism of the system was found to be due to cyclic horizontal loads generated by a pressurized air injection system. The results allowed to conclude that this type of structure is highly vulnerable to horizontal loads and has very little capacity to activate alternative loas paths. The paper also describes a possible solution that could be used to improve the mechanical performance of this structural typology against horizontal loading, based on the lessons learned from the experience.

Oxidization characteristics and thermal miscible flooding of high pressure air injection in light oil reservoirs

Physical modeling, numerical simulation and field case analysis were carried out to find out the subsurface thermal oxidation state, thermal oxidation front characteristics and production dynamic characteristics of high pressure air injection thermal oxidation miscible flooding technology. The lighter the composition and the lower the viscosity of the crude oil, the lower the fuel consumption and the combustion temperature are. The thermal oxidation front of light oil and volatile oil can advance stably, and a medium-temperature thermal oxidation stable displacement state can be formed in the light oil reservoir under high pressure conditions. With strong thermal gasification and distillation, light oil and volatile oil are likely to form a single phase zone of gasification and distillation with thermal flue gas at the high-temperature and high-pressure heat front, finally, an air-injection thermal miscible front. In light oil reservoirs, the development process of high-pressure air-injection thermal miscible flooding can be divided into three stages: boosting pressure stage, low gas-oil ratio and high-efficiency stable production stage and high gas-oil ratio production stage. Approximately 70% of crude oil is produced during the boosting pressure stage and low gas-oil ratio high-efficiency and stable production stage.

Development Of A Turbulent Boundary Layer Over An Air Cavity

The turbulent boundary layer development over an air cavity is studied experimentally using planar Particle Image Velocimetry. An identification technique based on the gradient of correlation values was employed to detect the cavity interface which revealed a well-defined, asymmetric bump-like geometry at the spanwise-uniform region of the cavity. No separation was observed at the leeward side of the cavity, owing largely to the high boundary layer thickness to maximum cavity thickness ratio (δ/tmax ≈ 11.8). The shape of the cavity was found to dictate the turbulent boundary development by subjecting it to alternating streamwise pressure gradients: from an adverse to a favourable and back to an adverse, moving downstream along the cavity. Profiles o f mean s treamwise velocity and turbulent stresses show the alternate pressure gradients and air injection to be the dominant perturbations to the flow, with streamline curvature only having a minor influence, also based on previous observations.

Oxidizing agent‐assisted extrusion cooking of yellow peas and the techno‐functionality of the resulting extrudate flours

To enhance pulse flour techno-functionality, different oxidizing agents were used during extrusion cooking. Benzoyl peroxide (BP) (45 mg/kg), azodicarbonamide (ADA) (150 mg/kg), and pressurized air (injection pressures of 200 and 400 kPa) were employed at three different extrusion temperature profiles, and their effects on techno-functional quality of resulting yellow pea (YP) extrudate flours were investigated. Oxidizing agents and extrusion temperature impacted water solubility (WS), water-binding capacity, emulsion capacity (EC), emulsion stability (ES), and pasting properties of YP extrudate flours. Oxidizing agent effects were extrusion temperature-dependent. At a die temperature of 95℃, BP and ADA addition significantly increased EC and ES, while air injection at 400 kPa increased WS, ES, cold, and trough viscosities. Manipulation of pulse flour techno-functionality through oxidizing agent-assisted extrusion has proven to be an effective approach to manufacture novel ingredients that can be used in a wide variety of foods. Practical applications High protein and fiber content of pulses make them an attractive ingredient for new product development strategies in the food industry. Nevertheless, there can be significant quality challenges when using pulses in food products due to their less than ideal techno-functional properties which lead to quality defects in reformulated products. Extrusion cooking has been employed to modify pulse flours and to develop pulse-based ingredients with superior techno-functionality. The use of oxidizing agents during extrusion cooking can be another means of addressing these techno-functionality issues, without the need of additional equipment for the industry. In this study, depending on the oxidizer type and extrusion temperature, oxidizing agent-assisted extrusion cooking has proven to improve several techno-functional quality attributes of yellow pea flour. The same technology can be employed to improve the techno-functionality of different pulse flours.

Investigation on the limiting oxygen concentration of combustible gas at high pressures and temperatures during oil recovery process

High pressure air injection is an important method for oil recovery. In this process, due to exist of combustible gas and oxygen, the wellbore explosion accident may happen. To solve the problem, the optimal method is to control the oxygen concentration in the wellbore. Thus, it needs to investigate the limiting oxygen concentration (LOC) of combustible gas at high pressures and temperatures. In this study, experiments were carried out to investigate the LOC of combustible gas at pressures from 1 MPa to 15 MPa and temperatures from 40 °C to 120 °C. The influence of pressure and temperature on LOC was analyzed. The results showed that the LOC of combustible gas declines a lot as the initial pressure and temperature increases. At 15 MPa and 120 °C, the LOC drops up to 8.21%. It was also found that the LOC is exponentially and linearly dependent on initial pressure and temperature, respectively. Besides, the empirical formulas were proposed to calculate the LOC at different conditions. Based on the present results, the standard for controlling the oxygen concentration in the wellbore was confirmed. As a result, the allowable oxygen concentration in the wellbore at different conditions can be determined. Therefore, the results of this study can help to avoid the wellbore explosion and ensure safety during air injection process.

Oxygen corrosion of N80 steel under laboratory conditions simulating high pressure air injection: Analysis of corrosion products

High pressure air injection (HPAI) is an effective and economic improved oil recovery process, however, the corrosion of tubing and casing in the injection well is one of the serious safety concerns which have limited its widely application. Until now the structure and composition of corrosion products formed in HPAI condition are still not fully clear because of the difficulty in simulating such a harsh environment in laboratory. In this work, the corrosion rates of N80 steel under laboratory conditions with different oxygen content (21–5% vol%), total pressure (50–20 MPa) and temperature (120–70 °C) simulating HPAI in Dagang oil field in China were investigated using weight loss measurement. The corrosion products on N80 steel surface were analyzed using scanning electron microscopy coupled with energy dispersive spectroscopy (SEM + EDS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the corrosion rate decreased with the decreasing oxygen partial pressure and temperature. Furthermore, for all the conditions, the corrosion rate was incredibly high in the simulating HPAI environment, consequently, large amount of corrosion products were formed. The corrosion products showed a two-layer structure: outer layer and inner layer. The outer layer was porous, brittle and easily flaked off, while the inner layer was more compact than outer layer. However, both layers could not effectively lower the corrosion rate of N80 steel in HPAI environment. The corrosion products were composed of hematite (α-Fe2O3), goethite (α-FeOOH), magnetite (Fe3O4) and trace amounts of Ca(OH)2 and CaO. In addition, the structure and composition of corrosion products did not change with the temperature and oxygen partial pressure in the simulating HPAI conditions.

High pressure air injection kinetic model for Bazhenov Shale Formation based on a set of oxidation studies

Modeling of high pressure air injection (HPAI) is a complicated process, where many parameters must be taken into account. One of the key aspects is to build a chemical reactions model for HPAI process, especially in oil shales, which is still under development. For successful HPAI implementation in the field, numerous laboratory and numerical studies must be done.The purpose of this study was to develop a HPAI kinetic model for Bazhenov Shale Formation by applying a complex approach including a set of oxidation experiments and complementary numerical simulation.Oxidation behavior of Bazhenov shales was studied by accelerating rate calorimetry (ARC), ramped-temperature oxidation (RTO) and pressurized differential scanning calorimetry (PDSC). Based on obtained data and experimental observations, the basic set of chemical reactions was proposed. It can represent an oxidation behavior and chemical changes of both oil and kerogen, occurring in HPAI process in Bazhenov Formation. Moreover, reactions kinetic model was developed. During validation of the numerical model with the experimental results, it was revealed that kerogen or its derivatives undergo oxidation in several temperature regions. Therefore, it should be represented by individual pseudo-components, which reflect their different physical-chemical properties. As a result, a minimum set of kerogen pseudo-components in solid phase that are necessary for describing oxidation reactions in low temperature region (LTR) and high temperature region (HTR) and pyrolysis reaction was determined.The application of experimental results has made it possible to substantially reduce the uncertainties revealed in air injection modeling in oil shales. Proposed chemical reactions and their kinetics were validated by experimental data and thus can be used as a start model for high-pressure air injection modeling in oil shales.

Thermal Methods for Enhanced Oil Recovery: Laboratory Testing, Simulation and Oilfields Applications (ThEOR2017)

About Theor 2017It is our great pleasure to invite you to the II International Workshop “Thermal Methods for Enhanced Oil Recovery: Laboratory Testing, Simulation and Oilfields Applications”, ThEOR2017, which will be held in Kazan, Russia. ThEOR2017 is aimed to bring together key researchers and professionals from the industry, R&D centers and academic institutes to share experiences, perspectives, insights and the latest developments in thermal methods for EOR from all over world. The workshop’s program contains four days of training courses, plenary and key-note lectures, technical and poster sessions on cutting edge issues related to petroleum industry. At the same time, we tried to keep the balance between traditional and emerging fields, fundamentals and applications, modeling and experiment, academic interests and industry needs.The first conference ThEOR2016 has gathered more than 150 registered participants from 45 different organizations all over the world. This fact gives us enormous satisfaction and pride, as this event attracted interest of the community and can be continued in 2017.Sections1.Steam Assisted Gravity Drainage (SAGD), including solvent-aided and reagent-aided processes2.Catalytic Compositions for In-Situ Oil Upgrading (development, characterization and usage of different catalysts for pyrolysis, oxidation, hydrocracking, aquathermolysis of fossil fuels)3.Modeling and Simulation of Thermal EOR, including development of new software4.Production of Unconventional Resources (tight oil, shale oil, tar sands, etc.)5.Kinetic and Thermochemical Studies of In-Situ Chemical Processes6.New Equipment for Testing and Field Application of Thermal EOR7.In-situ Combustion (wet and dry combustion)8.Laboratory Tools for Studying of Thermal EOR9.High Pressure Air Injection (HPAI)10.Ecological Impact of Thermal EOR11.Cyclic Steam Stimulation (CSS)12. Electromagnetic Heating EOR13.EOR Field Applications14.Steam Flooding15.Flue Gas EOR

Feasibility of spontaneous ignition during air injection in light oil reservoirs

Air injection in light oil reservoirs is a complicated EOR (enhanced oil recovery) technique. Till now, the mechanisms of HPAI (high pressure air injection) are still under controversy. Some researchers state that the heat generation is not important for the recovery of light oil during HPAI, and consider the LTO (low temperature oxidation) is only to remove the oxygen but not to establish the combustion front, while others claim that the spontaneous ignition and sustainable combustion front are the key aspects for a success HPAI project. The experimental trials on spontaneous ignition of HPAI to light oil are reviewed and discussed in this paper. This paper also investigates the spontaneous ignition feasibility through both analytical method and numerical method. The review shows that the combustion tube test is used to study combustion sustainability but not suitable to study the spontaneous ignition, and the thermal experiments can only be applied as screening tool to select favorable oil for HPAI in terms of feasibility of spontaneous ignition while further core flooding tests are still required. This paper also proposes an approach which associates thermal experiments with numerical simulation method to investigate the spontaneous ignition of a light oil. A case study is performed with a Wolfcamp light oil, the simulation study shows that the LTO of this light oil cannot induce spontaneous ignition during air injection in both core flooding experiment and field application. The analytical approach is very sensitive to the kinetic data which is used to estimate the ignition delay time. As a result, the analytical method may be misleading in terms of the feasibility of spontaneous ignition, if inappropriate data are selected.

Effect of low-temperature oxidation of light oil on oil recovery during high pressure air injection

ABSTRACTIn this research, the low-temperature oxidation (LTO) behavior of light oil was evaluated via isothermal oxidation tube experiments. Also, the core model together with nuclear magnetic resonance (NMR) analyzer was implemented to explore the effect of LTO of light oil on oil recovery during high pressure air injection (HPAI). The results indicated that the polycondensation reaction of light components (C5-C6) and intermediate ones (C7-C17) for light oil was the preferred reaction path during LTO process. The oxygen addition was the primary oxidation reaction in LTO, but the LTO reaction presented a more favorable trend for the bond scission with the oxidation degree. Because of the gas flow steering resulting from property variation of light oil and heat release determined by LTO, the total oil recovery yielded by air flooding was 8.82% higher than nitrogen flooding.

The properties and kinetics characteristics of oxidised ultra-heavy oil in the coke deposition process

High pressure air injection (HPAI) is generally considered an efficient method to drive crude oil. In this paper, the results showed that the oxidation of heavy oil was able to reduce the oxygen content. After the high pressure oxidation, the physical properties of crude oil changed, and it became inflammable. Due to the formation of coke, the viscosity increased sharply and the hydrogen/carbon (H/C) atomic ratio decreased. By using an environmental scanning electron microscope, it was observed that the structures of the original oil were reconstructed and the surface was covered by a large number of inflammable globular structure with a diameter of 500 nm that were believed to effectively reduce the flame point of the oxidised oil. Based on the kinetic analysis, under HTO conditions, deposited coke caused the reaction to be more exothermic, activating the subsequent deposition and forming a stable oxidation front. [Received: January 14, 2016; Accepted: September 18, 2016]

The properties and kinetics characteristics of oxidised ultra-heavy oil in the coke deposition process

High pressure air injection (HPAI) is generally considered an efficient method to drive crude oil. In this paper, the results showed that the oxidation of heavy oil was able to reduce the oxygen content. After the high pressure oxidation, the physical properties of crude oil changed, and it became inflammable. Due to the formation of coke, the viscosity increased sharply and the hydrogen/carbon (H/C) atomic ratio decreased. By using an environmental scanning electron microscope, it was observed that the structures of the original oil were reconstructed and the surface was covered by a large number of inflammable globular structure with a diameter of 500 nm that were believed to effectively reduce the flame point of the oxidised oil. Based on the kinetic analysis, under HTO conditions, deposited coke caused the reaction to be more exothermic, activating the subsequent deposition and forming a stable oxidation front. [Received: January 14, 2016; Accepted: September 18, 2016]

The mechanism of the oxidation of light oil

ABSTRACTHigh pressure air injection (HPAI) has been proven as an enhanced oil recovery process in many fields. The reactions associated with crude oil and oxygen are the main difference between HPAI and other gas injection processes. In order to study the reaction schemes, different experiments, such as the combustion tube, kinetic cell, accelerating rate calorimeter, and thermogravimetry analysis, are conducted. In this paper, a reaction scheme is build that includes three classes of reactions: low-temperature oxidation, cracking, and combustion. We then use simulation models with the proposed reaction scheme included to history match the data from a combustion tube experiment and a kinetic cell experiment. It is demonstrated that the reaction scheme is valid. Based on this study, lumping the crude oil into four pseudocomponents brings a good matching result, and the kinetic parameters are obtained through matching the experimental results.

Optimization of heat transfer and efficiency of engine via air bubble injection inside engine cooling system

The advanced thermal management in the internal combustion engines has a key role in the overall performance, which directly effect on the total fuel consumption and engine exhaust emissions. Recently, a number of valuable studies have been carried out to evaluate and improve cooling system performance by using various technologies such as sliding vane rotary pump, ultimate cooling, double temperature circuits, integration, electrically-controlled valves and advanced algorithms for flexible control of the cooling system components, but, in this study an innovative strategy is presented to achieve this purpose. With a novel point of view, air-water mixture is employed to use in the cooling circuit. For this reason, at the point of coolant inlet to the engine, a constant pressure air injection mechanism was established, which can be adjusted at the desired flow rate by computer. The air injection strategy has two significant advantages. In the warm-up stage more air injection into the coolant fluid cause to a rapid heating of engine components by creating air layers around the cylinder. In the after warm-up state, however, air injection is decreased to a specified level for producing turbulence flow only in order to heat transfer enhancement and cooling purposes. Obtained results revealed that the periodic air injection by using special strategy can improve fuel economy and also decrease engine pollutant emissions.

A novel insight of laboratory investigation and simulation for high pressure air injection in light oil reservoir

Due to the precipitous dropping oil price, air injection becomes advantageous to other gas flooding techniques as a result of the low cost. Therefore, this work targeted a light oil reservoir and comprehensively investigated the potential of air injection in this reservoir using experimental and numerical simulation, from which some new insights into high pressure air injection were recognized. Oxidation kinetics of the crude oil were first established using Thermal Gravity Analysis (TG)/Differential Thermal Gravity (DTG), and further validated through a set of history matching. The results indicate that the intense oxidation reaction consumed a great volume of oxygen forming only CO2. Temperature peak occurred after reacting 23 h, revealing that the oxidation reaction is an exothermic process under reservoir conditions and spontaneous combustion might take place. Reservoir dip angle is a crucial parameter governing the oil production and updip injection is generally suggested. Earlier air injection leads to more noticeable incremental temperature effect and also higher ultimate recovery. Air injection seems notably appropriate for rhythm reservoirs. Given a mature reservoir, high-permeability zone would cause gas breakthrough and thus considerably detract the air injection performance. Temperature-resistant polymer gel or foam is suggested to plug thief zones promoting oxidation reactions and air sweep efficiency.

Experimental investigation of thermal decomposition of Bazhenov formation kerogen: Mechanism and application for thermal enhanced oil recovery

Bazhenov formation is the key kerogen-bearing oil shale reserve in Russia. Therefore, as recovery from conventional reservoirs decreases, the need to find enhanced oil recovery (EOR) techniques in order to develop Bazhenov formation becomes evident. Despite the huge amount of reserves, oil shales in Russia have not been developed extensively due to the absence of suitable recovery technique. High pressure air injection (HPAI) is one of the recovery techniques that has a potential to become the main recovery method. In order to design this method correctly, different aspects must be investigated. To do that, thermal analysis methods should be implemented. In this work, thermomicroscopy, simultaneous thermal analysis and open system pyrolysis were performed to investigate kerogen conversion process.The optical thermomicroscopy showed changes in the macrostructure and the morphology of the Bazhenov shale sample during heating up to 720°C with the heating rate 10°C/min. It was demonstrated that the conversion of kerogen into hydrocarbons caused increasing in sample porosity. Geometrical characteristics of pores such as size and shape were determined, dynamic events during the heating were investigated. During the air purge, intensive fracturing occurred along the voids formed due to oxidation of organic matter at 450°C. The simultaneous thermal analysis (STA) revealed linear relationship between TOC and heat value for single well samples. Interval of oxidation reactions was determined.Bulk-kinetic parameters (activation energy and frequency factor) were calculated by numerical inversion of pyrolysis data using the Kinetics2015 optimization software. Fixing the A factor to a 2×1014s−1 and using a spacing of 1kcal/mole in the discrete activation energy distribution were investigated. In result, solution for different samples along the well was rather stable comparing to traditional approach.

Implementation of pressurized air injection system in a Kaplan prototype for the reduction of vibration caused by tip vortex cavitation

Blade tip cavitation is a well-known phenomenon that affects the performance of large-diameter Kaplan turbines and induces structural vibration. Injection of pressurized air has been found to yield promising results in reducing those damaging effects. In this work, the results of an experimental test of air injection on a 9.5-m-diameter Kaplan turbine are reported. Experiments were performed for several load conditions and for two different net heads. Accelerations, pressure pulsation and noise emission were monitored for every tested condition.Results show that, at the expense of a maximum efficiency drop of 0.2%, air injection induces a decrease on the level of vibration from 57% up to 84%, depending on the load condition. Such decrease is seen to be proportional to the air flow rate, in the range from 0.06 to 0.8‰ (respect to the discharge at the best efficiency point).