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Oil shale is a potential strategic resource with large reserves and widespread application potential, and in situ combustion is an effective method to exploit oil shale. However, the interaction mechanism between pyrolysis and combustion reaction is still not clear; hence, the evolution of components and temperature in the reservoir is hard to describe, limiting the application of in situ combustion in oil shale. In this paper, we established a comprehensive reactive flow model to describe the in situ combustion in oil shale, and the in situ combustion process is analyzed in detail. During the in situ combustion, only pyrolysis reactions occur in the matrix system, while combustion reactions only occur in the fracture system. Under the interaction mechanisms of these two kinds of chemical reactions, in situ combustion upgrading can be carried out continuously. The produced oil and hydrocarbon gas per unit volume of oil shale are 0.06 m3/m3 and 12.5 m3/m3 , respectively, but about 83.3 m3/m3 of CO2 is produced that needs to be stored subsequently when air is directly injected into the reservoir to maintain the combustion reactions. Increasing oil content and air injection rate can increase the cumulative oil and gas production, but there are optimal values above which the increase rate slows down. The energy conversion efficiency of in situ combustion is 49.5, and it is much higher than the energy conversion efficiency in other in situ upgrading methods for oil shale. [ABSTRACT FROM AUTHOR]

Abstract In-situ conversion process (ICP) can be an effective technique for the development of oil shale and ultra heavy oils, in which kerogen or heavy components will be cracked into light oils and gases via underground heating and pyrolysis, and the retorted oil can be recovered via conventional extraction techniques. Heating of targeted formation can be conducted using electrical heaters installed in vertical or horizontal wells, while a slow heating rate and lack of driving energy for oil production are the main issues of the conventional in-situ conversion process. In this study, an innovative nitrogen injection assisted in-situ conversion process (NAICP) was proposed to increase formation heating rate and improve oil production from oil shale. Reaction models of kerogen pyrolysis at elevated temperature were established, along with models for the evolution of porosity and permeability of oil shale formation during the pyrolysis process. The performance of nitrogen injection on the effectiveness of the ICP was then investigated through reservoir numerical simulation. The simulation results show that additional nitrogen injection can improve the heating rate of the oil shale formation, enhance oil production and energy efficiency in comparison with the conventional ICPs, which can be attributed to enhanced heat convection, pressurization effect and gas driving mechanism. Nitrogen injection assisted in-situ conversion process can be a promising technique for the exploitation of oil shale. Highlights • Nitrogen injection assisted in-situ process for oil shale was proposed. • The advantages of nitrogen injection assisted in-situ process were studied. • The mechanism of nitrogen injection to improve oil production performance was studied. • The effects of heating temperature and nitrogen injection rate were studied. [ABSTRACT FROM AUTHOR]

In-situ upgrading (ISU) via downhole heating is an innovative and efficient technique for oil shale and ultra heavy oil recovery. However, low heat transfer in the ISU process can limit its application in large scale. Therefore, efficient air injection assisted ISU process (AAISU) was proposed. In this study, reservoir stimulation of the AAISU process was performed to investigate the effect of air injection on heat transfer, oil production performance and energy conversion efficiency, as compared to the original ISU process. The influences of important engineering aspects were investigated, including well pattern, well distance, heating temperature and air injection rate for the field application of the AAISU technique. The reservoir stimulation results indicate that heat transfer rate can be significantly accelerated by air injection, and the average reservoir temperature during the AAISU can be increased to 250 °C after one year, in comparison with that of 116 °C in the ISU process. The contribution of air injection on the total heat transfer can be approached to 60%. Air injection can also enhance oil recovery and improve the energy efficiency. Heating temperature and air injection rate also have significant impact on the performance of the AAISU process. For a 15 m × 15 m × 10 m geo-model, increase of the heating temperature from 250 °C to 350 °C and air injection rate from 500 m3/day to 1000 m3/day can result in improved energy efficiency. However, heating temperature above 350 °C and the air injection rate over 1000 m3/day can led to a decreased energy efficiency, due to higher energy consumption on electrical heating and high rate of air injection. • Air injection assisted in situ upgrading process (AAISU) of ultra heavy oils was proposed. • The effect of air injection on heat transfer was studied. • The influence of well pattern and well distance on the AAISU was investigated. • The influence of heating temperature and air injection rate on the AAISU process was evaluated. [ABSTRACT FROM AUTHOR]

The geothermal resource in depleted high-temperature gas fields is abundant, and CO 2 is more suitable to exploit geothermal energy from these gas fields due to its high mobility and thermal physical properties. However, all the related mechanisms, operation processes, and economic analyses have not been comprehensively analyzed yet. To assess the technical and economic feasibility of this method of geothermal exploitation, a 120 °C depleted gas reservoir was selected to build geological and numerical models for analyzing its gas composition, temperature, and pressure during the whole process, including enhanced gas recovery, pressure build-up, and pure geothermal exploitation, based on existing wells. The results reveal that the CO 2 injection during EGR and pressure build-up can affect the reservoir temperature, and the optimization analyses indicate the heat mining rate can be maintained about 10 MW th for 30 years. The thermodynamic cycle analyses show that a power of 132.7 kW can be obtained if the organic Rankine cycle system with R134a is adopted, and the cost of geothermal power generation is about 0.45 $/(kW∙h) when the CO 2 price is 12 $/t. However, if the produced CO 2 directly drives the turbine, the power can increase to 718.5 kW and the cost reduces to 0.1$/(kW∙h). Image 1 • A gas reservoir was assessed and chosen for geothermal exploitation by recycling CO 2. • EGR, pressure build-up, and geothermal exploitation were analyzed as a whole process. • Geothermal exploitation optimization was conducted based on the whole process. • ORC and CO 2 direct cycle were proposed and analyzed for geothermal power generation. • Economic analysis shows geothermal power generation cost can be low as 0.11 $/(kW∙h). [ABSTRACT FROM AUTHOR]

Air injection for in situ combustion (ISC) and air injection assisted cyclic steam stimulation (AACSS) techniques have a good application prospective in the development of heavy oils, while the explosion risk in the process of air injection is of great concern and has restricted the application of the technology. So that injection of oxygen-reduced air has been proposed to eliminate and control the explosion of oil/gas mixture with air. In this study, low-temperature oxidation experiments of heavy oil samples were conducted using a small batch reactor under the pressure of 5–15 MPa and oxygen content of 5%–15% at 225 °C in order to investigate the effect of oxygen content and pressure on the reaction rate and heat release during oxidation of heavy oil with oxygen-reduced air. The variations of temperature and pressure during the oxidation reaction were measured in the experiment, and the explosion phenomena of heavy oils were observed in the air with high oxygen contents (oxygen content more than 15%). The reaction rate and the exothermic heat of the reaction were calculated based on the pressure and temperature curves and using an improved heat loss model. The experimental results showed that reducing the oxygen content (e.g. reduced oxygen less than 10%) in air can effectively prevent the explosion of oil-gas mixtures, and the reaction rate and heat release during heavy oil oxidation are linearly proportional to pressure and oxygen content in the injected air when oil is in excess. The results of this study indicate that the heat generated in oil oxidation, which is important for the ISC and AACSS processes, can be controlled by pressure and oxygen content of the injected air, which can lay a good foundation for the oxygen-reduced air injection technique, especially for its application in deep heavy oil reservoirs, in which injection of oxygen-reduced air at high pressure can offset the effect of low oxygen content on heat release. • The injection of oxygen-reduced air at high pressure for the deep heavy oil reservoirs was proposed. • The effect of oxygen-reduced air on preventing the explosion of oil-gas mixtures was proved. • The effects of oxygen partial pressure on the reaction rate and total heat released in heavy oil oxidation were studied. [ABSTRACT FROM AUTHOR]

In-situ upgrading (ISU) via down-hole heating can be an effective technique for exploitation of heavy oil reservoirs. In order to increase the heat transfer rate and reservoir energy, air or gas injection assisted in-situ upgrading process (AAISU) has been proposed. In the AAISU process, and also in the traditional in-situ combustion (ISC) process, complex thermal cracking reactions of heavy oils occur along with low temperature oxidation (LTO) in the presence of air or oxygen at high pressure conditions. In comparison with the conventional oil cracking or pyrolysis in refinery (without or with less oxygen), it has been concerned that low temperature oxidation of oil components during air injection may have some impacts on their cracking reactions. In this study, thermal cracking experiments of ultra-heavy oils in small batch reactor as well as thermogravimetry analysis (TGA) experiments have been conducted under high pressure in the presence of air and nitrogen to simulate the AAISU process. The starting temperature for cracking, the products of cracking reactions, reaction rate, and the influence of temperature and pressure on reaction kinetics have been investigated in order to reveal the influence of LTO on cracking reaction and to clarify the confusions. The experimental results demonstrated that the presence of air has some kind negative effect on the cracking of oils compared with that in the presence of nitrogen. For a typical ultra-heavy oil, the minimum starting temperature for cracking is around 350 °C in the presence of air at 3.2 MPa for a reaction time of over 30 days, while the starting temperature is around 325 °C when nitrogen is present. Less light oil components produced and lower cracking reaction rate (about 20–30%) were observed when air was present in comparison with that of nitrogen at the same pressure. That means higher activation energy is required to activate the thermal cracking reaction of oils when air or LTO reactions are involved. • Thermal cracking experiments of ultra heavy oil in air and N 2 have been done. • Kinetic modeling of thermal cracking reaction were conducted. • The effect of LTO reaction on the thermal cracking reaction was proposed. • The effects of temperature and pressure were studied. [ABSTRACT FROM AUTHOR]

• Air injection assisted in situ upgrading process (ISU) of ultra heavy oils was proposed. • The effect of air injection on heat transfer was studied. • The influence of low temperature oxidation on oil cracking was investigated. • The benefit of air injection on energy efficiency of the ISU process was evaluated. In situ upgrading (ISU) via underground heating and thermal cracking is regarded as an effective technique for exploiting ultra heavy oils, in which the heavy oil components can be cracked into light oils and gases for production. However, the slow heating rate via conduction from wellbore to oil formation is the main issue concerned in the conventional ISU process. Herein, an air injection assisted ISU technique (AAISU) is proposed to improve heat transfer by gas convection and along with the thermal effect of oil oxidation due to the oxygen in the injected air. Air injection can also provide extra energy for oil production and reservoir pressure maintenance. To illustrate the advantage of the proposed AAISU technique, thermal cracking experiments of ultra heavy oil samples in the presence of air under high pressure were conducted to investigate the reaction mechanisms and to establish the kinetics models of the oxidation and cracking reactions. Moreover, reservoir numerical simulation studies are performed to evaluate the effects of air injection during the ISU process. The experiment results indicate that the ultra heavy oil can be effectively cracked into gases, light oils and coke-like substances in the presence of air at temperature over 350 °C. The activation energy of the thermal cracking derived is around 248 kJ/mol. The numerical simulation results show that the heat transfer rate can be effectively enhanced by air injection because of the heat convection of the air flow and the thermal effect of the low temperature oxidation reactions of oil components. The total oil recovery factor can be increased via air injection with improved energy conversion efficiency from 6.51 GJ/GJ to 8.42 GJ/GJ. [ABSTRACT FROM AUTHOR]

Using physical simulation models, rainfall-induced landslides have been simulated under various rainfall intensities. During these simulations, we have monitored the physical and mechanical behaviors of the landslide over the slip surface at different heights of the model slopes, as well as taking the whole slope to identify its deformation and failure processes. The results show that the rainfall duration corresponding to the initiation of the debris landslide and is exponentially related to rainfall intensity. Corresponding to the three intervals of the rainfall intensity, there are three types of slope failure modes: (1) the small-slump failure at the leading edge of the slope; (2) the block-slump failure but sometimes there are large blocks sliding down; and (3) the bulk failure but sometimes there is the block-slump failure. Based on the total rainfall-lasting time and the associated proportion of failed mass volume, the early warning of debris landslide can be classified into five grades, i.e., red, orange to red, orange, yellow to orange and yellow, which correspond to the five slope failure modes, respectively. [ABSTRACT FROM AUTHOR]

Mechanical excavation or blasting generates stress waves that rapidly dissipate in the surrounding high-geostress rock, causing microdynamic disturbances. These disturbances trigger microcracks within the damaged rock, even inducing engineering disasters. However, the fracture behaviors and mechanisms of rockmass subjected to excavation disturbances under three-dimensional geostress remains unclear. Therefore, this study proposed a true triaxial static–dynamic combined loading method to capture the entire process of tunnel excavation damage and the continuous fracturing induced by microdynamic disturbances. True triaxial static–dynamic combined tests and numerical simulations using PFC3D-GBM were employed systematically to analyze the influence of principal stresses σ 1 , σ 2 , and σ 3 on the disturbances mechanical behaviors of the gabbro. The disturbance failure under true triaxial stress indicated a three-stage pattern of deformation: deceleration, constant velocity, and acceleration. With increased σ 1 or decreasing σ 2 and σ 3 , the disturbance bearing capacity of the gabbro significantly decreased. Moreover, an increase in σ 1 and σ 3 corresponded to an increased proportion of intergranular shear cracks, whereas an increase in σ 2 resulted in a notable increase in intragranular tensile cracks. Small-magnitude events tended to disperse during the deceleration and constant-velocity stages, whereas larger-magnitude events were concentrated during the acceleration stage. The AE parameter b -value initially increased and then decreased during disturbance fracture process. The excavation disturbance of the tunnel intensified the depth of the damage zone and the energy released, thereby increasing the risk of a catastrophic deep fracture. • Increasing σ 1 or decreasing σ 2 and σ 3 reduced significantly gabbro's disturbance bearing capacity. • The intragranular tensile cracks of the mineral grains were dominant during disturbance failure. • Increasing σ 1 and σ 2 or decreasing σ 3 gradually increased AE parameter b -value during disturbance failure. • The excavation disturbance of the tunnel exacerbates rock deformation and deepens damage zone, increasing the potential for tunnel-related disasters. [ABSTRACT FROM AUTHOR]

Geological strength index GSI , disturbance factor (D), material constant m i , and uniaxial compressive strength σ ci of the intact rock are essential input parameters IPs of the Hoek–Brown H − B criterion. Mechanical parameters MPs of the engineering rock mass, including elastic modulus E , cohesion c , and internal friction angle φ estimated by the H–B criterion, and the predicted excavation response of surrounding rock, including the displacement and excavation damage zone EDZ based on the MPs, are of high relevance with the four IPs of the H–B criterion. In this paper, the deep and huge underground cavern excavated in basalt from a hydropower station under construction in the southwest of China is used to analyse the sensitivity of the IPs on the MPs, the displacement, and EDZ of the surrounding rock mass. Firstly, the H–B criterion is applied to estimate the MPs, among which the IPs are obtained from a series of in situ and laboratory tests, including borehole camera observation, wave velocity test, uniaxial and triaxial compression tests, and so on. Secondly, the sensitivity relationships between IPs, MPs, and prediction results of displacement and EDZ are established and described quantitatively by the sensitivity factor (si). Results show that the MPs of the rock mass are more sensitive to GSI and D ⋅ GSI and σ ci are high-sensitivity parameters affecting the displacement and EDZ. Finally, the variations in the estimated MPs and associated prediction results concerning excavation response, which are caused by the uncertainties in the determination of the IPs, are further quantified. This study provides a straightforward assessment for the variability of the rock mass parameters estimated by the H–B criterion. It also gives a valuable reference to similar geotechnical engineering for the determination of rock mass parameters in the preliminary design. [ABSTRACT FROM AUTHOR]

Air injection into oil reservoirs for improved oil recovery is a proven technique in oilfields, while the safety of the process has been concerned over the possible explosion of natural gas at high pressure conditions. The combustion and explosion characteristics of natural gas-air mixture are also important for natural gas engines. In this study, ignited explosion experiments of methane-air mixtures have been conducted using a cylinder chamber under pressure up to 32 MPa, in which the influences of pressure on the explosion and flammability limits have been investigated. The experimental results indicate that the lower flammability limit of methane decreases slightly at elevated pressures, while its upper flammability limit increases significantly over 3 MPa, and the theoretical limiting oxygen concentration required for explosion is gradually reduced, posing greater explosion risks. At ambient temperature, the measured explosion limit range for methane in air is 2.93%–60.75%vol at 30 MPa, in contrast to 4.95% and 15.51%vol at atmospheric pressure, and the corresponding theoretical limiting oxygen concentration at 30 MPa can be reduced to as low as 5.86% from around 10% at 0.1 MPa when ignited using heated tungsten wire. A high pressure explosion limits model for methane-air mixture has been proposed based on the experimental data. • Ignited explosion experiments of methane at pressure up to 32 MPa was conducted. • The influences of pressure on the LFL, UFL and theoretical LOC was investigated. • A high pressure explosion limits model for methane-air mixtures was figured out. • The LFL of gases decreased with increasing the calorific values and combustion heat. • The pressure effect on the LFL could be attributed to the combustion heat. [ABSTRACT FROM AUTHOR]

Columnar jointed basalt (CJB), characterized by a hexagonal joint network, poses a great challenge for geotechnical designs due to its time-dependent propensity to deformation and damage. In this paper, the geometrical structure and failure modes of CJB exposed on a dam foundation is presented. The study was conducted during the unloading excavation of the Baihetan dam, China, aiming to provide a more in-depth understanding of the CJB damage mechanism. A systematic field test for the columnar jointed rock mass was also carried out on the dam foundation using ultrasonic P wave measurements. It was found that the excavation damage zone of the CJB not only showed time-dependent damage both in depth and degree but also showed spatially inhomogeneous distribution of the damage degree. Sequential regression analysis further demonstrated the time-dependent behavior of the CJB. Successive observations using digital borehole cameras in the field showed that the unloading damage mechanism of the Baihetan CJB was mainly related to the time-dependent opening of the initially closed joints. Several reinforcing methods for improving the stability of the damaged CJB, including the injection of liquid cement and the installation of the prestressed rock bolts, are discussed. [ABSTRACT FROM AUTHOR]

CO 2 foam can be used for mobility control and blocking gas breakthrough channels during CO 2 injection for enhanced oil recovery, and specially designed and formulated foam agents are needed to enhance the stability of the foams especially under reservoir conditions. In this study, the performance of CO 2 foams with various surfactants has been investigated, including amide based surfactants (e.g. coconut monoethanol amide (CMEA), lauryl diethanol amide (LDEA)), amine based surfactants (cocoacyl propyl dimethylamine (UC 11 AMPM)), sodium dodecyl sulfate (SDS) and its mixtures with the amides and amines. A visualized high pressure and high temperature foam meter and a sand-pack flooding setup were used to assess the foam's performance, along with the surface tension measurement of the surfactant solutions in the presence of CO 2. The influence of temperature, the surface tension of the aqueous solution, and SDS on CO 2 foam performance of amide and amine based surfactant were investigated. The experimental results indicate that the amide or amine based surfactant alone all show poor foam performance, while their CO 2 foam stability and their mobility control capability can be greatly enhanced at high temperature up to 100 °C with the salinity of 50000 mg/L. After adding SDS to CMEA, LDEA, or UC 11 AMPM, an excellent synergistic effect can be formed among the molecules, which can reduce the surface tensions of individual surfactant solution and enhance the stability of CO 2 foams. • The CO 2 foam performance of amine and amide based surfactants were tested at HTHP conditions. • Stable CO 2 foams can be generated by amine surfactant with SDS at temperature up to 130 °C. • The synergistic effect of amine based surfactant with SDS has been demonstrated for CO 2 foams at high temperature. [ABSTRACT FROM AUTHOR]

The columnar jointed basalt (CJB) in the left-bank dam foundation of the Baihetan hydropower station, China, is a special jointed rock mass with a columnar cylinder structure. A field investigation revealed three types of joints developing in the CJB: (1) columnar joints, (2) internal steeply dipping joints (ISDJ), and (3) internal gently dipping joints (IGDJ). A uniform distribution survey lines method (UDSLM) is proposed to describe quantitatively the spatial structural features of CJB exposed in limited outcrops. The visible spacing of internal joints in CJB is found to evolve; joint spacing decreases with increased degrees of weathering, disturbance, and time after unloading excavation. Combined with the actual structure, the formation mechanism of CJB is discussed from the structure of a single column to the whole rock mass, based on the contraction hypothesis. A columnar joint tensor, which is closely related to the CJB deformation characteristics, is established for describing the complex columnar joint network in the three-dimensional space based on the measurement data with the UDSLM. In addition, the structural characteristics of intact columns were exposed through dismantling cuboid specimens of 0.5 m edge lengths and 1.0 m height, verifying the rationality of the UDSLM and the evaluation of structural characteristics of CJB in the Baihetan dam foundation. This in situ study presents an in-depth understanding of CJB structure and provides meaningful guidance for excavation and supporting reinforcement for the CJB exposed on the Chinese Baihetan dam foundation. [ABSTRACT FROM AUTHOR]

Cyclic steam stimulation (CSS) has been widely applied as an effective technique for heavy oil reservoirs, but it is increasingly concerned in recent years for its limited oil recovery performance, low energy efficiency, high water consumption and great environmental footprints due to greenhouse gas emissions. The hybrid injection of air and steam in terms of low temperature oxidation (LTO) is an innovative technique for the development of heavy oil reservoirs, which can be operated based on the conventional well configuration and was firstly proposed as an enhanced oil recovery (EOR) process for mature heavy oil reservoirs encountered with low reservoir pressure, poor steam sweep efficiency and high water cut. In this study, numerical simulation study is conducted to evaluate the potentials of air assisted cyclic steam stimulation (AACSS) process as an alternative to CSS technique for heavy oil reservoirs. Effects of oil viscosity, LTO reaction and operated air to steam ratio on the well performance are examined for typical heavy oils. The results indicate that AACSS process can effectively improve oil recovery, enhance the energy efficiency and reduce CO 2 emissions in comparison with CSS alone, which can be attributed to the thermal effect due to oil oxidation, reservoir repressurization and gas driving offered by air injection, and the AACSS process can be more economically and environmentally attractive than CSS alone. The production performance of AACSS process for ultra heavy oil reservoirs can be more pronounced in comparison with that of ordinary heavy oils, and the LTO characteristics of different oils and the feasible air to steam ratio are reservoir specific for field operation. [ABSTRACT FROM AUTHOR]

Rock spalling is one of the most frequent break modes and a serious stability challenge for the supporting design during engineering excavation under high geo-stress and hard rock conditions. The primary objective of this work was the field investigation and experimental testing of basalt spalling in more than 50 cases in a large underground cavern in China. The field characteristics of basalt spalling, including the spatial distribution of events, its micro-surface morphology, the joint effect, the time-dependent relation to the excavation, the spatial distance to the opening face, and the supporting condition, are first presented via detailed statistical investigation. An in situ study of the full spalling process, including the surface development of rock slabbing and inner cracking extension of the surrounding rock, was recorded in a time series via a borehole camera, displacement measurement, and in situ photos. These experimental results exposed the time-dependent development of surface spalling performance and the inner cracking evolution of rock mass ahead of spalling. All of these actual spalling cases in a large underground cavern and the corresponding statistical analysis provided meaningful evidence for the tensile failure mechanism and prevention measures for rock spalling. [ABSTRACT FROM AUTHOR]

Amine-surfactant can be converted to be cationic surfactant in water solution saturated with CO 2 , which can enhance their foaming ability and foam stability, and the foaming ability can be switched off or on by CO 2 or replacing with N 2 with heating, so the amine-surfactants have great potential for smart mobility control to gas channeling during CO 2 flooding. In this study, three alkyl propyl dimethylamines with different carbon chain length (R = 11, 17 and 22, named as UC 11 AMPM, UC 17 AMPM, and UC 22 AMPM, respectively), were investigated as CO 2 responsive foam agent, and their foam performance under static and dynamic condition was tested up to 130 °C and 10.5 MPa using a visualized foam meter and a sand-pack flooding setup. The influence of carbon chain length on bulk solution viscosity, surface activity, foaming ability, and foam quality on bulk foam stability were measured and analyzed. The experimental results indicate that the three amine surfactants all demonstrate well CO 2 responsibility and CO 2 foaming ability, and show well switchable by CO 2 and N 2 at high temperature and high pressure (HTHP) condition. With the carbon chain length increasing, the surfactant shows higher absorption on the foam films, and bulk solution has a higher viscosity, which can decrease de-foam velocity and enhance the foam stability, as a result, the surfactant shows higher mobility control ability. • The CO 2 foam performance of three amine surfactants were tested at HTHP conditions. • The influence of carbon chain length on foam performance, bulk viscosity, surface active and foam quality was analyzed. • Alkyl propyl dimethylamines have well mobility control ability to CO 2 at high temperature-tolerance (up to 130 °C). [ABSTRACT FROM AUTHOR]