Physical Simulation Experiments Research Articles

Structural characteristics and formation mechanisms of Bayanhushu Sag in Hailar Basin, north‐east China: A study based on structural physical simulation

The Hailar Basin, located in north‐east China, is a typical continental rifted basin that contains oil and gas. The basin formation process comprised several stages of construction and reformation with complex formation mechanisms. The Bayanhushu (BYHS) Sag is a secondary structural unit in the south‐west Hailar Basin with a significant resource potential, but its current poor exploration and insufficient understanding of the structural evolution characteristics are restricting further oil and gas exploration. Therefore, the study of the structural evolution of the BYHS Sag plays a pivotal role in the future exploration and development of oil and gas. There are different hypotheses on the formation mechanisms and structural evolution of the BYHS Sag. To further understand the evolutionary history of the BYHS Sag, a structural physical simulation experiment was used based on the structural interpretation and geometric analysis of a seismic section. Inversion validation was then undertaken by the 2DMove equilibrium profile recovery technology. It was found that the formation process of the BYHS Sag was mainly controlled by the western Adunchulu Fault. Faults on the section developed in succession from top to bottom. The fault plane experienced multiple changes, thus forming a special seat‐shaped structural pattern. Structural inversion occurred twice during the evolution of the sag. The compressive stress during the tectonic inversion mainly acted in a SE direction. It is inferred that this was related to the subduction of the Palaeo‐Pacific Plate under the Eurasian Plate and the intermittent compression caused by the transmission of stress of the arc–continent collision.

Flow mechanism of production decline during natural depletion after hydraulic fracturing of horizontal wells in tight oil reservoirs

Currently, there are problems regarding decreasing production rates and low recovery factors in the field test during depletion development of staged fractured horizontal wells in tight reservoirs. However, the production decline mechanism during the depletion development process is not clear at present; so, it is necessary to further explore the pressure distribution law and production decline mechanism in order to guide subsequent tight reservoir exploitation and to improve the recovery factor. In this study, a multi-point pressure measurement physical simulation experiment system, a pressure distribution mathematic model, and a mathematic productivity model were developed. After fitting the model with the experimental data, the production decline mechanism during depletion development of tight oil reservoirs was analyzed from four aspects. The results show that in lower permeability tight reservoir facies, the pressure propagation distance is shorter. The pressure gradient of tight oil reservoirs during depletion development is four times higher than that of low permeability reservoirs. As the permeability decreases, the seepage resistance coefficient increases exponentially. The seepage resistance in tight oil reservoirs is nearly six times higher than that in low permeability reservoirs. Compared with traditional enhanced oil recovery (EOR) techniques, inter-fracture flooding has obvious mechanism advantages in the EOR of tight reservoirs.

Simulation of multi-cluster fracture propagation by hydraulic fracturing in shale reservoir

China is rich in shale oil resources. With the continuous breakthrough of exploitation technology, shale oil has become an important strategic oil resource in China. As it turns out, Multi-cluster fracturing technology of horizontal well is an efficient method to produce shale oil. However, most of the research on the initiation and propagation of fractures in shale reservoirs is based on numerical simulation. In this paper, through the improved true triaxial hydraulic fracturing physical simulation system, the rock multi-cluster fracturing physical simulation experiment, obtain the fracturing process parameters, and analyze the mechanism of fracture generation. The numerical model of fracture is established to analyze the interfracture interference phenomenon and optimize the segmenting and clustering problem. The results showed that: The larger the fracturing fluid viscosity is within a certain range, the smaller the probability of fracturing fluid leakage is, and the more conducive to the formation of fractures; Cluster spacing has a great influence on fracture propagation. Compared with double clusters, three clusters of fractures are more conducive to forming effective fractures. This paper comprehensively considers the multi-field coupling effect in the fracturing process, deeply understands the mechanical parameters of natural fractures, improves the understanding of the regularity of mechanical parameters of natural fractures under the multi-field coupling effect of fluid-solid-temperature, and provides further guidance for the optimization design of fracturing parameters.

Analytical model of fracture propagation in carbonate reservoirs

The fracture propagation length and propagation angle can be calculated quickly using the fracture propagation analytical model. But for fractured-vuggy carbonate reservoirs, the addition of a discontinuous medium in the analytical model makes it difficult to obtain accurate results via calculation. In this paper, an analytical solution model for fracture propagation in carbonate reservoirs is established. On the premise of ensuring calculation accuracy, the discontinuous displacement algorithm is used to introduce the fracture and hole characteristics into the calculation. The model comprehensively considers the interference of in-situ stress, fracture tip stress, and stress field of fractures and holes on fracture propagation. The calculation accuracy of the model in a fractured-vuggy carbonate reservoir is verified by a true triaxial fracturing physical simulation experiment. The model can be used for subsequent research on the fracture propagation law of carbonate reservoirs containing discontinuous bodies, such as fractures and caves.

Experimental study on the guiding effect of tectonic coal for coal and gas outburst

The guiding effect of tectonic coal on outburst was analyzed by combining typical outburst cases, and the physical simulation experiment was carried out to reproduce the influences of tectonic coal area on the outburst. Then, the guiding action mechanism of tectonic coal on the outburst was investigated. This study concludes that the threshold value of critical gas pressure for outburst in the group with local existence of tectonic coal area was lower than that in the group without local existence of tectonic coal area. Under the gas pressure of 0.30 MPa in the group with local existence of tectonic coal area, the outburst hole was located inside the tectonic coal area, a part of pulverized coal was not thrown out yet, and the coal pillar experienced failure only nearby the outburst mouth. As the gas pressure increases, the failure degree of the coal pillar become higher and higher until complete failure. Based the experimental results, the outburst development sequence was changed due to the existence of the tectonic coal area. Once the sealing conditions were destructed, the outburst firstly develops in the tectonic coal area. Nevertheless, a large quantity of outburst energy would be suddenly released in the outburst process of the tectonic coal area, so sufficient energy was supplied to transport the coal in the tectonic coal area to a farther distance, while the residual outburst energy can just result in the outburst of a small quantity of coal masses in the normal area. This research will be of great scientific significance for explaining the tectonic coal-induced change of outburst starting and development sequence.

Selection of emulsification system for high temperature and high salt reservoir and evaluation of reservoir adaptability

In this study, based on X reservoir conditions from Dagang Oilfield, China (temperature: 104 °C, salinity: 26376 mg/L), from the perspective of reducing interfacial tension and emulsification performance, IAPES (isododecyl alcohol polyoxyethylene ether sulfonate) with high temperature and high salt resistance was selected and its performance was evaluated. Finally, physical simulation experiments were conducted to form an in situ viscosity-increasing emulsion to enhance oil displacement efficiency. The experimental results show that when the concentration of IAPES is 0.3 wt%, the ability to reduce the interfacial tension of oil and water is significant and it has good antiaging stability. In addition, the lower the shear rate, the higher the water content, and the worse the emulsion stability. Finally, physical simulation experiments show that IAPES can increase the recovery rate by at least 28% in a larger permeability range. Under the experimental research conditions, the injection process parameters-injection speed (0.4 mL min−1) and injection volume (0.3PV) were optimized.

Effect of molecular carbon structures on the evolution of the pores and strength of lignite briquette coal with different heating rates

Briquette coal (BC) is the material produced from coal and gas outburst experiments. Preparing BC via the heating and pressure method can improve its low mechanical strength in physical simulation tests and gas outbursts while increasing experimental reliability. However, the influence of heating rate on the mechanical properties and microstructure of BC and the reasons underlying the evolution of its mechanical strength analyzed in terms of carbon molecular structure still need to be further studied. In this study, we developed an experimental system to prepare BC. The influence of the pore and carbon molecular structure on the BC uniaxial compressive strength was examined via a rock mechanics test system, scanning electron microscopy, nuclear magnetic resonance, specific surface area analysis, X-ray diffraction, and Fourier transform infrared spectroscopy. The results indicated that when the heating rate was increased from 3 to 6 °C/min, the briquette structure changed from looser smooth sheets to rough agglomerates of coal. The pore types observed after four different heating treatments (3, 4, 5, and 6 °C/min) on briquette samples were primarily micropores and mesopores. At 5 ℃/min, the number of micropores in the briquette sample were significantly higher than that observed in other samples. All samples exhibited similar adsorption curves as those of inverted “S”-type coal samples, which can be regarded as a combination of Type IV isotherms. With increasing heating rates, the La and Lc first increased and then decreased, with a decrease in Aar/Aal, whereas Hal/H and CH2/CH3 increased. The strength of coal samples under the increasing heating rates were 5.62, 6.43, 7.07, and 5.99 MPa, respectively. The heating rate of 5 ℃/min was found to be the appropriate heating rate for briquette preparation. These results can provide an excellent alternative raw material required for large-scale physical simulation experiments for the prevention and control of coal and gas outbursts.

Research on Roof Fracture Characteristics of Gob-Side Entry Retaining with Roof Cutting and Non-pillar Mining in Thick Coal Seam, China

Based on the S1201-2 large height mining in the 2–2 coal seam of Ningtiaota colliery with on-site microseismic measurement, physical simulation and theoretical analysis methods, this paper explores the rule of roof movement in thick coal seams with roof cutting and non-pillar (hereinafter referred to as RCN-P) mining, so as to obtain scientific and effective theoretical basis for entry support and to summarize the regional structural characteristics and dynamic periodic fracture characteristics. As can be seen from microseismic events, the entry roof is featured by "two zones and one line" along the horizontal direction, namely, the crack generation area, the roof movement area. Additionally, and the obvious lateral breaking of the entry roof on the coal wall is a typical feature of the thick coal seam with RCN-P mining. The roof is vertically divided into "three zones", the crack generation area, the roof movement area and the crack development area. The roof cutting activity mainly affects the overburden activity within the basic roof height range, which is also the roof movement area. In addition, the distribution frequency and the intensity of microseismic events indicate the roof periodic breaking characteristics. The "breaking pressure relief,” “advanced crack development,” and “the limit breaking state” of roof breaking corresponds to the initial, middle, and final stage of breaking in the periodic weighting process, respectively. Compared with the normal mining, the RCN-P mining reduces the periodic weighting length and increases the pressure strength. As is shown in the physical simulation experiment, the basic roof and the cutting control layer in the "regional structural characteristics" constitute the “large” and “small” structures with RCN-P mining. The basic roof key layer is the core to control the stability of the strata, and the breaking process from the cantilever beam to the short masonry beam of the roof-cutting control layer is the main cause of the entry stress. Correspondingly, the basic structure model of “short masonry-hinged” roof was proposed and the calculation method of support was established for the entry with RCN-P mining in thick coal seam, providing a research foundation for scientific and effective rock formation control.

Carbon Dioxide Flooding Technology for Ultra-Low Permeability Reservoirs to Greatly Enhance oil Recovery

Low-permeability reservoir pressure is difficult to maintain, CO2 miscible flooding is difficult to achieve for low-permeability reservoirs, and near-miscible flooding is a promising technology for enhancing oil recovery for low-permeability reservoirs. Aiming at the complex geological and development characteristics of China’s continental low-permeability reservoirs, the system elaborated Sinopec’s CO2 flooding theory, reservoir engineering optimization design, injection and production engineering and anti-corrosion technology, production CO2 recovery and utilization, and other technical developments and field tests. Happening. Based on the research of CO2 flooding mechanism, this paper takes a well-known oil field H 28 reservoir as the target, and conducts an indoor evaluation of CO2 flooding methods through core physical simulation experiments. The pilot test of CO2 flooding field shows that CO2 flooding can significantly increase the productivity of ultra-low permeability oil wells, improve the development effect of oil wells, and meet the needs of ultra-low permeability reservoir development, and has broad application prospects.

Study on fault sealing mechanism based on rock physics simulation experiment

At present, the development of fault oil and gas reservoirs has become an important way for to increase oil and gas productivity in China. However, due to the complex internal stress state of rock faults, the phenomenon of oil and gas escape is prone to occur during the mining process, so it is of great significance to study the sealing mechanism of the rock fault. For this reason, based on the theory of fault sealing, rock physical simulation experiments of fault sealing were carried out, and the mechanism of fault sealing and its main controlling factors were deeply studied. The results show that the rock fault dip and the argillaceous content of the fault filling are the main controlling factors for fault sealing; The essence of fault lateral sealing and vertical sealing is the sealing of the displacement pressure difference; As the dip angle of the fault increases, the lateral sealing of the fault becomes better, while the vertical sealing becomes worse; with the increase of the argillaceous content of the fault filling, the lateral and vertical sealability of the fault becomes better.

Experimental Study on the Initiation and Propagation of Multi-Cluster Hydraulic Fractures within One Stage in Horizontal Wells

Competitive propagation of fractures initiated from multiple perforation clusters is universal in hydraulic fracturing of unconventional reservoirs, which largely influences stimulation. However, the propagation mechanism of multi-fractures has not been fully revealed for the lack of a targeted laboratory observation. In this study, a physical simulation experiment system was developed for investigating the initiation and propagation of multi-cluster hydraulic fractures. Different from the traditional hydro-fracking test system, the new one was equipped with a multi-channel shunting module and a strain monitoring system, which could guarantee the full fracture extension at each perforation clusters and measure the internal deformation of specimens, respectively. Several groups of true tri-axial fracturing tests were performed, considering the factors of in situ stress, cluster spacing, pumping rate, and bedding structures. The results showed that initiation of multi-cluster hydraulic fractures within one stage could be simultaneous or successive according to the difference of the breakdown pressure and fracturing fluid injection. For simultaneous initiation, the breakdown pressure of the subsequent fracture was lower than or equal to the value of the previous fracture. Multiple fractures tended to attract and merge. For successive initiation, the breakdown pressures of fractures were gradually increasing. The subsequent fracture tended to intersect with or deviated from the previous fracture. Multiple fractures interaction was aggravated by the decrease of horizontal stress difference, bedding number and cluster spacing, and weakened by the increase of pump rate. The propagation area of multiple fractures increased with the pump rate, decreased with the cluster spacing. The strain response characteristics corresponded with the initiation and propagation of fracture, which was conducive to understanding the process of the fracturing. The test results provide a basis for optimum design of hydraulic fracturing.

Study on the Key Influential Factors on Water Huff-n-Puff in Ultralow-Permeability Reservoir

Ultralow-permeability reservoirs are difficult to effectively develop using conventional technologies, so it generally produces using horizontal wells’ volume fracturing. Besides, oil wells also face the problems of short stable or no production period, rapid decline rate, and extremely low development degree. Water huff-n-puff of oil recovery technology is a quality and efficiency technology to improve the development effect of an ultralow-permeability reservoir. A systematic study on the related key influential factors of water huff-n-puff is done for improving the development effect of ultralow-permeability reservoirs. This manuscript studied the key influential factors of water huff-n-puff and related improving recovery methods using physical simulation experiments to put forward a beneficial condition for water huff-n-puff, combining with the field practice. The results show that imbibition and energy supplement are main mechanisms of water huff-n-puff; imbibition oil recovery is influenced by rock wettability, permeability, boundary condition, fracture length, water injection speed, and imbibition solution type; the stronger the water-wet reservoir rock is, the bigger the core permeability is, the longer the fracture length is, the larger the contacting area between imbibition solution and rock is, the stronger the ability of imbibition solution to change wettability, and to reduce the interfacial tension force between oil and water is, the better effect of water huff-n-puff is. So far, the field practice of water huff-n-puff has been conducted in 38 wells; the success rate can reach 92.1%, cycle oil increment of single well can reach 972 t, and accumulated oil in evaluation period can reach 36936 t, which further proves that water huff-n-puff can achieve a good effect. The effective methods to improve the development effect of an ultralow-permeability reservoir are changing reservoir physical property and supplying reservoir energy. Higher reservoir energy is good for oil flow, and better physical property can improve the displacement effect and imbibition function of injection solution. Artificial fracturing, higher injection-production ratio, and pretreatment of temporary stoppage fracturing are good methods to improve the development effect of water huff-puff.

Instability Control of Roadway Surrounding Rock in Close-Distance Coal Seam Groups under Repeated Mining

In order to solve the problems of roadway stability and easy instability under repeated mining of close-distance coal seam groups, the mechanism and control technology of surrounding rock instability under repeated mining were studied via indoor testing, field testing, physical similarity simulation experiment, and numerical simulation. The results show that the surrounding rock of roadway has low strength, low bearing capacity, and poor self-stabilization ability, and it is vulnerable to engineering disturbances and fragmentation. Affected by the disturbance under repeated mining, the roadway surrounding rock cracks are developed and the sensitivity is strong, and it is prone to large-scale loose and destroyed. The location of the roadway is unreasonable, and the maximum principal stress of the roadway is 3.1 times of the minimum principal stress, which is quite different. Thus, under a large horizontal stress, the surrounding rock undergoes long-range expansion deformation. On the basis of this research, the direction and emphasis of stability control of roadway surrounding rock under repeated mining of coal seam groups in close-distance are shown. A repair scheme (i.e., long bolt + high-strength anchor cable + U-shaped steel + grouting) is proposed, and reduces the risk of roadway instability.

Experimental Comparative Investigation of Hot Solvent/Steam-Assisted Gravity Drainage in Oil Sand Reservoirs.

Hot solvent-assisted gravity drainage (HS-AGD) is an effective way to exploit oil sands and heavy oil both economically and environmentally. The visualized microscopic seepage experiments and two-dimensional (2-D) macroscopic simulation experiments of HS-AGD are carried out, and the results are compared with that of steam-assisted gravity drainage (SAGD) in detail for the first time in order to compare their development effects of the oil sand reservoir. MacKay River oil sand bitumen is taken as an oil sample in the experiments, with n-hexane as the solvent. Micro seepage characteristics of the hot solvent and steam and the remaining oil distribution of the solvent and steam drive are investigated through microseepage experiments. The expanding process of the solvent/steam chamber and production performance of HS-SAGD and SAGD are investigated through macrosimulation experiments. The study found that the sweep efficiency of hot solvent is higher than that of steam at the same temperature due to the small interfacial tension between the condensed solvent and heated bitumen. Due to the severe gravity segregation, the steam accumulated at the top of the model during the 2-D physical simulation experiment, which results in the huge heat loss at the top of the model. The temperature of the steam chamber is significantly lower than that of the solvent chamber. The oil recovery of 200 °C hot solvent vapor is twice as much as that of 300 °C steam owing to the different drainage mechanisms of the HS-AGD and SAGD. In SAGD, only heat transfer reduces the viscosity of oil sand bitumen. The components of oil produced in SAGD have little difference compared with that of the original bitumen. In HS-AGD, both mass transfer and the sensible heat transfer reduce the viscosity of oil sand bitumen. The in situ asphaltene precipitation induced by heated-solvent extraction also upgrades the bitumen. The results of component analysis show that in HS-AGD, the content of heavy components in the oil sand bitumen is obviously reduced. This paper aims to reveal the oil drainage mechanism of HS-AGD and SAGD from the macroscopic and microscopic view and to provide theoretical guidance for the field application of this technology.

Practice of high-intensity volume fracturing in the Shaximiao Formation tight sandstone gas reservoirs of the Qiulin Block, central Sichuan Basin

In order to solve the difficulties in the volume fracturing stimulation of Middle Jurassic Shaximiao Formation tight sandstone reservoirs in the Qiulin Block of Central Sichuan Basin and explore the adaptability of high-intensity volume fracturing technology, we selected the outcrop samples of Shaximiao Formation tight sandstone in the Qiulin Block to carry out the physical simulation experiment of true triaxial hydraulic fracturing. On this basis, horizontal well cluster perforation was optimally designed by using the production prediction model of staged multi-cluster fracturing horizontal wells. Then, based on the liquid control and proppant increase mode, three rounds of pilot tests were carried out on the tight sandstone reservoirs in this area. And the following research results were obtained. First, natural fractures in the Shaximiao Formation tight sandstone reservoir of the Qiulin Block are undeveloped, and hydraulic fractures are morphologically dominated by symmetric double-wing fractures, so complex fracture networks can be hardly formed. In addition, the reservoir is of medium to strong water sensitivity, so conventional volume fracturing is not adaptive to the reservoir stimulation in this block. Second, the connotation of high-intensity volume fracturing technology is to carry out multi-cluster perforation in each section to form multiple independent double-wing fractures and to implement the proppant injection mode of liquid control and proppant increase to reduce the inflow fluid while ensuring the high-intensity proppant injection, so as to reduce the damage of inflow fluid to the formation. Third, there are 10 fracturing sections in Well Qiulin 207-5-H2, with 7–12 clusters in each section, and the displacement is in the range of 16–18 m3/min. According to the fluid control and proppant increase mode, 12146 m3 slick water and 4170 t proppant are injected in total. The tested production rate and absolute open flow of natural gas after the fracturing are up to 83.88 × 104 m3/d and 214.05 × 104 m3/d, respectively. Fourth, with the decrease of cluster spacing, the cumulative gas production increases gradually, but when the cluster spacing is less than 15 m, the increase amplitude of cumulative gas production decreases. Fifth, when the proppant injection intensity is lower than 6 t/m, the tested gas production per kilometer of stimulated section in a horizontal well overall presents an increasing trend with the increase of proppant injection intensity. When the proppant injection intensity is higher than 6 t/m, however, the tested gas production per kilometer of stimulated section does not increase significantly with the increase of proppant injection intensity. Sixth, as the included angle between the borehole trajectory and the direction of maximum horizontal principal stress increases, the tested gas production per kilometer of stimulated section overall presents an increasing trend. When the hydraulic fracture is nearly perpendicular to the borehole, the effective drainage area is the largest and the tested gas production per kilometer of stimulated section is also the highest. In conclusion, the fracturing mode of high production well has a borehole trajectory of large included angle, perforation cluster spacing of 10 m or so, proppant injection intensity of 5 t/m and large-displacement slick water + continuous injection of combined particle size proppant.

Effect of heating on the molecular carbon structure and the evolution of mechanical properties of briquette coal

Preparing briquette coal (BC) via the heating and pressure method can improve its low mechanical strength in physical simulation tests and gas outbursts while increasing experimental reliability. However, the influence of temperature on the mechanical properties of BC, microstructure, and the reasons underlying the evolution of mechanical strength of BC analyzed in terms of the carbon molecular structure of coal still need to be further studied. For this purpose, a rock mechanics test system, scanning electron microscopy, X-ray diffraction, and Fourier Transform Infrared Spectroscopy were used to analyze the optimum temperature for BC preparation and identify how the heating temperature affects the molecular carbon structure of BC to increase its strength. The result showed that as the temperature increases from 150 °C to 600 °C, the aromatic d002 layer spacing decreased, whereas the aromatic layers of La and Lc increased, leading to an order degree increase of coal molecules. In addition, Hal/H and CH2/CH3 decreased, whereas Aar/Aal increased, indicating that the aliphatic chains shortened and the aromaticity increased, ultimately improving the strength of BC. However, when the temperature exceeds 300 °C, the pores suffers damage, increasing the width of the pores and crack. Through uniaxial compression, the mechanical strength of BC first increases and then decreases. Moreover, the mechanical strength, Poisson's ratio, and elastic modulus at 300 °C were close to that of raw coal (RC). These results provide guidance to prepare excellent raw materials needed for large-scale physical simulation experiments to prevent and control coal and gas outbursts.

Formation Mechanism of Mud Volcanoes/Mud Diapirs Based on Physical Simulation

The formation of mud volcanoes/mud diapirs is directly related to oil and gas accumulation and gas-hydrate mineralization. Their eruptive activities easily cause engineering accidents and may increase the greenhouse effect by the eruption of methane gas. Many scholars have performed much research on the developmental characteristics, geochemistry, and carbon emissions of mud diapirs/mud volcanoes, but the formation mechanism of mud diapirs/mud volcanoes is still controversial. Mud diapirs and mud volcanoes are especially developed in the northern South China Sea and are accompanied by abundant oil, gas, and gas-hydrate resources. Based on the mud volcanoes/mud diapirs in the northern South China Sea, the physical simulation experiments of mud diapir/mud volcano formation and evolution under different fluid pressures and tectonic environments have been performed by loading a fluid-input system in traditional sandbox simulation equipment. The genetic mechanism of mud diapirs/mud volcanoes is revealed, and a fluid-leakage model of mud diapirs/mud volcanoes under different geological conditions is established. We believe that in an overpressured environment, the greater the thickness of the overlying strata is, the greater the pressure or power required for the upward migration of muddy fluid to penetrate the overlying strata. Tectonic activity promotes the development of mud volcanos/mud diapirs. To a certain extent, the more intense the tectonic activity is, the more significant the effect of promoting the development of mud volcanoes/mud diapirs and the larger the mud diapirs/mud volcanoes become.

Experimental Study on Well Placement Optimization for Steam-Assisted Gravity Drainage to Enhance Recovery of Thin Layer Oil Sand Reservoirs

SAGD (steam-assisted gravity drainage) technique is one of the most efficient thermal recovery technologies for exploiting Mackay River thin layer oil sand reservoirs. However, when making use of the traditional SAGD technique (the production and injection well are located on the same axis with the horizontal well spacing of 0 m), the steam chamber development is usually insufficient because of the high longitudinal sweep rate of steam, which seriously influences the SAGD performance for developing thin layer oil sand reservoirs especially. It is extremely important to find an economical and practicable method to promote the steam chamber development in thin oil sand reservoirs in the process of SAGD production; optimizing well placement is a reliable method. In this paper, an improved well placement method is proposed to enhance production performance of traditional SAGD, which is changing the horizontal well spacing to place the production well below of the side of the injection well (two wells are not located on the same axis). Three groups of 2D visualization experiments with different horizontal distances between two wells (0 cm, 10 cm, and 20 cm) were carried out, respectively, to observe the development and change of the steam chamber development, and to explore the EOR mechanisms. On the 2D experiment basis, optimal horizontal distance was selected to perform 3D physical simulation experiment to study and verity the production mechanism systematically. The results of 2D visualization experiment showed that the final oil recoveries of experimental groups with 10 cm and 20 cm horizontal distances were 7.6% and 2.3% higher than those of traditional SGAD (horizontal distance was 0 cm), respectively. Combined with 3D experimental results, the change in the horizontal relative position of two wells makes the steam first spread laterally between injecting and producing wells; thus, the lateral development of the steam chamber was promoted, and changes of temperature field also display that the lateral sweep area of steam was increased obviously and the form of steam chamber is changed. Meanwhile, the generation of appropriate horizontal well spacing can combine the effect of steam displacement and gravity drainage better and improve the sweep efficiency of steam. Nevertheless, the overlarge horizontal well spacing will also hinder the steam chamber development because the strength of steam overlap is weakened. Furthermore, the findings of this study help for better understanding that changing the horizontal well spacing can promote the lateral development of steam chamber, which can be used to enhance the oil recovery of thin layer oil sand reservoirs especially.

Numerical and experimental study on thermo-mechanical processing of medium-carbon steels at low temperatures for achieving ultrafine-structured bainite

A combination of experimental and numerical approaches was applied for constructing a dynamic model for thermomechanical processing, which was used for simulating laboratory rolling and cooling, and for designing a cooling path to enable phase transformation from austenite to ultrafine (∼50–100 nm) bainitic laths. Physical thermomechanical simulation experiments were used for calibrating the numerical models. Hot rolling and water cooling experiments were conducted and they were numerically simulated. The calibrated numerical models were used for simulating the main processing stages affecting the final microstructure evolution during a laboratory scale processing, i.e. the low temperature (500 °C) ausforming and subsequent cooling schedules leading to the decomposition of austenite into bainite and martensite. The fitted model parameters and simulation results are presented for the laboratory rolling and two different cooling paths: (i) air cooling to 350 °C temperature with subsequent holding for 1–1.5 h, and (ii) water cooling close to martensite start temperature, and furnace holding for 1–1.5 h. Microstructural analysis was carried out using scanning electron microscopy combined with electron backscatter diffraction as well as X-ray diffraction and the structures were corroborated with mechanical properties evaluated in respect of hardness, tensile and impact toughness properties. The achieved mechanical properties and microstructures were further interpreted with the numerical simulation results. The results show that the calibrated numerical simulations provide an effective tool for designing suitable thermomechanical processing paths leading to desired microstructure.

Structural deformation characteristics of the Lower Yangtze area in South China and its structural physical simulation experiments

Abstract The analysis of structural deformation characteristics in the Lower Yangtze area of South China is of great significance to petroleum exploration and development in this area. Based on the geological and geophysical data, the structural deformation characteristics of the Lower Yangtze area are systematically discussed. Structural physical simulation experiments are further conducted to model the typical structural deformation systems and to discuss their dynamic mechanism. The results show that the hedging system is characterized by “asymmetric opposite hedging in the south and north” of the study area. The structural deformation on the northwest of the hedging system mainly occurred in the Middle-Late Triassic and was controlled by southeaster compression in the Indo-Chinese Period. The deformation of southeastern side of the hedging system mainly occurred in Middle-Late Jurassic and was controlled by northwester strong compression in the early period of the Yanshanian Movement. According to the development and evolution sequence of the hedging structure system in the Lower Yangtze area, Wuxi area has weak structural deformation and has not undergone intensive transformation in later periods. Also, the other factors of petroleum accumulation, including the source rock, reservoir, and sealing conditions, are superior, which make a potential area for exploration.