Physical Simulation Experiments Research Articles

Three-Dimensional Stochastic Distribution Characteristics of Void Fraction in Longwall Mining-Disturbed Overburden of Inclined Coal Seam

Abstract Fractures in the overburden induced by mining disturbances provide a channel for fluid flow between the surface and the underground. Mining-induced strata movement and fracture distribution are influenced by the gravity and dip angles of rock seams. In this paper, a new three-dimensional theoretical distribution model for void fraction in each partition of overlying rock strata disturbed by inclined coal seam mining was constructed. Based on the theoretical determination model, the three-dimensional random distribution characteristics for void fraction were obtained by combining the random distribution law of void fraction obtained by similar physical simulation experiments and image processing techniques. Theoretical deterministic models, stochastic theoretical models, and similar physical simulations all show that void fraction distribution in the tendency direction of the coal seam shows a bimodal asymmetric distribution with high and low peaks and a symmetric distribution in the strike direction. The void fraction of the overburden in the central part of the mining area is smaller than that of the surrounding area. The results of the theoretically determined model and stochastic model of the void fraction for the strata with different mining lengths and different coal seam inclinations were compared with the results of similar simulation experiments, respectively. The results are in agreement, further verifying the practicality of the model.

Experimental Study on Production Characteristics of Bottom Water Fractured-Vuggy Reservoir

To assess the high-pressure production characteristics of double-vug reservoirs with bottom water in the Tahe Oilfield, in this study, a high-pressure physical simulation experiment apparatus is built for double-vug reservoirs with bottom water. The high-pressure production characteristics of “vug-fracture-vug” reservoirs under different bottom water characteristics are studied experimentally, and fracture-vug relationship and oil recovery rates are explored. According to the findings, the oil recovery rate significantly affects the development effect of double-vug reservoirs with bottom water, and bottom water provides sufficient energy for reservoir development. Furthermore, considering the possible occurrence of water invasions, the production rate control must receive close attention in the development process to avoid strong water channeling. Constant-pressure bottom water has sufficient energy and can quickly replenish vug energy. Therefore, the recovery percentage and bottom water invasion and retention volume of fractured-vuggy reservoirs with constant-pressure bottom water are higher than those of fractured-vuggy reservoirs with constant-volume bottom water. Under the appropriate control of production factors, the presence of bottom water can noticeably improve the development effect of fractured-vuggy reservoirs.

Experimental Study of Diffusion and Formation Mineral Change in Supercritical CO2 Huff and Puff Process of Shale Reservoir

Abstract In order to understand the diffusion during CO2 huff and puff in the development of shale oil and its influence on the formation, expansion and viscosity reduction experiments of shale oil-CO2 system, CO2 extraction experiments, and CO2 huff and puff physical simulation experiments were conducted. The diffusion characteristics of CO2 during huff and puff and their effects on formation minerals were studied by chromatographic analysis and X-ray diffraction analysis of artificially fractured natural cores. Research indicates that CO2 huff and puff technology is an effective method to enhance the recovery of shale reservoirs after fracturing. By injecting CO2, the light components of shale oil can be effectively extracted; when the amount of injected CO2 is 50%, the saturation pressure of shale oil increases to 27.72 MPa, and the expansion coefficient increases by 27.9%, the viscosity reduction rate of shale oil can reach 58.97%, and the density reduction rate is 10.02%; under the soaking well pressure of 50 MPa, when 0.5PVCO2 was injected and the well stuffed for 8 hours, the CO2 was fully dissolved in the shale oil, and the continuous increase of the injection slug had a little effect on the CO2 diffusion. During the CO2 huff and puff process, CO2 would dissolve in the formation water and fracturing fluid and reacts with dolomite in the reservoir rock, consuming a large amount of dolomite in the reservoir, and the dolomite mineral content of core sample decreased from 30.1% to 2.6%.

Law of Countercurrent Energy Dissipation of Fresh Air by Coal and Gas Outburst Shock Waves

Accidental coal and gas outbursts lead to major disasters in coal-producing countries and are difficult to mitigate. To elucidate the energy dissipation law for coal and gas outburst shock waves in a complex ventilation network of mines, a coal-and-gas-outburst-energy-propagation simulation and parameter determination test device were developed and used to perform physical simulation experiments under different outburst strength conditions. These experiments were combined with numerical simulations to obtain the propagation law of outburst shock waves in a roadway and analyze different outburst intensities according to the gas counterflow criterion, highlighting the hazard characteristics of shock waves in a fresh air tunnel. The results showed that when a shock wave passed through the turning roadway, its intensity and speed were greatly attenuated and that the overpressure value of a shock wave in a straight roadway was greater than that at a corner. Upon encountering a rigid wall, the superposition of the incident and reflected shock waves increased the peak overpressure of the shock wave per unit area. When a fresh air roadway is near the coal uncovering position in rock drift (“Shimen”) a corresponding counter-backflow device should be installed at the location of the connecting road near the fresh air roadway, under the condition that normal ventilation is not affected. These numerical simulation results are consistent with general experimental trends, indicating that the analyses conducted in this study are reliable and can provide a theoretical basis for the prevention of secondary disasters due to coal and gas outbursts in mines.

Study on Reasonable Formation Pressure Maintenance Level for Middle-Deep Reservoirs in the Bohai Sea

Reasonable formation pressure maintenance level is significant to high-efficient development of oilfields. In order to study the effects of overlying strata pressure on permeability, oil-water phase permeability curve, and oil displacement efficiency, a physical simulation experiment is designed. Based on the experimental results and the oil-water phase flow theory, the production equation and the mathematical model of oil displacement efficiency considering stress sensitivity are established. And the productivity changes with pressure drop under different permeability are plotted. Then, the permeability coefficients calculated by quantitative characterization of stress sensitivity under different formation pressures are introduced into the numerical simulation model to quantitatively determine the reasonable formation pressure maintenance level of different reservoir properties. Experimental and theoretical studies show that the permeability decreases continuously with the increase in effective overlying strata pressure. In a low permeability reservoir, the more permeability decrease is caused by the increase in effective overlying strata pressure. When reservoir pressure is restored by water injection, the permeability loss is irreversible. With the increase in effective overlying strata pressure, the producer productivity decreases obviously, and the effective seepage capacity and oil displacement efficiency decrease. For reservoirs with permeability below 100 mD and high stress sensitivity, high formation pressure level should be maintained. For reservoirs with permeability of more than 300 mD, lower formation pressure is acceptable in the initial stage. The results are consistent with the actual production characteristics, which effectively guide the establishing of reasonable oilfield development strategy. It has important guiding significance to the oilfield development plans and development of the middle-deep oilfields.

Wettability of Tight Sandstone Reservoir and Its Impacts on the Oil Migration and Accumulation: A Case Study of Shahejie Formation in Dongying Depression, Bohai Bay Basin

The migration and accumulation of oil in tight sandstone reservoirs are mainly controlled by capillary force. Due to the small pore radius and complex pore structure of tight sandstone reservoirs, the capillary force is very sensitive to wettability, so wettability significantly affects oil migration and accumulation. However, the study of oil migration and accumulation in tight sandstone reservoirs often needs to combine multiple methods, the process is complex, and the research methods of wettability are not uniform, so the mechanism of wettability affecting oil migration and accumulation is not clear. Taking the tight sandstone of the Shahejie Formation in the Dongying sag, Bohai Bay Basin, as the research object, the wettability characteristics of a tight sandstone reservoir and their influence on oil migration and accumulation were analyzed by means of a pore permeability test, XRD analysis, micro-CT experiment, contact angle tests, spontaneous imbibition experiments, and physical simulation experiments on oil migration and accumulation. The results show that the reservoir is of the water-wet type, and its wettability is affected by the mineral composition. Wettability in turn affects the spontaneous imbibition characteristics by controlling the capillary force. Oil migration in tight sandstone reservoirs is characterized by non-Darcy flow, the oil is in the non-wetting phase and subject to capillary resistance. The key parameters to describe the oil migration and accumulation characteristics include the kickoff pressure gradient, the critical pressure gradient, and ultimate oil saturation. Wettability affects oil migration characteristics by controlling the capillary force. The more oil-wet the reservoir is, the more favourable it is to oil migration and oil accumulation and therefore the higher the reservoir’s ultimate oil saturation is.

Evaluation of the Effectiveness and Adaptability of a Composite Water Control Process for Horizontal Wells in Deepwater Gas Reservoirs

Deepwater gas fields have high bottom water energy and a high risk of seeing water. Higher requirements are put forward for the water control process to control the water effect. This article is based on the actual background and well design of the X gas field in the South China Sea and on three sets of physical simulation experiments and three sets of numerical simulation experiments. An analysis and comparison of the water control effect of a combination of continuous packer, continuous packer and variable density screen tube, and their adaptability evaluation in deepwater gas reservoirs were performed. The results obtained from the numerical and physical simulations are consistent. The experimental results show that the water control process of a continuous packer is mainly based on the water-seeing and water-blocking ability. It is less capable of extending the time to produce water in the horizontal section. However, its water-blocking ability is strong and is able to seal the water spot quickly. It extends the total production time by 12.29% and increases the total gas production by 5.96%; the combined water control process of the continuous packer and variable density screen tube can effectively play their respective advantages of water control. The combination of the continuous packer and variable density screen tube can effectively be advantageous of their respective water control processes, enabling the gas–water interface to advance in a balanced manner, extending the water-free gas recovery period by 11.61%, extending the total gas production time by 15.76%, and increasing the total gas production volume by 13.75%. Both water control processes have good applicability in deepwater gas fields and have certain sand control capability. It is conducive to the one-time completion operation for the commissioning of deepwater gas fields.

Physical simulation experiments of structural deformation and hydrocarbon accumulation in piggyback thrust wedges

The piggyback thrust wedge is a common structural style in thrust belts, which controls the lateral arrangement of hydrocarbon distribution. This study conducted physical simulation experiments of structural deformation and hydrocarbon accumulation by using the piggyback thrust wedge as the geological model. According to the experimental results, hydrocarbon accumulation patterns were obtained by investigating the evolutionary history of the fault-sealing performance while inverting the coupling relationship between the fault evolution and hydrocarbon accumulation. The structural deformation experiment showed that the evolution of the fault displacement and dip angle was characterized by gradual and staged progress. The kaolin smear continuity deteriorated with fault evolution. The soybean oil accumulation experiment demonstrated that the fault-sealing performance in the rear of the thrust wedge was inferior to that in the front. Hydrocarbon migration was mostly lateral in the front of the thrust wedge and mainly vertical in the rear. Quantitative analysis showed that the fault-sealing performance had three closed, partially closed, and open stages, which gradually deteriorated with the structural evolution. Three hydrocarbon accumulation patterns were identified in the piggyback thrust wedge: synchronized, adjustment, and non-adjustment patterns. The evolution of the structural deformation and faults-sealing performance resulted in spatio-temporal differences in hydrocarbon accumulation in the thrust belts of central-western China. The front of the thrust wedge had a higher exploration potential than the rear.

Microstructures and mechanical properties of three medium-Mn steels processed via quenching and partitioning as well as austenite reversion heat treatments

In order to establish the processing conditions to achieve the target of tensile strength of ∼1500 MPa with minimum retained austenite (RA) fraction of 10% in hot-rolled 3–4%Mn medium Mn steels, three hot-deformed steels with the base composition of Fe-0.3C–1Si-0.5Al (concentrations in wt.%) and variable contents of Mn (3 or 4 wt%), Ni (1 or 2 wt%), Mo (0.2 or 0.4 wt%) and V (0 or 0.2 wt%) were processed using both the quenching and partitioning (QP) and austenite reverse transformation (ART) treatments. Physical simulation experiments were carried out using small cylinders in a thermomechanical simulator and the differences in the amounts, morphology and stability of RA were established by using X-ray diffraction and electron probe microanalysis, besides standard material characterization techniques. Both processes provided relatively high fractions of austenite; up to 26 vol% using the QP route and up to 49 vol% by the ART treatment under specific conditions. These amounts were comparable to those predicted for the conditions of thermodynamic equilibrium using the ThermoCalc software. The stability of RA for various conditions was estimated in terms of the average carbon contents of the RA. In QP process RA had mostly blocky-like appearance while in ART process the majority of RA was lath-like shaped. The hardness measurements revealed that the level of hardness in the QP structures was higher than that of the ART samples, possibly affected by RA fraction but more obviously as a result of martensite tempered at relatively low temperatures in the QP and at high temperatures in the ART treated structures. Preliminary tensile properties were measured on selected laboratory hot-rolled samples and results analyzed based on RA characteristics. The preliminary results indicated that the target is achievable in the laboratory scale through both the processing routes.

Three-Dimensional Physical Similarity Simulation Experiments for a Transparent Shaft Coal Pocket Wall in Coal Mines.

To explore the variations of the loading, deformation, and loss and to determine the mechanical state, loss characteristics, and stability for the shaft coal pocket wall in coal mines under a dynamic-static load, this paper innovatively attempts to conduct a three-dimensional physical similarity test of a transparent material shaft coal pocket, as well as the experiments of loading and unloading coal in the shaft coal pocket using different bulk storage materials 80 times. Then, the deformation, pressure, the surrounding rock, and the flow pattern of the silo wall were discussed considering the existence of the warehouse wall support. The characteristics of shaft wall deformation and surrounding rock stress cracks during the unloading were analyzed with the help from multiple integrated test systems such as strain gauges, pressure sensors, borehole peeps, and other comprehensive test systems. The results indicated that different dispersion particles have a significant impact on the strain of the shaft wall. When using the coal particles as storage materials, the overpressure coefficient of the shaft wall is up to 1.95 times higher than using dry sand particles. The particle size and internal friction angle of the bulk particles impact significantly on the deformation of the wall, where the cohesive force among the dispersed particles produced by the compaction effect has a certain influence on the side pressure of the silo wall. During the unloading process, coal particles were easier to obtain an arching phenomenon than dry sand particles. In addition, the number of bulk arching could be significantly reduced under the conditions of the warehouse wall support. The “weak rock stratum” in the surrounding rock plays a major role in controlling the deformation and failure development of the shaft wall. The three-dimensional physical simulation experiment of the transparent shaft wall truly reproduces the field engineering practice, and the physical simulation results are verified by numerical simulation analysis.

Experimental Study and Thoughts on Safety and Environmental Protection of Steam Flooding of Horizontal Wells in a Heavy Oil Reservoir.

Based on similarity criteria, a scaled physical model was fabricated, containing four horizontal wells and representing one-fourth of an inverted nine-spot steam-flooding pattern. A series of steam-flooding physical simulation experiments were carried out after different types of integral cyclic steam stimulations. Temperature distributions in the model reservoir, overburden, and substratum were measured at intervals. Production performance could be exactly recorded. The spatial temperature distribution of the whole model could be obtained. The phenomena of steam channeling, steam override, and secondary heterogeneity were observed obviously. The results that show steam flooding would cause steam overlap and characteristics of steam channeling during steam flooding after different types of integral cyclic steam stimulation were very distinct. The reservoir would have different temperature fields and different flow resistance because of different integral cyclic steam stimulations. The formation fluid would bypass the place with large flow resistance (low-temperature zone), forming secondary heterogeneity and affecting the displacement effect and thus the development effect. The recovery percent of a heavy oil reservoir developed by steam flooding was low, and it would bring some potential safety risks and problems with environmental protection. The experimental results provide theoretical support for further study of the mechanism of steam flooding.

Plugging Performance of a New Sulfonated Tannin Gel System Applied in Tight Oil Reservoir

In this study, based on the high-temperature characteristics of Western China tight oil reservoirs, a phenolic-larch tannin, temperature-resistant, plugging agent was synthesized by changing the mass fractions of larch tannin, double cross-linking agent, and accelerator. Young’s modulus of the dispersed gel was directly measured by an atomic force microscope, and the macroscopic plugging performance was evaluated by a physical simulation experiment of the artificially fractured natural core from Western China tight oil oilfield, thereby establishing a mapping relationship between the two. Research indicates that the formula of the high-temperature-resistant tannin system optimized by the experiment is 3.0 % sulfonated tannin + 3.0 % formaldehyde cross ‐ linking agent + 1.0 % phenol cross ‐ linking agent + 0.05 % MnS O 4 accelerator ; the mechanical strength of the tannin gel and its plugging performance have a linear relationship. When Young’s modulus rises from 18.74 to 63.89 KPa, the plugging rate rises from 94.11% to 97.44%.

Study on the Effect of Microstructure Gradients Caused by Heat Gradients on Hydrogen Embrittlement Sensitivity in Heavy Forgings

The hydrogen embrittlement problem of alloy steel heavy forgings not only has the common properties of general hydrogen embrittlement, but also has the characteristics brought by its scale characteristics. The research of hydrogen embrittlement, combined with its characteristics and commonness, is of vital importance for the service safety of engineering structures. The temperature field and microstructure distribution in the machining process were investigated through the simulation of a finite element. On this basis, the physical simulation experiments were carried out to obtain the microstructure of heavy forgings in radial directions. The hydrogen embrittlement sensitivity was characterized by electrochemical hydrogen charging and slow strain rate tests (SSRT). The microstructure and fracture morphology of the samples were characterized to explore the law and mechanism of hydrogen embrittlement sensitivity gradient distribution along the axial direction. It is helpful to understand the hydrogen embrittlement of heavy forgings in order to guide engineering practice.

Physical Simulation Experiment Study on Injection-Production Characteristics of Off-Surface Reservoirs in Sapu Oil Layer in Xing-6 Area

Based on the information of dense cored wells and by analyzing off-surface reservoir data, we determined the type of off-surface reservoirs in the area, the characteristics of off-surface reservoir microfacies (vertical), the planar distribution characteristics of off-surface reservoirs, the off-surface reservoir rocks, facies features, etc., so as to extract the matching pattern of the well pattern and the sand body. Based on the core analysis of the cored well area and the analysis of the matching relationship between injection and production between wells, five typical conventional off-surface and high-quality strip sand body injection and production modes between wells were extracted, and five corresponding core physical models and numerical models were constructed. Through physical simulation displacement experiment and numerical simulation displacement experiment analysis, the research shows that conventional off-surface reservoirs can be exploited separately and can be produced well, with a recovery rate of 40%; conventional off-surface reservoirs are combined with high-quality strip sand bodies (Lu et al., Journal of Xi’an Shiyou University (Natural Science Edition) , 2019, 34(4): 60–66 and Cheng et al., Oil and Gas Reservoir Evaluation and Development , 2019, 9(02): 56–59). Under mining conditions, there is serious interlayer interference; under the condition of tandem mining of conventional off-surface reservoirs and high-quality strip sand bodies, the production level of off-surface reservoirs can be effectively improved. Experiments show that the contribution rate of conventional off-surface reservoirs reaches 62% under the conditions of pin-out mining with high-quality sand bodies. The research results lay the foundation for further refined development and improvement of the economic and effective utilization of reserves.

Quantitative evaluation of hydrocarbon lateral diversion migration through the oil-source fault

As the hydrocarbon vertical migration pathway, fault and its relationship with reservoir extremely affect the distribution and accumulation of oil and gas in the non-hydrocarbon generating layers. On both sides of the fault, there are frequently various sand bodies with different oil and gas enrichment. However, previous researches on this differential charging are mostly based on empirical formulas or physical simulation experiments instead of fluid seepage mechanism. This paper proposed a quantitative evaluation method on the basis of Darcy's law to analyze the difference in hydrocarbon lateral diversion capacity through the oil-source fault. It can be seen that in the process of hydrocarbon migrate along the oil-source fault, hydrocarbons could divert laterally into sand bodies through the fault when there is a downward pressure gradient. Conversely, hydrocarbons can only migrate upward along the fault. Furthermore, among the four normal fault-sand body configuration types, the opposite fastigium type is most favorable for hydrocarbons laterally charging whereas the fastigium type is the least beneficial. The ratio of resistance in the sand body to in the fault was proposed as the evaluation index ( Fs) to quantitatively evaluate the hydrocarbon lateral diversion migration capacity, on the assumption that hydrocarbons in different sand bodies are sourced from the same source rocks. The index ( Fs) is directly proportional to the lateral diversion capacity of hydrocarbon, and is complex controlled by many geological factors including the reservoir permeability, fault-sand body contact length and dip angles of sand body and fault. And the higher the value of Fs, the stronger the hydrocarbons laterally divert ability of the sand body. In addition, the calculation result of well GG16102 in the Beidagang buried hill coincide well with the oil and gas interpretation result indicating that the method is practical to quantitatively evaluate the laterally diverting hydrocarbons of sand bodies under the condition of fault-sandstone configuration.

Research on Simulation Analysis of Physical Training Based on Deep Learning Algorithm

Aging is the trend of the global population in the 21st century. Physical degradation of the elderly and related care is a major challenge in the face of an aging society. Exercise can delay physiological aging and promote the metabolism of body functions. Although aging is an irreversible natural law, proper physical training can help prevent aging. Therefore, relevant personnel attach great importance to the training of physical fitness. To this end, a 12-week elderly functional fitness training experiment was conducted with elderly residents in a village in Nanjing. In the detection process, the gait analysis system is mainly used for the subject’s motion detection and recording and records the data into the gait analysis software system based on the improved deep learning algorithm for sports training simulation analysis. After completing the physical training simulation experiment, the RTM model is used for simulation analysis. The results were evaluated. The evaluation data show that the homogeneity test results of the designed physical training simulation experiment are very reasonable. Since the result is much larger than 0.10, it can be inferred that the results of the physical training simulation analysis have been expected and also meet the national GB/T 31054–2014 standard requirements.

Study and Field Trials on Dissolvable Frac Plugs for Slightly Deformed Casing Horizontal Well Volume Fracturing.

In view of the need for slightly deformed casing horizontal well fracturing, a new dissolvable frac plug has been designed using mechanical and material methods. The results of simulation analysis, laboratory research, and field test verification show that: (1) the integral half-split inlaid tooth slip and implicit-shaped single sealing element are adopted. Under the same pressure resistance index, the outer diameter and the total length of the frac plug are small, the inner diameter is big, and the pressure resistance downhole passing performance are better. (2) Using dissolvable magnesium-based powder, hydrogenated butadiene-acrylonitrile rubber, fluororubber polyglycolic acid, and the cellulose fiber and coating process, the solubility of the frac plug is observed to be more reliable. (3) Using the structural modular design, the frac plug has outstanding advantages in terms of on-site installation, preventing early seat sealing, self-separation after release, less dissolvable matter, and so on. (4) The finite element simulation and physical simulation experiments show that 121.36 mm ID casing with the frac plug can tolerate deformation in the range of 15 mm, which meets the needs for the casing usage. The temperature increases from 60 to 98 °C, and the maximum sustainable pressure differential decreases from 7.6 to 11.2 h at 70 MPa without any leakage. The frac plug can be completely dissolved within 300 h, and the final size of the residual solid is less than 0.3 mm. (5) In field trials of eight wells, through the minimum set of change inner diameter of 107 mm, the maximum wellhead fracturing pressure is 83.9 MPa in the formation, with the temperature ranging from 36 to 110 °C, and the plug can be completely self-dissolved in 16 days (the shortest time that has been found). After the fracturing operation with this dissolvable frac plug, the average daily oil production is maintained as high as 10.45 t with 22,000 m3 gas production. A new dissolvable frac plug tool solves the problem of continuous fracturing in slightly casing variable wells, restores the full bore of the wellbore after staged fracturing, and realizes the production of oil and gas wells without drilling plug and the smooth implementation of follow-up measures. It has a wider application range, higher efficiency, safety, and economy. It is of great significance for industrialized staged fracturing of unconventional oil and gas horizontal wells and platform wells.

Investigation on gas drainage effect under different borehole layout via 3D monitoring of gas pressure

Gas drainage using underground boreholes is an important method for the prevention of coal gas mine accidents. To study the influence of the increase in the number of boreholes on the gas drainage effect using a self-developed device for gas drainage, pressure sensors and branch boreholes were installed in a coal body. Four groups of physical simulation experiments of gas drainage were conducted. The results revealed that with an increase in the number of boreholes, the attenuation rate of the gas pressure is accelerated; the decrease in the gas pressure accelerates in the form of a natural logarithmic function. The effective drainage area, with characteristics of dynamic expansion and variations, has an approximately circular expansion with the borehole as the center. It gradually increases with the advancement of drainage. The relationship between the effective drainage area and the extraction time is in the form of a power function. With an increase in the number of boreholes, the effective drainage area increases more rapidly, and the regional outburst elimination occurs earlier. The effective stress and matrix shrinkage effect exhibit a coupled influence on the permeability, which causes the permeability evolution to exhibit periodic variations. At the early stage of drainage, the effective stress plays a major role, which reduces the permeability. At the middle and late stages of drainage, the matrix shrinkage effect plays a major role, which increases the permeability. With an increase in the number of boreholes, the attenuation rate and recovery amplitude of the permeability increase.

Increased wave load on the Gudong seawall caused by seabed scour

Seawalls play an important role in protecting coastal regions by resisting storm surge and waves. However, a seawall can cause seabed erosion at its foot by facilitating hydrodynamic scouring, but the effects of seabed scour on wave loads have not been sufficiently considered. This paper presents a case study of increased wave load on the Gudong seawall in the Huanghe delta, China, which has experienced serious erosion since its construction. Four commonly-used empirical formulas, which were also verified through our physical simulation experiments, were used to calculate the wave loads on the crown wall of the seawall based on observed wave conditions. The maximum allowable wave load on the seawall was also determined based on historical extreme sea conditions that caused severe damage to the seawall. Our calculations showed that, the wave load has increased at least two times since the seawall's construction due to seabed scour, and the present crown wall cannot resist large waves with a 5-year return period, much less than the 50-year return period it was designed to resist. This suggested that the reasonable design of seawalls needs to adequately consider seabed scour as well as the associated increase in of wave loads.

Cyclic In-Situ Combustion Process for Improved Heavy Oil Recovery after Cyclic Steam Stimulation

Summary Cyclic in-situ combustion (ISC) is a novel process with great potential for thermal enhanced oil recovery (EOR). In this study, a 3D physical simulation experiment of cyclic ISC after cyclic steam stimulation (CSS) was carried out for the first time. The mass loss during heavy oil oxidation was studied by thermogravimetry (TG) and the preheating temperature of sandpack was determined by differential scanning calorimeter (DSC). The oxidation process of heavy oil in a porous medium was investigated by a heavy oil static oxidation experiment. The development characteristics and EOR mechanism of cyclic ISC after CSS were studied through 3D physical simulation experiments and the characteristics of the coking zone was studied by scanning electron microscope (SEM) and computed tomography (CT). The results of the thermal analysis indicate that three different regions were observed with increasing temperature: low-temperature oxidation zone (LTO), fuel deposition zone (FD), and high-temperature oxidation zone (HTO). When the temperature reaches 480°C, the mixed oil sand has the most exothermic effect and the high-temperature oxidation reaction is the most vigorous. The results of the 3D physical simulation show that steam channeling and steam overlay in CSS reduced the swept volume of steam and heat usage rate. During the cyclic ISC, the oil bank can overcome the heterogeneity of the oil reservoir caused by steam channeling and steam overlay, which makes the combustion front move forward smoothly. Cyclic ISC can greatly increase the temperature of the zone near the well, and upgrade the crude oil through cracking to reduce the viscosity of heavy oil. The foaming oil formed by the dissolution of flue gas improves the fluidity of the crude oil. The oil recovery of CSS is 19.3%, and the oil recovery of cyclic ISC increased by 13.2%. SEM and CT show that flake black solid coke was attached to the surface of the sand at the coking zone. The coking zone is a porous medium structure with a porosity of 35.14%, which has little effect on the oil recovery in the process of cyclic ISC.