Abstract:The South China Sea has been confirmed to be rich in oil and gas resources as well as natural gas hydrate resources. However， hydrate-bearing formations are often encountered during the drilling of oil and gas wells. Cementing is a critical step in oil and gas development. In deepwater drilling， the heat released during cement hydration can potentially induce hydrate decomposition， compromising formation stability and even affecting cementing quality. This study utilized numerical simulation methods， focusing on the hydrate-bearing formation at the SH7 site in the Shenhu area of the South China Sea GMGS-1 project. A numerical model for cementing was established to analyze the issues caused by cement slurry invasion into hydrate-bearing formations and the impact of cementing process parameters. The study found that an increase in cement hydration heat release rate significantly advanced the onset of gas and water influx， as well as increased its volume. The cementing pressure differential had a minor impact on the influx phenomenon， but it suppressed the influx when exceeding a certain threshold. Prolonging the pressure maintenance period significantly delayed the initiation of influx and reduced its volume. Therefore， it is recommended in practical engineering to use low-heat cement， extend the pressure maintenance period， and avoid excessively high cementing pressure differentials in the early stages to minimize hydrate decomposition and mitigate the occurrence of influx. This research provides a theoretical foundation for the cementing of hydrate-bearing formations， which is of great significance for enhancing the safety and efficiency of cementing operations.

Abstract:The packer expansion tube technology has been developed for the hydrogeological exploration of wall protection and the repair of damaged well pipes. It has the characteristics of low cost and simple process. In order to make the packer meet the requirements of the expandable tubular， the expansion mechanism of the packer is improved. The packer is used to test the pipes with different properties. According to the test results， 316L stainless steel is selected as the expandable tubular. In order to study the relationship between the expansion pressure and the change of the length and thickness of the expansion tube during the two expansions of the packer-type expansion tube， the expansion process of the expansion tube with ?219mm and wall thickness of 4mm was numerically simulated by Abaqus software to obtain the change of stress and strain during the expansion process， and field tests were conducted on the expansion tube with the same specifications. Then， by comparing the simulation results with the test results， it is concluded that the simulation results are basically the same as the actual demonstration results， indicating that the numerical simulation results can be used as a reference for the actual construction.

Abstract:Hydraulic fracturing technology plays a key role in the efficiency exploitation of low-permeability oil-gas and geothermal reservoirs. In order to study the extension pattern of hydraulic fractures within the hot dry rock， the effect of flow rate and viscosity of fracturing fluid and horizontal geo-stress difference on the morphology of hydraulic fractures were investigated by the cohesive zone method （CZM）， and the combination of the above fracturing process parameters was optimized using orthogonal tests. The results show that the flow rate of fracturing fluid has a significant effect on the length of the hydraulic fracture， whereas the viscosity of the fracturing fluid has a significant effect on the width of the hydraulic fracture. The increase in the flow rate and viscosity of the fracturing fluid promotes the emergence and extension of branching fractures. When the horizontal geo-stress difference is 1MPa， the model established in this paper can obtain the best fracturing modification effect under the conditions that the fracturing fluid flow rate is 0.004m3/s and the viscosity is 0.07Pa·s； As the flow rate and viscosity of fracturing fluid exceed 0.004m3/s and 0.07Pa·s respectively and continuously increase， a decrease in the length and width of the hydraulic fracture will occur. Therefore， it is considered that the continuous improvement of fracturing effect cannot be achieved blindly by increasing the flow rate and viscosity of the fracturing fluid during the actual fracturing process. The findings of this study are expected to provide significant support in predicting the fracture extension behavior and the optimizing the fracturing parameters during the exploitation of hot dry rock geothermal resources.

Abstract:The structural and performance characteristics of turbodrill are analyzed. In the high-temperature and high-pressure and highly abrasive strata of deep and extra-deep wells， turbodrill has basically become the only alternative downhole dynamic drilling tool to increase speed and reduce costs in composite drilling. In view of the exploration and development requirements of deep resources， the advantages and existing problems of the existing turbodrill core-extraction technology are described. In addition， numerical simulation technology is used to analyze and study the influence of drilling fluid flow state on the coring effect in the process of simultaneous coring with turbodrill. It is recognized that turbodrill can withstand high temperature and is suitable for full drilling and coring in deep and extra-deep wells， but its high rotational speed is also easy to cause core wear and rock column spiral in the process of coring drilling. Improving the high-speed single motion performance of the core-drilling tool， lengthening the spiral stabilizer of the turbodrill and using the reverse spiral spoiler are beneficial to reduce the vortex motion at the bottom of the hole and the axial velocity difference between the inner and outer tubes of the core， so as to improve the core quality of the core-drilling tool.

Abstract:In response to the frozen issues in subway tunnel constuction in Tianjin area where complex hydrogeological conditions exist such as water-rich sand strata， the laboratory model experiments and numerical simulations was combined to investigate the effects of cold media tempreture and seepage velocity on the frozen temperature field， closure time， and growth of frozen walls in sandy soil. The research findings indicate that as the cold media tempreture decreases， the temperature gradient near the frozen pipe becomes steeper， and the frozen wall becomes thicker. In the presence of seepage， the seepage water transports cold energy from the upstream to the downstream， which impedes the development of the frozen wall to the upstream direction while promotes it to some extent to the downstream direction， which leads to the uneven thickness of the frozen wall. Ultimately， the shape of the frozen wall changes from circular in still water to heart-shape. In practical engineering， measures such as grouting to reduce the seepage velocity and lower cold media temperature can be used to cut the clolure time and finally ensure a smooth frozen process.

Abstract:Hot dry rock （HDR） is a clean and renewable energy resource developed mainly through enhanced geothermal systems （EGS）. In EGS engineering， geothermal drilling technology is required for the construction of both injection wells and production wells， and the formation fracture and wellbore collapse during high temperature and high pressure drilling are important problems in hot dry rock drilling construction. Under the action of temperature difference， the temperature stress is generated between mineral particles due to the difference of thermal expansion and cold shrinkage characteristics of mineral particles of rock， which results in thermal fracture of rock mass. In this paper， with the help of RFPA numerical simulation software， the fracture propagation of granite in wellbore under the action of cold shock during hot dry rock mining is studied. The results show that during the cold shock process of the wellbore model， as the increase of the cold shock time， the tensile stress on the rock surface first increases to a peak value and then decreases slowly. The fracture growth can be roughly divided into early， middle and late stages. In the early stage， an annular tensile stress zone appears around the wellbore and uniform micro-cracks begin to appear. In the middle stage， with the increase of time， the tensile stress zone gradually spreads to the periphery of the wellbore and the fracture spreads outward beyond the tensile stress zone. In the late stage， the tensile stress gradually decreases to less than the tensile strength of the model， and the fracture propagation slows down until it stops. The confining pressure， well diameter and temperature have a significant impact on the damage effect of the surrounding rock under cold shock. Among them， the temperature and well diameter promotes the growth of cold shock fracture while the confining pressure inhibits it.

Abstract:Taking a foundation pit project in Wuhan as the background， based on two observation indicators of water level drop inside and outside the confined layer pit and surface settlement outside the pit， a three-dimensional numerical calculation model for group well precipitation was established using finite element software ABAQUS for numerical simulation analysis. By setting multiple calculation conditions， under the conditions of different insertion ratios of suspended waterproof curtains λ， the influence of different well screen lengths L and filter tube burial depth H on the water level drop inside and outside the confined layer pit and surface settlement outside the pit was studied. The research results indicate that the impact of the insertion ratio λ on the water leverl drops inside and outside the confined layer pit and the surface settlement outside the pit is much greater than that of the length L and burial depth H of the well screen in the dewatering well. When the λ is between 0.4 and 0.6， there is a large space to increase the water level drop inside the confined layer pit or decrease the water level drop outside the confined layer pit by changing the length L and the burial depth H. When λ exceeds 0.6， it has a good water blocking effect. The buried depth H of the well screen should be located at the top of the confined aquifer， and the length of the well screen L should not exceed 0.4 times the thickness of the confined aquifer M.

Abstract:The dip angle in the red bed of eastern Sichuan is generally relatively small， a special form of sliding-buckling deformation appears on the gently inclined and consequent slope. Compared to bedding-sliding， its failure、 stress and deformation are more complicated. In this paper， based on the analysis of the deformation mode of Xinglang slope， the damage process and micromechanical analysis are carried out based on PFC2D software. The results revealed that： due to the topographic features of steep at the top and slow at the bottom which affected by the wide and gentle folding in east Sichuan， the main reason why the sliding-buckling happens is the stress concentration caused by the gentle rock stratum and old landslide accumulation. The deformation damage mode is analyzed into three stages： sliding-bending bulging， strong bending-bulging and sliding-buckling. The displacement of the model damage nephogram shows that the rock body faults and shear characteristics due to stress concentration exist at the location of monitoring point 2 in the middle of the landslide， which is consistent with the actual investigation. The central buckling of the landslide shoed compressive stress concentration before sliding， when the stress concentrated to certern degree， the rock mass sheared along the bedding or the break zone， by which the deformation pattern of sliding-bending-buckling formed. The research has referrencical significance for the stability evaluation of other similar slope.

Abstract:Natural gas hydrate is an efficient and clean energy， which is widely distributed in the sedimentary strata in the South China Sea. China has successfully carried out two trial productions in 2017 and 2020 respectively. However， due to the special occurrence conditions of marine natural gas hydrate， there are still some problems with single well trial production， such as small production range and short time for high and stable production. In order to improve the exploitation range of hydrate， the two-dimensional hydraulic fracturing numerical model is studied based on cohesive units. Through simulation， the half length and width of cracks of the two models 100m×100m and 20m×20m are compared. It is concluded that at the injection pressure of 30MPa， the half length of the crack is 6m for both models， and the maximum width is 5.8mm and 5.5mm respectively. The more accurate experimental results can be obtained by constructing a larger model. Moreover， the variation law of fracture width with injection time is studied. With the continuous increase of injection pressure and injection volume， the initial fracture width increases rapidly， and then the fracture propagates “step by step” under the action of in-situ stress and injection fluid pressure. The research has been successfully applied in the hydraulic fracturing model analysis of unconventional energy reservoirs such as shale gas， coalbed methane， and provides some technical guidance for marine natural gas hydrate reservoir hydraulic fracturing .

Abstract:The depressurization method is a common approach for the exploitation of natural gas hydrate reservoirs in marine areas. This method can induce complex multi-physical coupling responses in the near-wellbore reservoir， leading to pressure changes， temperature variations， hydrate decomposition， deterioration of reservoir mechanical properties， and formation subsidence. This study utilizes a fully-coupled hydro-thermo-mechanical numerical model to analyze the mechanical property deterioration and subsidence characteristics of marine natural gas hydrate reservoirs caused by depressurization in horizontal wellbores， characterizing the multi-field coupling response laws of the horizontal wellbore and surrounding reservoir， and identifying the influencing factors of reservoir mechanical property deterioration and subsidence. The simulation results indicate that the affected area of reservoir pressure and temperature changes is much larger than the decomposition front of hydrates， and the distribution of effective normal stress varies significantly in different directions. The deterioration area of cohesion induced by depressurization is highly correlated with the plastic zone and hydrate decomposition zone. The subsidence characteristics in the shallow and deep areas relative to the horizontal wellbore exhibites distinct features. The simulation results provide a reference for the stability analysis of marine natural gas hydrate reservoirs during depressurization exploitation through horizontal wellbores.