| Sichuan Basin is one of the major oil and gas producing areas in China.The exploration and development potential of the Sinian Dengying Formation carbonate reservoir in Gaomo block in the basin is huge,but the stratum is broken,pores,caves and fractures are developed,and there are leakage problems in different degrees in the drilling process,which seriously restricts the exploration and development of oil and gas resources.In this thesis,the core,thin section,physical properties,seismic and logging data and other data are used to systematically study the carbonate reservoir of Dengying Formation of Sinian in Gaomo area of Sichuan Basin.The results show that:(1)Dengying Formation in this area belongs to carbonate platform deposition,and pores,caves and fractures are developed in the reservoir section,and the pore types include intergranular dissolved pores,intergranular dissolved pores,intragranular dissolved pores and lattice pores.Karst caves can be seen in the stratum,mainly small and medium-sized caves,with few large caves.The middle fractures are developed,mainly including structural fractures,pressure-dissolved fractures and structural expansion-dissolved fractures,with a high degree of development.It belongs to the fracture-pore reservoir with low porosity and low permeability.(2)The leakage of reservoir sections in this area is complex,and the leakage of different sections is different,and some sections are serious.The leakage is mainly crack leakage,and it also has karst cave leakage.The main types of fracture leakage are natural fracture leakage and induced fracture leakage.(3)Through the research on the mechanism of lost circulation,it is found that the main mechanism of lost circulation is that there are many formations,uneven formation pressure,staggered distribution of high-pressure formations and low-pressure formations,and there are many sets of pressure systems.In the process of drilling,the wellbore pressure cannot be well controlled to keep a balance with the formation pressure.When drilling high-pressure formations,it is easy to fracture the low-pressure formations,resulting in the loss of lowpressure formations.At the same time,the formations are broken,and there are not only a lot of natural fractures in the formations.There are still a lot of artificially induced fractures,and the used drilling and completion fluid can’t meet the need of plugging,so it can’t plug the fractures successfully.Under the influence of high pressure difference,the fractures in the formation will open or communicate with each other,causing leakage and even aggravating the severity of leakage.(4)Combined with the leakage characteristics of the work area,using PFC2 D numerical simulation software,the wedge joint is selected as the research object,and the particle size of plugging material is optimized.(5)Through numerical simulation experiments,it is found that the plugging success rate increases with the increasing ratio of plugging particle size to crack opening.When the ratio of plugging particle size to crack opening is greater than 1/3,there is a great chance to form effective plugging.(6)The bridging methods of plugging material particles in cracks are mainly single-particle bridging(when the ratio of plugging particle size to opening degree of wedge-shaped cracks is greater than 2/3),double-particle bridging(when the ratio of plugging particle size to opening degree of wedge-shaped cracks is between 1/3 and 1/2),parallel double-particle bridging and multi-particle bridging(when the ratio of plugging particle size to opening degree of wedge-shaped cracks is less than 1/3).(7)For micro-cracks,the particle size of plugging material can be greater than or equal to the opening degree at the crack entrance;For cracks with large opening degree,the particle size of plugging material should be at least 1/3 of the opening degree at the crack entrance to ensure a high plugging success rate. |
