Study On Optimization Of Mud Entry Depth Of Deepwater Riser

Problems concern about deep-water conductor stability is the one key technical difficulty for deep-water oil & gas exploitation. As the first casing running into the shallow marine sediment, deep-water conductor will not only protect the wellbole surface but also play as a critical supporting component in the deep-water wellhead system. Properly choosing a setting length for deep-water conductor is the most attractive engineering problem during all the time in offshore oil industry because it could directly affect the well completion quality and the drilling cost. Unnecessary long setting depth of conductor would waste the steel material which leading to cost increase, however, if the conductor was setting too shallow in the formation, drilling fluid could fracture the shallow marine sediment during second drilling, or even more serious, causing mechanical safety problem in wellhead and affect the wellhead stability. All in all, preciously predicting the conductor setting depth have a significate impact and enormous reality worth for deep-water resource development in South China Sea.By widely searching the related documents in and out of our country, combined with the reality situation of deep-water drilling and completion in south china sea, emphasis was put on designing a conductor setting depth which could ensure the drilling fluid safely circulate during second drilling and fulfilling the couple mechanical balance of the horizontal & axial loading on the top of conductor at the same time. This thesis conduct the following main studys:(1) Shallow marine sediment mechanical property study. Based on the researching documents refer to shallow marine sediment mechanical property, studys about the parameters which could represent the mechanical characteristic of shallow marine sediment like elasticity modulus, poisson's ratio, bulk modulus, shear modulus and confined-vertical stress, together with the related calculation and testing methods was conducted. By utilizing the unit weight parameter, calculating the overburden stress distribution in one block of South China Sea, using the empirical correlation equations to calculate the formation horizontal stress distribution which could support the formation fracture stress prediction in the following chapter.(2) Complete the study of deep-water conductor minimum setting depth calculation method in the condition of safely circulation of drilling fluid. By building the shallow marine sediment wellbore mechanical elastic-plasticity mathematical analysis model and implementing the study of model sensibility, finding out that the radical stress raise to maximum value on the interface between elastic zone and plastic zone, and the tangential stress monotonous increasing from wellbore to further points in all formations. Relying on this discovery, integrating with super-pore pressure concept, calculating the vertical fracture pressure in elastic-plasticity zone, and based on the principal that the liquid pressure at bottom of wellbore must be less than the formation fracture pressure, apply this theory to analysis one block of south china sea using iteration method and diagrammatize method, the result indicate that the minimum setting depth is 35.6m when the wellbore pressure hit 15.4MPa and formation tension stress is 0.02MPa.(3)Conducting the deep-water conductor setting depth numerical simulation study. According to the princepal of riser mechanical balance and bending moment balance, building the fourth order differential equations for riser mechanical analysis, employ the finite-differential method successfully solving this equation. Using MATLAB analysis module, promote a numerical simulation study in one imaginative 1300 meters water depth block in south china sea, analysis the horizontal displacement and bending moment on the riser. Conducting a 2-dimensional and 3-dimensional numerical simulation study in ABAQUS analysis environment, reveal the Von Mises stress and shear stress distribution and loading features along the conductor and wellbore during the loading step, and also obtaining the judging rule of conductor ultimate bearing capacity by analysis couple loading upper boundary numerical simulation model.(4) Example study on one block of South China Sea. Utilizing deep-water conductor setting depth finite element analysis theory from this thesis, incorporate with the actual formation parameters and well structure parameters in one block of south china sea, calculating the wellhead loads and the suitable conductor length for four different wells. According to the soil mechanical features in this particular block, assuming the elasticity of soil is 10MPa and divided the formation into 13 layers to build the numerical simulation model for the soil, the simulation result indicate that the critical conductor setting depth is 36.0m. Obtaining the conductor bearing capacity curve by calculating a series of models in different conductor setting depth, using diagrammatize method analysis the finite element analysis result then achieving the conductor setting depth. Compared with all the factors, obtaining the final optimizing 24 inch and 36 inch deep-water conductor setting depth in South China Sea and also suggesting the most suitable setting depth should be 49.0m for this area.In summary, above study give a scientific evidence for deep-water conductor setting depth calculation method in South China Sea, and also providing meaningful suggestions for future South China Sea high-efficient, safe and stable deep-water resource development in some degree.