Study On Application Of Pressure - Controlled Drilling Technology For Marine Stratigraphy In Northern

The role of marine strata in northern Sichuan by tectonic stress in different directions, results in uneven distribution of pressure systems and the pressure which change a lot in each direction. The highest pore pressure is130MPa. The information of drilling show that stratum contain hydrogen sulfide and the safe mud density window is narrow, the pressure window is about6-8MPa, where the circular annulus pressure loss is about5MPa. The conventional drilling technique results in downhole complex easily, with the feature of a large amount of leakage and dealing difficultly. The issuers of security well controlling are prominent. Owned by the state science and technology major projects (2011ZX05021-006), it is the part of the research content.According to the drilling data analysis, stratigraphy in Northern Sichuan contains hydrogen sulfide. The test shows that the content of hydrogen sulfide in well-A is11.60g/m3, which in well-B is11.10g/m3. The safe mud density window in marine strata is too narrow, which in well-A and well-C is about8MPa, well-B about7MPa. The circular annulus pressure loss is about5MPa. The window margin is around2-3MPa, which means leakage risks. Because the bottom hole pressure in deep well must be attached5MPa, the exceeding formation pressure window margin will lead to leakage risks, in accordance with well-control industries standard about natural gas well with high pressure sulfur-containing. Therefore, the conventional drilling technique results in downhole complex easily in marine strata northern Sichuan. The effective method is managed pressure controlling for the issuers of the narrow density window. By reducing the fluid density and regulating wellhead back pressure to adjust downhole pressure equivalent density at the range of safe mud density window in layer, downhole pressure can be balanced. In order to meet the demands for well controlling of high pressure sulfurous gas, wellhead back pressure should be adjusted during drilling. Combining improved model with adjacent real drilling data, designing the programme of managed pressure control for well D marine strata in northern Sichuan, the following conclusion can be drew:designed additional downhole pressure of drilling fluid density value is1～2MPa. Equipment operating under normal conditions, wellhead back pressure is controlled at3-4MPa after circulate stopped. When pump cycles, hydrostatic pressure of the drilling fluid and annular pressure loss act on the downhole at the same time, to meet the demands for well controlling of at least5MPa positive pressure difference; If there is a special case of out control equipment, downhole can always keep positive pressure difference, avoid hydrogen sulfide overflow, and meet the requirements of safely drilling.Well-D use the technology of MPD in marine strata northern Sichuan, and formation pressure window is about6MPa, Where the circular annulus pressure loss is about5MPa. Real drill fluid density is slightly below formation pressure coefficient, to control the wellhead back pressure within7MPa, keeping the downhole equivalent density of2.05～2.20g/cm3during entire drilling. No complex things happen in downhole. In normal situation, the technology of MPD can solve the issue of narrow density window effectively in marine strata northern Sichuan. However, if there is a special case of out control equipment, overflow will occur due to hydrostatic column pressure cannot balance formation pressure. Choosing the improved model of sulfurous gas MPD pressure controlling as the project design, the downhole is safer. It provides a new thinking for system of drilling later and makes popularization and development of MPD technology significant in marine strata northern Sichuan.