Research On Wellbore Stability In Complex Formation Of MS Oilfield

With the acceleration of oil exploration and exploitation all over the world, the formation during drilling tends to be more and more complex, rather than conventional mudstone or sandstone, which leads to the wellbore collapse problem more prominently. Once the borehole is not stable, it may cause all kinds of complexity and accidents, which will seriously affect the drilling cycle, significantly increasing the cost, and even bringing adverse effect on later drilling construction. Therefore, to make sure wellbore stability is very crucial considering both manpower and resource.The formation in the studied area MS oilfield spans broadly, from top to bottom in turn drilling in sand shale, salt layer and carbonate formation. The lithology in Upper Fars group is sand mudstone where formation strength is weak, and borehole wall is prone to collapsing when mudstone is immersed in drilling fluid. Lower Fars group information is salt-gypsum rock with shale, where exist abnormal high pressure, and the salt stone creep slippage resulting in tight hole and sticking. Meanwhile, the salt dissolves resulting in borehole wall collapse, which adds the probability of sticking. Under the Lower Fars formation, the lithology is mainly carbonate rock. The fracture grows and obtains unconformity surface that easily cause loss. The formation lithology is so complicated that easily collapsed layer, high pressure layer and easily leakage layer instantaneously appear. So the borehole instability problem is outstanding, which leads to tough construction in the process of drilling.This thesis integrates logging, geologic, drilling and completion datum of the region researched, and summarizes a set of suitable methods for the evaluation on wellbore stability, through engaging in lots of research work, on the basis of domestic and foreign scholars, and combining with regional stratigraphic characteristics and difficulties of drilling engineering. This thesis has acquired main technical results as follows.(1) Firstly, this paper explains geological tectonic characteristics of the block, describes the lithology, counts the complexities in the drilling process, and analyzes the main causes of borehole instability. The main problems facing in the research area are tight pull, sticking and lost circulation.(2) Then this paper comprehensively analyses all kinds of pore pressure prediction methods, and establishes the suitable model of calculating formation pore pressure according to concrete formation conditions. Upper Fars and Lower Fars group formation adopt the Eaton method, while carbonate formation under Lower Fars group adopts the effective stress law, because carbonate attributes to chemical deposition without the characteristic of normal compaction law. Multiple pressure systems coexist in the region researched. In Upper Fars formation, pore pressure coefficient varies from0.928to1.154, belonging to normal pressure. In Lower Fars formation, pore pressure coefficient varies from0.942to2.304, so it exists abnormal high pressure in this group; Under Lower Fars formation, pore pressure coefficient varies from1.003to1.11, belonging to normal pressure.(3) Rock mechanics parameters have been achieved by conducting the uniaxial and triaxial strength tests, and using well logging datum and empiric model. The experiments show that the formation strength in salt-gypsum rock is very strong. Both in the salt rock and limestone formation, the cohesive force is very high, varying fro5.36～19.48MPa, and internal friction angle is also very big.(4) Because of abnormal fault developing in internal structure, the relative magnitude of earth stress can be assessed:σH> σv>σh. The single point ground stress is caculated by the experiment of Acoustical Reflection Kaiser effect. Then the peper calculates the tectonic stress coefficient through the Huang’s model:ω1=0.7332, ω2=0.4665. Finally, the continuous earth stress profiles are established by logging datum.(5) The thisis evaluates borehole stability from the angle of mechanics, analyses the circumferential stress distribution, determines the appropriate strength failure criterion, establishes the formation collapse pressure and fracture pressure prediction models and then calculates the variable rule with the well depth changes. Research results show that the collapse pressure profile calculated is consistent with the hole diameter changes, and the maximum error between measured and predicted formation fracture pressure is9.821%, which has high precision and satisfies the engineering requirement, which also demonstrates that predicted methods and results are of reliable. Finally, drilling fluid density is checked by combining with field drilling conditions and the hole enlargement coefficient, which can recommend reasonable drilling fluid density window to later construction work of the studied block, avoiding to cause drilling complexities and accidents, and improving the drilling time efficiency.