Probability Of Drilling Fluid Leakage Under Potential Leak - Leakage

Lost circulation, which happens frequently as oil and gas exploration and exploitation turn to deep complicated formations, is one of the most severe down-hole accidents during drilling and completion. Lost circulation not only hinders drilling safely and effectively by causing massive mud loss and inducing other down-hole accidents, but also causes the most severe formation damage, which hinders the precise evaluation of pay zones. Previous study showed that the "prevention first and combined prevention and curation" strategy should be adopted when dealing with lost circulation problems, and it’s necessary and important to locate the lost zones precisely before a successful lost circulation treatment.For the purpose of improving the efficiency of lost circulation treatment, the thesis chose fractured and cavernous carbonate formation in the12th district of Tahe oilfield as research objects, and studied lost circulation mechanism, comprehensive evaluation of lost circulation possibility, lost circulation pressure and lost circulation characteristics on the basis of rock mechanics, fracture mechanics by adopting the fault tree analysis method and genetic analytic hierarchy process as research tools. Finally, the following conclusions are achieved:Lost circulation mechanism and lost circulation reasons are clearly understood by analyzing characteristics of lost circulation in the12th district of Tahe oilfield. This district is a typical fractured and cavernous carbonate formation, and the main reasons for lost circulation are the fractures and caverns in the formation which are controlled by tectonic faults and the corrosion of underground water. The average total loss volume of typical lost wells are more than400m3, and the average loss rate during the first several hours after lost circulation varies from10m3/h to50m3/h. Lost circulation in this district are often accompany with travel empty and as a result, are classified as large fracture-cavern loss circulation type.The thesis discussed the strategy for lost circulation prevention and curation by adopting the fault tress analysis method. It’s concluded that the necessary and sufficient conditions for lost circulation are the combination of lost circulation space, lost circulation tunnel and lost circulation differential pressure by analyzing the geology and engineering reasons for lost circulation. Qualitative analysis of the lost circulation fault tree model showed that the only practicable ways to prevent or cure lost circulation are designing a proper well structure to seal lost zones and improving the ability to seal lost tunnels of drilling fluid.A comprehensive determination method for lost zone prediction was formed with the core method of the genetic analytic hierarchy process. The comprehensive evaluation factor system for potential lost zones was first constructed on the basis of lost circulation mechanism, and the arrangement weight of each factor was then analyzed by adopting the genetic analytic hierarchy process. Finally, the comprehensive value for potential lost zone determination was achieved, which the bigger the value is, the lager average lost rate and lost possibility of lost circulation of potential lost zone would be.The lost circulation model for fracture propagating loss was modified on the basis of fracture mechanism. The induced fracture loss is due to the formation being fractured, the fracture propagating loss is due to the reopen and propagating process of micro fracture, and the large scale fracture-cavern loss is due to drilling fluid flows into fracture and cavern spaces directly with differential pressure. The lost circulation pressure model for fracture propagating loss was modified on the basis of fracture mechanism, and the whole lost circulation model will be an important reference for defining a better safe mud window.The thesis drew an outline of the strategy for predicting potential lost zones. It’s important to comprehensively determine the potential lost zone with larger possibility of lost circulation rate by comprehensively consider the combination of geological and engineering data. It’s possible to determine the potential lost zone with basic fluid mechanics by monitoring the stand pip pressure change and the unusual drilling fluid return from annular space. Besides, it’s better to evaluate the characteristic of the lost circulation tunnel by well logging in order to make more accurate decision for lost circulation treatment.