Main Factors Of Lost Circulation Diagnosis Based On Fault Tree Analysis

Lost circulation is one of the most serious downhole complex accidents in the process of oil and gas drilling and completion.With the development of oil and gas exploration and development to deep complex strata,lost circulation occur frequently.As a result,it leads to a large number of working fluid loss and induced other downhole accidents,hinder the safe and efficient drilling.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 diagnose the main cause of lost circulation in a successful lost circulation treatment.The thesis introduces the fault tree analysis method for the purpose of diagnosing the main cause of lost circulation and improving the efficiency of lost circulation treatment,choses fractured formation in the KF and KL district as research objects,summarizes the characteristics of lost circulation in the district,establishes the fault tree model of lost circulation,diagnoses the main cause of lost circulation.According to the diagnosis results of lost circulation,to carry out indoor experiment of controlling lost circulation,optimize the plugging formula of drilling fluid,and successfully applied to the wells test in the work area.The main achievements and knowledge of the paper are as follows:The characteristics and mechanism of lost circulation are clearly studied,and the basic events leading to the occurrence of lost circulation are sorted out.The lost circulation characteristics of fractured reservoir drilling fluid in the work area are analyzed,and the lost circulation factors of drilling fluid are clarified according to the statistical data.Lost circulation is the result of the combined effect of geological factors and engineering factors.Lost circulation space,lost circulation tunnel and lost circulation pressure are the necessary conditions for the occurrence of lost circulation,and then the basic event leading to the occurrence of lost circulation is determined.This paper introduces some basic theoretical knowledge of the fault tree analysis method,including the basic composition symbol of the fault tree,the characteristics of the fault tree and the principle of action,the basic analysis procedure of the fault tree and the advantages and disadvantages of the fault tree analysis method in order to provide the basis for the establishment of tree model.The fault tree model of lost circulation is established,and the main reason of lost circulation is diagnosed in the basic event.The basic event of the lost circulation will be connected with the logic gate to establish the fault tree model.Based on the model qualitative analysis of the main causes of lost circulation through the minimum cut method and the minimum path set method,we get 30 minimum cut sets and 3 minimum diameter sets.On the basis of qualitative analysis,the structural importance,probability importance and critical importance of the basic events of lost circulation are determined,and the main reason of lost circulation is determined quantitatively.The results show that the formation of porosity,fracture development and formation pressure are the main causes of the lost circulation.The drilling speed and drilling fluid performance are the main engineering factors leading to the lost circulation.It is effective to prevent and control the lost circulation by determining the reasonable density of drilling fluid,optimizing the plugging material,optimizing the drilling fluid performance and improving the pressure capacity of the plugging layer.According to the main cause of the lost circulation in the work area,a plugging drilling fluid formula is formed.After the drilling fluid is modified,the pressure capacity of the 500?m crack specimen is increased to 10MPa,and the return rate is almost 72.9%,which is in accordance with the general production requirements.The modified drilling fluid is used in the field.Using drilling while plugging technology in the construction of strict not only controlling the drilling speed,greatly reducing the amount of lost circulation and drilling cycle,but also verifying the application of the tree analysis method for the main reason of the lost circulation in the feasibility and accuracy of diagnosis.