The Leakage Mechanism Of Deviated Wells And Plugging Effect Evaluation Experiment

Loss circulation is one of the most serious downhole complex conditions that affect the safety of drilling operation.Because of its complexity,loss circulation,which has not been fully resolved,has become the major engineering problem of oil exploration and development at home and abroad.In drilling engineering,considering t he high risk of leakage problem,precaution comes first and the leak stoppage is supposed to be complementary.The key to prevent loss circulation is to understand the mechanism of it,and predict the leakage pressure.This paper analyzed the basic characteristics and mechanism of different leakages from their phenomena and conditions.Based on the different leakage mechanisms,well leakage can be divided into three types: the permeab le loss,large fracture cavity leakage and fracturing leakage.After comparing the relationship between the formation fracture pressure and the leakage pressure,the fracturing leakage was selected as the research object in this paper.A fracture initiation model of crack leakage loss was proposed,based on the wellbore stress distribution of surrounding rock in deviated wells,which followed the tensile fracture criterion and effective stress law.On the basis of the crack initiation model,considering the stress state of the micro cracks,the micro cracks link-up criterion was derived.Based on the Penny fracture model,the influential factors of fracture growth were obtained by taking into account the non-invaded zones in fractures.Above all,the leakage mechanism of deviated well and the corresponding stress condition at each stage of the leakage loss were concluded.The calculation model of leakage pressure was programmed using the Matlab software.This software can calculate the pressure at each stage of the leakage loss.In certain field applications,drilling with high density mud is necessary and two methods have been suggested to avoid leakage.One is to choose the appropriate drilling direction,which can make the pressure for fracture initiating and linking up lager.The other was using non-invaded mud during the drilling,which can make the crack expansion pressure increase.When the well leakage happens,a reasonable plugging plan to seal the leakage layer is required.The key to a successful lost circulation scheme is the blocking the leakage passage.A set of high temperature and high pressure dynamic core leak plugging tester was designed to simulate the real formation plugging cycle under different conditions.In order to test its practicability,this device has been used to evaluate the performance of drilling fluid practically used in an oil field.The core taken from the field with artificial fractures was the study object.After using the drilling fluid containing the plugging agent,the leakage of the core was sealed successfully.Then pierced the plug using base mud without plugging agent and recorded the test data.Finally,based on the test data,the model of to evaluate the relationships between the pressure bearing ability and its affecting factors,such as the fracture porosity and confining pressure,was established.Based the results,a recommended plugging program was proposed for the oil field in question.