Hole Stability And Drilling Fluid Optimization Technical Research For BZ13-1 Oilfield

Shortage of petroleum resources has been a main problem in the world because the geologic reserve of oil and gas reduces fastly.The onshore area which has never been explored shrinks,while the estimated offshore reserve of oil and gas is considerable since the oceans occupy 70%of the total area of the Earth.Recently,the focus of the oil and gas exploitation in the Bohai Oilfield shifts from the Neocene to the Eogene and thus the well depth increases gradually because the offshore production has lasted for many years.The well problem and complications,such as the small safety window of mud weight,lost circulation, well kick,stuck pipe and wellbore instability have been experienced in the formations of the Eogene.The Bozhong(BZ)13-1 Oilfield started to be exploited from the year 2009 in order to realize the total oil and gas production of 30 million cubic meters per year in Bohai Oilfields of the China National Offshore Oilfield Corporation(CNOOC).The early drilling operations experienced some downhole complications such as severe lost circulation,well kick,gas invasion,stuck pipe and wellbore instability and therefore the drilling rate was very low.The lost time for solving these complications in 3 exploration wells reached 21.56%of the total drilling time.The BZ13-1 Oilfield is one of the most geologically complicated oilfields in the Bohai Bay and has a very small safety window of mud weight.Therefore,the proper drilling fluid systems are badly required to combat these complications to ensure safe and smooth drilling operations.In order to understand the mechanism of lost circulation,well kick and caving,it is necessary to do some research into the formation lithology and find out the pore pressure,crustal stress and formation intensity and then draw a profile of the pore pressure, lost circulation pressure and caving pressure.These data can be used for the optimization of the casing procedure of a well to reduce the drilling risk and enhance the drilling rate.The research into the mechanism of the wellbore stability and the optimization of the drilling fluid systems not only ensures the smooth exploitation of the BZ13-1 Oilfield,but also helps to accelerate the development of the domestic lost circulation technology and can be worked as a reference to the exploitation of the similar oilfields in Bohai Bay.The research purpose is to study in stability of borehole wall for BZ13-1 oilfield, optimizing drilling fluid system for reservoir unsteady formation of BZ13-1 oilfield to increase formation pressure bearing ability.And we can use larger range mud density because of formation pressure bearing ability increasing.Ensure BZ13-1 oilfild development to be successful completion.