Research And Application Of Water-based Drilling Fluid System For Horizontal Well In Yanchang Oilfield

Horizontal well technology is an important means for the development of new oilfields,the deep development of old oilfields,and the improvement of ultimate recovery of difficult-to-use reservoirs.The quality of drilling fluid system is the key to the success of horizontal well drilling.In order to solve the problems of low well production,high cost,and rapid production decline,the horizontal well technology have been carried out in Yanchang Oilfield.The failure of drilling fluid technology seriously hindered the development of horizontal well technology.Based on a large number of literature,this paper summarizes the research status and progress of horizontal well drilling fluid technology at home and abroad.Starting from the geological characteristics of horizontal wells in Yanchang Oilfield,the mechanism of wellbore instability in horizontal wells is summarized,and the technical requirements and countermeasures of horizontal well drilling fluid are put forward.Through laboratory experiments,the drilling fluid formula for horizontal wells in Yanchang Oilfield is determined.The main achievements of this paper are as follows:(1)Yanchang Oilfield is a typical low and ultra-low permeability reservoir with shallow burial depth and low temperature,quartz content in rocks ranges from 17.6% to 72.3%,and dolomite content is relatively small.Illite is the main clay mineral,accounting for 37.9%~87.8%,and the density of rock is between 2.48 ~ 2.67g/cm3.(2)Reservoir rocks have high compressive strength and strong elastic deformation;the friction angle between bedding plane and shale block is basically the same,but the cohesion of bedding plane is much smaller than that of shale block;indentation hardness of the core is between 182.38 and 341.11 MPa.(3)The mechanism of shale wellbore instability in Yanchang Oilfield is the result of mechanical factors,physical and chemical factors,drilling mechanical disturbance and its comprehensive efforts.The basic formula of water-based drilling fluid is established,that is,the first section is bentonite drilling fluid;the second vertical section is solid-free polymer drilling fluid;the second inclined section is low solid and strongly inhibited polyjuan drilling fluid,and the second horizontal section is low damage and low friction drilling fluid.(4)Field application of water-based drilling fluid in horizontal wells shows that the depth,length of horizontal section and drilling cycle of horizontal wells have been improved,and the construction quality has been improved significantly.