Study On Flat-rheology Synthetic-based Drilling Fluid For Deepwater Drilling

Deepwater synthetic-based drilling fluids are faced with technical difficulties such as lost circulation in narrow safe density window formations,emulsion instability in wide temperature range,and low temperature rheology control.Therefore,the development of deepwater oil and gas in China was restricted.This paper focuses on the technical difficulties of deepwater synthetic-based drilling fluid.The stability of W/O emulsions,and the significant influences of emulsifiers and organic clays under deepwater wellbore temperature were studied.The mechanism and method of flat-rheology control of deepwater synthetic-based drilling fluid were proposed.The organic clays and rheology modifier for deepwater synthetic-based drilling fluid were developed.The key treatment agents such as high-efficiency emulsifier,were selected,and the flat-rehology synthetic-based drilling fluid was optimized.Based on the analysis model of temperature field in deepwater wellbore and the newly developed calculation method of ECD of synthetic-based drilling fluids,the precise control of ECD of deepwater synthetic-based drilling fluid can be achieved.Field applications showed that the drilling fluid successfully solved the technical problem of serious lost circulation in narrow density window formations in deep water.Based on molecular dynamics simulation and experimental verification,the mechanism of the synthetic-based drilling fluid emulsifier was revealed.The emulsifying efficiency is related to the amount of adsorption and molecular morphology of emulsifiers at the oil-water interface,and the non-bond interaction.The relationship between the morphology and distribution of emulsion droplets,the adsorption and dispersion of solid particles in W/O emulsions and their interactions with the‘flat rheology'of W/O emulsions were revealed.It is found that the morphology,distribution and interaction force of the W/O emulsion droplets formed by the emulsifier is one of the key factors to achieve the‘flat rheology'performance of the synthetic-based drilling fluid.The emulsifier used in flat-rheology synthetic-based drilling fluid was optimized based on molecular dynamics simulation combined with experimental method.Based on the experimental analysis of the adsorption-desorption at high temperatures and dispersion characteristics of organic clays at low temperatures,a high temperature adsorption-desorption model of organic clays was established,and a new method for controlling the rheological properties of synthetic-based drilling fluids was proposed.The non-bond interaction between the emulsion droplets and the drilling fluid components at a low temperature can be weakened by using proper emulsifier and adjusting the adsorption capacity and morphology of organic clay at the oil-water interface.On this basis,the flat-rheology synthetic-based drilling fluid with uniform particle size and stable performance can be prepared using rheology modifier which can form an anti high temperature crosslinking structure.The structure of emulsifier is the basis of‘flat-rheology',and the temperature resistance of the organic clay determines the maximum temperature for‘flat-rheology'.Based on the theories of energy conservation,momentum conservation and heat conduction,a mathematical model for predicting the temperature field around wells is established.The temperature distributions in the wellbore at different depths from wellhead to the bottom in the string and annulus were obtained.PVT experiments were carried out with Grace M7500 high temperature rheometer to investigate the influences of temperature and pressure on the density of the synthetic-based drilling fluid.A mathematical model for predicting the static density of the drilling fluid in the wellbore was established.Based on a large number of high temperature and high pressure rheological experiments of drilling fluids,the effects of temperature and pressure on the rheological properties of drilling fluids are analyzed.A mathematical model for ECD calculation of flat-rheology synthetic-based drilling fluids was established finally.The flat-rheology synthetic-based drilling fluid for deepwater drilling has been developed.The drilling fluid has excellent electrical stability,low temperature rheological property,shale inhibition and hydrate inhibition properties.It can maintain stable yield point,gel strength and6r/min reading in the range of 2-150?,thus effectively reducing the ECD.This new drilling fluid has been successfully used in five ultra-deepwater wells in the South China Sea.The maximum water depth is 2619 m,which is the largest water depth of deepwater wells in Asian-Pacific region.The field measured?ECD is only 0.03-0.04 g/cm~3,which is better than that(0.06 g/cm~3)of the advanced flat-rheology synthetic-based drilling fluid of foreign companies.This drilling fluid solves the technical problems such as lost circulation caused by the difficulty in controlling the rheology of drilling fluids in large temperature range in deep water.What's more,this is the flat-rheology synthetic-based drilling fluid technology with own intellectual property rights.