Application Of Low-density Oil-based Drilling Fluids In Low-pressure Reservoirs In Towers

Based on the difficulties of reservoir protection in Tazhong 11 well area drilling, such as strong water block effect and low formation pressure coefficient of Silurian sandstone, by researching the drilling fluid system applied for low formation pressure reservoir protection in home and abroad it come to realize that using low-density full-oil-based drilling fluid is an economical and feasible way to protect reservoir. By single agent synthesis and filtering, low-density full-oil-based drilling fluid system is constructed. The drilling fluid system take diesel as base fluid and the added agent is produced in domestic. The density of the drilling fluid system can be as low as 0.90 g/cm3, its temperature tolerance can be as high as 120?150?, its cuttings-water contamination resistance can be 10?20%, its cuttings roller recovery can be higher than 95%, and core permeability recovery value can be higher than 90%. In all, the system can afford good results.The practical application situation for one vertical well and one horizontal well in Tazhong 11 well area shows that this low-density full-oil-based drilling fluid have some features such as stable performance, simple maintenance, high ROP and so on. All the performance of the system can meet the demand for drilling fluid planning and reservoir protection. Oil-based drilling fluid can be recycled and reused, so it brings lower drilling cost and less environmental contamination. The drilling fluid system have good reservoir protection effects as below:skin factor in well completing test is lower than 0; static core permeability recovery value is higher than 90%; dynamic core permeability recovery value is higher than 75%; water loss in 120? is less than 8 ml and both two wells have higher production in well completing test than that of neighboring well. Its mechanism for enhancing the ROP and protecting reservoir includes several aspects as below:First, its low density makes sure slightly underbalanced drilling and balanced drilling, can reduce the chip-hold down effect and bottom hole pressure difference to enhance ROP, at the same time the invaded formation solid quantity from drilling fluid can be reduced. Second, higher ROP reduces the soaking time for wellbore, the drilling fluid system has lower HTHP filtration rate and both these can decrease pollution radius. Last, essentially speaking diesel as the filtration can avoid water blocking damage.