Research on and Application of Near-Oil-Based Drilling Fluid System

In recent years many studies have been conducted on near-oil-based drilling fluids to overcome the problems encountered in using oil-based drilling fluids such as high formulation cost and difficulties in addressing the oily cutting treatment problem. Based on the “near-oil-based” designing idea, a near-oil base fluid ZYBL was developed. ZYBL exhibits these features such as hydrophobicity through filming, water absorption through low water-activity reverse osmosis, ultra-strong inhibitive capacity and high lubricity etc. A near-oil-based drilling fluid was formulated with 20% ZYBL as the continuous phase and other additives of different functions such as rheology additives, filtration control agents, plugging agents, inhibitive agents and borehole wall strengthening agents. This near-oil-based drilling fluid has the working mechanism and properties that are similar to those of an oil-based drilling fluid, and is environmentally friendly. The density of this near-oil-based drilling fluid can be adjusted between 1.15 g/cm³ and 2.55 g/cm³. When the density of the drilling fluid is 1.15 g/cm³, the water activity is 0.651. This near-oil-based drilling fluid functions normally at temperatures up to 180 ℃ Laboratory experimental results show that the primary recovery rate of cuttings is 99.8%, the extreme-pressure coefficient of friction is 0.034, the mud cake adhesion coefficient is 0.0524, the API filtration rate is 0 mL, and the HTHP filtration rate is 6.6 mL. The near-oil-based drilling fluid exhibits good contamination resistance and reservoir protection capacity. An EC₅₀ value of 139,700 mg/L means that it has no bio-toxicity. This near-oil-based drilling fluid in several aspects, such as shale inhibition, lubricity and reservoir protection etc., is similar to an oil-based drilling fluid. The cost of formulating this near-oil-based drilling fluid is significantly lower than that of formulating an oil-based drilling fluid. Field application of this near-oil-based drilling fluid on 55 wells in Xinjiang, Chuanyu, Zhongyuan oilfields and in northeast China has proven its advantages in borehole wall stabilization, lubrication, pipe sticking prevention, drilling rate enhancement, bottomhole temperature reduction through circulation, and low overall cost. Near-oil-based drilling fluid represents the mainstream development trend of water-based drilling fluids at home and abroad, it can be used in tough working conditions such as high-temperature deep and ultra-deep wells, long horizontal wells for shale oil and gas, and horizontal wells penetrating highly water-sensitive mudstones; it also enables green, safe, economical and efficient drilling, accelerates the achievement of the “replacing oil with water” technical goal, and delivers remarkable economic and social benefits with broad prospects for popularization and application.