When the drilling fluid is subjected to a filter press test, the volume of filtrate passing through the filter medium and the mud cake formed. The smaller the filtration loss of the drilling fluid, the easier it is to form a low-permeability, flexible, thin and dense filter cake, which is beneficial to stabilizing the wellbore and protecting the oil and gas layer.
Factors affecting the filtration loss are: (1) The filtration loss increases with time and is proportional to the square root of time. (2) When the filter medium is constant, the filtration loss will change in proportion to the square root of the pressure difference. (3) As the temperature rises, the filtrate viscosity decreases and the filtration loss increases. (4) If the mud cake is thin, tough and dense, the filtration loss will decrease. (5) If the clay particles in the drilling fluid are properly dispersed, it will be beneficial to reduce the filtration loss.
The filtration loss is divided into static filtration loss and dynamic filtration loss. Generally, only the static filtration loss is required. The static filtration loss is divided into API filtration loss and high temperature and high pressure (HTHP) filtration loss. API filtration is the filtration of drilling fluid at room temperature, 690kPa pressure difference, and 30 minutes. High temperature and high pressure filtration is the filtration of drilling fluid at 150℃, 3450kPa, and 30 minutes.
High temperature and high pressure conditions have a great influence on the performance of drilling fluid. It changes the dispersion state of clay in the drilling fluid, causes the treatment agent to degrade and cross-link, and also changes the viscosity of the liquid phase and the interaction between the liquid phase and the well wall, thereby affecting the stability of the drilling fluid performance and the stability of the well wall. Therefore, deep well drilling fluids are particularly emphasized to have a properly low high temperature and high pressure filtration, which is generally required to be between 10 and 20mL.