Establishment and Field Application of a Method for Evaluating Plugging Efficiency of a Drilling Fluid in Fractured Formations

The Ordovician system and the Chuanxi marine carbonate formation in Shunbei oil and gas field are developed with broken formations, and borehole wall collapse have been frequently encountered during drilling. To stabilize the borehole wall with the density of the drilling fluid, it is necessary for the drilling fluid to perform well in plugging the fractured formations. Methods presently in use for evaluating the plugging capacity of a drilling fluid use porous filtration media which cannot represent the fracture development characteristics of fractured formations. To simulate the filtration behavior of a fluid in a fractured formation, a plugging capacity evaluation method was developed using slotted filter papers, thin metal sheets with acid-itched microfractures as well as 3D printed disks with fractures of various sizes, and using the thin metal-sheets with acid-itched microfractures method, the high temperature polymer sulfonate drilling fluid (mixed with oil) used on the well SHB83CH was evaluated for its microfracture plugging capacity and was then optimized. The results of the evaluation show that, for the acid-itched metal sheets method, metal sheets with 30 μm microfractures can be batch produced, and thus has the advantage of significantly higher test precision over the 3D printed disk method (the width of the fractures has deviation >50%). The number of the fractures is positively exponentially correlated with the total volume of filter loss, the width of a fracture has a quadratic function relationship with the volume of spurt filter loss, and the total volume of filter loss has a negative quadratic function relationship with the width of a fracture. An increase in the width of a fracture results in a more significant increase in the volume of the spurt filter loss, while the total volume of the filter loss tends to remain constant. The volume of the spurt filter loss can be used to evaluate the rate of mud cake formation and the effect of the mud cake on plugging the microfractures, and the total volume of filter loss can be used to evaluate the compactness of the mud cake. Field application showed that the use of the optimized drilling fluid reduced the percent hole enlargement to less than 5%. The evaluation methods established in this study provide a reliable technical means for screening plugging materials for fractured formations.