Abstract: A mud loss control fluid was formulated to control mud loss under pressure during drilling the fractured Ordovician reservoirs in the Block Tahe 10, where mud losses prevailed in the past drilling operations. This mud loss control fluid was formulated with optimized sized acid soluble materials. In determining the sizing of the particles of the lost circulation materials (LCM), the amount of mud losses was used as the main evaluation index based on the sizes of the fractures found in the formations of the reservoir, and the response surface methodology (RSM) was used for optimized design of the sizing of the particles. Laboratory evaluation results showed that the particle size distribution has a continuous and wide-spanned distribution curve. Fractures of any sizes can be plugged with enough particles of matching sizes. Three mud loss control (under pressure) fluids for different sizes of fractures were formulated using the particle sizing technique. These fluids performed very well in controlling mud losses and pressure bearing capacity was greater than 8 MPa. The volume of mud loss through 3 mm fractures was less than 200 mL, and the volume of mud loss through 2 mm and 1 mm fractures was less than 50 mL. More than 99% of the LCM was dissolved by acids, while 98.4% of the particles in the layers of LCM formed on the surfaces of the borehole wall can be acid dissolved.