Factors Affecting Loss of Kill Mud Used in Carbonate Rock Reservoirs

The carbonate gas reservoir in Changqing oilfield isfull of fractures and vugs, different zones have quite different characteristics, andsevere inhomogeneity exists in the formations. Fracturing stimulation only makes the fractures in the reservoir much more complex. In the late period of development, workover operation performed on a well killed with a kill mud has always been facing with high volume of mud losses and factors of the mud losses are not well defined. To find out the affecting factors of the mud losses, numerical simulation of 5 factors has been conducted using the Warren-Root dual-permeability model by analyzing the geological data and production data of the typical wells and by fitting the water production rate, the gas production rate and the average formation pressure. The results of the numerical simulation show that the most important factors affecting mud losses are the positive differential pressure of the fluid column inside the well, the viscosity of the kill mud and the permeability of the fractures, of which only the differential pressure and the permeability are controllable. By moderately increasing the viscosity of the kill mud and decreasing the positive differential pressure of the fluid column inside a well, mud losses can be well controlled.The problem is that the high viscosity will result in difficulties in pumping the kill mud and formation damage from adsorption and detention of the kill mud. Viscosity increase of a kill mud can be achieved by increasing its viscoelasticity; this not only increases the resistance to the flow of kill mud into formations, it also renders the kill mud pressure bearing capacity. An idea of using elastic gel as the kill mud with temporary plugging capacity in Changqing is presented in this paper based on the research achievements.