An Integrated Method for Evaluating Lost Circulation Risk for Shale Gas Drilling in Sichuan Basin

Abstract

Accurate prediction of lost circulation fracture width and lost circulation pressure is the basis for preventing lost circulation and curing the losses. They are controlled by many factors, such as in-situ stress, wellbore fluctuating pressure, rock properties, natural fractures, and so on. Researchers have put forward a variety of calculation models of lost circulation pressure for fractured formations; however, in the field practice, it is still mainly based on the minimum horizontal principal stress. The lost circulation of fractured formation seriously jeopardizes the safety and progress of drilling and production. At present, the success rate of curing the losses is low, one of the important reasons is that the fracture width cannot be accurately predicted. It is difficult to determine the wellbore strengthening method and particle size of the lost circulation materials due to unknown fracture width. Various prediction models of fracture width are used to calculate the fracture width of the loss layer, and the influence of the factors such as the loss time, drilling fluid properties (viscosity and yield value), and the angle between the fracture and the wellbore on the calculated fracture width is studied. For the lost circulation pressure, the acceptable loss rate in the drilling process is given. On the premise of the calculated fracture width, the lost circulation pressure under the acceptable loss rate is determined. The results show that the values of fracture width and lost circulation pressure predicted by the three methods are close to each other, and the prediction results of the three methods can be averaged in the field application to provide reference for the following work. The fracture width is sensitive to drilling fluid viscosity, bottom hole pressure, and other parameters, so the value of these parameters should be accurately measured in the field to reduce the uncertainty. The results also show that the larger the angle between fracture and wellbore, the more serious the lost circulation will be.