Effects of Oil-Based and Water-Based Drilling Fluids on Fracture Propagation Pressure

Lost circulation and wellbore instability have been encountered in drilling the Permian fractured formations in the sag west to the well Pen-1 in the Junggar Basin. To understand how oil-based/water-based drilling fluids affect the propagation pressure of a fracture, a comparative analysis was conducted on the effects of oil-based/water-based drilling fluids on the propagation pressure of the fracture. Physical simulation experiments on the propagation of core fractures were conducted through rock mechanical test, analysis of rock physical parameters as well as drilling fluid property test, and the law of the integrated action of the rock properties and drilling fluid properties on the propagation of fractures was systematically investigated. The conclusions of the experiments are as follows: ① fracturing the fractured cores with oil-based and water-based drilling fluids to observe the magnitude of the propagation pressure of the fractures, and it was found that the propagation pressure of the fractures measured in tests with oil-based drilling fluids is remarkably higher than that measured in tests with water-based drilling fluids. ② It was revealed that the drilling fluid viscosity is the primary factor controlling the propagation pressure of a fracture, and its importance exceeds the filtration property of the drilling fluid. High drilling fluid viscosity hinders the effective propagation of the pressure inside the fracture, hence significantly increasing the propagation pressure. ③ It was clarified that rock permeability is the prerequisite for the drilling fluid filtrates to form a “pressure drop zone” at the tip of a fracture and hence to promote the propagation. For oil-based drilling fluids, although having strong wetting capacity, their high viscosity and extremely low filtration rate work together to suppress the formation of the “pressure drop zone”, thereby masking the wettability potential. ④ Based on the experimental results, the contribution of each factor to fracture propagation pressure is quantified, the importance of these factors is listed as this: drilling fluid viscosity > rock permeability > filtration property > brittleness index > wettability > porosity. This research reveals the core mechanisms of oil-based and water-based drilling fluids in affecting the propagation pressure of a fracture, clarifies the key control factors and their relative importance, and provides an important theoretical support to the scientific selection of drilling fluid types and the formulation of control strategy for wellbore stability in safely drilling fractured formations.