Drilling Mud Loss in Naturally Fractured Reservoirs: Theoretical Modelling and Field Data Analysis

摘要

Real-time analysis of drilling mud loss data is an effective tool to characterize the size, distribution and permeability of natural fractures. The estimated properties of natural fractures can then be used to modify the drilling fluid properties and design remedial mud loss treatment to stop further drilling fluid loss. Moreover, characterization of natural fracture provides crucial information for the design and application of reservoir stimulation operations. In this paper, we present a theoretical model to simulate the invasion of the drilling fluid into natural fractures. Mathematical modelling was performed for two types of rheology constitutive models which are commonly used for drilling fluids, namely Bingham Plastic and Herschel-Bulkley. The fluid leak-off into the formation is modelled using Carter's model. Moreover, a simplified local elasticity equation is used for the fracture width variation due to fluid pressure. A forward finite-difference numerical scheme is used to solve the pressure drop equation. The developed model is then validated against well-known analytical solutions and numerical simulations. Parametric studies were conducted to evaluate the impact of the fracture deformability, formation leak-off properties, drilling fluid rheology, and the pore fluid contribution. Finally, the output of the developed model is compared with the field data of a welldocumented mud loss incident. The obtained results indicate that the incorporating the effect of the fluid leak-off may significantly increase the rate of mud loss volume. Therefore, for highly permeable formations (such as carbonates), considering the impact of the fluid leak-off effect seems necessary for a realistic characterization of the mud loss problem. Fracture deformability affects the mud loss volume, with higher deformability leading to a larger mud loss volume. Also, the numerical results confirmed that the rate and magnitude of mud loss volume is significantly affected by the rheological properties of the drilling fluid, in particular the yield stress. In contrast, the effect of pore fluid seems relatively negligible for typical formation fluid types. Comparing the results of our simulation with mud loss field data shows that the developed model can accurately predict the rate and total volume of mud loss into the fractures. Moreover, it shows that incorporating the impact of the fluid leak-off and the fracture stiffness may significantly affect the fracture width prediction. Therefore, these factors should be considered to obtain an accurate estimate of the natural fracture aperture.