A Robust Model to Estimate the Mud Loss into Naturally Fractured Formations

Abstract

Drilling through naturally fractured formations results in multiple complications; fracture ballooning and loss circulation are predominant among them that requires more understanding of the process. In order to better understand the mechanism behind fluid loss into the natural fractures a new model has been developed which addresses the shortcomings of pre-existing models. The paper presents a new comprehensive analytical model; represented by partial differential equation that can be used for various fluid rheologies and thus, eliminating the need for different models for different fluids. The model also takes into consideration of fracture deformation with constant and variable leak off rates into the formation. The proposed model was validated by reducing it to the preexisting models by incorporating their assumptions into it. The model has also been validated using synthetic as well as multiple field data. Sensitivity analyses were performed to determine the effects of fracture normal stiffness, leak off rate, mud-yield stress, aperture and other parameters on fluid losses. The new model presented demonstrates that fracture deformation and leak off pressure play important roles in the determination of fluid loss into the formation. The results have shown that by neglecting the fracture deformation or leak off results in the under estimation of fluid losses. It has also been found that the fluid Yield stress plays an important role in preventing fluid loss into the formation. Sensitivity analysis shows that once the pressure potential falls below the yield stress value, the fluid migration into the fracture is stopped. The novelty of the proposed model lies in the fact that it can be used for different drilling fluids and also more generic. It takes into account fracture deformation and leak-off into the formation which lead to more accurate estimation of fluid losses than the existing models.

New Leakage Model for Naturally Fractured Formations Considering the Effects of Dual-System Hydro-Mechanical Coupling

“…Other parameters are shown in Table 1. When κ = 0 md, the calculation results were compared with Bychina's [20] finite difference method to verify the correctness of the new model. As shown in Figures 7-9, when κ = 0 md, the calculation results of the proposed model are consistent with those of Bychina's finite difference method, which verifies the reliability of the model.…”

“…Owing to the continuous pressure and mass distribution on the contact surface, the boundary conditions of the contact surface are given by Eqs. (19) and (20).…”

“…When drilling mud loss occurs in a naturally fractured formation, this can be a penny-shaped natural fracture with an infinitely large radius [11,18,20,21] (see Figure 2(a)). The initial flowing net pressure inside the fracture p N ′ causes fracture deformation and leakage and correspondingly produces the reaction pressure p S ′.…”

“…Other researchers have proposed an elastic equation to incorporate the impact of the fracture deformability, which relates the fracture width variation to the local pressure through the fracture normal stiffness [13][14][15][16][17][18][19]. To analyze fluid leak-off, Bychina conducted threshold processing for this leak-off process; that is, when the well-bottom differential pressure is greater than a fixed value, the fluid seeps into the formation at a certain leak-off velocity [20]. Razavi et al described this with the Cater model [21].…”

Technical system for mud loss analysis and diagnosis in drilling engineering to prevent reservoir damage

“…Bychina et al 27 presented a new comprehensive analytical model that can be used for various fluid rheologies and thus, eliminating the need for different models for different fluids. The model also takes into consideration fracture deformation with constant and variable leak‐off rates into the formation.…”