Near-wellbore processes in naturally fractured or weakly consolidated formations

Abstract

Drilling and completion in naturally fractured or weakly consolidated formations could be challenging. While drilling through naturally fractured reservoirs, severe lost circulation is a major contributor to the nonproductive time. It may also cause significant issues in subsequent well completion and reservoir production. Quantitative prediction of the drilling mud loss in a natural fracture is therefore critical for selecting prevention and remedial strategies to control lost circulation. Meanwhile, better understanding of how weakly consolidated formations respond to fluid injection is of fundamental importance to many engineering applications such as hydraulic fracturing, water flooding and drill cuttings reinjection. To be able to predict where the injected fluid goes and what kind of opening geometry is created in the subsurface is essential to the design and optimization of these engineering applications. Theoretical models are formulated in this work to determine the drilling mud loss in a natural fracture intersected by a wellbore. We first consider the transient response for two particular cases when the natural fracture is either parallel or perpendicular to the vertical wellbore axis. We adopt the constant height and the radial fracture geometry models. Complex rheology of the drilling mud and natural fracture deformability are both accounted for in these models. Next the ultimate drilling mud loss from an inclined wellbore intersected by an arbitrarily oriented natural fracture is modeled. The drilling mud invasion profile can be predicted based on the yield stress and the geometrical configurations of the wellbore and the natural fracture. The solution, validated by published experimental data, can also be used to estimate natural fracture permeability based on mud loss data in the field. A numerical strategy based on Discrete Element Method (DEM) is proposed to model the injection process of a high viscosity fluid into an unconsolidated or weakly consolidated formation. Novelty of this numerical study is in the modeling methodology for the fluid-grain interface. Fluid front advances only when the gap between two neighboring particles exceeds a threshold value, a criterion which could be considered as similar to taking into account of the effect of surface tension. Constant rate of fluid injection is modeled and parametric analysis is conducted on the critical opening size, the confining stress, the initial borehole size, the injection flow rate, the friction angle and the stress anisotropy. Numerical results on the borehole breakdown pressure are then compared with those from the bifurcation analysis for the cohesionless case and elasto-plasticity solutions for the cohesive case.