Abstract

Circulation loss is a major issue while drilling through naturally fractured reservoirs. Quantitative prediction of the drilling mud loss is therefore critically needed for the control of lost circulation. While it is usually assumed that steady-state rheology of a drilling mud can be described by a yield stress and power law shear thinning, it is not uncommon that a drilling mud exhibits Newtonian or even shear thickening behaviors in the high shear rate range (shear rate ?? > 1000 1/s). High shear rate rheology governs fluid flow behaviors at early time and is therefore critical to control of lost circulation. In this work, effect of rheology, in particular, the high shear rate rheology, on drilling mud loss in a single natural fracture is investigated. A piecewise rheological model incorporating a yield stress and power laws for both the low shear rate and high shear rate is employed. The natural fracture is assumed to be nearly perpendicular to the wellbore and is initially closed. The problem formulated is solved numerically using an explicit moving mesh algorithm. Effects of the rheological parameters and the leakoff coefficient on the mud loss behaviors are analyzed.

1. INTRODUCTION

Severe lost circulation is a common contributor to the nonproductive time in drilling, in particular in naturally fractured reservoirs. Lost circulation 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, real-time monitoring of the mud loss in overbalance drilling has commonly been employed as a diagnostic tool for formation characterization [1-3].

Various theoretical models for mud invasion in a single isolated natural fracture have been developed in the literature to solve for the natural fracture permeability based on the mud loss log data [e.g., 4-8]. The basic elements in these theoretical models include assumptions for the fracture orientation and geometry, the fracture deformation law, the leakoff behavior, whether the fracture length is finite or infinite, and whether flow along the fracture is transient or steady state. Drilling mud is generally assumed to be a single-phase fluid with non-Newtonian rheological characteristics such as yield stress and power law shear thinning, e.g., rheology of Herschel-Bulkley type [9]. Extension of these models to include the shut-in phase also allows investigation of various mud or formation fluid flow scenarios [10].

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