Abstract

This article deals with the comprehension of the rheological mechanisms relevant to drill in fluid invasion through reservoir rocks, aiming the design of a solids free non-invasive fluid. The strategy to develop this concept was to flow different kinds of fluids (Newtonian and different polymeric solutions) through consolidated inert porous media. Experimental results indicate that Newtonian fluids, CMC solutions and low concentration XC and PHPA solutions were properly fit by Darcy law. On the other hand, the highly concentrated solution presented relevant deviations. Such deviations tended to increase with the increase of deformation rate (or differential pressure). This fact indicates that there is a need for applying viscoelastic constitutive models to account for the extra friction losses. An expression for resistive force estimation as function of the first normal stress difference correlated properly with the experimental results.

Introduction

Designing drill in fluids which can guarantee minimum invasion into the reservoir rock is a must for open hole completion wells. The industry has proposed several ideas to deal with the problem, most of them based on adding bridging agents to the fluid formulation. Such agents would block pores near the well bore and, consequently, prevent additional fluid to invade the rock.

Several authors (Xiao et al.1 Peng et al.2, Carlson et al.3, Longeron et al.4, Clark et al.5, Gallino et al.6, Audibert et al.7 and Navarrete et al.8) present relevant theoretical and experimental studies on the filtration properties of water based fluids. Those tests were run both in dynamic and static conditions for several types of fluid, pH, solids size, shape and concentration, pressure and shear stresses.

Lomba et al.9 introduced a discussion on additional mechanisms, besides bridging, which could minimize fluid invasion. Among them, there is a topic on how polymers of different rheological properties would behave while flowing through a non consolidated sand bed in a static filtration apparatus specially designed to evaluate invasion. The authors concluded that shear viscosity was not the only factor which governs invasion, since some less viscous fluids presented less invasive behavior, for the same conditions, than high viscosity fluids. The authors postulate several hypotheses on the role of the viscoelastic properties of such fluids on the invasion behavior.

The role of rheology on fluid invasion in the reservoir is still not clearly stated. Main points which arise are: to what nature of efforts the fluid is submitted when flowing through the porous media? Which shear rates characterize the well-reservoir boundary? Which rheological properties govern the invasion phenomenon? The main goal of this part of the study is to establish an experimental and theoretical program aiming the answer to these questions.

A different concept is proposed for controlling invasion in this article: designing a polymer based fluid which would generate extremely high friction losses when flowing through a porous medium without generating extra losses while flowing in the well. In this case the fluid would present proper flow and solids transport properties in the well and would not invade the rock formation.

Background

Consider a static filtration experiment, where a non Newtonian fluid, when submitted to a constant pressure differential, flows through a porous medium previously saturated with the same fluid (Fig. 1). Neglecting hydrostatic effects, the collected volume (V) along the time (t) can be predicted, according to Darcy's Law by the following expression:

  • Equation 1

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