Abstract

The role of filtrate rheological properties on drill in fluid invasion control is mentioned by some authors as alternatives to bridging agents. Extensional viscosities and normal stress differences are some of the rheological properties of a drill in fluid which can provide extra friction in the flow through porous media, thus preventing invasion. Most of these studies present either experimental single phase validation or only include a theoretical and numerical simulation approach.

The present paper intends to add the reservoir engineering's point of view to the design of non-invasive polymeric solids free drill in fluids. An innovative experimental study, including injection experiments and CT scanning evaluation (X-Ray/2D), was carried out to investigate the role of rheology from different polymeric solutions on the invasion of different rocks - fluid reservoir systems.

Tests included two different types of polymer (one viscous and one viscoelastic); two different levels of differential pressure (in order to induce shear and extension governed flows), two levels of resident oil viscosity and different rock surface conditions (hydrophilic and hydrophobic).

Experimental results were evaluated through sensitivity models describing fluid invasion. Relevant insights and guidelines arose from the analysis of invading polymeric fluid behavior against expected shear rates and fluid's flow rates into the porous media as the response of the formation's properties and imposed gradient pressure.

Calculated data and laboratory measured data were compared and discussed in order to identify dominant parameters on drilling fluid invasion's process. Invasion depth, contact time and saturation profiles were carefully analyzed. A comprehensive analysis, based on CT scan images, of the invasion flow patterns (plug or fingering) is detailed and results indicate that viscoelastic properties may prevent invasion in heavy oils.

Introduction

Drilling fluids need to control formation pressure and maintain well-bore stability, seal permeable formations, minimize reservoir damage and allow an adequate evaluation of the formation. Moreover, its properties needs satisfy performance requisites such as borehole cleaning, filtration and rate of penetration.

Designing a proper fluid for a specific situation demands understanding of the drill-in fluid properties, particularly rheology, and its performance under the operation's conditions. For instance, the viscosity of a drilling fluid must be maintained within certain limits: low-shear-rate viscosity needs to be high enough to transport cuttings out of the wellbore efficiently, while high-shear-rate viscosity needs to be as low as possible to maintain pump ability, remove cuttings from beneath the bit, and minimize equivalent circulating density of the mud (Growcock and Harvey, 2005).

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