Abstract

Formation damage is a concern when attempting to control fluid loss during drilling operations. The usual approach is to use bridging agents and high polymer loadings to reduce fluid loss. However, these additives contribute to formation damage. In this paper we explore the fluid loss characteristics of xanthan-based fluids, including starch and calcium carbonate, during the drilling process. Fluid loss while drilling is a complex process where fluid is lost underneath the bit and through the surface of the wellbore. A unique laboratory scale drilling simulator was used to determine the leakoff and formation damage of xanthan-based drilling fluid formulations. The fluid was circulated as in a conventional drilling operation, through the microbit and up the annulus under overbalanced conditions. Thin section analysis and environmental SEM were performed on rock samples to identify the different components of the fluid system. Sandstones up to 1,000 md were used. Cleanup sequences, which include enzymes and oxidizers, were also evaluated after the cores were drilled. The cleanup efficiencies were compared to conventional QC-testing techniques used by operators for filtercake removal.

The results showed that most of the fluid is lost underneath the bit in a continuous spurt condition while drilling. The filtercake formed during drilling does not pose a resistance to flow during production, but poses a strong resistance during leakoff. Xanthan gum contributes to fluid loss reduction, while combinations of CaCO3, starch and xanthan gave the lowest leakoff and formation damage. Wellbore soaking procedures do improve production after drilling.

Introduction

The issue of formation damage has always been a concern when attempting to control fluid loss during drilling operations. The use of bridging agents and high polymer loadings to reduce fluid loss has been the common approach. However, these additives have the potential to contribute to formation damage.1 Xanthan gum has been used extensively as a viscosifier in drilling, drill-in and completion fluids because of its unique rheological properties.2 In this paper we explore the fluid loss characteristics of xanthan-based fluids, including starch and calcium carbonate during the drilling process.

Fluid loss while drilling is a complex process where a significant part of the fluid is lost underneath the bit under continuous spurt conditions while drilling.3 The spurt loss appears to occur where new surface area is being generated, i.e. where the rock is being crushed and removed. During the initial spurt loss, an internal filtercake is formed, which eventually leads to and external filtercake. The composition of the filterake is made up of bridging agents (drill fines, CaCO3, and starch) and viscosifying polymer (xanthan gum, HEC) that covers the porous wellbore. This filtercake is beneficial since it can significantly reduce the fluid loss rate preventing further damage to the wellbore. During the spurt phase, however, fluid enters the formation resulting in potential damage. The depth of the invasion and the reduction in permeability in the invaded zone will determine the skin and overall effect on production.1,4

A significant effort has been placed into removing the external filtercake by means of soaking the wellbore with breaker solutions intended for the bio-polymer and the bridging agents.5-9 The idea is to dissolve the filtercake to reduce potential damage during production.

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