Abstract

A novel reservoir drill-in fluid (RDF) has been developed that utilizes viscoelastic surfactants (VES) as the primary viscosifier and as a consequence, eliminates the need to use biopolymers for viscosity. Biopolymers have been the standard RDF viscosifier for a long time. They provide stable rheological properties required for drilling and completing the reservoir section, but they have been difficult to remove as very few chemical breakers can effectively destroy the polymer chain. The VES-based viscosifier can be easily removed by a variety of non-aqueous materials, including produced hydrocarbons making this new RDF a viable alternative for a variety of openhole completion projects, particularly low-temperature and injector applications.

This new VES-based RDF system not only suspends soluble bridging material, but supplies the rheological properties necessary to transport cuttings out from the wellbore. Fluid-loss control is achieved with a novel polysaccharide starch that is compatible with the surfactant used in the system. With the addition of this particular starch, the VES-based RDF exhibits similar characteristics to a biopolymer-based RDF which also utilizes polysaccharides and soluble bridging agents, and is stable enough to withstand conventional water-based RDF contaminants.

This paper will detail the laboratory phase involved in the development of this new system. The testing confirmed that the properties normally associated with conventional biopolymer-based RDF systems are also achieved with this innovative VES RDF system. Additional testing revealed solids contamination effects and the effect that the addition of lubricants had on this unique system. Further investigation will also show the effect of a water-based filter cake breaker system currently being used in completion operations. Results of return permeability testing will also be discussed in this paper.

The development of this new RDF system showed that VES technology currently being applied in other aspects in oil and gas completion operations can now be applied in drilling the reservoir, and that this unique system can achieve the same functionality as conventional water-based RDF without the use of biopolymers.

Introduction

Biopolymers have been used extensively in reservoir drill-in fluids (RDF) to drill the reservoir sections in oil and gas wells. Either xanthan gum or scleroglucan have been used successfully in the majority of water-based RDFs to help transport drill cuttings out of the hole and to suspend solid material in the fluid system when velocities are low or when circulation comes to a halt (Zamora et al. 1993; Powell et al. 1991). Along with a starch and a soluble bridging material, the biopolymer becomes part of a filter cake whose function is to prevent invasion into a permeable formation.

In several parts of the oil and gas industry there is the concern about the invasion or leak off of biopolymers into the reservoir causing impairment to production or injection (Audibert et al. 1999; Krilov et al. 1996; Dalmazzone et al. 2004). Several professional papers mention that this impairment is a result of the polymer molecule settling or being adsorbed on or into the reservoir rock pore spaces.

It has also been argued that any formation damage caused by biopolymers can be mitigated with clean-up treatment or that the concentration of biopolymers in the drill-in fluids is small especially when used with the starch and bridging agents (Navarrete et al. 2000).

The debate on the impact of biopolymers on reservoir impairment made it clear that an alternative viscosifier should be examined as a replacement for biopolymers in some RDFs.

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