New Visco-Elastic Surfactant Formulations Extend Simultaneous Gravel-Packing and Cake-Cleanup Technique to Higher-Pressure and Higher-Temperature Horizontal Open-Hole Completions: Laboratory Development and a Field Case History From the North Sea.
Authors: Daniel S. , Morris L. , Chen Y. , Brady M. E. , Lungwitz B. R. , George L. , van Kranenburg A. , Ali S. A. , Twynam A. , Parlar M. 
Published: December 2007

Abstract
Filter-cake cleanup in open-hole gravel-pack completions has traditionally involved several stages: gravel-packing, pulling out of the hole with the work string, running in hole with production tubing, running in hole with coiled tubing, circulating the excess carrier fluid from inside the base pipe and spotting a breaker solution. Furthermore, in cases where removal of both the polymeric components and the CaCO3 bridging agents is necessary, an enzyme or an oxidizer soak has typically been followed by an acid treatment. This process is time consuming and costly, in addition to being non-optimal in terms of uniform cake-cleanup in long open-hole completions, due to rapid reaction between acid and CaCO3 particles. Recently proposed simultaneous gravel-packing and cake-cleanup method incorporating breakers into the gravel-pack carrier fluid have been demonstrated to be an efficient technique through more than a dozen field applications, as evidenced by higher productivity compared to offset wells completed with conventional techniques. Because of the inherent risk in simultaneous gravel-packing and cake-cleanup with water-packing technique, carrier fluids containing breakers for filtercake removal have been used in conjunction with shunt-packing, which has been demonstrated through both field applications and large-scale yard testing not to rely on either the filter-cake or the formation properties. However, because shunt technique relies on viscosity for gravel placement, application of the simultaneous gravel-packing and cake-cleanup technique with polymer-free visco-elastic-surfactant (VES) carrier fluids has been limited to wells requiring a density of about 9.8 ppg (for circulating packs) and a bottom hole circulating temperature less than 200°F.
In this paper, we present new surfactant formulations, which have unique chemistry that allows them to form visco-elastic surfactant solutions in high-density brines and remain stable at elevated temperatures. Densities up to 13.8 ppg with a rheological profile suitable for shunt-packing at bottom hole circulating temperatures exceeding 250°F are achievable with this new surfactant, covering a large majority of the gravel-pack applications. It is further demonstrated that this new surfactant system does not only allow the use of certain chemicals that can be used for CaCO3 dissolution in conjunction with a gravel-pack process, but also requires these chemicals as co-surfactants in order to develop a visco-elastic structure. In addition to discussion of the unique chemistry of this VES formulation, data pertaining to proposed gravel-pack applications are presented. These include rheology, gravel suspension properties, filtercake dissolution characteristics, and retained permeabilities. A field case history utilizing the new VES fluid incorporating filter-cake cleanup chemicals is also detailed.