Visco-Elastic Surfactant (VES) fluids are polymer-free fluids that generate viscosities suitable for fracturing operations without the use of polymer additives. VES fluids do not form polymer filter-cake, and thus, viscous resistance of the fluid flowing through the rock matrix primarily governs fluid loss. This has historically limited the application to fracturing reservoirs with low permeabilities. A new VES fracturing fluid has been developed for use in high permeability reservoirs and successfully pumped in the Gulf of Mexico. The fluid exhibits enhanced fluid efficiency while still maintaining the high proppant pack conductivity associated with the lack of polymer damage. In this paper, laboratory test results for the new fluid are presented along with three high-permeability case histories. The estimated reservoir permeabilities were as high as 167 mD and reservoir heights ranged from 30 -90 feet. In all cases the entire propped fracture design was successfully placed. Introduction Visco-elastic surfactants have been used in oil and gas wells for fracturing stimulation for over ten years1. During this time the technology has evolved from a niche application in gravel packing2 to a mainstream range of applications where clean proppant packs and gravel packs are desired3. In the latest form presented here, a VES fluid utilizing a new surfactant has been formulated and optimized for high permeability operations. Fluid Rheology Viscosity of a VES fluid is created by self-assembly of surfactant molecules in an aqueous solution. Hydrophobic tails of surfactants associate and orient to create rod shaped structures commonly referred to as micelles. Entanglement of these flexible micelles imparts viscosity to the solution as shown in Figure 1. A variety of surfactant types can be used for formulating VES fluids, including anionic surfactants, cationic surfactants, and zwitterionic surfactants4. The new VES fluid presented here utilizes a specially formulated zwitterionic surfactant that creates stable micelles with unusual high-temperature stability. Figure 2 presents the fluid rheology as a function of temperature for different levels of surfactant package concentration. The surfactant package creates useful rheology within concentrations ranging from 3.5% to 6%. The exact surfactant concentration depends on the bottom hole temperature and desired fluid viscosity. Fluid Breaker The VES fluid can break to water-like viscosity by exposure to liquid hydrocarbons or dilution with reservoir brines. Additionally, a new encapsulated breaker has been developed for the zwitterionic VES described here. The encapsulated material uses a polyelectrolyte to disrupt the surfactant micelles and lower the fluid viscosity even in dry gas wells where there is neither brine nor liquid hydrocarbon to assist the breaking process. The effectiveness of this breaker has been measured in a series of proppant pack conductivity tests demonstrating greater than or equal to 95% retained proppant pack permeability. Laboratory Fluid Loss Measurements To model the fluid loss properties, the VES system was injected into 12-inch long sandstone cores at a constant driving pressure of 1000 psi. The cumulative fluid volume flowing into the core was measured as a function of time and the total fluid loss coefficient vs. permeabilities was plotted in Figure 3. Also shown in Figure 3 is the typical trend for total fluid loss coefficient of crosslinked polymer fluids. The results indicate that the fluid loss coefficient for the VES system is comparable to that achieved with crosslinked polymers.

Summary Openhole sand-control technique selection has been a topic of interest since the late 1990s and was discussed most comprehensively in SPE 85504 (Price-Smith et al. 2003), which proposed guidelines for selection between standalone screens (SASs), α/β packing, and shunt-tube packing. Proposed guidelines were based on formation characteristics such as formation strength, particle-size distribution (PSD), mineralogy across the well path; risk factors involved in execution as well as reliability/longevity; and cost considerations. From a PSD standpoint, their guidelines were based on the criteria proposed earlier in SPE 39437 (Tiffin et al. 1998), whereas risk evaluation was based on the technologies available at that time. Since then, significant advancements were made in understanding sand-retention mechanisms and failure modes of SASs, and in technology development to extend the limits of openhole gravel packing. These combined with the field experience in the last decade certainly warrant re-examination of their guidelines, which is the objective of this paper. In this paper, we begin with a critical review of the current sand-control technique-selection methodologies for openhole completions, including the way some of the risk factors are being evaluated to eliminate a given completion technique. On the basis of the technologies developed in the last decade, we propose a new approach for selecting sand-control technique, along with techniques/tools for proper evaluation of the risk factors. The proposed approach significantly extends the application limits of SASs and α/β packing compared with what was proposed in SPE 85504 (Price-Smith et al. 2003).

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A weighted, all-oil fluid system was developed to activate oil-swell packers and maintain compatibility with water-based filtercakes. This fluid system has enabled the successful installation of multiple openhole gravel pack completions to date.Traditionally, base oils, such as diesel, serve as activating fluids. The low density of the base oil risks compromising well control and/or borehole stability. There are products available to viscosify the base oil and suspend weight material for some applications, but jobs most require special shearing equipment operating at temperatures above safe limits for base oil. Other products are required at significant concentrations that risk adverse reactions with the water-based filtercake and filtrate. Invert emulsion systems offer greater density and viscosity, but they are incompatible with the water-based filtercakes, risking formation damage. This new fluid system utilizes a new viscosifying agent that easily mixes using rig equipment along with an acid-soluble, treated weight material to maintain a dispersion in oil. All of the products are micronized to reduce any risk of plugging downhole completion equipment. The all-oil system is spotted in the open hole, allowing the swell packers to expand. The system may either remain in the wellbore to flow back with production or circulate out of the wellbore prior to gravel packing. Flexibility in additive concentrations aids to effectively displacing the fluid in and out of the well. This paper discusses the development process of this all-oil system in the laboratory and the challenges overcome to deliver a rig-practical solution. A review of field planning and execution is included for the initial wells employing the system. Zonal Isolation EquipmentAdvances in openhole completion technology have resulted in new hardware to aid in improved production performance. Openhole packers are utilized to isolate sections of the reservoir (Freyer and Huse 2002). Isolation aids to block production of undesirable fluids, such as water in the production interval or shale sections where risk of formation collapse may result in plugged completion equipment. Openhole packers also function to separate the production interval into separate sections of varied permeability or pressure drawdown requirements. In conjunction with inflow control devices, the

The Kraken field development was planned and executed through several batch drilling and completion phases to allow a review of each phase and incorporate the lessons learned in the next phase. All producers were drilled with an oil-based reservoir drilling fluid (OB-RDF). The lower completion with shunted sand control screens was installed in conditioned OB-RDF, followed by displacements to water-based fluids after the packer was set. Gravel packing was performed with a visco-elastic surfactant fluid, and a breaker treatment was spotted across the open hole prior to isolating the open hole with a fluid loss control valve. This paper discusses the design, execution and evaluation of the lower completion phase for the development of the Kraken field in the North Sea. This includes detailed reservoir evaluation, methodology followed for sandface completion selection, steps taken to improve efficiency through lessons learned and continuously extend well lengths to be gravel packed in a low frac-window environment, and well performance results in this 24-well field development, with focus on the 7 out of 13 oil producers, detailing 3 of them.