A new reservoir drilling fluid system utilizes a non-biopolymer, acid- or enzyme-soluble polymer that serves both as viscosifier and fluid-loss additive when combined with activated magnesium oxide and a divalent-cation-based brine. The new fluid shows a unique shear-thinning rheological profile featuring relatively low, high-shear-rate viscosity along with relatively high, low-shear-rate viscosity. This behavior is highly unusual in high-density, brine-based reservoir drilling fluids. A result of this behavior is that effective hole cleaning is provided without generating excessive high-shear-rate viscosities that lead to disproportionate equivalent circulating densities. The new fluid system is based on the higher density, divalent-cation-containing brines (utilizing CaCl2, CaBr2, CaCl2 / CaBr2, ZnBr2 / CaBr2 and ZnBr2 / CaBr2 / CaCl2) in the 11.5 to 17.5 lb/gal density range. The total amount of the sized CaCO3 bridging particles is kept relatively low, 13 to 35 lb/bbl, so that thin, chemically removable filter cakes are produced. The size distribution of these bridging particles is designed according to the ideal packing sequence for optimizing sealing and producing a minimally invading (well productivity enhancing) fluid. Introduction A number of technical advances in the petroleum industry have created cost-effective methods for the exploration and development of deep oil and gas reservoirs. One result of these developments is an increased demand for higher density reservoir drilling fluids (RDF's). However, the density attainable for economically viable, brine-based reservoir drilling fluids is limited under current technology.1,2,3,4 Some limitations are based on the fact that current biopolymer-CaCO3-brine-based reservoir drill-in fluids utilize viscosifiers that are either incompatible with the higher-density brines or require special mixing equipment / techniques and complex formulations.1,2 In other cases, the cost of a base brine compatible with currently available biopolymer viscosifiers is such that the final drill-in fluid is priced out of consideration.4,5 This paper presents a newly developed biopolymer-free fluid system that uses conventional high-density base brines to fulfill the density requirement, a low concentration of bridging-solids, and a new viscosifier / fluid-loss package to produce an easily blended drill-in fluid with exceptional rheological and filter cake qualities. Most brine-based reservoir drilling fluid systems used today consist of five primary components: base brine, pH control additive, biopolymer-derived viscosifier, starch-based fluid-loss additive, and bridging particles.5,6 Containing no biopolymers, such as xanthan7,8 gum or scleroglucan,9 the new non-biopolymer reservoir drilling fluid (NBRDF) system uses a single acid- or enzyme-soluble starch that fulfills the role of both viscosifier and fluid-loss additive when combined with a divalent-cation-based brine and a highly activated magnesium oxide. This fluid delivers a unique shear-thinning rheological profile that provides effective hole-cleaning without generating excessive high-shear-rate viscosities that lead to disproportionate equivalent circulating densities.10,11 The new fluid system is based on the higher density, divalent-cation-containing brines in the 11.5 lb/gal to 17.5 lb/gal density range. Brine-based fluids based on calcium chloride, calcium bromide, and zinc bromide brines provide several advantages. Formulating RDF systems in heavier brines minimizes the solids concentration required to weight-up to a high density. Keeping the solids low results in a lowering of the plastic viscosity. Buoyancy, or the upward pressure exerted by a fluid against particulates in the fluid, reduces the demands upon the viscosifying additives for particle suspension and cuttings removal.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents a case history of the completion procedures applied to a four-well gas reservoir development in the Norwegian sector of the North Sea where open-hole gravel packing was utilized as the sandface completion technique.The chosen technique to gravel pack the wells was based upon Alternate Path Technology, which incorporates shunt tubes with nozzles on the outside of the gravel pack screen. These shunts create an alternative flow path, allowing slurry to bypass premature bridging and fill any voids beyond the bridge. The use of shunt tubes gives the opportunity of achieving a complete gravel pack without the requirement of a filtercake-sealed wellbore. This introduces the possibility for the filter cake to be removed during the packing operation. As a result, a low-solids drilling fluid that deposits a chemically degradable filter cake can be utilized.The reservoir pressure in this field required relatively highdensity, reservoir drilling and gravel pack fluids (1.65 SG / 13.75 lb/gal). This paper describes the design and development work involved in formulating a mutually compatible, calcium bromide-based reservoir drilling fluid and a Viscoelastic Surfactant (VES) based gravel pack fluid and the subsequent successful application in a subsea field development.

Low-solids, brine-based reservoir drilling fluids (RDFs) are widely accepted as beneficial to optimizing compatibility with the completion design while minimizing fluid-related formation damage. Traditionally, the maximum density attainable with a lowsolids fluid has been limited because of either the prohibitively high cost of the required base brine or the poor performance of viscosifying biopolymers in a dense, divalent cationic environment.An RDF has been developed that exhibits unusually high quality rheological behavior in high-density calcium-and zinc-based brines without the aid of a biopolymer. The new fluid shows a unique shear-thinning rheological profile that features a relatively low high-shear-rate viscosity along with a relatively high lowshear-rate viscosity (LSRV). This behavior is highly unusual in high-density, brine-based RDFs. A result of this behavior is that effective hole cleaning is provided without generating excessive high-shear-rate viscosities that lead to excessive equivalent circulating densities (ECDs).The first field trial of this fluid was on the reservoir section of Well 34/10 I-1-AH in the Gulfaks Satellites Development in the Norwegian sector of the North Sea. Fluid properties during pretesting, mixing, drilling, and completion of this section are detailed in this paper.

Low-solids, brine-based reservoir drilling fluids are widely accepted as being beneficial with respect to minimising fluid-related formation damage and optimising compatibility with the completion design. Traditionally, the maximum density attainable with a low solids fluid has been limited - either due to the prohibitively high cost of the base brine required, or the poor performance of viscosifying biopolymers in a divalent cation environment. A reservoir drilling fluid has been developed that exhibits unusually high quality rheological behaviour in high-density calcium- and zinc-based brines, without the aid of a biopolymer. The first field trial of this fluid was on the reservoir section of well 34/10 I-1-AH, in the Gulfaks Satellites Development in the Norwegian Sector of the North Sea. Fluid properties during the pre-testing, mixing, drilling and completion of this section are detailed in this paper. Introduction The use of specialised water based drilling fluids for drilling reservoir sections has become common practice over the last decade.1–7 Fluids based on a clear brine to achieve required density and a small concentration of chemically removable solids as a bridging agent8–10 (calcium carbonate, sodium chloride) have gained wide acceptance in the industry. The maximum density achievable with this type of drilling fluid has been limited, either by the chemistry or the economics of potential base brines.1–7 Biopolymers, such as xanthan gum,11,12 scleroglucan13 and welan gum are usually added to this type of reservoir drilling fluid to provide the viscosity and gel structure necessary to suspend solids (bridging agent, drill cuttings, etc). The polymers are selected for the rheological profile imparted to the drilling fluid - high low-shear rate viscosity (LSRV), for solids suspension in static fluids and low high-shear rate viscosity, to reduce required pump pressure.14 Full polymer hydration is feasible in all types of monovalent brine (potassium, sodium and caesium based). However, the performance of this type of polymer is compromised in the presence of a high concentration of divalent ions.1–3 Using a variety of specialised procedures (high shear, elevated temperature, pre-dispersion in fresh water), these polymers can be made to viscosify divalent brines up to a point. However, once the brine reaches a density where the divalent ion content is too high, or the volume of accessible "free" water is too low, no amount of treatment will generate a rheological profile suitable for high performance reservoir drilling. As a result, the maximum density achievable with low solids reservoir drilling fluids has been limited to the maximum density achievable with a cost-effective monovalent base brine, or a sufficiently dilute divalent brine. Research in this area has focussed on identifying viscosifying polymers that will hydrate in high hardness environments, or alternative methods of viscosification, such as hydroxyethyl cellulose or silica flour.3 Success with these novel types of fluid has been limited - not only with respect to drilling performance, but potentially compromising formation damage or filtercake clean-up under field conditions. A novel reservoir drilling fluid system has been developed that uses a combination of a modified starch fluid-loss control agent and a high surface-area grade of magnesium oxide for pH control. The combination of these two components in high salinity divalent brines, (calcium chloride, calcium bromide, magnesium chloride and zinc bromide), produces a suitable rheological profile for a drilling fluid.1,2 Furthermore, the chemical nature of the components enhances the efficacy of chemical treatments to break down fluid viscosity or remove residual filter cake.1

An established non-biopolymer reservoir drill-in (NBRDF) system which was developed in very early 2000 for high-density drilling applications – approximately 11.5 to 17.5 lbm/gal – was recently improved to provide functionality in low-density drilling applications – as low as 9.5 lbm/gal. Global implementation of this system over the last several years has demonstrated not only flexibility with respect to drilling and completing in diverse reservoirs but also its application as a workover system. In addition, this system was recently optimized by incorporating compatible chemistry to mitigate atypical damage mechanisms. As such, several case histories are presented to demonstrate the system's broad functionality with respect to density, completion type, reservoir, and logistics, as well as its capacity to reduce near-wellbore/formation damage. This non-biopolymer reservoir drill-in fluid (NBRDF) system demonstrates relatively high tolerance to solids and reservoir fluids as well as ease of hydration when mixing on the rig. Routinely the hydraulics are consistent and predictable even as no biopolymer is utilized. This system is compatible with the incorporation of an inhibitor, specifically, a scale inhibitor to mitigate calcium carbonate and minor sulfate scaling. This system recently incorporated a sized calcinated magnesium compound (MC2) and sized magnesium complex (MC3) which promotes viscosity, specifically low-shear rate viscosity, at a lower density range that was not achievable before. The initial concepts of buoyancy of solids and the exclusion of a biopolymer as incorporated in relatively high-density brines formed the basis for the development of this system. These concepts are discussed first to provide a background for the preceding recent improvements and case histories. Each case history presents a different set of objectives whereby pre-planning assessments were implemented to address and mitigate perceived risks as related to the fluids. The methods employed include assessments for rheology, scaling, hydraulics, displacements, compatibility and formation damage. The drilling and completion results for these projects exhibit a wide range of applications as well as flexibility with respect to required density, completion hardware and reservoir type. The procedures utilized for each project are evaluated with respect to specific drilling and completion targets and include the iterations/modifications required; subsequently the field learnings are also discussed.