A New Biopolymer-Free, Low-Solids, High-Density Reservoir Drilling Fluid: Laboratory Development and Field Implementation
- Robert L. Horton (M-I L.L.C.) | Kim O. Tresco (TBC-Brinadd) | James W. Dobson Jr. (TBC-Brinadd) | Gry Kristvik Bye (Statoil) | Dave A. Knox (M-I L.L.C.) | Charles F. Svoboda (M-I L.L.C.) | William E. Foxenberg (M-I L.L.C.) | Taylor C. Green (M-I L.L.C.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2004
- Document Type
- Journal Paper
- 29 - 39
- 2004. Society of Petroleum Engineers
- 4.2 Pipelines, Flowlines and Risers, 2.7.1 Completion Fluids, 2.4.5 Gravel pack design & evaluation, 3 Production and Well Operations, 2.1.7 Deepwater Completions Design, 5.5.2 Core Analysis, 1.11.4 Solids Control, 1.14 Casing and Cementing, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.8 Formation Damage, 4.2.3 Materials and Corrosion, 2 Well Completion, 5.4.10 Microbial Methods, 6.1.5 Human Resources, Competence and Training, 4.3.4 Scale, 1.11 Drilling Fluids and Materials, 4.1.2 Separation and Treating, 6.5.4 Naturally Occurring Radioactive Materials, 2.4.3 Sand/Solids Control, 1.6 Drilling Operations, 2.2.3 Fluid Loss Control, 5.1 Reservoir Characterisation, 4.3.1 Hydrates, 5.2 Reservoir Fluid Dynamics
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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 low-solids 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 low-shear-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.
The use of specialized water-based drilling fluids for drilling reservoir sections has become common practice in the last decade.1-6 Fluids based on a clear brine to achieve the required density and a small concentration of chemically removable solids as a bridging agent (calcium carbonate, sodium chloride, potassium chloride) have gained wide acceptance in the industry.7-9 The maximum density achievable with this type of drilling fluid has been limited by either the chemistry or the economics of potential base brines.1-6 Biopolymers, such as xanthan gum,10,11 scleroglucan,12 and welan gum, are usually added to this type of RDF to provide the viscosity and gel structure necessary to suspend solids (bridging agent, drill cuttings, etc.). For the purposes of this paper, the term "biopolymer-free" refers to the absence of a polymer derived from biofermentation. The polymers are selected for the rheological profile imparted to the drilling fluid - high LSRV for solids suspension in static fluids and low high-shear-rate viscosity to reduce the required pump pressure.13
Full biopolymer hydration is feasible in all monovalent brines (potassium, sodium-, and cesium-based). However, the performance of a biopolymer is compromised in the presence of a high concentration of divalent ions.1-3 Using a variety of specialized procedures (high shear, elevated temperature, and predispersion in fresh water), traditional biopolymers can be made to viscosify divalent brines to a point. However, once the brine reaches a density at which 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 RDFs has been traditionally limited to the maximum density achievable with a cost-effective monovalent brine or a sufficiently dilute divalent brine.
A new nonbiopolymer reservoir drilling fluid (NB-RDF) system has been developed that produces a suitable rheological profile with a specially derivatized starch and a high-surface-area grade of magnesium oxide in combination with high-salinity divalent brines (calcium chloride, calcium bromide, zinc bromide, and two- and three-salt blends).1,2 The chemical nature of the components enhances the efficacy of chemical treatments to break down fluid viscosity or remove residual filter cake.1
The starch is unique in that it can serve as both a viscosifier and a fluid-loss additive. It appears to function in this dual capacity only when combined with highly reactive magnesium oxide (HRMgO) and a dense, divalent-cation-based brine. This dual-function starch (DFS) is a specially processed, high-molecular-weight, branched-chain starch derivative that generates elevated LSRVs and controls high-pressure/high-temperature (HP/HT) filtrate loss in inexpensive divalent salt brines. For the purposes of this paper, the derivatized starch is considered to be biopolymer-free. The distinction is important because the DFS is readily decomposed in mild acid, whereas biopolymers (or biofermentation-derived polymers, to be precise) are not so readily decomposed in mild acid. An added benefit of the DFS is the reduction in the number of components required to formulate the system, making it simpler and less expensive.
The base brines for the new fluid system are higher-density, divalent-cation-containing brines in the 11.5- to 17.5-lb/gal density range. Brine-based fluids of this type provide several advantages. Formulating RDF systems in heavier brines minimizes the solids concentration required to weight up to a high density. Lower suspended-solids content reduces the plastic viscosity (PV). Buoyancy, or the upward pressure exerted by a fluid against particulates in the fluid, reduces the particle-suspension and cuttings-removal demands upon the viscosifiers.
Particularly in the 11.5- to 12.8-lb/gal density range, the divalent-cation-containing brines are considerably less expensive than the monovalent alternatives based on NaBr/NaCl- and NaBr-based brines.
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