A New Biopolymer-Free, Low Solids, High Density Reservoir Drilling Fluid

Abstract

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.

A New Biopolymer-Free, Low-Solids, High-Density Reservoir Drilling Fluid: Laboratory Development and Field Implementation

“…[1][2][3][4][5][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][8][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.…”

“…[7][8][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][2][3][4][5][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.…”

“…However, the performance of a biopolymer is compromised in the presence of a high concentration of divalent ions. [1][2][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.…”

“…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.…”

Strategy for Designing an Optimum Drill-In Fluid Utilizing Rock and Petrophysical Properties

“…The methods that are utilized to determine an appropriate bridging solids distribution vary. [1][2][3][4] Often a petrophysical method is employed and involves the utilization of optical and/or electron microscopes, core permeameters, and/or porosimeters to analyze core material. Other data such as grain size, sorting, pore type, mineralogy, and texture can be derived from these instruments.…”