Optimizing the Selection of Bridging Particles for Reservoir Drilling Fluids

Abstract

Abstract Most water-based reservoir drilling fluid systems used today are comprised of four primary components: the base brine, viscosifier, fluid-loss additive, and bridging particles. With the exception of minor adjustments of loading levels, the first three components normally do not change.The two primary types of bridging agents include calcium carbonate and sodium chloride salt. Some companies offer as many as seven different grades or sizes of each type of bridging particle. For the first time, the "ideal" pigment-size-distribution theory used widely in the paint industry has been transferred to practical oilfield use. This paper discusses the method and its use in selecting the optimum blend of bridging particles, focusing on an ideal packing sequence for minimizing fluid invasion. The authors examine the various procedures for optimizing sealing, as well as a management maintenance system. The paper expands on Abrams' Median Particle-Size Rule by going beyond the size of particle required to initiate a bridge. In the discussion, the authors examine the ideal packing sequence for formulating a minimally invading (non-damaging) fluid. Introduction Aside from minor adjustments of solids loading, the base brine, viscosifier and fluid-loss control additive in a traditional reservoir drilling fluid system change little from their original composition. However, the fourth primary component - the bridging particles - is used in a wide range of grades and sizes, depending on the anticipated extent of fluid invasion that must be thwarted. Thus, designing proper particle-size distribution is the first step towards formulating a minimally invading, non-damaging fluid. Historically, Abrams' rule1 has been used for this purpose. This rule states that "the median particle size of the bridging material should be equal to or slightly greater than 1/3 the median pore size of the formation." It goes on to suggest the concentration of the bridging solids must be at least 5% by volume of the solids in the fluid. In terms of particle size this means, for example, that 50µ particles should be effective at sealing pores up to or around 150µ in diameter. However, Abrams' rule only addresses the size of particle required to initiate a bridge. The rule does not give optimum size or address an ideal packing sequence for minimizing fluid invasion and optimizing sealing. While Abrams' rule has been the principle guideline for selecting particle size and concentration, others have used a "shotgun" approach to provide a broad and "all-encompassing" particle-size-distribution range. This approach assumes a single size distribution, often apparantly random, will cover the full range of pore throats or permeabilities that are likely to be encountered.2 However, this paper considers an ideal packing approach to the sealing problem, with the aim being to reduce formation damage and enhance the performance. This theory approaches reservoir drilling fluids from a reservoir-specific perspective. The theory works equally well for water-based (WBM) and oil-based (OBM) reservoir drilling fluids. Ideal Packing Theory - A Definition "Ideal packing" can be defined as the full range of particle-size distribution required to effectively seal all voids, including those created by bridging agents. This subsequent layering of bridging agents results in a tighter and less invading filter cake. The Ideal Packing Theory (IPT) introduced here takes a graphical approach to determine the optimum particle-size distribution of bridging material for given formation characteristics. The IPT uses either pore sizing from thin section analyses or permeability information, combined with particle-size distributions of the bridging material, to determine the Ideal Packing Sequence (IPS).