Applications of Weighted Acid-Soluble Workover Fluids
Authors: Chesser B.G. , Nelson G.F. 
Published: December 2007

This paper describes the composition and properties of a completion and workover fluid that can be weighted to 18.0 lb/gal (2157 kg/m) with an acid-soluble weighting agent. Several field case histories where these fluids were used in high-density gravel-packing operations are described. Property measurements demonstrate the stability, and highly pumpable flow characteristics of the weighted pumpable flow characteristics of the weighted fluids.

Introduction
Clear, solids-free brines have been used extensively in completion and workover operations, and, where applicable, these have proven quite effective. There are several reasons, however, why a solids-free brine may not meet the requirements of a particular completion or remedial operation. It then is necessary to add solids to improve or to impart certain properties to the fluids - i.e., improved hole cleaning. Rig hydraulics may be inadequate to clean cuttings or sand satisfactorily from the well using the brine alone; therefore, it is necessary to add a viscosifier to increase the brine's lifting capacity. At other times, the addition of bridging materials may be required to control loss of brine to the formation. Excessive loss of brine to the formation may result in formation damage, failure to maintain sufficient hydrostatic head, or unacceptable brine costs.Another reason for adding solids to a clear brine is to increase fluid density to control formation pressures. However, this is not widely practiced. Saturated sodium chloride provides a fluid density of 10.0 lb/gal (1198 kg/m) and calcium chloride brines are usable up to 11. 7 lb/gal (1402 kg/m) at ambient temperatures above 60 deg. F (15.6 deg. C). In the past few years, calcium bromide brines with densities up to 15.0 lb/gal (1797 kg/m3) have been used, but because of the high cost of this brine, its use cannot always be justified. Because of these limitations of the clear brines, other means would be needed to meet the higher density requirements of completion and workover. This paper describes the requirements, composition, and properties of a solids-laden brine system that has proven successful in field application.

Requirements of a Solids-Laden Brine
Solids-laden fluids often are considered undesirable as completion and workover fluids because these fluids could transport particles into the formation, resulting in particle plugging. This effect has been shown to exist by particle plugging. This effect has been shown to exist by several investigators and is a source of formation damage. However, several design criteria can be used to prevent, or at least minimize, particle invasion. prevent, or at least minimize, particle invasion. One design consideration is to form a particle bridge at the sand face as quickly and effectively as possible. This can be done by selecting the particle-size distribution and concentration of solids for the workover or completion fluid. Abrams concluded that to minimize particle invasion, the bridging materials should be equal to or greater than one-third the median pore size of the formation and should be present in quantities of at least 5% vol of the solids.Ultrafine particles are more apt to be transported into the formation as discrete particles, such as would occur in a deflocculated state. For this reason, a flocculating environment is desirable since the fines tend to exist as aggregates and are more likely to be retained by the bridging particles. Salt brines are, of course, highly flocculating to particles. Salt brines are, of course, highly flocculating to dispersed solids and are, therefore, highly desirable as the water phase.
JPT
P. 35