A New, Nondamaging, Acid-Soluble Weighting Material
Authors: Sloan J.P. , Brooks J.P. , Dear S.F. 
Published: December 2007

Iron carbonate, a new acid-soluble weighting material, promises significant reduction of filter cake buildup and particulate invasion formation damage. It is soluble in dilute hydrochloric and formic acid, has gravity and hardness similar to barite, is compatible with oil and water muds, and can be used to weight muds up to the 19-lb/gal range.

Introduction
Often, the productivity of a formation is damaged by the invasion of undesirable mud solids. This is especially true when higher weighted fluids using barite are employed, since these fluids contain a high percentage of insoluble solids. The resulting loss of production has been attributed to four primary causes:particle invasion into the production zone;filter cake buildup particle invasion into the production zone;filter cake buildup on the wellbore adjacent to the production zone;invasion of the fluid phase into the production zone; andinvasion of whole mud into the production zone.
Particle invasion into the production zone can block pore holes, causing a loss of either injection or return permeability, or both. Filter cake buildup on the wellbore can also lead to injection permeability loss. This in turn reduces penetration of acidizing fluids that are designed to stimulate production.Fluid phase invasion leads to swelling of clays in the formation or to water blockage caused by differences between the interfacial tension of the formation fluid and the invading fluid.Invasion of whole mud along fracture planes of a producing formation can also cause irreversible damage deep within the formation; thus a combination of particle invasion, filter cake buildup, and fluid phase damage occurs within the formation.This paper describes a laboratory study of iron carbonate-based fluids in which two of the primary causes of formation damage - particle invasion and filter cake buildup - were investigated. Their damaging effects were virtually eliminated. As a weighting material for fluids up to the 19-lb/gal range, iron carbonate appears to offer excellent potential for minimizing formation damage during workover operations, in the initial penetration of new pay zones, and when perforating.

Theory and Background
Glenn and Slusser have shown that mud-particulate invasion can penetrate to appreciable depths in the porous zone, causing penetrate to appreciable depths in the porous zone, causing substantial reduction in permeability. Insoluble weighting materials may also allow deposition of a damaging filter cake in the wellbore. The combined effects of the insoluble filter cake and invading particles can reduce the effectiveness of well stimulation techniques.The development of improved fluids has concentrated on solids-free and calcium-carbonate based formulations. One of the major problems of these systems is their upper weight limitation, Those systems employing calcium carbonate as a weighting material are used for workover and completion because of their acid solubility. With these fluids, both the filter cake and the particles that have invaded the porous formation can be dissolved, particles that have invaded the porous formation can be dissolved, thus allowing maximum penetration into the formation. However, because of the relatively low density of calcium carbonate (2.7 gm/cc), fluids made with this material are limited in weight to about 14 lb/gal.
JPT
P. 15