Abstract

Great care must be exercised in the formulation and use of completion and workover fluids; otherwise extreme damage can be done by "non-damaging" fluids. Excessive loss of brines, polymers, or bridging solids to a producing strata can result in irreversible formation damage. Rapid formation of an effective filter cake on the formation face with the resultant low filtration rate and minimum solids penetration can significantly reduce the potential for formation damage.

Laboratory data indicates that low filtration rates achieved with the formation of a thin, competent filter film composed of properly sized bridging particles and hydrocolloid polymers will most effectively particles and hydrocolloid polymers will most effectively prevent formation damage. The deposited filter film prevent formation damage. The deposited filter film acts as a downhole filter, screening out unwanted solids from the small volume of filtrate actually entering the formation.

Introduction

Fluids and solids invading the production zone cause formation damage. The most successful method of minimizing damage is to establish a low permeability filter film at the wellbore face as rapidly permeability filter film at the wellbore face as rapidly as possible. The filter film should be easily broken down when the well is put back on production. The importance of using non-damaging completion and workover fluids has long been recognized and close attention is being given to the problem in the field operations. However, some factors in designing and applying these fluids are often overlooked; the result is a "non-damaging fluid" causing formation damage. One of the critical factors in designing non-damaging fluids is obtaining surface bridging on the formation face with minimum indepth solids penetration. This can only be done by proper selection of bridging particle sizes in relation to the formation pore sizes. particle sizes in relation to the formation pore sizes. The use of acid degradable additives in a wellbore fluid does not assure protection for a producing zone. Such additives merely make removing formation damage more practical or possible.

The most commonly used completion fluid additives are the hydroxyethyl cellulose (HEC) polymer and sized calcium carbonate bridging solids. The objective of this study was to investigate the interaction of calcium carbonate particle size distribution and hydrocolloid polymers. Previous studies have related surface bridging and particle size distribution in conventional drilling fluids.

Brines filtered through 2–5 micron filters are used extensively in completion and workover operations. Filtered brines are best used where formation permeability is not significantly affected by liquid permeability is not significantly affected by liquid invasion or where fluid loss to the formation is minimal. However, field tests have indicated the need for maintaining exceptionally clean equipment on the rig to prevent the filtered fluid from picking up rust, scale, prevent the filtered fluid from picking up rust, scale, or other debris and carrying it downhole.

The addition of properly formulated fluid loss control additives to the brine protects the formation from both solid and liquid invasion. The formation of a thin filter film on the face of the wellbore will essentially filter out all of the solids down to and including colloidal size particles. Further fluid loss is significantly reduced. The filter film then is a more effective method of solids removal than surface filters, but success with this approach depends upon the ability of the fluid to quickly establish a filter film on the formation surface.

POLYMER ADDITIVES POLYMER ADDITIVES Certain polymers will raise the viscosity and improve the carrying capacity of water. (The polymers most commonly used in the oil fields are long chain molecules which hydrate in water — hydrocolloids.) Other polymers will lower fluid loss when used with bridging solids. Polymers are in the colloidal size range and help in the filtration control, once the primary bridge has been established by the coarser primary bridge has been established by the coarser inert bridging particles.

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