Abstract

The minimization of polymeric damage is an important factor to the conductivity and productivity of a well. In many cases this damage is attributed to unbroken gel residue and/or dynamically formed filter cake on the formation faces. Recently, removal treatments have been developed and improved for the polymeric damage resulting from fracturing, gravel packing, drilling and workover operations. The best method, from a chemical standpoint, of monitoring the extent of polymer damage and its subsequent degradation is through the examination of flowback samples. Flowback analysis provides valuable information regarding well cleanup progress at various intervals and may be used as a quantitative profile for the amount of polymer treatment load recovered. Samples are analyzed before and after treatments to determine the total carbohydrate content, which is a gauge of polymeric damage produced by guar, cellulose, starch, xanthan and other polysaccharides. Although high carbohydrate levels are a symptom of damaged wells, it is misleading to conclude that lower carbohydrate content equates to a lesser degree of damage. Other factors, such as bacterial presence, enzyme activity and size distribution of polymer fragments contribute significantly to the results of flowback analysis.

This paper presents an improved method to effectively analyze flowback samples as well as a guideline for applying these new procedures. Detailed laboratory protocols are presented that include tests for carbohydrate content, molecular weight distribution, enzyme/bacteria detection and viscosity measurements. This improved flowback analysis provides a method to detect polymer damage downhole and may be used to evaluate polymer load recovery. Several field studies are also included to demonstrate this new comprehensive procedure.

Introduction

Produced waters have long been recognized as an important source of information about reservoirs. Oil field waters are analyzed for various physical and chemical properties including pH, specific gravity, content of iron, bicarbonate, chloride, sulfate and other inorganic anions and cations. The characteristics of produced waters from several typical wells are presented in Table 1. Chemical analysis of formation waters is useful in production problems, such as identifying the source of intrusive water, planning waterflood and saltwater disposal projects and treating to prevent corrosion problems in primary, secondary and tertiary recovery. An previous method of quantifying cleanup following the fracturing of a well was to report load water recovery. Chlorides, sulfates and/or specific gravity of the flowback were tested and compared with properties of the formation water. These methods attempted to quantify fracture load recovery, but was only a measure of the water load and gave no information about polymer return.

Natural, water soluble polymers have a long history of use in oil field applications due to their unique fluid rheology characteristics, proppant carrying ability and high temperature stability. Applications include drilling, fracturing, enhanced recovery, completion and workover operations. However, the fact that these same polymers can leave behind unbroken gel filter cake and insoluble residues has been well established. Polymer filter cake is deposited on formation faces or within the fracture during pumping and/or upon fracture closure. At times the concentration of the filter cake becomes so high that breaker additives are unable to thoroughly degrade it. Insoluble residues and high molecular weight fragments are polymer degradation products that are no longer soluble and, therefore, fall out of solution. These degradation products can settle within the proppant pack and impair permeability. Since the damage produced by natural polymers can have a negative effect on well productivity, it is important to ensure that most of the polymer is returned after a treatment. Flowback waters are now being recognized as a valuable source of information regarding polymer damage. Pope reported that a more quantifiable approach to describing fracture cleanup is performed by determining the amount of guar returned from the fracture during flowback. Brannon/Tjon-Joe-Pin and Tyssee/Vetter also used analysis of return waters to support arguments made by their studies regarding polymers. This paper presents an improved method to effectively analyze flowback samples for polymeric damage. P. 485^

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