Improved Flowback Analysis: A Conclusive Evaluation for Polymer Damage
Authors: Flippen Michelle C. , Yang B. Helena 
Published: December 2007

The removal of polymers utilized in oil field applications is important to the conductivity and productivity of a well. Unbroken gel residue and/or dynamically formed filter cakes on the formation face are two forms of damage resulting from fracturing, gravel packing, drilling, and workover operations.

Monitoring the extent of polymeric damage and its clean up through removal treatments is best achieved by analysis of flowback samples. This form of analysis can provide valuable information regarding polymer degradation downhole and be used as a quantitative profile for the amount of treatment load recovered. Flowback samples can be analyzed before and after treatments to determine the total carbohydrate content, which is a measurement of sugar concentration, in pounds per thousand gallons. Guar, cellulose, starch, xanthan, and other polysaccharides used as viscosifying agents are examples of complex sugars. 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, breaker 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 for polymer damage. Detailed laboratory protocols are provided that include tests for carbohydrate content, molecular weight distribution, enzyme/bacteria detection and viscosity measurements. This improved flowback analysis provides a method to evaluate polymer load recovery and to detect any polymer damage downhole. Several field studies are also included to demonstrate this comprehensive analytical procedure and how it supplies a more conclusive post treatment evaluation.

Introduction

Natural, water soluble polymers are widely used in the oil and gas industry as additives in drilling, fracturing, enhanced recovery, completion and workover operations. The wide use of these polymers can be attributed to their unique fluid rheology and particle transport characteristics. However, the fact that natural polymers are responsible for reducing productivity through the impairment of formation permeabilities is well established. Two forms of damage produced are unbroken gel filter cakes on the formation face and insoluble residues within the proppant pack. Filter cakes are a dense mass of polymer and can be deposited during pumping and/or upon fracture closure. At times the concentration of polymer is so high that traditional breaker concentrations are unable to thoroughly degrade it. Insoluble residues and high molecular weight polymer fragments are degradation products that are no longer soluble and fall out of solution. These by-products of breaker reactions can settle within the proppant pack and reduce permeability. Since the damage produced by natural polymers can have a negative impact on well productivity, it is important to ensure that most of the polymer load has been recovered. Flowback waters have previously been used as a source of information regarding load recoveries after the fracturing of a well. One method of quantifying clean up was to measure chlorides, sulfates and/or specific gravities of flowback samples and compare it to properties of the formation water. However, this method served only as a measure of the water load recovered and gave no information pertaining to the amount of polymer returned. An alternative method was to monitor the viscosity of the flowback samples over time. Reduced viscosity observed in return flow fluids was considered proof of optimum gel degradation. However, a problem arises when a formation's naturally produced water dilutes flowback samples, leading to a misinterpretation of adequately broken gels. Recently, flowback waters have been recognized as a valuable source information in the evaluation for 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. Tysee and Vetter also used analysis of return waters to support arguments made by their study regarding water soluble polymers.

This paper presents an improved method to effectively analyze flowback samples in effort to evaluate for polymeric damage. This procedure can be applied towards wells suspected to have polymer damage, as well as be used as a quantitative tool after fracturing or polymer removal treatments. P. 433^