Determination of the Effect of Formation Water on Fracture Fluid Cleanup Through Field Testing in the East Texas Cotton Valley
Authors: Willberg D.M. , Card R.J. , Britt L.K. , Samuel M. , England K.W. , Cawiezel K.E. , Krus H. 
Published: December 2007

A field study in east Texas showed that formation water production influences polymer recovery from hydraulic fractures. This study was conducted on 10 wells located in the Cotton Valley Taylor formation. A typical fracture stimulation design included between 9,000 to 14,000 bbl of zirconium crosslinked guar gel. Detailed chemical analysis of flowback samples was used to identify the effect of formation water on polymer recovery. Polymer and chloride concentrations were measured. Water produced during flowback averaged 52 5% of the fluid pumped on the jobs, while polymer recovery during flowback averaged 35 + 6% of the amount pumped. Polymer concentration in the flowback fluid from all the wells declined over time, as chloride concentration increased. This is attributed to the production of formation water. Flowback rate has a minimal effect on polymer recovery from these wells. The results of this study are compared with those previously obtained from another low permeability formation, specifically the Codell formation in Colorado. Flowback analysis is an important tool to determine how specific reservoir conditions influence fracture cleanup.

Introduction

Understanding how hydraulic fractures clean-up is essential for improving well stimulation. Residual gel can damage fracture conductivity, shorten effective fracture half- length, and limit the productivity of the well. The drive to develop fluids, additives, and procedures that minimize this damage continues to be a dominant theme in fracture fluid development programs. For example, reduced-polymer and viscoelastic surfactant (VES) fluid systems minimize damage by reducing or eliminating the polymer required to execute a treatment. Fluid additives have been developed to improve polymer recovery, including encapsulated breakers and aggregate dispersants. Aggressive procedures using fibrous material to maintain proppant flowback control have also been developed to maximize fluid recovery.

Still, our understanding of the fundamental physical and chemical processes governing fluid recovery from hydraulic fractures is immature. Fracture cleanup is a complex problem, and many parameters - fluid system, job design, flowback procedure, and reservoir conditions - can influence polymer and fluid recovery efficiencies. Often specific products and methods that work well in one reservoir have little effect in other situations.

Well productivity is the ultimate measure of treatment effectiveness. However, it does not provide direct information on fracture cleanup. A well with poor cleanup in a relatively rich pay zone may give superior performance compared to a well which had excellent cleanup but was in a relatively poorer pay zone. Additional information besides well productivity is needed to determine how the various reservoir and treatment parameters influence fracture cleanup.

Systematic analysis of fluid and polymer returns after a treatment is completed is the only method to quantify fracture cleanup. In this paper this is referred to as flowback analysis. Water production and polymer returns are determined as a function of time and/or flowback volume. The concentration of other chemical species, such as chloride ions, is also determined to provide data on the mechanisms of fluid recovery and on fluid/formation interactions.

Considering this is a basic approach, surprisingly little information is available in the published literature on actual flowback analysis field studies. Only load water recovery is commonly recorded after a fracturing treatment. However, as will be discussed below, load water provides insufficient information to quantify fracture cleanup, especially if the formation produces water.