To maximize well productivity, it is essential to maximize fracture cleanup. A field study in the Codell formation of Colorado was conducted to examine the effects of guar removal from hydraulic fractures on gas production.
The conventional method of quantifying cleanup from a hydraulic fracture has been to report load water recovery; however, this value is affected by any formation water that might be produced. A more quantifiable approach to describing fracture cleanup has been performed in this study by determining the amount of guar returned from the fracture during flowback A 12-well study was performed by sampling flowback fluids during cleanup. The concentration of guar in each sample was determined using a colorimetric technique allowing the total amount of polymer recovered over the flowback period to be calculated.
Under equivalent reservoir conditions (pressure, permeability, etc.) and fracture conditions (width, proppant loading and distribution, etc.), physically, it is reasonable to expect higher production rates from wells which have produced back more guar since a larger volume (porosity) will be available for flow. Under low permeability reservoir conditions such as those in the Codell (0.01 md), this guar removal will need to provide added length to show an increase in production. This concept is illustrated with field data. For example, wells whose fractures produced 600-700 lb of guar (180,000 Ib proppant) produced gas at rates of 35-40 MSCF/D whereas wells whose fractures produced 1100-1200 lb of guar produced gas at rates of 70-80 MSCF/D, most likely indicating a cleanup over a longer length of the fracture.
In addition, the effects of flowback rate on load water recovery, guar concentration, and guar recovered are illustrated.
Attempts to understand the effects of load recovery on well productivity have been made over the years. In 1975, Claude Cooke wrote that "residue from guar polymer is the most important material presently used in fracturing fluids that can cause fracture conductivity reduction!" In the years since this comment, little has been done to address the effects of polymer. Recently, the development of new breaker technology and its successful field application has led to renewed interest in the area of polymer load, polymer type, residue and fines recovery.
Fluid cleanup or load recovery can impact well productivity dramatically. A number of parameters have been shown to play an important role in fracture fluid cleanup or load recovery. The most important of these include relative permeability, fluid viscosity, proppant fines, and gel residue. The most common effect deals with the reduction in the formation relative permeability to the hydrocarbon phase by the injection of a water based fracturing fluid.
In reservoirs where relative permeability impairs well performance, alternatives to conventional water based fracturing fluids such as carbon dioxide and/or nitrogen foams (minimal water component), and hydrocarbon based fluid systems are considered to reduce or eliminate the production impairment during well cleanup.
Fracture fluid viscosity also plays an important role in well performance during fracturing cleanup! It has been shown that viscosities in excess of 50 cp seriously impair well performance. Breakers are utilized in our fracturing fluid systems to reduce viscosity following fracture treatments to aid fracture fluid cleanup. Breaker technology has made significant advances in recent years. We should ensure that we utilize these technological advances to optimize our well performance.
In addition to the effect of relative permeability and fracture fluid viscosity on well performance, the other performance impairment comes from material left in the proppant pack itself. This material can significantly affect well performance throughout the life of the well. Materials of concern to the pack include both formation and proppant fines as well as gel residue. Reduced proppant pack permeability means detrimentally affected well performance. The effect of formation and proppant fines on porosity reduction can be conceptualized readily; the effect or significance of polymer residue is much more difficult to appreciate, although equally devastating to well performance.