In hydraulic fracturing, the temperature at which a polymer is to be used determines the break mechanism and the breaker used for its degradation. The polymers most commonly used in fracturing were degraded under a variety of conditions and their resulting flow impairment characteristics were measured in 20/40 mesh sandpacks. Enzyme breaker was used for the low temperature tests (49 °C), oxidizing breaker was used for the intermediate temperature tests (82 °C) and thermal degradation processes were used for the high temperature tests (135 ° – 204 °C).
The polymers evaluated were derivatized cellulose (0% w/w residue), derivatized guar gum (l-2% w/w residue), intermediate residue guar gum (5–6% w/w residue) and guar gum (8–10% w/w residue). The relative residue volumes for these polymers were determined and compared to their actual flow impairment values. The effect of polymer loading on the relative flow impairment was also determined for these polymers as a function of break temperature.
A variety of water soluble polymers are available which can provide the viscosity necessary to perform a fracturing treatment. In addition to providing viscosity for sand transport, the polymers must also break and leave a minimum amount of flow impairment to both the formation and created fracture. It is known, however, that a large number of these polymers used in fracturing leave an insoluble material (residue) after breaking. This insoluble material may influence flow in both the propped fracture and formation by simply blocking or restricting pore spaces. A number of researchers have shown that this is undoubtedly the case with a variety of polymers 1,2,3.
A numerical correlation was made by Gooke2 between the volume of polymer residue left by various polymers and the amount of permeability reduction in 20/40 mesh sand using a modified version of the Kozeny equation. The results of this indicated that when the residue volume produced from oxidizing breaker at 93 °C is increased (polyacrylamide > derivatized cellulose < guar gum), the reduction in fracture conductivity is also increased. A more recent study4 using hydroxypropylguar gum, hydroxyethylcellulose and carboxymethylcellulose degraded under high temperature conditions (149 °C) showed that although a considerable quantity of polymer was left in the 20/40 mesh sandpack, a very small decrease in Permeability, if any, was detected using low polymer loadings.
There are several factors involved in the degradation of these polymers which could potentially affect the amount of residue remaining from each polymer: breaker type, breaker concentration, break time and break temperature. The type of breaker used to degrade a polymer (i.e., enzyme, oxidizer or thermal processes) is very important. Since each breaker type operates under a different mechanism to degrade the polymer, there is a different set of factors associated with each breaker which could affect the amount of residue or potential flow impairment seen with a single polymer. For this reason, flow impairment resulting from the most popular gelling agents used in hydraulic fracturing was evaluated under low temperature break conditions (enzyme breaker), intermediate temperature break conditions (oxidizing breaker) and high temperature degradation conditions (thermal breaking).
