Technical Committee Reservoir Engineering
Stability of high molecular fracgels under HTHP conditions in gas wells
Previous experience in frac operation showed that in many cases fracgel recovery is incomplete.
This leads to a decreased productivity of the stimulated well. One assumed reason is
an insufficient degradation of the fracgel due to a decrease in the effectiveness of the added
breaker induced by interactions with added production chemicals. It was the purpose of this
project to investigate the effectiveness of breakers for degradation of high molecular guarbased
fracgels in the presence of selected production chemicals in low pH, high temperature,
high pressure (HTHP)-environments. The present work covers the polymer analytical characterization
of basic components for fracgel production and the synthesis of a reference fracgel
with a comparable property profile of a field product.
The method which was developed to investigate the influence of breakers and production chemicals on the properties of the reference fracgel and its degradation, accounts for the fracgel strength, the absolute molar mass distribution of soluble and the fraction of insoluble residues. In the temperature range of 120°C to 150°C, thermal fracgel degradation is pronounced. Ammonium peroxodisulfate (APS) is the most effective investigated breaker. Compared to APS, sodium bromate shows a significantly lower breaker activity. The temperaturestabilizing agent sodium thiosulfate decelerates fracgel degradation and reduces the effectiveness of APS without annihilating it. Above 90°C, with increasing degradation time and temperature, a part of the deployed polymer precipitates and forms a water-insoluble residue. At 150°C, its fraction increases up to 30% after 48 h. High concentrations of electrolytes favour the formation of insoluble residue. A time-dependant minimum value of the percentage of insoluble residue occurs. It can be influenced by addition of sodium thiosulfate.