Acid Fracturing of Deep Gas Wells Using a Surfactant-Based Acid: Long-Term Effects on Gas Production Rate

Abstract The deep, tight carbonate formations in Saudi Arabia are ideally suited for acid fracturing treatments. Various types of acids such as regular, in-situ gelled, and emulsified acids, have been used in order to achieve optimum fracture length and conductivity. Acids used for these treatments were based on 28 wt% HCl. A mixture of 15 wt% HCl and 9 wt% formic acid was used in wells completed with super Cr-13 tubulars. A high pH borate gel was pumped in stages to reduce leak-off and maintain the bottomhole pressure at values greater than the fracturing pressure of the formation. A new generation of viscoelastic surfactant-based acid was implemented in the field. A short term production evaluation based on the initial production (post flowback) from these wells could not clearly distinguish between the benefits obtained from the viscoelastic diverting acid versus the in-situ gelled acid.1 However, the wells treated with viscoelastic surfactant based acid did clean out in a shorter period of time. The main objective of this study was to compare the "long term" production results obtained from five wells treated with the surfactant-based acid with off-set wells treated with in-situ gelled acids. This paper provides for the first time long term production data evaluation following acid fracturing using surfactant-based acids. It also compares the performance of two main acid systems based on field data. The evaluation of acid fracturing treatments with in-situ gelled acid systems and surfactant-based acid systems suggest that there was no significant variation in the design and the execution of both the viscoelastic and in-situ gelled acid treatments. Among the 5 wells treated with surfactant-based acid, 4 of them showed better long-term production performance over a 2-3 year period compared to their offset wells. In the short-term (nearly 3 months production), 2 wells treated with viscoelastic acid showed no difference in production performance compared to their offset wells.