Geochemical Evaluation Mitigates Potential Productivity Loss in Gorgon Field, Offshore Western Australia

Abstract

Gorgon's gas well completions range in temperature from 144C to 163C, cover 900 meters of section, and top depths range from 3500 to 4000 m tvdss. Reservoir sands vary from 5–65 m thick. Gorgon Triassic Mungaroo Fm sands contain abundant authigenic kaolin clay as pore filling and grain replacements. Prior to completion operations, formation integrity was a concern due to expected completion brine losses, high temperature, and multi-year stand time after completions. Native kaolin minerals were speculated to be capable of dissolving and back-precipitating zeolites because initial completion brine pH was designed at 9.8. Zeolite growth can be a porosity-loss process if they precipitate at the expense of higher density parent minerals. This near-wellbore formation damage risk warranted further investigation. Laboratory experiments were designed and run at 160C for 4 weeks, and SEM analyses were conducted pre and post flood. Experiments also were modelled with Geochem Workbench. All eight experiments were made up with brine from Gorgon supplier materials, and pH was varied from 9.8 to near neutral. Zeolite growth was confirmed in high pH lab runs and shown viable in chemical modelling. Post-flood perms range from 101% to 72% of initial. Zeolite morphology and chemistry are consistent with analcime, a Na zeolite type. Lab tests were finished in time to consider results before Gorgon completions. Brine pH, a critical factor to formation reactivity, was moderated to a near neutral for the perforating fluid as a result of this testing. Field procedures were modified further to allow the neutral pH fluid in the 7” liner and 7 5/8” tubing during perforation, while the 9.8 pH brine remained above the production packer for corrosion control. Completion perforation operations were conducted successfully. Although wells are yet to be put on full production, post-perf flowback did not indicate formation damage. In summary, this set of geochemical experiments illustrated a potential downside risk clearly enough that completion plans were altered. We believe that the integrity of the gas-producing formation was maintained through thoughtful geochemical planning, execution, and knowledge sharing.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015