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

Abstract

Gorgon's eight gas well completions range in temperature from 144C to 163C, and cover 900 meters of section in each well. Reservoir sands vary in thickness from 5-65 meters. Gorgon Triassic Mungaroo Formation sands contain abundant authigenic kaolin clay as pore filling and grain replacements. Prior to completion operations, formation integrity was questioned because of anticipated completion brine losses, high interval temperature, and multi-year stand time after well completions. Native kaolin minerals were speculated to be capable of dissolving and back-precipitating zeolites because intial 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 was thought to warrant further investigation. Laboratory experiments were designed and run at 160C, and SEM analyses were conducted pre and post flood. Experiments also were modelled with Geochem Workbench. All eight experiments were run with brine made up from Gorgon supplier materials, and pH was varied from 9.8 to near neutral. Zeolite growth was confirmed in high pH fluid lab tests and shown to be viable in chemical modelling. Post-flood perms range from 101% to 72% of initial. Although zeolite was produced in only 1 of 8 lab tests, the tests were conducted in only 4 weeks which is well less than Gorgon completion soak time. Zeolite morphology and chemistry are consistent with analcime, a Na zeolite type. Lab tests were finished in time to consider results before Gorgon wells were completed. Brine pH, viewed as a critical factor to formation reactivity, was moderated to a near neutral pH 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 will remain above the production packer for corrosion control. Completion perforation operations were conducted successfully. Although wells are yet to be put on full production, flowback showed no signs of formation damage. In summary, this set of geochemical experiments illustrated a potential downside risk for the field clearly enough that completion plans were altered. We believe that the integrity of the gas-producing formation and the revenue stream was maintained through thoughtful geochemical planning, execution, and knowledge sharing.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017