The Pugang gas field, one of the biggest inland gas discoveries of the last decade in China, has hydrogen sulfide (H2S) concentrations as high as 10%, making surface- and downhole-facilities design and execution challenging. Testing these wells also is difficult because of the surface risks inherent in conventional testing. Well testing using production-logging tools recently has been introduced in this field and has been of tremendous benefit to the operator in identifying the inflow zones and in performing well-test analysis.
The Pugang gas field, discovered in 2002, is the second-largest gas field in China and the largest sour field discovered in the last decade. This field is north of Chongqing in the Sichuan basin. The reservoir fluid in this field is dry gas with high H2S (14.4%) and carbon dioxide (CO2) content (9.91%). The field is flanked on the west with a gas/water contact at 5140-m true vertical depth subsea. With no voidage-replacement mechanism in place yet, the production of water because of the contact movement is also a concern for the operator. Therefore, production logging as a reservoir-monitoring and -surveillance tool has been used since 2010, and more than 18 wells have been tested to date.
Gas in the Pugang field is mainly contained in the Lower Triassic Feixianguan and Upper Permian Changxing formations. These carbonates are open-platform carbonates, mainly platform-margin shoal and reef sediments. The Feixianguan formation is composed mainly of carbonate platform-margin shoal and evaporitic environments. The main porosity types in the Feixianguan formation include intercrystalline, solution-enlarged intercrystalline, solution vugs, and compressional microfractures. Normally, the solution porosity is connected by fractures, which gives rise to the high permeability observed in this formation....
Production-Logging Tools Facilitate Well Testing in Challenging Environments
15 January 2016