Underground gas storage project Breitbrunn/Eggstätt - Applied geosciences in planning, steering, and formation evaluation of the storage wells

From February 1999 up until December 2000 one pilot hole and subsequently six storage wells were drilled into the Tertiary C and D Chatt Sands at 2000 m TVD which form part of the depleted Breitbrunn/Eggstätt gas field. The storage wells have been completed with 7 cemented liners placed in 8.5 horizontal sections extending up to 960 m. For proper targeting of the 12.25 build-up sections structural mappings based on 13 historical exploration and production wells had to be performed. Anticipated lithological marker depths in overburden and target thus derived have been confirmed with less than 1.5 m TVD depth error using RWD real-time data acquired in the subsequent drilling operation. Extended rock mechanical and hydraulic analyses on rock samples taken from the extensively cored Chatt Sand sections in the pilot well served as a basis for the optimal orientation of the horizontal sections, proper choice of the mud system, and the final perforation strategy. Due to the heterogeneity of the deltaic Chatt Sands the storage wells had to fully transect their gross thickness. Drilling out of the sands into the unstable marly clays above and below had to be avoided. At last, keeping in mind the complex lithological composition of the sands deposited from the adjacent Alpines, high demands had to be met by appropriate LWD geosteering equipment. Real-time data and images front azimuthal resistivity and density measurements were used to keep the well trajectories in the target. Final petrophysical interpretations and borehole stability analyses were based mainly on time-lapse LWD drill and ream run data. These data aided hi identifying mud invasion as well as fracture and breakout initiation dynamics. For the proper placement of perforation intervals, their orientation and shot density, oriented porosity cut-offs derived from the LWD data as well as compressive strength data derived from calibrated pipe conveyed full wave sonic logs were incorporated. Two remaining questions relevant to long term storage and field performance which are related to both, the theory of sonic logs and Boolean reservoir modeling, remain unanswered so far. Novel interpretation methods for their solution are currently evolving.