Challenges to Geosteering and Completion Optimization of Horizontal Wells in the Cotton Valley Formation, East Texas

Summary Cotton Valley tight gas sands in the East Texas basin of North America consist of very-fine-grained, well-sorted quartz arenites and subarkoses that are overprinted by significant diagenetic processes. Stratigraphic variations in rock type control gas production. Hydraulic fracturing delivers economic gas production. We drilled horizontal wells more than 4,000 ft long consisting of more than 12 hydraulic-fracturing stages. The good gas-producing rock-type reservoirs are usually less than 14-ft thick and exhibit strong diagenetic overprint. Low gas prices challenge the use of sophisticated geosteering logs. By use of a very-basic gamma ray (GR) log and very-limited offset-pilot-well data, chasing such thin and variable sand bodies over a distance of 4,000 ft in a marginal marine sedimentary environment is daunting. Apart from this, horizontal wells that target thin layers present unique challenges to completion optimization. We acquired quad-combo log data, including azimuthal-density image data, by use of a logging-while-drilling (LWD) tool. We performed extensive log modeling by combining the horizontal well logs with logs from the two offset vertical wells, and achieved consistent interpretation. Log modeling has helped the post-drill well diagnosis in geosteering, completion design, and production performance. It has also supported formation evaluation. This study will highlight an integrated-study work flow in tight gas sands by use of openhole and cased-hole data. It will demonstrate the geosteering challenges, explain the log-modeling process, and display the formation-evaluation results. Geomechanical study, together with hydraulic-fracturing data and production-log data, will be used to confirm that good gas-producing rock types are easier to fracture, and they contribute better to production.