--> --> Occurrence of volcanic ash beds in the Wolfcamp and Spraberry of the Midland Basin and their potential challenges for drilling

AAPG Southwest Section Annual Convention

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Occurrence of volcanic ash beds in the Wolfcamp and Spraberry of the Midland Basin and their potential challenges for drilling

Abstract

Multiple instances of volcanic ash beds are identified during active drilling within the Wolfcamp shale and Spraberry formation in the Midland Basin of west Texas. These ash beds were initially observed in vertical cores using ultraviolet light and can range between 1 cm to 15 cm (0.5 ft), with most 3 cm or less. During drilling of recent horizontal wellbores, operations has logged cuttings containing 5% ash to greater than 80% ash per sample. Volcanic ash is identified in wells located throughout the entire N-S length of the central portion of the Midland Basin on both the western and eastern margins. Stratigraphically, the majority of volcanic ash occurrences are within the Wolfcamp A and Wolfcamp B operational units. A possible source for these volcanic ash-fall deposits is a Permian-aged volcanic arc located in present-day NE Mexico. Recent drilling and operational issues have occurred in some high-percentage ash-bearing zones that include tight wellbore, wellbore pack-off, directional control issues, and increased drag while setting casing. In some instances, drilling issues were severe enough to require sidetracking the well or setting casing short of planned total depth. Possible problems caused by volcanic ash may include preferential break-out due to low rock strength, sloughing in the wellbore, ineffective wellbore cleaning, and altering the drilling fluid. These adverse effects may slow down drilling performance. Studies are ongoing to identify regions and target zones that contain a high occurrence of ash and to assess the severity of potential challenges during drilling and completion operations.