Successfully Managing Drilling-Fluid Losses in Multiple Highly Depleted Sands

摘要

Successfully drilling through depleted sands to reachrndeeper targets often demands a delicate balancing actrnbetween maintaining mechanical stability in normally andrnover-pressured shales, while controlling fluid losses duernto hydraulic fracturing. A host of considerations go intornthe planning process, including whether furtherrnproduction from the depleted zones is planned.rnFor a porous formation, it is generally accepted thatrnhydraulic fracturing pressure is partially dependent uponrnformation pore pressure.1,2 However, the modelsrncommonly used to predict both fracture initiation andrnfracture propagation are often unable to predict thernchanges in hydraulic fracturing pressures due torndepletion. The authors believe this is due more to therndifficulty in predicting the data required to link porernpressure variations to stress, rather than a deficiency inrnthe existing models. More significantly, these modelsrntypically assume no flow, and thus are not valid wherernthere is significant fluid loss to the formation.rnDue to the prohibitive cost of a new North Sea well, arnstability analysis was performed to determine thernfeasibility of sidetracking from an existing well and thenrndirectionally drilling through two (2) highly depleted sandrnformations with a single hole section.rnThe case study presented compares the originalrnmechanical stability and hydraulic fracturing estimatesrnwith the actual mud weights used to successfullyrncomplete this difficult drilling operation. Descriptions ofrnhole instability and fluid losses are presented along withrnthe drilling fluid parameters used to combat them.rnMethods used for estimating formation pore pressure, insiturnstresses and formation rock properties are alsorndescribed. In addition, possible explanations for therndifferences between the planned and actual stabilityrnestimates are explored, with future recommendations forrnsimilar operations.