Abstract

Estimating the recoverable oil and gas from an organic shale refrac or what should be more correctly called a "recompletion" is consistently the top concern of students in the refrac and new completion "best practices" industry courses taught by the author. The term "refrac" applies to bullhead diverter treatments that re-stimulate existing perforations and these are no longer a "best practice" in most cases (Barba 2024). Once mechanical isolation is employed and legacy perforations are sealed off permanently the treatment becomes a "recompletion" of previously unstimulated portions of the reservoir. There are two main methods recommended to estimate these stranded volumes. The first method involves an estimate of oil or gas in place from open hole logs and applying the expected post recompletion total recovery factor to those volumes. The cumulative production to date is subtracted from the total post recompletion expected EUR (estimated ultimate oil or gas recovery). It has the advantage of an estimate of recoverable hydrocarbons with a 100% cluster efficiency completion where operators may not have used "best practices" to maximize cluster efficiency. The first technique can have limitations when open hole log data coverage is poor, producing heights are uncertain, or if there is significant heterogeneity in the local geology. The second method uses offset modern completions in analog oil or gas in place portions of the field to estimate the future recompletion total EUR/ft. The expected total EUR is then adjusted for the recompletion candidate gross perforated interval to forecast the total post recompletion recovery. The cumulative production to date is again subtracted from that value. It can be used in areas with limited well log control if reservoir properties are relatively homogeneous between the recompletion candidate and the producing new wells. The main drawback to the offset EUR/ft method is when operators do not employ "best practices" to maximize cluster efficiency. This study compares the two estimates in areas with extensive open hole log coverage and production data and discusses the advantages and disadvantages of each method.

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