Root-Cause Analysis of Drilling Lost Returns in Injectite Reservoirs

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 174848, “Root-Cause Analysis of Drilling Lost Returns in Injectite Reservoirs,” by Scott Buechler, Jing Ning, Nidheesh Bharadwaj, Kaustubh Kulkarni, Caleb DeValve, Carsten Elsborg, Timothy Head, and Arnout Mols, ExxonMobil, prepared for the 2015 SPE Annual Technical Conference and Exhibition, Houston, 28–30 September. The paper has not been peer reviewed. The work presented combines a fundamental-physics approach with field data to identify the root cause of drilling failures in Paleocene and Eocene injectite-sand intervals. Study results showed that unfavorably oriented sand/shale interfaces, which can occur more frequently at the top of an injectite reservoir, can reopen at equivalent circulating densities (ECDs) below predicted fracture gradients, but above the minimum stress, and can result in massive losses. Theoretical Approach and Application Diagnosing narrow-margin drilling challenges in injectite sands has proved to be extremely difficult because of the convoluted nature of available drilling data, statistically small data sets, and often contradictory “apples-to- apples” comparisons made on the basis of simple metrics. Two mechanisms have seemed probable: (1) suboptimal hole cleaning leading to packoff and subsequent lost returns, or (2) lost returns leading to suboptimal hole cleaning and subsequent packoff. Wellbore Cleaning. In this work, wellbore cleaning is evaluated with a proprietary transient-hydraulics and –hole-cleaning model. This model is based on first-principle equations of mass and momentum transport for drilling fluid and cuttings along the wellbore in the annular space between the drillpipe and casing (or open hole). The key physical effects during the cuttings-removal process are captured through constitutive models for frictional pressure drop in a circulating mud, heat transfer between the formation and drilling fluid, cuttings deposition/ erosion, and particle/liquid drag force. The partial-differential equations of mud and cuttings transport are discretized along the wellbore, and the entire system of equations is solved simultaneously in an implicit manner for each timestep. With this approach, the efficiency of hole cleaning can be characterized for these wells. Additionally, the effect of both lost returns and breakout can be incorporated into the models.