Formation damage caused by overbalanced drilling with water-based mud (WBM) is inevitable as a result of mud filtrate invading the near-wellbore formation. The invasion radius is critical to the multiphase flow when the well is put on production. It contributes to the total skin that hinders the hydrocarbon production. Furthermore, the response of the logging tools may be affected as a result of such invasion, rendering many inaccurate calculations in formation evaluation. To evaluate the skin caused by mud-filtrate invasion, it is important to determine the radius of invasion. A thorough literature review indicated that no practical and reliable method with solid theoretical basis to quantify formation damage is available. Former studies assumed that single-phase drilling fluid displaces reservoir fluid during the invasion. The neglecting of residual reservoir fluid in the invaded zone will introduce error to invasion-radius estimation. This work takes the residual reservoir fluid into account; thus, the estimation of invasion radius is more accurate.

This work proposes a practical model to determine the depth of mud-filtrate invasion near the wellbore drilled by WBM. The distribution of mud-filtrate saturation in the near-wellbore region is also calculated by using drilling-operation parameters, mud-filtration-test data, relative permeability, and drilling time. With the accurately determined invasion radius and known wellbore radius, reservoir permeability, and damaged-reservoir permeability, one can evaluate skin factor more accurately. With the knowledge of invasion volume and radius, one can design the wellbore-cleanup procedure appropriately. The proposed model allows engineers to predict the well performance and to diagnose wellbore problems by checking any deviation from the predicted production. This study also can assist with the correction of parameters inferred from log measurements, thereby reducing the over- and/or underestimation of log-derived parameters used in various formation-evaluation calculations.

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