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Formation damage can significantly defer hydrocarbon production. Formation damage is one of the key parameters used to evaluate the quality of various operations during the life of a well. The evolution of formation damage over time is rarely studied. This paper presents the pressure transient analysis (PTA) used to derive a correlation between skin factor increase and the length of time for which a well was left exposed to drilling fluid. Multiple field examples are analyzed and modeled comparatively to evaluate the magnitude of the time-lapse formation damage. A quantitative relationship between the increase in mechanical skin as a function of exposure to drilling mud is constructed by a polynomial fitting. This gives us an insight on the how the well skin increases with exposure to drilling mud. Several mathematical models have been developed to model formation damage mechanisms. However, it is cumbersome to isolate and compute the individual mechanisms contributing to observed damage. Formation damage results from a combination of the pore size distribution, fines migration, fluid compatibility problems, wettability, presence of inorganic salts deposition, insufficient well cleanup, organic substance deposition, multiphase flow effects, etc. The proposed approach uses the skin factor derived from pressure transient analysis (PTA) to track the time-lapse evolution of formation damage during exposure time to mud, from drilling to production. The methodology has been applied to various reservoir rocks to quantify the impact for a specific rock type. Results from pressure transient tests conducted immediately after drilling, casing, and perforating are compared with production tests performed multiple years after drilling confirm that there is a mathematical relation between the formation damage, type of drilling and completion fluid, formation rock type, and exposure time to such fluids. The formation damage increase is more severe for longer duration of exposure, and also more severe for higher formation permeability. |
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