Historically, an opinion has been formed that invasion of the mud filtrate in the near-wellbore zone introduces a positive skin factor. Indeed, the magnitude of the positive skin factor can high, even in completely penetrated wells. And yet, thousands of pressure buildup curves annually recorded in Tatneft PJSC support the inverse trend – average distribution of the total skin factor approximates to -3.7. The underlying reasons can be diverse. First of all, one must differentiate between the total skin factor determined form pressure buildup curves and its constituents, geometrical and mechanical skin factors. A large number of production enhancement operations, including well stimulation and fracking, performed by the operator introduce the negative skin factor. Average distribution of the mechanical skin factor approximates zero, significantly differing, thus, from the total skin factor. This fact also raises suspicions of experts, who reason that the parameter characterizing the bottomhole zone should not be but positive. This discrepancy can be explained by a simplified interpretation of the mechanical skin factor. In reality, it is influenced by a number of drivers that can either increase, or decrease the skin factor. Positive contribution to the mechanical skin factor may be the result of friction losses from formation to the pressure gauge, etc.; negative contributions result from non-linear-viscous oil properties, physicochemical interaction of fluid with the open surface of pores, mechanical damage of open hole completions in incompetent formations, natural fracturing of sandstone reservoirs, behind-the-casing flows, active water-saturated intervals, etc. Considering the diversity of forces influencing the skin factor, the cause of any particular skin factor magnitude should be found out using all available reservoir and well data.
It should be noted that the persisting myth of the bottomhole zone serious damage is demolished by actual surveys. The results of earlier experiments on comparison of formation damage resulting from conventional and underbalance perforation and modeling of flow to perforations confirm the trend. In majority of cases, the mechanical damage caused by drilling fluids is removed by adequate perforation and completion.
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