Study of Skin Damage in Unconsolidated Sandstone by Computed Tomography

摘要

It is well known that the porous media invasion by components of drilling fluids decreases near wellbore permeability and it is one of the factors that generate poor well productivity or injectivity. Permanent changes on the original permeability decrease well flow capacity and only can be evaluated after a well production test. The quantification of the difference, the damage, is defined as the mechanical skin. Among others, two particular factors contribute for mechanical skin during the drilling process: reduction on the oil or gas saturation by the drilling fluid filtrate and reduction on the effective porous throat size by bridging or aggregation of solids and other components. The measure of the damage, its extent and remedial treatments has been studied since a long time using the API linear flow cell and filtration tests. However, the results obtained with this equipment do not allow an effective quantification that could be pplied directly to the field mainly because of the test geometry. In this study, a technique to quantify physically the damage has being evaluated, using a physical simulator composed by a X-ray transparent radial flow cell, hollow cylinder synthetic sand core sample saturated with mineral oil after full water saturation, the X-ray Computerized Tomography (CT) and a procedure to evaluate the permeability return. The invasion profile for water-base drilling fluid components were mapped after drilling simulation and the residual damage was quantified after an oil production period. The results of this study showed that: permanent changes occurred on the porous media structure, which incorporated drilling fluids components at the grain and oil saturation was dramatically reduced. Both damages were not removed with the reverse flow of oil, generating a relative oil permeability reduction that was calculated in terms of mechanical skin associated.