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 applied 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.
The resistance to flow and consequently increase of the differencial pressure needed for fluid drainage in a porous media is controlled by the porous geometry, fluid viscosity and fluid velocity. This relationship for linear or radial flow is well defined by the Darcy's law. An additional resistance to flow around the wellbore is given by changes in the porous size due to confinement forces or invation of particles or drilling fluid components. These changes will depend on the drilling, completion and production practices.
Van Everdigen 1 defined skin as the additional resistance to flow that reduces the capacity of production in oil wells. Hawkins 2 formulated a mathematical equation that relates the skin effect with the affected radius and permeability of the damage zone. Hurst 3 et al showed mathematically how to overcome the difficulties found in the application of negative skins. They assumed that the effective radius of the wellbore went larger than the radius of the well in subject and modified the existent mathematical solutions to include this effect. Also, a positive skin can be treated in the opposite way. Person 4 et al concluded that the skin damage for a formation around the wellbore in horizontal wells can severely reduce the production of gas, especially when the ratio between the length of the well and the surface of drainage is small and the vertical permeability is high. Parn-Anurak and Engler 5 developed a method that simulates the invasion of the drilling fluid in a two-phase system and evaluates the damage in the wellbore. The model proposed includes filtrate invasion, filtercake formation, and relative permeability components. The filtrate and filtercake models were developed based on the equations of mass balance and Darcy and the system was solved numerically to characterize filtrate invasion. The simulation results anticipate the distribution of filtrate, indicate the maximum depth of invasion and also the invasion profile for the drilling fluid components near the wellbore. The method improved the characterization of the filtrate and the damage estimation along a horizontal well.
The principal objective of this paper is to propose a methodology to evaluate the mechanical skin in a radial flow. The paper analyzes the damage generated by the invasion of a water based polymeric drilling fluid into the synthetic core samples and estimates the skin factor and the permeability reduction.