Abstract
Skin factor is often regarded as a constant in most of the mathematical model for well test analysis in oilfields, but this is only a kind of simplified treatment with the actual skin factor changeable. This paper defined the average permeability of a damaged area as a function of time by using the definition of skin factor. Therefore a relationship between a variable skin factor and time was established. The variable skin factor derived was introduced into existing traditional models rather than using a constant skin factor, then, this newly derived mathematical model for well test analysis considering variable skin factor was solved by Laplace transform. The dimensionless wellbore pressure and its derivative changed with dimensionless time were plotted with double logarithm and these plots can be used for type curve fitting. The effects of all the parameters in the expression of variable skin factor were analyzed based on the dimensionless wellbore pressure and its derivative. Finally, actual well testing data were used to fit the type curves developed which validates the applicability of the mathematical model from Sheng-2 Block, Shengli Oilfield, China.
Highlights
Skin factor is a parameter which is used to evaluate the degree of influence of formation permeability around the wellbore by external factors.[1,2] The conception of “thin-skin mathematical model” was initially proposed to adjust the error of bottom hole pressure between theoretical calculation and measured data by van Everdingen.[3]
The variable skin factor derived was introduced into existing traditional models rather than using a constant skin factor, this newly derived mathematical model for well test analysis considering variable skin factor was solved by Laplace transform
1) This paper established a well test analysis model of considering variable skin factor with the model expressed as a relationship between skin factor and time by defining two parameters that are β and viD
Summary
Skin factor is a parameter which is used to evaluate the degree of influence of formation permeability around the wellbore by external factors.[1,2] The conception of “thin-skin mathematical model” was initially proposed to adjust the error of bottom hole pressure between theoretical calculation and measured data by van Everdingen.[3]. Larsen and Kviljo studied cleanup effects which could change skin factor.[18] The variable skin factor was represented by a simple hyperbolic function of time which was absent in theoretical evidence. It can be used universally to conduct well test in single oil phase flow, especially in super-low and -high permeability oil reservoirs, their permeabilities around well area change significantly in short time for these two kinds of reservoirs. In effect, these results supplement the content of modern well test analysis
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