On the determination of the skin factor and the turbulence term coefficient through a single constant gas pressure test

The best correlation for evaluating the turbulence term coefficient, D( mu) , for transient flow in gas wells under constant flow rate conditions is cu rrently that of Lee et al. [Lee, R.L., Logan, R.W., Tek, M.R., 1987. Effect of turbulence on transient flow of real gas through porous media. SPE Form . Eval. (March), 108-120]. Several authors have used this correlation to ca lculate the production decline for wells that produce under constant bottom hole pressure conditions, but it is not clear if this is comet. When exploi ting any gas reservoir, the flow rate and the average reservoir pressure de crease with time. This process makes D(mu) increase, whereas D(mu)q(sc) dec reases with time; in contrast, when a well is produced under constant flow rate conditions, D(mu)q(sc) increases with time. Due to this fact, it can b e of considerable practical interest to analyze the behavior of the high ve locity pseudoskin factor, D(mu)q(sc), for gas wells that produce under cons tant bottomhole pressure conditions. This paper presents the results of a s tudy aimed at obtaining a correlation of both the mechanical skin factor, s , and the turbulence term coefficient, D(mu), for constant bottomhole press ure conditions through the analysis of a single test. Results obtained thro ugh a radial real gas flow simulator, carefully validated for pseudosteady state conditions were used as a database for the development of the new cor relations. The skin effect considered varies from 0 to 10. Two different ga ses are considered. Other variables studied are the constant pressure, the reservoir size r(e)/r(w), and formation permeability. The correlation is pr esented by means of straight-line fits of s(t)/[s(1 - r(w)/r(d))] in the ve rtical axis and D(mu(i))q(sc) in the horizontal axis. The correlation coeff icient was 0.9976. The error obtained when one evaluates the skin factor, s , using the correlation presented in this work is less than 10%. The error is substantially less when production rates are low. These rates strictly c orrespond to the case of low permeability reservoirs, where the constant pr essure boundary condition considered is this study closely apply. (C) 2000 Elsevier Science B.V. All rights reserved.