Experimental Study and Modeling of Gas Trapping in Foam Flow in Porous Media Using a Gas Tracer Technique
Creators: name: Li, Zhen
Publisher: name: The University of Texas at Austin
Year: 2008
Subjects: Foam
Gas trapping
Gas tracing

description: Foam has been used worldwide to improve acid placement in matrix acid treatments and reduce gas fingering/channeling in gas injection. Gas trapping is a crucial component of foam mobility and diversion for successful foam applications. This dissertation was aimed at a better understanding of gas trapping in foam flow and evaluating the accuracy of the existing gas tracer techniques for trapped gas determination. For this purpose, a one-dimensional (ID) model describing tracer mass transfer during steady-state foam flow was developed, which was then used to determine flowing gas fraction. The latter could be done by optimally fitting the model to effluent tracer concentration profiles that were obtained by either gas chromatography (GC) or X-ray CT image analysis. It was found that the flowing-gas fraction (X8r) decreased gas injection rate, increased with liquid injection rate, and that the corresponding mass transfer coefficient (aske) increased with liquid injection rate. For the first time, the statistical unce1tainty of the ID model parameters (Xgr and aske) was analyzed. 95% confidence inte1vals of Xgf obtained during fitting the ID model to the gas-chromatography measured tracer profiles ranged from 0.004 to O.I43. 95% confidence intervals of X8r in foams from Nguyen et al. 's study also showed a similar range of 0.03 to 0.076. The uncertainty of Xgr obviously increased with increasing liquid injection rate. For relatively wet foam, the unce1tainty appeared to increase with decreasing gas injection rate. 95% confidence intervals of the tracer mass-transfer coefficient for both GC and X-ray-CT-image-analysis detennined tracer effluent profiles showed the same trend that the unce1tainty of the estimated mass-transfer coefficient increased as liquid injection rate increased. The sensitivity sh1dy of residuals with respect to flowing-gas fraction and effective mass transfer coefficient was sh1died and thus the uncertainty intervals of X8r and a5ke were plotted in the format of surface response contours. The uncertainty intervals of Xgr and aske equivalent to 95% confidence intervals from statistical analysis are in good agreement. This indicated that the uncer1ainty obtained by statistical analysis can represent the true uncertainty of flowing-gas fraction and mass transfer coefficient, assuming the other assumptions made in the statistical the01y hold. A three-dimensional (3D) model of tracer mass transfer was also developed. The model qualitatively fit the in-situ tracer distribution profiles and quantitatively the reconstructed effluent profiles obtained from X-ray CT image analysis. However, the uniqueness of the fitted values of the flowing-gas fraction was not guaranteed. Furthermore, the 3D model was used to test the accuracy of currently used tracer methods for determining flowing-gas fraction. The most important findings were that none of these methods were able to distinguish between the two cases with flowing-gas fraction varied by a factor of two and that in-situ distribution of tracer strongly influenced the accuracy of the tracer methods.