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The effects of the unswept zone mobility (kh/u) on steam injectivity were investigated. Six falloff tests were run in a steamflood near Maricopa, California. The tests were analyzed using the radial composite model with the aid of the Barua-Horne and the Kappa Saphir well test analysis software packages.
Computer aided matching of the radial composite model with steam injection well falloff data provided remarkable agreement between field data and model simulation. It was possible to find skin, wellbore storage, steam zone mobility and compressibility, and the mobility and compressibility of the zone ahead of the stern front. Low mobility ahead of the steam zone was found to limit steam injectivity in several field cases.
In 1979 Eggenschweiler et al found that the radial composite model had an unexpected property which made it suitable for thermal injection well falloff pressure transient analysis. Wellbore storage and skin effects lasted for short time (seconds) then there was a semi-log radial flow period for about one hour which contained the swept zone properties, followed immediately by a pseudo-steadystate Cartesian straightline which contained the swept zone volume for gas injection with gravity override. The pseudo-steady period appeared to last for about five hours. Thus, the properties of this pressure transient test were ideal. The test could be completed in a short time.
Inspection of many sets of field test data indicated that the procedure was appropriate for both insitu combustion and steam injection. However, the swept volume usually appeared too large for both kinds of tests.
Onyekonwu found it was necessary to reduce the apparent combustion zone volume by half. Numerical simulation showed the pressure test measured both the combustion zone and a high mobility zone volume in the steam plateau in combustion. Walsh et al showed that the steam zone volume was controlled by a very large adiabatic compressibility due to steam condensation.
Messner and Williams applied the pseudo-steadystate method to steam injection falloff interpretation. A notable result of their study was that it was sometimes difficult to identify the correct pseudosteady line.
Eggenschweiler et al observed that there was a decades-long transition zone between the initial semi-log straightline for the swept zone and the second semi-log straightline for the mobility of the region ahead of the swept zone. As a result, they concluded that it was likely that producing well interference would preclude measuring the mobility of the region ahead of the swept zone. However, Barua and Norne considered computer regression matching of thermal falloff data with the radial composite model and concluded that both the mobility and porosity-compressibility product in the swept zone and ahead of the swept zone could be found as well as swept zone radius, wellbore storage and skin effect. This procedure was better for finding swept volume than the Cartesian straightline method.
The results of Barua and Horne were impressive because experience had shorn it was often difficult to identify the pseudo-steady line correctly. They also showed comparisons between field data and simulated data which were in remarkable agreement. Thus, computer aided analysis by nonlinear regression showed it was possible to find the mobility ahead of the swept zone even though no semi-log straightline was evident.
This observation was of critical importance to the present study.
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