On the Subject of Reserve Estimates in Water-Drive Gas Reservoirs

Abstract

In his article in the JPT Forum, William Hurst resumes his discussion of our paper on reserve estimates in water-drive gas reservoirs. We can only make reference to our previous reply and add the following comments.1. The aims of our paper were:to show that the simultaneous determination of gas reserves and aquifer characteristics from the past history of waterdrive gas reservoirs can result in more than one set of solutions; andto evaluate the actual indetermination range of gas reserve estimates for some reservoirs that have a well known past history, where a water-drive had been ascertained, as they had been partially invaded by the water encroaching from edge partially invaded by the water encroaching from edge or bottom aquifers. 2. The detailed geological description of the gas reservoirs of the Po Valley, Italy, was presented in 1957 at the "Symposium on Gas Fields of Western Europe". A feature that is common to almost all of these reservoirs is their being in contact with aquifers of good permeability. 3. Contrary to William Hurst's statement that "the authors claim that they have water-drive reservoirs, but with their computer programs, they are unable to establish this effect", the presence of a water encroachment (calculated according to the van Everdingen-Hurst method) is clearly shown (Ref. 2, Table 3) in all sets of solutions presented for the reservoirs considered. The actual occurrence of a water encroachment into the reservoirs in question 2 has been proved by the following facts:a. In three of the reservoirs (BM-A, R-A, S-F)new wells have been drilled in the last few years. It has been ascertained that the water/gascontacts, at the moment the wells have been drilled, were 220 ft, 65 ft and 1 10 ft, respectively, abovetheir initial position in the three reservoirs inquestion.b. In BM-A field, 3 years after shut-down thereservoir pressure increased by 361 psi from 1,252to 1,613 psia, the initial reservoir pressure being1,927 psia.c. Some time after the production period consideredin our paper a marked deviation from linearityeventually occurred in the p/z vs Gp curve of allthe reservoirs considered. 4. Hurst's discussion seems to imply that linearity in the initial part of the p/z vs Gp curve is enough to prove a depletion-type of performance. That this does not prove a depletion-type of performance. That this does not apply has already been shown by others. There is no ground to support William Hurst's interpretation that the reservoirs in question are depletion-type and, as an unlucky consequence, to validate his "indication that considerable gas reserves may still exist" in those gas reservoirs of the Po Valley which have been abandoned. Of course, we are confident we are able to distinguish between a well needing only a work-over and a well that has been completely invaded by the encroached water (incidentally, we never wrote "logged up"). P. 561