Analysis of Negative Revisions to Natural Gas Reserves in Texas

Early problems in overestimating effective porosity in some deep Delaware Basin carbonate reservoirs in the Permian Basin, particularly in District 8, resulted in noticeable negative revisions once these issues were resolved. However, the net negative volume of revisions for the Permian Basin (Districts 8, SA, and parts of 7B and 7C) was nearly an order of magnitude less than that for the Gulf Coast Basin. The largest negative revisions of total natural gas reserves were concentrated in the Gulf Coast within Texas Railroad Commission Districts 2, 3, and 4. District 4, with the largest volume of negative revisions, accounted for 56 percent of all negative revisions in Texas for the period 1966-1979. The total for the three districts equaled that of the whole state for the same period. Negative revisions of non-associated gas reserves in Districts 2, 3, and 4 accounted for more than two-thirds of the total gas negative revisions for the entire state from 1966 through 1979. Large negative revisions were mainly due to a combination of interrelated factors. Among these, the original overestimation of natural gas reserves in the Texas Gulf Coast, fueled by optimism from market-related incentives, was significant. These estimates underwent insufficient critical review and reassessment, as supplies greatly exceeded demand. Continued high Reserves to Production (R/P) ratios into the 1960s further delayed reassessment. Technical variables such as water saturation, reservoir heterogeneity, and recovery factors, as well as non-technical variables including economic climate and regulatory controls, were analyzed. Concern should be raised regarding the quality of reserve estimates declared during times of excess supply, as these reserves would not have undergone the test of extended maximum demand. However, there have been more frequent reviews of actual recoverable reserves over the last five years. Continued careful review of technical factors and awareness of the impacts of economic and regulatory environment changes suggest that extensive negative revisions over the next 10 to 20 years can be avoided.Bureau of Economic Geolog

Analysis of Negative Revisions to Natural Gas Reserves in Texas

The role of negative revisions in the large-scale decline in natural gas reserves in Texas during the late 1960s and through the 1970s was examined. Analysis of the factors that contributed to the negative revisions determined that no single element was responsible. However, (1) continued high levels of production, (2) original optimistic estimates of gas in place and recovery factors, (3) market-related factors that encouraged overestimation of reserves, and (4) unusually high reserves-to-production ratios (> 15) that obscured the underlying weakness in reserves combined in the Texas Gulf Coast to drastically reduce booked reserves of natural gas. Negative revisions totaling more than 20 trillion cubic feet during the period were found to have resulted mainly from ambiguities in the degree of reservoir heterogeneity, in calculation of water saturations, and in drive mechanism, along with the overestimation of reserves due to optimism encouraged by market-related incentives. The much-reduced reserves-to-production ratios that now exist, along with continued closer monitoring of technical, economic, and regulatory factors that affect gas reserves, indicate that a return of extensive negative revisions over the next 10 to 20 years is avoidable.Bureau of Economic Geolog

Opportunities for Additional Recovery in University Lands Reservoirs -- Characterization of University Lands Reservoirs, Final Report

In 1984, The University of Texas System funded a Bureau of Economic Geology project, "Characterization of University Lands Reservoirs," to assess in detail the potential for incremental recovery of oil from University Lands reservoirs by extended conventional methods. The objectives of the 5-year project were to quantify the volumes of unrecovered mobile oil remaining in reservoirs on University Lands, to determine whether the specific location of the unrecovered mobile oil could be delineated through integrated geoscience characterization of individual reservoirs, and to develop strategies to optimize recovery of this resource. Unrecovered mobile oil is mobile at reservoir conditions but is prevented from migrating to the wellbore by geologic complexities or heterogeneities. This final report describes results of the 5 years of research conducted on University Lands reservoirs. One hundred and one reservoirs, each of which has produced more than 1 million stock tank barrels (MMSTB) of oil, were included in a resource assessment and play analysis undertaken (1) to determine the volumes and distribution of all components of the University Lands resource base and (2) to select reservoirs for detailed analysis. These reservoirs collectively contained 7.25 billion barrels (BSTB) of oil at discovery, have produced 1.5 BSTB, and contain 200 MMSTB of reserves. Ultimate recovery at implemented technology is projected to be 24 percent of the original oil in place; thus, 5.5 BSTB of oil will remain after recovery of existing reserves. Unrecovered mobile oil (exclusive of reserves) amounts to 2.2 BSTB, and immobile, or residual, oil totals 3.3 BSTB.Bureau of Economic Geolog

Water Management by the Electric Power Industry

Center for Water and the Environmen