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Many studies have been made relative to the gas resource potential of the Rocky Mountain region, but a search of the literature has revealed few forecasts as to what gas may be actually producible under a variety of wellhead price conditions. This study provides such forecasts and shows how the undedicated Rocky Mountain gas resources could compete with the Permian and Anadarko basins in the national gas market. The principal analytic tool used in this study was a model developed by ICF-Lewin Energy, Inc. entitled "The Oil and Gas Replacement Cost Analysis Model" (REPCO).1 This system provides the user with a resource base that is disaggregated by geographic, depth, resource quality, and certainty that is served by a flexible decision oriented software package. The system was designed to assist producers, pipelines, distribution companies and large-volume industrial consumers to respond to the challenge of changes in the regulatory climate and falling product prices. The largest single factor on the horizon for the realization of the Rocky's deliverability potential is the Thermal Enhanced Oil Recovery (EOR) market in California. The need there for long term, reliable supplies of natural gas has been the subject of great interest to pipeline companies who are competing for that market. The geographic proximity of the Rocky Mountain region together with the large resource base makes the area the prime candidate for providing that supply.

The first horizontal re-entry gas well was successfully drilled and put on line in the Sierra Chata field in the Neuquén basin in Argentina. This article compares the model forecast with the results obtained. The objectives were:to add gas reserves andto incorporate horizontal technology that will allow access thin or low permeability sands, in remote areas that would be economically marginal with vertical wells. Recent interpretation of 3-D seismic and geologic data indicated that the "D" Sands were better developed to the northwest of the vertical well SCh-17. In order to access gas reserves postulated to exist there, it was planned to drill a horizontal re-entry well utilizing the existing vertical well, SCh-17 as a re-entry conduit. From the well SCh-17 a re-entry was made at 244 m from the deviation point (KOP) in 1600 m MD, drilling slightly up-dip to 2975 m MD, using with a downhole motor and a MWD logging system. Shale-silt lithologies within the sand members affect the vertical transmissibility (TZ) between the "D" Sand members. Sensitivity runs were made using a reservoir simulator built to test the effect of such variables on the horizontal well productivity. For the most probable case, an initial flowrate of 17.7 MMscfd (502 Mm3/d) was predicted. A final cumulative production of 7.3 BSCF (206.7 MMm3) of gas was predicted by the numerical simulation model. Six months of production history confirmed the forecast and assumptions made in the well proposal and also encourage new horizontal prospects. This year, two additional horizontal wells have been proposed to be drilled in the Sierra Chata field applying some of the concepts proven by the subject well. At least one of which will include an additional deviation of the horizontal wellbore to intersect multiple target zones. Introduction The success obtained with horizontal wells in reservoirs with some similarities to the Mulichinco Sands1,2, suggested the possibility of applying horizontal well technology to the development of the Sierra Chata gas field. The Sierra Chata gas field occupies the southeast corner of the CNQ10 (Chihuidos) exploration block located in the Neuquén Basin, (see Figure 1). The field is located on the flank of the Chihuidos anticline which is approximately 100 km long by 50 km wide and is oriented in a NW-SE direction across the exploration block bearing its name. It is estimated that the origin of the structure dates back to the Jurassic with the last flexure having occurred in the Tertiary. Within the geographic column present in the subsurface, the Mulichinco and Quintuco horizons have confirmed hydrocarbons, with only the former exhibiting commercial quantities to date. The Figure 2 represents the stratigraphic column of the Neuquén Basin, including the Mulichinco formation. Several companies have drilled numerous exploratory wells in the area during the last twenty years. The interpretation of 2-D seismic identified a characteristic response of reservoirs having good porosity, which led to the drilling of the exploratory well SCh x-1 in 1993, and subsequent discovery of commercial quantities of natural gas at Sierra Chata3. The SCh x-1 well produced with an initial flow rate of 22,605 MSCFD (640 Mm3/d) of dry gas with a 1,000 psig wellhead pressure (WHP). To date thirty vertical and one horizontal well have been drilled in Sierra Chata, including six exploratory wells (two of which were unproductive).