Alcohol-free micellar solutions which yield high solubilization ratios (15-30 cc/cc), high final oil recoveries and eliminate the use of alcohol, a costly chemical, have been formulated for application in chemical floods.A major difficulty was to preclude gels, liquid crystals, macroemulsions and precipitates along the composition path during a chemical flood. A screening strategy was applied to select among combinations of quite different surfactant structures. Corefloods were run with a blend of pure surfactants (C180CH2 o CH2-S03Na, 8eC16S03Na and 7eC14S03Na), an ethoxylated sulfonate (6C16(OCH2° CH2)2S03Na and a blend of an internal olefine sulfonate and a petroleum sulfonate. A slug size-efficiency curve obtained from corefloods with screened alcohol-free formulations, was compared to previous data with a conventional petroleum sulfonate/alcohol system (IRS 10-4l01lBA) at 65 0 C taken as our basecase.An equimolar blend of an internal olefin (C20-C24) sulfonate and a petroleum sulfonate gave a final oil recovery of 94% with a 13% Pvi slug size, 3v"/o surfactant concentration, at 80 0 C. When the slug size was reduced to 3% Pvi, the oil recovery still was 80%. Final results are compared to the basecase and other combinations tested and reported under comparable conditions.

Ten polyacrylamides of different molecular weight and in different forms (powder, broth, emulsions) and three biopolymers were tested in order to evaluate their suitability for a EOR project in Catriel Oeste oil field in Rio Negro province, with very high water salinity. Screening involved: a) Resistance (RF) and Residual Resistance Factor (RRF) through fritted glass filters and viscosity measurement of solutions with different polymer concentrations, b) RF and RRF in actual pay zones cores with the selected polymers, c) ageing test at reservoir temperature, d) and oil recovery flood tests with final selected poliacrylamide. Laboratory tests allowed a suitable polyacrylamide to be selected and an optimal concentration of the solutions to be injected in the field chosen, which have been used for pilot test simulation and detailed design. RF in actual cores gave satisfactory results.

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).

The first horizontal re-entry gas well was successfully drilled and put online in the Sierra Chata field in the Neuquén basin in Argentina. This articlecompares the model forecast with the results obtained. The objectives were:to add gas reserves andto incorporate horizontal technology that willallow access thin or low permeability sands, in remote areas that would beeconomically 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 todrill a horizontal re-entry well utilizing the existing vertical well, SCh-17as a re-entry conduit. From the well SCh-17 a re-entry was made at 244 m fromthe 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 verticaltransmissibility (TZ) between the "D" Sand members. Sensitivity runs were madeusing a reservoir simulator built to test the effect of such variables on thehorizontal well productivity. For the most probable case, an initial flowrateof 17.7 MMscfd (502 Mm3/d) was predicted. A final cumulative production of 7.3BSCF (206.7 MMm3) of gas was predicted by the numerical simulation model. Three years of production history confirmed the forecast and assumptionsmade in the well proposal and also encourage new horizontal prospects. Introduction The success obtained with horizontal wells in reservoirs with somesimilarities to the Mulichinco Sands1, 2, suggested the possibilityof applying horizontal well technology to the development of the Sierra Chatagas field. The Sierra Chata gas field occupies the southeast corner of theCNQ10 (Chihuidos) exploration block located in the Neuquén Basin, (see Figure1). The field is located on the flank of the Chihuidos anticline which isapproximately 100 km long by 50 km wide and is oriented in a NW-SE directionacross the exploration block bearing its name. It is estimated that the originof the structure dates back to the Jurassic with the last flexure havingoccurred in the Tertiary. Within the geographic column present in thesubsurface, the Mulichinco and Quintuco horizons have confirmed hydrocarbons, with only the former exhibiting commercial quantities to date. The Figure 2represents the stratigraphic column of the Neuquén Basin, including theMulichinco formation. Several companies have drilled numerous exploratory wellsin the area during the last twenty years. The interpretation of 2-D seismicidentified a characteristic response of reservoirs having good porosity, whichled to the drilling of the exploratory well SCh x-1 in 1993, and subsequentdiscovery of commercial quantities of natural gas at Sierra Chata 3.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 verticaland one horizontal well have been drilled in Sierra Chata, including sixexploratory wells (two of which were unproductive).