First Horizontal Well Opens New Gas Opportunities in the Sierra Chata Field, Neuquén Basin-Argentina

Abstract

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).