Horizontal Wells in Melones Heavy Oil Field
Authors: Reina Manolo 
Published: December 2007

Abstract
From 1994 to 1997, 66 horizontal wells, including 29 re-entries, were drilled and completed in the Melones field, This is a heavy oil field producer (10.5 API) since 1956, using, until 1994, 7 vertical well patterns with 620 and 720 meters spacing. Original Oil in Place in the Melones field was 9.5 MMMSTB, its estimated primary reserves were 0.7 MMMSTB, equivalent to a 7.3 percent recovery factor. Using horizontal wells, in some cases combined with steam injection, the recovery factor has been estimated at 14 percent. In 1994 the horizontal drilling campaign was initiated in two groups of the sands, where 80 percent of field reserves exist. Thus far, the results are very interesting; the maximum length reached of the horizontal lateral has been 2,000 feet, length limited only by drainage area of the existing vertical wells. The measured Productivity Index (PI) shows an exponential performance when it is related to the LH factor, which is equal to the effective length of the horizontal lateral (total length of the horizontal lateral minus length of the navigation in shale) times the oil bearing net sand thickness. The maximum PI determined was 9.4 STB/D/psi for a LH equal to 95,000 square foot. A similar performance of the PI was obtained for the drainage volume. Initial production rates, in the best horizontal wells, have been in the order of 1,700 STB/D, compared to 200 STB/D in the vertical wells. Drilling has not presented great problems since hole stability and sand production is in the order of one milligram per one liter of fluid sample. The horizontal wells are completed with slotted liners between 0.012 y 0.018 inches. This allows the use of Electrical Submersible Pumps in wells with PI's of more than 3.0 STB/D/psi. Other parameters evaluated at present to design the horizontal well completion, additionally to the length of the horizontal lateral, are: reservoir heterogeneity, sand thickness and structure, reservoir pressure, drainage of the area, and crude viscosity. The geologic model has been validated in most of the cases. In other cases, sand bed dip and the fault location has been determined with enough accuracy. Due to the success in this phase and with the experience obtained, we will continue increasing the field production potential through horizontal drilling. In accordance with simulation studies made for the area and partial production results obtained, horizontal well productivity increases with length of the horizontal lateral. Therefore, our 1997 program contains 1,800 feet horizontal wells in infill drilling zones and 3,000 feet horizontal wells in the reservoir area with distant wells.

Introduction
Horizontal drilling in the Melones field was initiated in 1994 with the re-entries to existing wells, and was continued with new wells in 1995. Until May of this year, we have completed 29 re-entries and 37 new horizontal wells. The 1994, 1995 and 1996 drilling programs, were located in areas of high well density with spacings varying between 620 and 720 m. These big spacings allowed to include horizontal wells, re-entries and new wells, with lateral lengths up to 1800 feet. The 1997 program includes 1800 feet wells, and additionally, wells with lateral lengths above 1800 feet, that we will locate in back yard areas. Geological and reservoir analyses results are been applied in actual drilling programs to improve production performance.