Two horizontal wells were drilled into a thick, steeply dipping reservoir at the Naval Petroleum Reserve No. 1, Elk Hills, California. The formation beds dip up to 60 degrees in some parts of the reservoir. Drainage of fluids, therefore, into the horizontal sections is along bedding planes and not across them. Under these conditions the cited importance of high vertical permeability in horizontal wells becomes immaterial. In the support of the placement, drilling, and completion of these horizontal wells, reservoir layering and characterization was developed in a detailed form.
The first horizontal well was a medium radius redrill. Drilling and completion problems associated with drilling a new horizontal well out of an old vertical well, drilled and completed in 1951, are given. The second horizontal was a medium radius grassroots well. Both wells have tangent sections at 45 degrees [.78 rad] and horizontal sections placed 40 feet [12 m] above the estimated water-oil contact. The gas/oil contact is about 250 feet [76 m] updip from the horizontal sections. Points of comparison are made between these two types of horizontal wells and offset vertical wells.
Both horizontal wells have shown significant productivity improvement over conventional, vertical wells. Typically, conventional wells are produced at several hundred barrels of oil per day to control excessive gas production from this pressure maintained reservoir; however, the horizontal wells produce over 1000 barrels [159 m3] daily at solution gas. The cost to drill and complete these horizontal wells was double conventional costs, but the improved oil recovery from them will amount to an estimated 10 million barrels [1.6 million m3] of additional oil per well, not otherwise recoverable from conventional vertical wells. The potential for improved oil recovery overall, therefore, from horizontal wells could lead to an ultimate oil recovery as high as 70 percent of the estimated 382 million barrels [61 million m3] originally in place (OOIP).