--> --> Burnett Ranch: A CO2 Flood on the Eastern Shelf of the Permian Basin, King County, Texas

AAPG Southwest Section Annual Convention

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Burnett Ranch: A CO2 Flood on the Eastern Shelf of the Permian Basin, King County, Texas

Abstract

The Burnett Ranch Field, located in King County, lies on the Eastern Shelf of the Permian Basin. Consisting of multiple—and in certain areas, stacked—reservoirs and multiple fields, the Burnett Ranch boasts a long and storied history, both with regards to its geology and petroleum system. The primary reservoirs in this area are Pennsylvanian-aged Strawn sandstones, with secondary Strawn limestones, Permian-aged Tannehill sandstones, Pennsylvanian- aged Bend conglomerates, Mississippian-aged Marble Falls-equivalent limestones, and the Ordovician-aged Ellenburger carbonates. The first marginally successful well was drilled in the field in the 1950s, but it was not until the 1970s when primary production sharply increased with the discovery of the major oil fields in the area. The Burnett Ranch changed operators multiple times thereafter, and secondary production efforts began in the 1980s. Hunt acquired the acreage in the early 1990s and initiated tertiary production in the 2010s, which continues to this day. The primary reservoirs in the field are the Twin Peaks Sands, the Strawn Lower Sand, the Strawn 5400’ Sand, and the Strawn 5400’ Lime. All four of these reservoir intervals are alternately flooded with both CO2 and water based on individual well production. There are currently 36 injectors and 46 producers, around which injection- centered patterns are drawn. The traditional five-spot injection pattern is not used in the field due to the considerations of topography, the use of existing wellbores, and the need to optimize locations with stacked reservoirs. The CO2 flood is currently progressing from the northern end of the field to the south. Injectors on the outskirts of the CO2 flood are used to create higher pressure in the downdip reservoir to keep the CO2 inside the hydrocarbon-bearing portion of the reservoir. Once injected into the reservoir, CO2 at certain temperatures and pressures will become miscible with the residual oil left behind after primary and secondary production. The resulting mixture of oil and CO2 has a reduced viscosity, which allows it to flow more easily through the reservoir. The purpose of alternating CO2 injection with water is to force the CO2 down new pathways through the reservoir—the water fills the CO2-created pathways and block them off from any CO2 injected thereafter. This is the mechanism by which the life cycle of a field that otherwise would have been abandoned is extended, allowing it to produce an additional 10-15% OOIP.