This field is productive in the Cotton Valley Limestone formation (also called the "Cotton Valley Lime") at a vertical depth of approximately 9,500 feet. (Carroll #1 was packed at 9.200 feet) Field development continued following the initial discovery in 1978 into the early 1980s, eventually encompassing an area 12 miles long and 5 miles wide (approximately 30,000 acres). Between 1978 and 1987, the Cheneyboro Field produced approximately 3.0 million Bbl of oil from 69 vertical wells, representing an average of approximately 45,000 Bbl per well. Some of the vertical wells have produced over 100,000 Bbl, indicating better drainage where the wells penetrated the fracture system. In 1987, the Tarrant County Water Authority expropriated approximately 20,000 acres of this field. Producing wells were plugged and abandoned to permit construction of the Richland/Chambers Creek Reservoir, a water supply project for Tarrant County and the City of Fort Worth, Texas.
The Cotton Valley Lime reservoir at Cheneyboro is highly fractured. The primary objective reservoir rock is an oolitic carbonate grainstone of Jurassic age that was deposited on a Paleozoic shelf break. Subsequent pullout of the deeper Louann Salt caused extensive fracturing. The salt withdrawal left the residual field structure as simple regional dip. Hydrocarbon trapping occurs asa result of the high degree of fracture density bounded by areas of non-permeability. Core and log analyses indicate the presence of 2.5 to 4.5% oil saturated matrix porosity in the field. Vertical wells in this reservoir produce 42(degree) API oil.
Horizontal drilling techniques will lead to initial rates and recovery efficiencies several times those experienced in the original vertical well completions. Since the majority of the field is under lake water, directional drilling from the shoreline was anticipated. Based on analogy to horizontal drilling in fractured limestone reservoirs in other areas, increased productive capacity and ultimate reserves are anticipated relative to historical, vertical per well averages.
When the field was first developed, several shallower zones tested hydrocarbons at commercial rates. Due to the expropriation of the field, these zones were not developed. The Cretaceous Rodessa, Glen Rose, Pettit and Travis Peak intervals may also prove productive in the field area.
Gas from these wells is expected to be connected to one of several pipelines in the area.
(Horizontal Drilling. Horizontal drilling begins with drilling in the normal manner (vertically) to a point above the objective formation. From that point, the hole is directionally deviated until the bit is drilling generally horizontally in the producing zone. Directional drilling technology has advanced to the point that the drill-bit can be kept in one geological horizon for many hundreds of feet away from the vertical well bore. It is no longer necessary to strike a localized fracture zone accurately with a vertical well. Instead, a well can be drilled horizontally through an area where fractured zones are known to exist with a greater chance of encountering the vertical fractures. A single horizontal well can encounter several localized fracture zones.
Horizontal drilling was first developed over 20 years ago, and has been used successfully in oil and gas fields as diverse as those located in West Virginia, the North Sea, Saskatchewan, Argentina, Prudhoe Bay and South Texas, to extract oil and gas where vertical drilling is impossible or uneconomical. Horizontal drilling has also increased production of oil and gas from fields with thin pay zones, low permeability sands, vertical fractured reservoirs, discontinuous formations and reservoirs with gas and water coning problems. High angle directional drilling has been performed extensively onshore in California to reach bottom holes in congested cities or harbors where vertical drilling would not be feasible. Horizontal drilling has been used extensively offshore to drill many wells from one platform. )
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