Development of the Cotton Valley Geopressure Zone in Panola County, Texas, Using Air/N2 Drilling and Openhole Panola County, Texas, Using Air/N2 Drilling and Openhole Completion Techniques
The Jurassic aged Cotton Valley Sands and overpressured Bossier Sands can be effectively drilled using air and/or nitrogen methods, logged, completed and open hole stimulated without ever exposing the wellbore to potentially harmful drilling fluids. This in turn increases well productivity and offers considerable cost savings productivity and offers considerable cost savings to the standard mud and liner setting techniques.
This paper will review the geology and drilling of the Deberry (Cotton Valley Lower) Field, utilizing air and/or nitrogen, open hole completion and open hole stimulation. The Cotton Valley (Taylor Sand) and Geopressure zone are located on the western flank of the Bethany Structure of Panola County, Texas. The field location and estimated limits are shown in Figure 1.
The Jurassic aged Cotton Valley Group of Panola County, Texas are typically regressive Panola County, Texas are typically regressive sand and shale depositions. Within the East Texas Basin the facies relationships of the Cotton Valley Group are terrestrial to the north and neritic towards the southern margins of the Sabine Platforms with hydrocarbon accumulation in both the clastic shoreline systems and in the erratic Bessier Sands which were deposited in marine environment.
The Cotton Valley Sands of the Sabine Platform occur above the Bessier Shale. The bottom most sand of the Cotton Valley group is known as the Taylor Sand. This Taylor Sand has been the objective of extensive exploration and development in recent years and the geology of which has been well documented in the literature by Collins 1.
It was found that thin sandstone or siltstone lenses have been deposited within the thick Bessier shale sequence. These sandstone beds, which will be referred to as Geopressure Zones, release gas under high initial pressures in excess of 6000 psi (420 kg/cm2), with gradients as high as 0.58 psi/ft. (13.41 kpa/m). These traps are stratigraphic, with pinchouts of permeability in individual beds, and the confinement of gas created by the thick shale sequences deposited above and below. These shale beds are also the probable source beds for the hydrocarbon accumulations. Table 1 is a summary of the parameters of the Geopressure Zone. Presley and Reed consider these Geopressure sandstones to be coarse-grained facies of submarine fan systems that accumulated along the margins of the Bessier marine basin. Supply is believed to be from landward shallow-water, fluvial-deltaic and/or barrier island systems, with the fans forming off the prodelta of the Cotton Valley shoreline. Trends of the Geopressure sands were presumably developed in structural lows during Haynesville time.
The Bossier fans filled these lows, since structural position of the lows between Smackover-Haynesville structural highs had probably been established by Bessier time. In the area of the Haynesville shelf, in the western part of the East Texas Basin, there is production of Geopressure zone gas from sandstones in structural troughs. These Geopressure sandstones are commonly less than thirty feet thick and are encased in shale. The encasement of these Geopressure sands by the Bossier Shale is believed to be the main factor in these sands being overpressured. During diagenesis of the Bessier Shale a "squeezing" effect was placed on these Bessier Sands due to compaction within the basin. These sands were unable to release the excess amount of pressure caused by differential compaction, due to the permeability barrier created by the Bossier Shale.