The ultimate goal in any business is to maximize profits. In the oil industry this is done by reducing costs and/or increasing production. Increasing production rates or increasing the recoverable reserves can achieve profits from production. In this study we look at how underbalanced drilling (UBD) and open hole completions changed the economics in the Bitterweed S. (Caballos) Field.

The Bitterweed S. (Caballos) Field was discovered in 1983, in Pecos County, Texas by Tenneco Oil Company. Currently, Riata Energy Incorporated, in order to improve the economics of its wells, is using underbalanced drilling and open hole completions, as well permit in the Caballos formations. This study will look at several wells drilled in the Caballos formation with conventional or underbalanced methods. Depending on the area the Caballos formation can be found from 4500' to 7700'.

Underbalanced drilling is used to improve well productivity, drilling performance, and an understanding of the reservoir. Well productivity is improved by minimizing formation damage caused by drilling mud, cuttings, fluids, and cement entering the formation. These items can cause skin damage and increase the cost of the well by requiring costly stimulation operations. It is believed that by minimizing formation damage reserves can be increased.

Drilling performance is improved by lowering the bottom hole pressure, which increases the penetration rate and reduces drilling costs by reducing drilling days. Lowering the bottom hole pressure also enhances bit performance. Using underbalanced drilling methods minimizes lost circulation problems and differential sticking.

By passed gas zones can be found or studied by being able to monitor the flow continuously as the well is being drilled. Uncontaminated cuttings can be obtained using underbalanced methods and evaluated within minutes of drilling.

The Caballos formation is a commercial gas zone, which has been drilled conventionally and underbalanced. In this study, the well that was drilled underbalanced is producing approximately twice as much as the surrounding wells that were drilled conventionally at a rate of 3.33 MMCF/D as compared to a rate of 1.8 MMCF/D, respectively. IP's in the field have been 10 times higher for underbalanced wells but production is currently limited by pipeline capacity out of the field.


Exploration and Production companies are continually trying to improve the bottom line by using new and old technologies. Underbalanced drilling techniques will help increase well production rates and maximize recoverable reserves. In many cases it can actually reduce drilling and completion costs. Added drilling costs associated with UBD are minimal and almost always are offset by higher rates of penetration and fewer drilling complications.

In conventional drilling, the drilling fluid is forced into the reservoir by the hydrostatic force exerted by the drilling fluid. This fluid intrusion into the reservoir causes damage to the formation near the wellbore. This formation damage limits the flow of oil and gas into the wellbore. The productivity of the well is lowered by this formation damage.

Underbalanced drilling lowers the hydrostatic pressure of the drilling fluid column so that the pressure in the wellbore is less than the formation pressure at all times. The lower pressure in the wellbore causes the oil/gas to flow from the formation into the wellbore. The lower wellbore pressure and the higher formation pressure prevent the drilling fluid from entering into the formation. This increases the production rates and increases the recovery of available reserves.

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