Rejuvenating the Buda Limestone Reservoir in Texas by Using Crude Oil and Nitrogen Injection in Underbalanced Regime: Case History
Authors: Davis G. , Wilcox G. , Arnone M. , Bruington S. 
Published: February 2016

Abstract
The naturally fractured Buda formation, which underlies the Eagle Ford shale play across central and south Texas, is experiencing resurgence in exploration and production activity as a result of improvements in horizontal and underbalanced drilling techniques. Formed during the Cretaceous period, the Buda formation was produced from vertical wells for decades, leaving aside the potential of crossing all the vertical fractures and associated recoverable hydrocarbons. However, many fractures in the Buda formation are underpressured because not filled with hydrocarbons from the overlying Eagle Ford formation. The application of Underbalanced Drilling in horizontals wells to drill Buda wells is today showing excellent results.
The underpressured nature of some fractures in the Buda dictates that wells be drilled underbalanced; drilling the formation conventionally typically results in issues such as considerable circulation losses and then associated kicks when drilling through depleted and filled fractures. In certain areas, the Buda formation also presents the challenge of hydrogen sulfide gas (H2S), which poses environmental and safety issues. In some cases, the Buda's fracture systems may be vertically extensive enough to establish hydraulic continuity with underlying formations, a situation that could result in extraneous water production.
This paper examines a case study employing Underbalanced Drilling (UBD) techniques to drill a well in the Buda Limestone formation. Before embarking on the project, the operator conducted a six-county assessment of the Buda, comparing wells that were drilled using a conventional mud system with wells that were drilled using underbalanced drilling techniques. A significant concern with drilling the wells conventionally was that the weight of the drilling fluid column would create pressure against the 5.0 pounds per gallon (ppg) or less from sub pressured fractures from Buda formation, which could result in severe to total loss of drilling mud. Even drilling this formation with freshwater applies 8.33 ppg of pressure for a column of fluid. The study revealed the UBD wells mitigated the loss of drilling fluid and allowed the producing section to be drilled successfully to total depth. Using UBD techniques also yielded significant improvements in production through its ability to prevent drilling mud and chemical additives from invading the drilled formation.
With the UBD approach, the operator was able to drill the production section of eleven wells (at today) to TD (Target Depth) using an open-hole completion and effectively control low-pressure zones. The approach resulted in benefits such as reducing rig time and nonproductive time (NPT), incurring minimal fluid losses, obtaining early production of crude while drilling, and characterizing the reservoir for future developments.
As the industry continually strives to meet the ever-growing demand for hydrocarbons worldwide, new technologies are seeing expanded application not only in emerging, but in mature conventional plays with untapped potential. UBD technology was combined with advanced MWD tools, mechanically operated wellbore isolation devices, multibore wellhead system and efficient four phase separation. This has provided the operators an opportunity to have a second look at the Buda formation and overcome steep challenges to produce wells more efficiently and economically than before.