Located in West Texas, the Yates field was discovered in 1926 and is one of the largest oil reservoirs in the United States. Its shallow reserves have been accessed mostly with vertical wells but, over the last few years, implementation of a horizontal well program has significantly affected the overall production performance of the field. The reservoir energy of the field is maintained by Nitrogen (N2) injection into the gas cap and field-produced water injection below the oil leg. Low reservoir pressure and highly productive rock in both the gas cap and the water zone present challenges to drilling operations whether underbalanced or conventional. These conditions caused wells drilled previously by conventional methods to incur severe lost circulation upon encountering a fracture system. This resulted in premature suspension of drilling operations and a consequent failure to achieve the desired lateral lengths
One such well had been prematurely terminated after using conventional drilling practices that resulted in massive losses and posed the risk of stuck pipe. It was selected as a candidate well for a pilot project to evaluate the use of underbalanced drilling techniques (UBD) using a close loop system with high pressure separation. Through proper planning, the use of UBD allowed the operator to increase the horizontal length of this well and significantly increase its production potential.
This paper discusses the well design and the underbalanced modeling considerations (taking into account detailed reservoir properties), which led to the extension of the existing horizontal leg to intersect additional fracture sets without losses, resulting in a significant production increase. The paper also describes the process used to address HSE issues of the project in the face of the potential presence of high surface levels of H2S while drilling. The drilling and production results that were achieved were significant to the future operational potential and viability of this mature field. Of equal importance was the success of the operation from a health, safety and environmental (HSE) perspective considering that the field has a 4 percent H2S producing gas cap, an environment in which the industry has been traditionally reluctant to use UBD techniques.