This paper describes a safe and successful horizontal drilling operation on an H2S producing well, using underbalanced drilling (UBD) techniques. The well had been prematurely terminated after using conventional drilling practices that resulted in massive losses and posed the risk of stuck pipe; but, through proper planning, UBD allowed the operator to increase the horizontal length of an existing well. Initial production rates were substantially higher than the rates observed before drilling the UB lateral extension.

The drilling and production results were significant to the future operational potential and viability of this mature field. Equally important was the prodigious success from a health, safety and environmental (HSE) perspective because the field has a 4 percent H2S producing gas cap. Traditionally the industry has been reluctant to use underbalanced techniques in this type of environment.

Located in West Texas, the Yates field was discovered in 1926 and is one of the largest oil reservoirs in United States. Its shallow reserves have been accessed mostly with vertical wells. Over the last few years, implementation of a horizontal well program has made significantly affected on the overall production performance of the field.

The field maintains its reservoir energy by N2 injection into the gas cap and from 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 maintaining underbalanced conditions. These conditions caused wells drilled previously by conventional methods to incur severe lost circulation on encountering a fracture system. The lost-circulation problem resulted in premature suspension of drilling operations. As a consequence, many wells did not achieve long lateral lengths.

This paper discusses the well design and the underbalanced modeling considerations (taking into account detailed reservoir properties). The paper also describes the process used to address HSE issues of the project in the face of the potential presence of high surface of levels H2S while drilling.

Extending the well length underbalanced and intersecting additional fracture sets without losses resulted in an eightfold production increase.


Since the Yates field (Fig. 1) was first discovered in the early 1900s, it has produced a cumulative 1.4 billion barrels of oil. Even though all of the wells in the field have crossed the productive interval of the San Andres dolomite, their production has been inconsistent because many of these vertical wells did not intersect the fracture system within the oil column, which is the main production mechanism in the field.

With fractures being the primary production mechanism for the field, the next step in the development strategy was the use of horizontal drilling to enhance production by increasing the probability that the wells would cross the fracture networks within the oil column. A number of existing wellbores that were mechanically stable and exhibited either low or no production were good re-entry candidates; therefore, the horizontal program was implemented and resulted, as expected, in a significantly higher overall field production.

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