Abstract

This paper provides results of a study comparing several parameters from 40 wells that were fracture-stimulated with conventional perforating techniques to 30 wells fracture-stimulated with overbalanced perforating (OBP). All wells are in the Bravo Dome Field and are producing from the same formation. The parameters used for comparison are (1) interval length and depth, (2) number of perforations and phasing, (3) perforating technique, (4) average and maximum bottomhole calculated treating pressure (BHTP), (5) proppant volume to job volume ratio, and (6) production results. There were two opinions on the benefit/risk of using OBP as an initial step in completing wells that may require fracture stimulation. One opinion suggests that OBP reduces near-wellbore tortuosity and creates a dominant fracture geometry, resulting in lower injection pressures and reduced chance of early screenouts. The other opinion suggests that OBP increases tortuosity and creates multiple fractures that could lead to high injection pressures and early screenouts.

Introduction

The Bravo Dome Field (BDF) is located in the southeast corner of Union County and the northeast corner of Harding County, New Mexico (Fig. 1, Page 7). The main reservoir in the field is the Tubb Sand at depths of 2,000 to 2,500 ft. Naturally occurring carbon-dioxide (CO2) gas has been produced from this field since the 1930s.

Most CO2 resources in Union and Harding Counties were developed in the 1980s when several oil fields in the west Texas portion of the Permian Basin were included in a pilot test of CO2 as an enhanced oil recovery drive mechanism. Amoco and Cities Service were the major operators involved in this work in the early 1980s. The number of CO2-producing wells in the BDF increased from 16 in 1982 to 258 in 1985. Although some wells will produce marketable volumes of CO2 without stimulation, most wells require fracture stimulation to produce at a commercial rate (0.5 to 2 MMscf/D). Average porosity in the BDF ranges from 10 to 20%. Productive net pays are generally 5 to 20 ft thick, with gross intervals of over 200 ft. Average permeability is 42 md.

This paper discusses completion programs executed in 1993 and 1995. Two distinct approaches were used in perforating: - The 1993 wells were stimulated by the underbalanced perforating method.

  • The 1995 wells were stimulated by overbalanced perforating.

The producing company and vendors made every effort to minimize cost and maximize production through innovative technology and team involvement.

On the 1993 project, treating crews used underbalanced tubing-conveyed perforating (UTCP) as a method of reducing near-wellbore skin damage. UTCP enables an immediate surge of formation pressure into the wellbore after perforations are shot; the surge is driven by the high pressure of the reservoir overcoming the lower wellbore pressure. This underbalance process was expected to (1) increase the number of perforations open to flow, (2) decrease fracture initiation pressure during the fracture treatment, and (3) increase reservoir flow into the wellbore during production. Results were disappointing. The stimulations placed more proppant behind pipe than had been placed in the past, but many injection problems and screenouts occurred.

The 1995 stimulations were conducted with OBP in an effort to increase injectivity and decrease the likelihood of early screenout. The operator was concerned that OBP would produce the exact opposite effect and that the high-energy surge associated with OBP would create multiple fractures along the segmented perforated interval. This complex multifracture system could possibly result in higher treating pressures if many narrow fractures could not accept a proppant-laden fluid.

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