Abstract

The CAR-x1003 exploratory well, drilled in the Caranda field in 1997, proved to be a discovery well of the Robor? Formation.

Test interpretation during completion was difficult because of high formation pressures and temperatures. In 2003, an additional test provided critical data for producing and estimating the reserves in this formation.

This paper shows the criteria involved in the design and interpretation of the more recent test and the use of those test results for estimating AOF and gas reserves through a Material Balance Equation (MBE) for overpressured reservoirs. The resulting estimates had a significant impact on the course of subsequent field development.

Introduction

The Caranda field is located in the Chaco-Tarija Basin, northwest of the city of Santa Cruz de la Sierra, in Bolivia. It produces gas and oil from shallow reservoirs (depths shallower than 2000 m) and deep reservoirs (depths greater than 4500 m). The location map is shown in Figure 1.

Exploration of deep field reservoirs began in 1997 with drilling of the CAR-x1003 well, which proved to be a gas discovery well of the Robor? Formation.

During well drilling and completion, several operating problems were encountered due to high pressures and temperatures (12,500 psi and 313 °F). Pressure tests were incomplete, which prevented appropriate test interpretation.

The CAR-x1003 well produced from the Robor? H reservoir. In January 2003, an additional pressure test was performed using an electronic gauge in an effort to more accurately characterize the reservoir and predict well-related reserves using the MBE approach.

Total K and S values were derived from conventional analysis of flow periods, but the test could not be fully validated before applying the non-darcy skin effect.

Geologic Setting

The Robor? Formation is composed of shales deposited in a littoral or shallow marine environment with intercalating shelfal sandstones to-ward the top and base. This formation comprises two reservoirs: Robor? H and Robor? G. Reservoir H consists of an interval from 4491 m to 4509 m, which is composed of four sandstone bodies intercalating with claystones. Net sand thickness is 11.8 m and consists of fine-grained to very finegrained quartz sandstones. The set of logs run on the well is shown in Figure 2.

Production History

The CAR-x1003 well has been producing from Reservoir H in the Robor? Formation since 1999. Production, however, has been intermittent and subject to requirements, since the composition of produced gas affects recovery of gas-associated liquids at the treating plant. Cumulative production is 2.2 Bscf. The well production curve is shown in Figure 3.

Pressure Test Analysis

A pressure test was performed using an electronic gauge in order to monitor well evolution and validate proven reserves located in the Robor? Formation. The test was designed to avoid high-pressure (12,500 psi) and high-temperature (313 °F) problems encountered while conducting the test during well completion and to determine whether non-darcy skin effect actually existed. To solve the high-pressure issue, special gauges were used to fit the specific borehole conditions.

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