FIRST BOLIVIAN HORIZONTAL WELL DRILLED

Mario Carazas Miranda, Yacimientos Petroliferos, Fiscales Bolivianos
Santa Cruz, Bolivia

In early 1991, the petroleum engineering department of the Bolivian national oil company, Yacimientos Petroliferos Fiscales Bolivianos (YPFB), drilled its first horizontal well, Well VGR-29 (SH).

The purpose of the well was to improve production and drainage efficiency in the Vuelta Grande field.

Another reason for this horizontal well was to investigate the flowing conditions and lithology of the gas-condensate reservoirs of the Tapecua and Cangapi formations.

The project was based in part on an analysis of the structural geology and reservoir conditions of 28 of the vertical wells (26 of which are producers) in the Vuelta Grande field.

The structural evaluation showed peculiar stratigraphic characteristics of the productive formations: large lateral variations of petrophysical conditions, varied distribution of permeable zones, and possible lenses with limited communication. These conditions were suitable for the application of a horizontal well.

The main zones of productive gas and condensate are in the base of the Yecua formation (Tertiary), in the Tapecua formation (Cretaceous), and in the upper part of the Cangapi formation (Triassic). The common gas/water contact is at a subsea depth of 5,740 ft (1,750 m) for the latter two reservoirs (Fig. 1).

The Tapecua was discovered in 1978 with the drilling of Well VGR-XL. Its gas-condensate reserves make it one of the most important reservoirs in the country.

The Tapecua formation has an average gross pay of 558 ft, Cangapi an average gross pay of 541 ft. The total gross sand of 1,099 ft has an average net permeable pay zone of 35%.

The Vuelta Grande field has been on line for 18 months with 16 wells producing 2,600 b/d of condensate and 90 MMcfd of gas. The field has a modern processing plant which makes 900 b/d of natural gasoline and a gas injection facility which injects about 80 MMcfd into 10 wells in the Tapecua and Cangapi formations.

The Vuelta Grande field is situated in the Department of Chuquisaca, Province Luis Calvo, 75 miles north of Argentina and 185 miles southeast of Sucre, the capital of Bolivia.

HORIZONTAL WELL

The actual field production is lower than expected based on the behavior of the reservoirs and the predictions from reservoir simulation models.

To study the feasibility of a horizontal well, stratigraphic correlations of six generic units were evaluated based on data obtained from electrical, radioactive, and acoustic logs.

The calculated parameters of effective porosity, hydrocarbon saturation, and clay volume varied vertically as well as horizontally in each unit, particularly in areas where permeable zones did not exist. The zone thickness concentrated in certain areas and did not follow patterns of continuity over a large part of the field.

Once the horizontal program was established by the drilling department, Emsco GC-500 rig No. 8 spudded Well VGR-29 (SH) on Jan. 6, 1991. The drilling operation went according to plan, with 13 3/8-in. casing set at 1,970 ft and 9 5/8-in. casing set at 7,915 ft after deviating the well to establish the final orientation.

On Apr. 11, 1991, the well was kicked off to build angle with an 8 1/2-in. bit and measurement-while-drilling services. Seven days later the well reached a final length of 9,535 ft (2,907 m) with a deviation angle of 87 (Fig. 2). This corresponds to horizontal displacement of 1,673 ft in the productive formation across a projected vertical distance of 282 ft.

The bottom of the well is at a subsea depth of 5,638 ft, about 100 ft above the common hydrocarbon/water contact of the Tapecua and Congapi reservoirs.

Based on the log analysis, the well was completed with a prepacked filter. Following completion, testing of the well began at the end of April.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.