Abstract

After three attempts in the last 6 years to fully characterize the reservoir in the Caballos Formation in the Puerto Colon Field, different approaches to explain the reservoir heterogeneity have been suggested, each of which involves a great deal of uncertainty. Of particular importance is the fact that the oil-water contact has been found at varying depths from well to well. This variation could not be satisfactorily explained structurally nor stratigraphically. Bouttete et al, 1993, on the basis of incomplete 2D seismic information integrated to an extensive set of geologic and engineering data stated that there are three different reservoirs in the Caballos Formation. In 1996, Calderon et al, using additional well and seismic data, concluded that there is a single tilted oil water contact in the Formation. There is strong evidence supporting each of these models but there are still some inconsistencies. Recently, Pe a et al, performed a thorough structural, stratigraphic and petrophysical review. This latest approach considered improved data management procedures and geostatistical modelling in order to produce a better picture of the reservoir. A rigorous validation and integration process under a project database allowed the workteam to improve the understanding of the system under study. By combining geostatistical modeling with geologic expertise it was possible to reduce much of the uncertainty and choose the most probable static and dynamic models of the reservoir. It was found that there are not sufficient elements in the static and dynamic models to account for the existence of three different reservoirs. A tilted OWC was accepted as the best representation of the reservoir for the available information.

Introduction

Puerto Colon Field is located in the Colombian Province of Putumayo close to the border with Ecuador (Fig 1). The Cretaceous Caballos Sandstones and Villeta sandstones and limestones (Fig. 2) are hydrocarbon productive, being the Caballos reservoir the primary development objective. Caballos reservoir is in average 10500 ft deep (measured depth). Initial reservoir pressure was 4705 psia at 9600 ft subsea. Thirty years after the start of the productive life of the reservoir, 28 MB of 30.5 API oil have been produced and due to a strong bottom water drive, current static pressure is only 350 psi below the original pressure. Reservoir pressure is well above the 1600 psi bubble point pressure. The Puerto Colon Field was discovered in October of 1965 with the drilling of well Acae-2. Up to the present 18 oil wells have been drilled and tested in the Caballos. The initial development program was performed between 1965 and 1970 when seven wells were drilled. A reservoir study for the field was conducted in 19731 using volumetrics and decline analysis to determine original oil in place and reserves. As a result of additional studies2,3,4, seven more wells were drilled between 1984 and 1988. The first Integrated Reservoir Evaluation5 began in 1991, since then the model has been kept updated and two major models proposed to characterize the static and dynamic properties of the Caballos accumulation. One important feature that has to be accounted for in the model is the deepening of the fluid contact from west to east. In the original model5,6, it was stated that there were three independent reservoirs in the Caballos Formation each of which having its own oil water contact. Currently, a tilted oil water contact is the accepted approach7,8. This water surface tilting was attributed to tectonic tilting.

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