Carbonate Reservoir Characterization Explains the Diverse Production Response in Caipe Field, Western Venezuela
Authors: Robaina R. , Martinez G. , Prieto M. , Aquino R. , Ruiz F. , Pimentel L. , Sierra J. , Gonzalez E. 
Published: December 2007

Abstract
The purpose of this study was to characterize the upper Cretaceous "O" Limestone reservoir of the Caipe Norte field of the Barinas Basin of western Venezuela. Geophysical petrophysical sedimentological and engineering data were integrated in the reservoir model proposed in the study. Three wells have been drilled in this reservoir since its discovery in 1994. Cumulative production amounts to more than 360 Mbls of medium gravity oil. Production comes exclusively from Well-1 which was the first well drilled. Well-1 was drilled based on 2D seismic data.
The motivation for the present study was the inconsistency in production behavior between Well-1 and the other 2 wells (Well-2 and Well-3), even though these were drilled at an 800-m interwell spacing.
Sedimentological analysis of a core from Well-2 shows a variety of lithologies and evidence of diagenetic overprint on the primary characteristics. Crystalline dolomitized carbonates show the best porosities. Sandy carbonates show the lowest levels of porosities, because the quartz grains are actually floating in the calcareous-mud matrix. The petrophysical evaluation shows that Well-1 (productive well) has the highest concentrations of dolomitized carbonates. Well-3 shows the lowest concentrations of crystalline carbonates and the highest concentrations of sandy carbonates. Furthermore, the seismic attribute analysis performed on the 3D seismic data indicated a differentiation in seismic response between the area where Well-1 was drilled and the areas where the other 2 wells were drilled. Nine attributes were analyzed, and their summary is presented in a "Seismic Similarity Map" that emphasizes the areal differences.
Based on these observations, it is interpreted that the seismic response variations are related to the matrix variations, and that the areas on the Seismic Similarity Map that appear similar to the area where Well-1 was drilled are potentially productive.
The operational and production analysis indicated a high probability of formation damage in Well-2 during cementation. This might be the real reason why this well has not come into production. Even though Well-2 does not show the same levels of porosity as Well-1, its core shows the presence of dolomites and vuggy porosity.

Introduction
Caipe Norte field is located in the Barinas Basin of southwestern Venezuela. The main productive intervals belong to the Cretaceous Escandalosa Fm. and the Middle Eocene Gobernador Fm. (Fig. 1). Basinwide, within the Cretaceous sequence, 2 members are responsible for most of the production, the "P" Sands and the "O" Limestone. in Caipe field only the "O" Limestone has shown production, along with the sands of the Middle Eocene Gobernador Fm. The "O" Limestone is the main target of this study. It was first completed in Well-1 in 1994 based on 2D seismic interpretation. The initial production test yielded 641 BPD of 26.4 API-gravity oil. A 3D seismic survey was later acquired.
Based on the success of Well-1, an appraisal and exploitation campaign led to the drilling of Well-2 and Well-3, between July 1994 and August 1995, at 800-m spacing. These 2 wells are presently suspended, and the multiple efforts to bring them into production have failed, even though the petrophysical evaluations show production potentials.
A characterization study with a multidisciplinary approach was then planned, in order to define the reasons for the potential mismatch between Well-1 and the other 2 wells.

Study Area
The study area covers about 20 Km2, 20 km Southwest of the city of Barinas in southwestern Venezuela (Fig. 2).
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