Abstract

Intercampo oilfield is characterized by heavy oil with mid-high permeability and high oil saturation, which is an unconsolidated reservoir driven by edge water and bottom water. This paper presents the oilfield's application of three decline analysis models to the 13 horizontal and 13 vertical wells of the oilfield respectively, as well as the comparison of the decline features between vertical and horizontal wells. The authors point out that the decline analysis models are not only applicable for vertical wells but also for horizontal wells. Although the water cut rising of horizontal wells are different from vertical wells, the authors emphasize the well type and reservoir properties are not associated with the decline analysis models. In regards to screening and comparing of decline analysis models; how to select and use the models; their application condition; and the models features are clearly presented in this paper.

The paper introduces two new decline analysis models for horizontal wells. They have been applied to predict production rate of the oilfields, reservoirs, and wells, as well as their future trends. The results accord well with the actual data from the oilfield. The classical models screened out and the new models were recommended analyzing the production performance of the oilfields and reservoirs for the types of wells.

Introduction

The Intercampo Oilfield, located in Zulia in the northeast part of Lake Maracaibo in Venezuela, was taken over by CNPC American Ltd. in 1998. The water depth of the lake ranges from 7 to 26 meters and the entire area is 39.56 square meters in extent. Most of the reservoirs are buried from 3560 to 7500 ft, with heavy crude-oil gravity ranging from 12.5 to 23.3 API. The reservoir is characterized by strong heterogeneity, substantial differences in physical properties, such as middle-heavy crude oil, edge water and bottom water [1]. CNPC American Ltd drilled 131 new wells, including 54 horizontal wells by the end of 2004, of which the first horizontal well was drilled in 1999. In the new wells, except for twenty seven ESP wells, 77 wells are produced by continuous gas lift.

The existing decline curve analysis techniques, which included three Arps models (exponential, hyperbolic, and harmonic, 1945), and Fetkovich model (1980), are derived empirically. But the Arps models are still a preferred method for forecasting the oil production and proven reserve. The methods have played a very important role in exploration and development of oilfields in the world [2–6].

Li and Horne (2001) developed their decline analytical model, called the Li-Horne model [7–8], and based on fluid flow mechanisms study. In fact, Li and Horne made a conceptual mistake in their reasoning of the Arps models. Our research found that the Arps' models does not have any association with fluid flow mechanism, reservoir types, and the fluid's characteristics, such as steady or unsteady flow and single or multiphase flow.

The two new decline analysis models are introduced in this paper; they are named the Orstrand-Weng model [2, 9] and T model [10]. By using the two models, the production decline analysis and reserve prediction both will be done.

The main purpose of the paper is to conduct a comparison among the four models of the Arps model, Li-Horne model, Orstrand-Weng model and T model for the application condition and results in the oilfields.

Reservoir Description

In the last decade, horizontal well technology has been employed widely in the production of heavy oil and extra heavy oil in the reservoirs of the Intercampo oilfield. The reservoirs are currently classified into three types. The first type includes two areas of BASUP and BAMED. These reservoirs are situated in the upper Miocene, characterized by heavy-oil, mid-to-high permeability, and high-saturation, unconsolidated sand reservoirs. In the second type, we have BAINF, LAGNA and LGINF, in the middle Miocene. They are middle-heavy crude oil, mid-to-high permeability, high-saturation, unconsolidated sand reservoirs. Finally, the third type B-2-X features, a middle-heavy crude oil, low porosity-permeability and high-oil-saturation sand reservoir, situated in the lower Eocene.

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