Article

Water-Alternating-Gas Pilot in the Largest Oil Field in Argentina: Chihuido de la Sierra Negra, Neuquen Basin

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Abstract

Two field pilot tests of immiscible water-alternating gas (WAG) injection are being conducted in Chihuido de la Sierra Negra, the largest oilfield in Argentina. Immiscible gas injection technology was selected because of its attractive incremental oil recovery potential for the two main reservoirs in this field. These are mature, waterflooded, undersaturated light oil sandstone reservoirs that are expected to reach a combined ultimate waterflood recovery factor of about 40 % OOIP. Scaled laboratory tests, pilot-scale simulation models, and pilot performance indicate that the immiscible WAG process can be expected to add between 3 and 8 % OOIP due to the contribution of several improved recovery mechanisms, namely oil swelling and viscosity reduction, and waterflood residual oil mobilization in three-phase flow. Another important mechanism that has been cited in immiscible WAG projects is improved volumetric sweep, either because of relative permeability effects or gravity segregation. The latter effect can be advantageous under particular circumstances, such as in clean formations with good vertical communication that may have undergone water underride during waterflood and may thus have unswept oil at the top of the reservoir. This paper presents a status report on the preliminary evaluation of the performance of the field tests. Although we focus on oil production response and sweep efficiency estimation, a comprehensive view is presented including all the relevant production, facilities, environmental and reservoir engineering issues associated with the pilot tests. The key to a successful evaluation of the process performance in the field is a quantitative assessment of the incremental oil production, volumetric sweep efficiency and compositional effects. These issues have proved to be more difficult than it was initially expected, due to the particular circumstances of the recent production history of the field and limitations in its routine measurement tools. However, careful data analysis, the introduction of unconventional measurement techniques and the use of numerical simulation have allowed to obtain performance indicators and to estimate the incremental production. These estimations are crucial for the decision analysis of project expansion to field scale, whose economics can be marginal due to high capital and operation expenses. Introduction Chihuido de la Sierra Negra (ChSN) field is located in the Neuquén Basin in west-central Argentina, 200 km northwest of the city of Neuquén. The field was discovered in 1968 and primary production began in 1979. Waterflooding started in 1993. Cumulative oil production is 82 Mm3 (517 million barrels) and cumulative water injection is 325 Mm3 (2 billion barrels). The stratigraphic column of Chihuido de la Sierra Negra is shown in Figure 1. The 6-km thick column is composed of several sedimentary cycles including clastic and carbonatic deposits of both marine and continental origin, affected at different times of the basin evolution by flooding and transgression events and a temporal disconnection as a final result. The producing reservoirs containing 60 % of the recoverable reserves are the Troncoso Inferior Member of the Huitrin formation, and the upper section of Agrio formation. (~ 1100 meters measured depth). The quartz sandstones of the Avilé Member, deposited in aeolian dune fields, contain almost 30 % of the ChSN reserves (~ 1300 meters measured depth). The depositional environments in the Troncoso Inferior Member include Aeolian dunes (5T and 4T intervals) and fluvial channels (3T and 2T). The middle portion of the productive column (Agrio superior) is formed by marine sandbars (3A, 2A, 1A and 0A). All the layers lack vertical communication between each other except 5T and 4T [1]. The Avilé sandstone has an originally saturated, 35°API oil and its main primary production mechanism was solution gas drive, with a minor influence of the expansion of a primary gas cap in the NE part of the field. Troncoso-Agrio reservoirs contained a slightly undersaturated, 33°API oil, produced also by solution gas drive. Both reservoir fluids are low viscosity and the rocks are water-wet, so response to waterflooding has been generally very good. Both reservoirs are currently with an important degree of undersaturation, which makes them attractive candidates for immiscible gas flooding.

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