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

摘要

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.