Abstract

Evaluation of the chemical flood potential in the Chihuido de la Sierra Negra field, Argentina, has been carried out using several different recovery scenarios. This field has been submitted to extensive waterflooding for several years, and surface facilities have been designed to recycle produced brine as injection brine. The use of produced brine, as chemical solution make-up water is therefore very advantageous from both an operational and economical point of view. However, the formation brine contains around 110,000 PPM Total Dissolved Solids with around 2,800 PPM divalent cations. This makes the selection of the proper surfactant extremely difficult. Processes such as Alkali Surfactant Polymer Flooding have been considered, however, the requirement of large amounts of fresh water as well as softening units in the field adversely affects the economics of this process.

This paper will discuss the development of a new anionic surfactant that provides solubility in high salinities and low interfacial tension at low concentration. The Chihuido de la Sierra Negra field history will be briefly described, and the laboratory screening and evaluation of the surfactants, including the interfacial tension properties, the adsorption, the core evaluation, and the performance of the flood will also be discussed.

The significance of this development is that it could lend feasibility to EOR projects that would be deemed uneconomical because of water treatment and handling costs.

Introduction

Chihuido de la Sierra Negra is currently the largest producing oilfield in Argentina. It is located in the Neuquen basin. Current production is around 12,000 m3/d of light oil (33–35 API) and 70,000 m3/d of water, with 700 producers and 550 injectors. Water injecion rate is approximately 90,000 m3/d The pay zone (lower Cretaceous) is made of several sandstone layers, with a gross thickness of around 200 m. The best producing intervals, from top to bottom, are the Troncoso Inferior (TRI), the Agrio Superior (AGRS) and Avile (AVI). The target layer for pilot implementation is the Troncoso 5T, which contains 40% OOIP, with low heterogeneity and good permeability (30 - 200 mD).

The recent increase in oil price has renewed interest towards the EOR techniques. REPSOL-YPF chose to investigate various EOR techniques for the future production of this mature field. It is believed that polymer flooding will not mobilize a great deal of extra oil with respect to water flood due to the favorable mobility ratio. An Alkaline Surfactant Polymer flood (ASP) was then considered as a result of some studies1 and pilot projects which gave promising results in China2, US3 and Venezuela.4,5 However, because of the high salinity and hardness of the injection and the connate brines, the use of alkali became economically impossible and technically difficult due to the high cost of water treatment, the sludge disposal and the potential scale problems in the formation. Furthermore, the stability of the commercially available polymer became a concern under the given conditions of the ASP system.

Because of the above mentioned concerns, although the reservoir temperature (55°C) and permeability are favorable for an ASP implementation in Chihuido, some unique chemistry is needed for the design of a Surfactant Polymer (SP) flood, to overcome the unfavorable high salinity brine and potentially high surfactant adsorption.

Field Description.

The Chihuido area (10,000 ha) is located in the northern part of the prolific Neuquen basin, 30km to the east of the first Andean frontal thrusted structures. The main regional hydrocarbon reservoir of Troncoso Inferior consists of laminated eolian sandstones and fluvial channels belonging to the Early Cretaceous Huitrin Formation (Rayoso Goup). The Agrio superior productive intervals belong to shallow marine barrier sandstones and Avile member is composed of an eolian environment. These two last productive members are located in the Agrio Formation (Mendoza Group) of the stratigraphic column.

The main source rock is represented by the Upper Jurassic Vaca Muerta formation and thick anhydritic layers of the Huitrin formation provide the principal seal.

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