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RESERVOIRS RESPONSE TO THE GAS INJECTION PROJECTS IN THE TALARA BASIN,

OFFSHORE FIELDS, NW- PERU

Mario Chvez Cerna Petro-Tech Peruana Summary The study area is located between the parallels 0410 to 0445, and the meridians 8105 to 8145, towards West of Greenwich Meridian (Fig.1), where the stratigraphic sequence recorded is approximately 23,000 feet of clastic sediments, characterized by a shaly-sand sequence which includes formations of Paleozoic, Cretaceous, Tertiary and Quaternary age (Fig. 2). Presently, there are 34 active gas injection projects located in four offshore fields named Pea Negra, Lobitos, Providencia and Litoral, where a total of 28 MMSCFD is injected, recovering 2826 BOPD of secondary oil, which represents the 26.5% of the total production of the field. The best producer reservoirs in offshore fields of the Talara Basin, which correspond to a Pull apart basin (Fig. 3), belong to Eocene age and they are locally named Mogollon, Pea Negra, Cabo Blanco and Helico in Pea Negra field; Basal Salina, Rio Bravo, Parias, and Helico in Lobitos field; Basal Salina in Providencia field and Parias in Litoral field (Fig. 4).

Finally, it is important to mention that in reservoirs with similar sedimentary characteristics, the enhanced recovery projects need to be implemented not too much long after completing the development of the reservoir. Introduction The gas injection is used to supply the natural energy of the reservoirs and keep pressure maintenance. Present technique consists in injecting gas through wells located structurally up dip of the block reservoir in order to enhance the total oil recovered from the wells located down dip the block reservoir. The gas injection projects offshore Peru began in early 1966, originally in order to use the excess of gas from the gas lift compressors. However, only a small amount of gas was introduced into the reservoir compared with the production rate. Since 1974, the gas injection projects in the Pea Negra field were implemented in order to increase the recovery factor and keep the pressure maintenance. Later, in 1975, response to the gas injection projects was observed. The production mechanism is gas drive solution, characterized by a fast increase of the Gas- Oil ratio, fast decline of the production rate and pressure. The water produced comes from high water saturation sands, where the oil production response is directly proportional to the net pay and the porosity, being affected by the clay volume, water saturation and size of the structural block. Geology Structural Framework Several tectonic episodes of normal faulting affected the Talara Basin from the Paleozoic to the Eocene, where the structural styles have a direct bearing on hydrocarbon systems (Fig. 5). Based on structural cross sections, the dip meter logs and stratigraphic correlations, and the detailed structural maps on tops of producer formations were prepared in order to define the framework and characterization of reservoirs (Fig. 6). Sedimentary Aspects Basal Salina This formation is composed of alternations of thick and thin beds of milky- white quartz, and gray to dark gray quartzite sandstone, medium to coarse grained, grading to conglomerate, poorly sorted, interbedded with dark steel-gray claystone. There is not recorded outcropping of this formation. However based on ditch cuttings, electrical logs, conventional and sidewall cores, the sedimentary environment are marine, forming turbidite sequences flowing from West to East (Fig. 7).

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Mogollon Present formation is outcropping in Quebrada Mogollon, and consists of medium grained to pebbly massive milky-white, hyaline and light green quartz sandstones, interbedded with brown- gray shales. Sedimentary environment corresponds to fluvio-deltaic system flowing from N towards S, with marked changes from fluvial deposits in the Mancora area, through re-sedimented conglomerate on a submarine slope in the El Alto- Pea Negra area to a major fan system with progressively finer deposits towards the South (Fig. 8). Pea Negra-Rio Bravo Formed by white-gray, fine to medium grained sandstone, calcareous, with shell fragment, interbedded with gray claystone and siltstone. Sedimentary environment corresponds to beach bar sands flowing from NE towards SW, and based on lithological characteristics and palynological determinations is being considered as the same unit called Pea Negra in the Pea Negra field and Rio Bravo in the Lobitos field (Fig. 9). Cabo Blanco Formed by massive medium to coarse and conglomeratic-grained sandstone, milky, hyaline and white quartzitic grains interbedded with light gray claystone. The depositional model indicates a fluvio-deltaic system flowing to the northwest, and it was deposited in the El Alto-Pea Negra area (Fig. 10). Parias Consists of massive medium of coarse-grained sandstone, white, milky-white quartz and dark grains, interbedded with beds of pebbles and reddish-brown, medium gray claystone. Towards the Lobitos area, the Parias becomes thinner and fine-grained sandstone. Sedimentary environment is fluvio-deltaic system flowing from East towards West. However, in the Lobitos area, it corresponds to a Crevasse splay deposits flowing towards North (Fig. 11). Conclusions In spite of the reservoirs complexity in the Talara basin, the gas injection is a good alternative based

in a detailed characterization of reservoir considering multidisciplinary teams. The 26.5% of the daily production in the offshore fields of Talara basin is a clear response of the gas

injection projects. The selective gas injection in thicker reservoirs optimizes the oil recovery in different characterized

layers and avoids the continuous loss of pressure. Technical & Economical Contribution A good reservoir characterization is a very important task to be done by a multidisciplinary team in

order to obtain the expected positive results of a gas injection project. Presently, wet gas is being injected in the active projects. However, in a near future, there are plans

to extract liquids from the gas and use the dry gas for the currently active and future gas injection projects. Further evaluations will be issued.

The Secondary Recovery Factor percentage of the Basal Salina, Mogollon, Pea Negra- Rio Bravo, Parias and Cabo Blanco reservoirs are 2, 3, 6, 6 & 13 % respectively (Fig. 12). The importance to continue developing reservoir characteristics prospects to optimize the Final Recovery Factor of each one producer formations in Offshore fields of the Talara basin, NW-Peru is based on these results.

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Bibliography Carozzi, A. (1975); Depositional Model for the Mogollon, Parias and Cabo Blanco clastic systems of

the Talara Basin, NW Peru. Belco Petroleum Corporation of Peru.

Carozzi, A. & Palomino, R. (1993); The Talara fore arc basin, NW Peru. Depositional models of oil-producing Cenozoic clastic systems. Journal of Petroleum Geology, volume 16(1) January 1993.

Chavez, M. (2004); Evaluation of the Lobitos offshore field, Talara basin. Petro-Tech Peruana S.A, Internal report. Chavez, M. & Janampa, H. (1994 2001). Petro-Tech Peruana S.A, Internal reports. Chavez, M. & Monforte, Y. (2002). Petro-Tech Peruana S.A, Internal reports. Chavez, M. & Montes, C. (2002). Petro-Tech Peruana S.A, Internal reports. Euribe, A. (1976); Glossary of Stratigraphic Units of NW Peru. Biostratigraphic Report 76-01 Belco Petroleum Corporation of Peru. Serra, O. (1985); Sedimentary environments from wire line logs.

Schlumberger.

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