fax 01-972-952-9435. AbstractThis paper presents a simplified economical analysis of PETROBRAS' experience with the pilot project for polymer mobility control in the Carmópolis field, and an evaluation of the world and Brazilian current scenario of this technology. The positive results of this pilot and the new world scenario of the oil industry, shown here, assure great potential of this technology, especially in a time marked by a rising oil prices and growing environmental and social conscience. The pilot project was aimed firstly to evaluate the fit of the polymer technology to Carmopólis field, considering the particular characteristics of the field regarding rock heterogeneities, oil type, water salinity, temperature, pressure, etc. A second goal was to obtain know-how in all the project phases like lab tests, design and operation in field scale and, finally, technical and economical analysis as a function of the additional oil recovered and the direct investment. This economic analysis is a starting point for more comprehensive economical evaluations. This economic evaluation closed the sequence and will be important to the possible future unfolds. This evaluation was done with a special criteria because of the operational typical difficulties of this kind of project. All the investment is justified by the fact of polymer injection to be, according the literature, the easiest application and one of the lowest cost of the incremental oil and besides, the onshore experience is the first step for the future offshore field application. This experience demonstrated that the injection process of polymer for mobility control is efficient even for heterogeneous reservoirs as Carmópolis field, and still, in a sustainable way, in other words, with increase of the oil production and reduction of the use of the water and consequentially of the water produced, and of the costs and of the environmental problems associated to this water, in agreement with the new world partner-environmental conscience.

Carmópolis Field in Sergipe/Alagoas Basin in northeastern Brazil is the country's largest onshore oil accumulation at 253 MMm3OOIP and a current total oil production of 2,880 m3/d. Discovered in 1963, it was quickly put into primary production. Water flooding followed in 1971 at the central portion of the field. The combination of adverse fluid mobility ratio, reservoir heterogeneity and the lack of proper selective injection, led to the quick decline of production. Immediately, a major program of selective plugging, stimulation and selective injection was able to stabilize production. Water flooding was then extended to the entire main block of the field. Well pattern was changed from five to nine-spot arrangement, with a corresponding downsizing in well spacing and injection rates. Carmopolis has also experienced several pilot projects for Enhanced Oil Recovery (EOR): polymer flooding, steam flooding and in situ combustion, respectively. This past history of Carmópolis Field and the significance of water flooding to oil production in Brazil with approximately 1,850 MMm3OOIP currently submitted to this method of recovery, led to the selection of Carmópolis as the target for one of the projects in the portfolio of PRAVAP - Petrobras Strategic Improved Oil Recovery (IOR) Program. The scope of this project included the review of the water flooding operation through improved reservoir characterization and flow simulation, as well as the investigation of other IOR methods that might reverse the production declining trend. This paper summarizes the outcome of this project that went from lab research to field testing and led to the approval of operational implementations worth US$ 34 million NPV. Introduction Water flooding has come a long way since its accidental "implementation" in the area around the city of Pithole, Pennsylvania back in 18651. By the mid fifties this improved method of recovery was responsible for more than 10% of the total oil production in the US. By 1986, this share was thought to be in the 50% range2. This scenario is not any different around the world, specially in major producing countries like the former USSR and the Middle East. A good example is the giant field of Ghawar in Saudi Arabia already partially under water flooding. The situation in Brazil is very similar. Close to 2,000 MMm3 of OOIP are currently under the influence of water injection. In the near future water injection rates will reach more than 500,000 m3/d in discovered fields of Campos Basin. In Marlim field alone in the same Basin, water injection is expected to peak around 100,000 m3/d. In fact, the history of water flooding in Brazil dates back to the early fifties and Carmópolis Field onshore Sergipe/Alagoas Basin, as shown in Figure1, is a very representative part of it. Discovered in 1963 and quickly brought on stream, it produces predominantly from the sandstone and conglomerate reservoirs of the Muribeca/Carmópolis formation and secondarily from the deeper Barra de Itiuba formation plus the fractured basement (see Figure 2). Accordingly, oil quality varies considerably throughout the stratigraphic column. General reservoir data is given on Table1 and a good description of the reservoir geology is given by C ndido and Wardlaw3. This paper focuses on the review of the Improved Oil Recovery (IOR) methods tested in Carmópolis field over the last 28 years, the recently concluded R&D project within PRAVAP - Petrobras Strategic IOR Program, and the corresponding field pilot implementations. Past History of IOR Applications in Carmópolis Field. A good review of the past IOR applications in Carmópolis Field is given by Correia4et al., Doria5 and Romeu6et al.

Summary Carmópolis field, in northeastern Brazil's Sergipe/Alagoas basin, is the country's largest onshore oil accumulation at 253×106 m3 original oil in place (OOIP) and a current total oil production of 2880 m3/d. Discovered in 1963, it was quickly put into primary production. Waterflooding followed in 1971 at the central portion of the field. The combination of adverse fluid mobility ratio, reservoir heterogeneity, and the lack of proper selective injection led to the quick decline of production; however, a major program of selective plugging, stimulation, and selective injection was able to stabilize production immediately. Waterflooding was then extended to the entire main block of the field. The well pattern was changed from five- to nine-spot arrangement, with a corresponding downsizing in well spacing and injection rates. Carmópolis also was subjected to an intense improved oil recovery (IOR) campaign with pilot tests on polymer flooding, steamflooding and in-situ combustion. The history of Carmópolis field and the significance of waterflooding to oil production in Brazil, with approximately 2000×106 m3 OOIP currently submitted to this method of recovery, led to the selection of Carmópolis as the target for one of the projects in the PRAVAP (Petrobras Strategic IOR Program) portfolio. The scope of this project included a review of the waterflooding operation through improved reservoir characterization and flow simulation, as well as the investigation of other IOR methods that might reverse the declining production trend. This paper reviews the IOR history of Carmópolis field and summarizes the outcome of the PRAVAP project that led to the approval of field implementations worth U.S. $34 million net present value (NPV). Introduction Waterflooding has come a long way since its accidental implementation in 1865,1 in the area around the city of Pithole, Pennsylvania. Less than a century later, this improved method of recovery was responsible for more than 10% of the total oil production in the U.S. By 1986, this share was thought to be in the 50% range.2 This scenario is not any different around the world, especially in major producing regions like the former USSR and the Middle East. A good example is the giant field of Ghawar in Saudi Arabia, already partially under waterflooding. The situation in Brazil is very similar. Close to 2000×106 m3 OOIP are currently under the influence of water injection. In the near future, water injection rates will reach more than 500 000 m3/d in the discovered fields of Campos basin. In that basin, water injection in Marlim field alone is expected to peak around 100 000 m3/d. In fact, the history of waterflooding in Brazil dates back to the early 1950's, and Carmópolis field onshore Sergipe/Alagoas basin, as shown in Fig. 1, is a very characteristic part of it. Discovered in 1963 and quickly brought on stream, it produces predominantly from the sandstone and conglomerate reservoirs of the Carmópolis/Muribeca formation and secondarily from the deeper Barra de Itiuba formation and the fractured basement (see Fig. 2). Accordingly, oil quality varies considerably throughout the stratigraphic column. General reservoir data is given in Table 1; for a good description of the reservoir geology, refer to Candido and Wardlaw.3 This paper focuses on the review of IOR applications in Carmópolis field over the past 28 years, the results of the recently concluded project within the PRAVAP portfolio, and the corresponding field pilot implementations. History of IOR Applications in Carmópolis Field Waterflooding. Waterflooding was first implemented in Carmópolis field in 1968 on a 65-ha inverted nine-spot pattern in the southern part of the field. The target was the 2.97×106 m3 OOIP in zones CPS-1, −2 and −3. This project was operated for 3 years at an injection rate of 0.01 PV/yr, with no significant results. Injection was then confined to zones CPS-1 and −2 alone; still, after 2 years of close monitoring, results were inconclusive. Eventually, poor performance led to project abandonment. Meanwhile, in 1971, waterflooding had been initiated at the main block of the field, where substantial reservoir depletion (40 kg/cm2) had led to an average well productivity decline of 30%. The project was designed as nine inverted nine-spot patterns occupying an area of 576 ha. The target oil was 42×106 m3 in zones CPS-1, −2, −3, and −4 of the Carmópolis/Muribeca formation, as well as the reservoirs in the Barra de Itiuba formation. The final estimated recovery factor (FR) was 25.3% for a projected injection rate of 1800 m3/d (200 m3/d/well). During this 8-year project, injection in the Barra de Itiuba formation and zones CPS-3 and −4 of the Carmópolis/Muribeca was suspended owing to the high oil viscosity in the former and the bad quality of the conglomerate reservoirs in the latter. Despite the difficulties in managing selective injection, the project was considered an overall success. Reservoir pressure was restored and well productivity increased. However, the price hike that culminated with the second oil shock of 1979, as well as the fact that the project life expectancy was estimated at 30 years under the original design specifications, led to an effort to upgrade the project design to anticipate production from that area of the field. The result was the conversion of the original nine-spot arrangement of the waterflooding operation to 44 inverted five-spot patterns of 8 ha each, with water injection concentrated in zones CPS-1 and −2 alone. Forty-four new injectors and 28 production wells were drilled and completed only in zones CPS-1 and −2 for a target oil of 15×106 m3. Former injection wells were converted into producers. Injection began in April 1978 at a rate of 100 m3/d/well totaling 4400 m3/d for the entire operation. Because of the downsizing in well patterns and the increase in injection rate, oil production in the area rose from 600 to 800 m3/d within 3 years. Water/oil ratio then began to increase sharply with massive water breakthrough in the producing wells. Originally thought to be simply inherent to the higher injection rates, this breakthrough was later found to be associated to the high mobility ratio of the fluid displacement and reservoir high-permeability streaks. Together with the decrease of the injection rate and the extension of completion to zone CPS-3, an ambitious program of injection well profile modification, through stimulation and selectivity plugging, was implemented. It ensured the maintenance of oil production at acceptable levels up to the mid-1980's. By 1986, however, with the profile modification treatments losing their effectiveness, production went into a sharp decline once again. Fig. 3 gives an overall view of the exploitation history in the main block of Carmópolis field.

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