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.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAn alternative method for measuring polymer concentration in core effluents during dynamic polymer adsorption determination tests in laboratory scale was developed. Laboratory tests were made by injecting polyacrylamide solutions at different concentrations in a water saturated sandstone core. Core effluent was measured for the shear rate necessary to cause a constant and high (70% of full scale) torque on a rotational rheometer. The measured shear rate was converted to concentration values by using a calibration curve. Polymer concentration was also determined by spectrophotometry for comparison.The curve obtained by the proposed isotorque method showed greater reliability and lower variations than the spectrophotometric method. The isotorque method, besides being simple in terms of laboratory work, is able to give better adsorption values as input for polymer flooding simulations.

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.

located at Reconcavo Basin in northeastern Brazil has been under production for more than fifty years. From the very beginning in 1947, exploitation has been plagued with problems that standard geology and reservoir engineering practices were not able to cope with. With an OOIP of 113 MMm 3 of 38º API paraffinic crude, current recovery factor stands at only 15%, in spite of the early implementation of waterflooding. Three major reservoir studies by different organizations, Petrobras included, were conducted over the years with significant impact on the geological model and appraisal of the original volumes, together with recommendations as to the variation in the injection and production patterns. They were implemented accordingly but with limited success and, other than the heterogeneity of the reservoirs in the main Sergi Formation, no suitable explanation was found for the poor recovery performance, not to mention a solution to the problem. The size of the target oil and the constant threat of field abandonment, led to the approval of a research project in late 1993 as part of the portfolio of PRAVAP, Petrobras Advanced Oil Recovery Program. Its goal was twofold: verify the assumption of paraffin precipitation in the reservoir as the cause for the low recovery factor, and eventually put to the test recovery strategies that would ensure the economical exploitation of the shallow deposits of paraffinic crude in Reconcavo Basin. This paper summarizes the major accomplishments of this project throughout the 3½ years of research and development and the corresponding EOR field implementations.

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