An offshore carbonate oil field in the Arabian Gulf is exhibiting asphaltene deposition inside the tubing of production wells completed in one of two main producible limestone reservoirs. This problem significantly reduces well profitability because of production loss and frequent asphaltene-removal jobs (solvent soaking). Furthermore, future full-field enhanced-oil-recovery (EOR) development-namely gas injection-is now planned and might have a risk of enhancing the asphaltene problem. Therefore, comprehensive study has been carried out not only to establish a less-frequent and more-effective remedy than the current action but also to evaluate a future risk of gas injection.The study was initiated with careful review of the fundamental measurements of asphaltene properties collected during the 20 years of production history-saturates, aromatics, resins, and asphaltenes (SARA); asphaltene contents; and asphaltene-onsetpressure (AOP). Subsequently, the mathematical modeling analysis using those properties was incorporated into the study for better understanding/predicting of asphaltene-precipitation behavior. This paper describes the integration/optimization of the asphalteneprecipitation-envelope (APE) modeling on the basis of all available laboratory data, and consequently suggests representative APE. The APE-model validity was evaluated by comparison with actual observation data in the problematic reservoir.On the basis of the mathematical models established, several sensitivities (i.e., mixing with injection gas and blending oils produced from the two main producible reservoirs) were investigated to assess impacts of the future EOR on asphaltene risk from the subsurface and surface points of view. Several types of injection gas were examined, and their risks were compared and identified. Consequently, the surface-facility design was adequately modified and optimized in order to minimize asphaltene risk influenced by gas injection.

Asphaltene study is now becoming a regular menu as a part of gas-injection studies (Kokal et al. Moghadasi et al. 2006). The asphaltene onset pressure (AOP) is one of the most important factors in understanding asphaltene precipitating behavior. The solid detection system (SDS) based on light-scattering technique has been quite popular and widely used in all over the world (Kokal et al. 2003(Kokal et al. , 2004Jamaluddin et al. 2000;Negahban et al. 2005;Gholoum et al. 2003;Garcia et al. 2001;Oskui et al. 2006;Gonzalez et al. 2007) to measure AOP. The simple experiments to measure AOP are usually conducted using a mixture of reservoir fluid and injection gas, and various gas-mixing volumes are assumed to be investigated. These various experimental specifications of gas-mixing volume are useful in understanding asphaltene risks during gas-injection projects. However, this type of investigation can show only a static asphaltene behavior, and sometimes might overlook true asphaltene risks.In the gas-injection pilot (GIP) project in an offshore carbonate oil field in the Arabian Gulf, the static asphaltene behavior was studied by the SDS using near-infrared (NIR) light-scattering technique. For this study, a single-phase bottomhole sample was collected from the same producing zone, but the sampling location was 90 ft shallower than the GIP area. Various combinations of mixtures were examined to measure AOP (i.e., reservoir fluid mixed with 0, 25, 37.5, 43.5, and 50 mol% injection gas). Furthermore, the numerical models were generated and calibrated with the experimental findings. To evaluate the asphaltene risks at the GIP area, the models were adjusted to the target oil composition by considering existing oil compositional gradient in the field. However, the modeling analyses showed that the operating conditions of producing wells are outside the estimated asphaltene-precipitation envelope (APE). This result was inconsistent with the field fact in which actual asphaltene deposits were observed and collected from the bottomhole of some wells in the GIP area. Thus we were obliged to recognize that our current experimental results of static asphaltene behavior overlooked the actual asphaltene risks. What is insufficient for realistic modeling? Our hypothesis is the dynamic asphaltene behavior.During a gas-injection process, the injected-gas composition is changed because of a vaporizing-gas-drive (VGD) mechanism, in which gas was enriched with the intermediate-molecular-weight hydrocarbons from reservoir oil. Our latest experiments investigated a static asphaltene behavior only; that is, it did not include this process. Therefore, the sensitivity analyses were motivated to realistically evaluate the actual APE, counting the VGD effects with the calibrated model. Various enriched-gas compositions were investigated in terms of how these enriched gases would affect APE. Consequently, it was found that the enrichment of intermediate components expanded the APE, and the operating conditions of asphaltene-problematic wells co...

An offshore carbonate oil field in the Arabian Gulf is exhibiting asphaltene deposition problem mainly inside tubing of production wells completed in one of two main producible limestone reservoirs. This problem significantly reduces well profitability due to production loss and frequent asphaltene removal job (solvent soaking). Furthermore, future full-field EOR development, namely gas injection, is now planned and might have a risk to enhance the asphaltene problem. Therefore, comprehensive study has been carried out not only to establish less frequent and more effective remedy than the current action but also to evaluate a future risk of gas injection.The study was initiated with careful review of the fundamental measurements, collected during the 20 years production history, of asphaltene properties, i.e. SARA (saturates, aromatics, resins and asphaltenes) analysis, asphaltene contents, AOP (asphaltene onset pressure) measurement, etc. Subsequently, the mathematical modeling analysis using those properties was incorporated into the study in order to develop APE (asphaltene precipitation envelope) for better understanding/predicting of asphaltene precipitation behavior. Therefore, this paper describes the integration/optimization of the APE modeling based on all available laboratory data, and consequently suggests representative APE. The APE model validity was evaluated by comparison with actual observation data in the problematic reservoir.Based on the mathematical models once established, several sensitivities, namely mixing with injection gas and blending oils produced from two main producible reservoirs, were investigated in order to assess impacts of the future EOR on asphaltene risk from sub-surface and surface point of views. Several types of injection gas were examined, and their risks were compared and identified. Consequently, the surface facility design was adequately modified and optimized in order to minimize asphaltene risk assisted by gas injection.

Three case studies pertaining to the precipitation risk of asphaltene, which comprise comprehensive evaluation from laboratory measurement to practical feedback into the field, are described. Two cases included aspects of gas injection, and the third corresponds to formation damage at a naturally depleted field. Asphaltene onset pressure (AOP) is a fundamental characteristic that represents in situ asphaltene behavior from a single-phase fluid sample; therefore, all evaluations were performed on the basis of laboratory-measured AOP data. The evaluations reported reveal how practical interpretation of laboratory-measured AOPs links appropriately to actual on-site phenomena. In the first two case studies, numerical asphaltene fluid models, calibrated using measured AOPs, were generated to evaluate asphaltene precipitation envelopes (APEs) by applying the cubic plus association equation of state. In the first case, a sensitivity analysis based on a numerical model of the original reservoir fluid was performed for several injection gases to investigate the impact of gas injection on APE behavior from the subsurface and surface points of view. In the second study, the variation of asphaltene deposits, actually observed at the gas injection pilot area, was explained by considering the vaporizing gas drive (VGD) process in terms of APE behavior. A sensitivity study was conducted to estimate how enriching the injection gas by this effect could expand the APE. An expanded APE could cause the variation of asphaltene deposits observed in the field: deposits were observed at certain times when enriched injection gas was accumulated but not when lean injection gas accumulated. Control of pressure depletion is considered an effective countermeasure to mitigate asphaltene precipitation in a naturally depleted field. For a field with a potential asphaltene deposition problem in the reservoir, it is recommended to maintain the reservoir pressure above the AOP. Such mitigation needs reliable AOP data. Once destabilized, solid particles of asphaltene grow continuously from the nanoscale (precipitation) to the microscale by aggregation. Our AOP measurements adopted the latest laboratory techniques. Three data sets were acquired using filtration, high-pressure microscopy analysis, and laser light scattering techniques. Despite using the same single-fluid sample, the AOP results were not identical because each measuring technique had different detection limits for the minimum asphaltene particle size. This paper demonstrates how we assessed the AOP data to determine the cause of asphaltene-induced formation damage by comparison to actual flowing bottomhole pressure behavior. This practical AOP measurement could suggest a more optimum pressure control target that would allow for maximum oil production while mitigating production potential loss as a result of shutdown for cleanup or removal jobs.

Asphaltene study is now becoming a regular menu as a part of gas injection studies 1–11. The asphaltene onset pressure (AOP) is one of the most important factors to understand asphaltene precipitating behavior. The SDS (solid detection system) based on light scattering technique has been quite popular and widely used in all over the world 1,7–9,12–15. The simple experiments to measure AOP are usually conducted using mixture of reservoir fluid and injection gas, and various gas mixing volume are assumed to be investigated. These various experimental specification of gas mixing volume are useful to understand asphaltene risks during gas injection projects. However, what this investigation can show is just a static asphaltene behavior, and sometimes might overlook true asphaltene risks. In the gas injection pilot (GIP) project in an offshore carbonate oil field in the Arabian Gulf, the static asphaltene behavior was studied by the SDS using NIR (neear infrared) light scattering technique. For this study, a single phase bottomhole sample was collected from the same producing zone, but the sampling location was 90 ft shallower than the GIP area. Various combination of mixtures (sampled reservoir fluid mixed with 0, 25, 37.5, 43.5 and 50 mol% injection gas) were examined to measure AOP. Furthermore, the numerical models were generated and calibrated with the experimental findings. In order to evaluate the asphaltene risks at the GIP area, the models were adjusted to the target oil composition by considering existing oil compositional gradient in the field. However, the modeling analyses showed that the operating conditions of producing wells are outside the estimated asphaltene precipitation envelope (APE). This result was inconsistent with the field fact, in which actual asphaltene deposits were observed and collected from bottomhole of some wells in the GIP area. Namely, we were obliged to judge that our current experimental results of static asphaltene behavior overlooked at the actual asphaltene risks. What is insufficient for a realistic modeling ? Our hypothesis is the dynamic asphaltene behavior. During gas injection process, the injected gas composition is changed due to vaporizing gas drive (VGD) mechanism, in which gas was enriched with the intermediate molecular weight hydrocarbons from reservoir oil. Our latest experimental investigation of static asphaltene behavior did not include this process. Therefore, the sensitivity analyses of the VGD effects were carried out with the calibrated model to realistically evaluate the actual APE. Various enriched gas composition were assumed, and the affects of these enriched gas on APE were investigated. Consequently, it was found that the enrichment of intermediate components expanded APE, and the operating condition of asphaltene problematic wells could be explained to be inside APE. Therefore, we concluded that the dynamic asphaltene behavior must be understood for a realistic risk evaluation in the gas injection project. Introduction Background and Histories The target field was discovered in 1963 and started production in 1967. It is currently operated by ADMA-OPCO. It produces from two carbonate reservoirs (A and B) and its oil is transported and processed at the plant near an island. To maintain reservoir pressure, the dump flood water injection started in 1972, followed by powered water injection in 1978 and the crestal gas injection in 2003. In addition to this project, gas injection pilot project at the flank area has been carried out at western flank area of the field.