Ready for Gas Injection: Asphaltene Risk Evaluation by Mathematical Modeling of Asphaltene Precipitation Envelope (APE) With Integration of all Laboratory Deliverables

Abstract

Abstract 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. Introduction Background An offshore carbonate field producing oil from two limestone reservoirs (Upper & Lower) has suffered asphaltene deposition problem, especially inside tubing of the upper reservoir. On the other hands, the lower reservoir has not encountered any remarkable asphaltene related problems so far. Asphaltene problem has been treated mechanically, chemically and/or operationally. Among available various remedies in each technology which have been widely applied in the world1–7, the most conventional ways have been applied in our target field. The selection of remedial action was based on operational and cost-wise reasons. Running gauge cutter is the main effective remedy for removing any asphaltene deposition inside tubing in the field. Xylene is also used to dissolve asphaltene adhering on tubing wall. In addition, horizontal well drilling, gas and water injection were applied to improve oil recovery. Those applications were mainly expected to minimize draw down and to maintain reservoir pressure, and they might play an auxiliary role to mitigate asphaltene precipitation. Moreover, the application of downhole asphaltene inhibitor squeezing8–12 was attempted to delay deposition of asphaltene in the tubing and reduce frequent asphaltene removal operation (gauge cutting and/or solvent soaking). However, the chemical inhibitor squeezing treatment was stopped because the adequate adsorption of squeezed inhibitor on carbonate rock surface could not be achieved (i.e. its effectiveness prolonged shorter than expectation). For the time being, more conventional way of continuous downhole inhibitor injection13–16 would not be a considerable choice in our case, due to its large investment for the workover to introduce macaroni injection lines into dual completed wells. In spite of these various efforts and long histories, the asphaltene problem has not been remedied yet and therefore, it was decided to improve the fundamental understanding of asphaltene precipitation behavior more correctly by integrating comprehensive interpretation of all available laboratory data and field records.