Experimental and Theoretical Assessment of the Asphaltene Precipitation Characteristics of the Sahil Field Under a Proposed Miscible Gas Injection Scheme

Abstract

Abstract As part of the feasibility assessment for an improved oil recovery project on the Sahil reservoir in the United Arab Emirates, it was desired to assess the potential for asphaltene formation arising from the miscible contact of a rich hydrocarbon injection gas and the Sahil reservoir fluid. In this work, experimental data for the upper and lower asphaltene onset conditions over a temperature range from 110– 255°F were measured for the reservoir fluid. In addition, three injection gas mixtures (15, 45, & 60 mole%) were also evaluated at the reservoir temperature of 255°F. The experimental data were measured using a Near Infrared (NIR) light scattering technique. Subsequently, the experimental data was used to tune an equationof- state (EOS) based model that represents the asphaltene phase as pseudo pure component solid. Results of the work demonstrated a significant effect of temperature and injection gas concentration on the asphaltene onset conditions for the reservoir fluid. It was also shown that the chosen model could accurately represent the measured behavior of the fluids over the range of conditions tested. Introduction The Sahil reservoir in the United Arab Emirates, was first brought on to production in 1973 and has been produced under primary depletion up to 1994. The reservoir performance indicated that there was no aquifer support. Hence, a pressure maintenance scheme in the form of peripheral water injection was implemented in 1994. Subsequently, a miscible rich gas injection trial was initiated in 1998 with four gas injection pilots, one gas injection pilot to the North of the field and three pilots in the central part of the field. Solids depositions were observed in many producing wells. A summary of the Sahil reservoir fluid properties is provided in Table 1. These properties confirm a typical black-oil reservoir fluid. Properties of note for the stock tank oil include a n-C5 insoluble asphaltene content of 1.3% (w/w), a wax content of 7.9% (w/w) (UOP 46–64) and a corresponding cloud point of 140°F (ASTM D-2500). These values are reasonably typical and would generally not indicate a great risk for wax or asphaltene formation under primary production. Generally, asphaltenes tend to remain in solution or in colloidal suspension under reservoir temperature and pressure conditions. They may start to precipitate once the stability of the colloidal suspension is destabilized, which is caused by the changes in temperature and/or pressure during primary depletion.1 On the other hand, asphaltene have been reported to become unstable as a result of fluid blending (co-mingling) of fluid streams2 as well as by gas injection during Improved Oil Recovery (IOR) operations.3–6 Due to these observations, and in view of the consideration of gas miscible IOR operations on the Sahil reservoir, it was felt that an assessment of the asphaltene formation potential for this reservoir may be advisable. For this reason, a work scope including experimental measurements and theoretical modeling was initiated. This work outlines the results of that effort.