Application of Single Well Chemical Tracer Tests to Determine Remaining Oil Saturation in Deepwater Turbidite Reservoirs

Abstract

Remaining oil saturation (ROS) and waterflood residual oil saturation (Sorw) are key parameters for reservoir modeling and waterflood management in a group of heterogeneous deepwater turbidite reservoirs. A large amount of laboratory special core analysis (SCAL) data indicated high Sorw values and a large target potential target for chemical EOR (enhanced oil recovery). Available SCAL data was not considered reliable. Acquiring additional core was considered to be too costly and too risky due to the highly deviated well paths required for new wells. Single Well Chemical Tracer Tests (SWCTT's) in producing wells were the only viable alternative. This paper describes – to our knowledge – the first applications of SWCTT in a deepwater setting. An early 2010 SWCTT showed ROS / Sorw to be much lower than expected but test interpretation was uncertain. The 1st SWCTT provided a valuable learning experience to improve test design and execution and to improve on significant logistical challenges in the deepwater setting. Using lessons learned we performed two additional SWCTT's in late 2010. The later SWCTT's included well integrity pre-tests and smaller completion intervals. Typical SWCTT volumes were ~5,000 bbl of seawater containing tracers with a depth of investigation of ~4 to 5 meters. All three SWCTT's indicated low Sorw values, ranging from 0.05 to 0.20 with a nominal average of 0.15. Similar results from all three SWCTT's indicate that microscopic displacement efficiency is very good; eliminating the option of chemical EOR. The current field development plan is focused on improving volumetric sweep efficiency. Properly designed and executed SWCTT's can be considered as large-scale "laboratory waterflood tests" at true reservoir conditions (e.g., live oil, wettability and stress history). Compared to conventional SCAL tests using small plug samples, SWCTT's see a much larger rock volume and avoid wettability alteration issues that plague modern cores taken with OBM (oil based mud). Though logistically challenging in deepwater, SWCTT's can be more cost- and time-effective than taking a new core.

Numerical Interpretation of Single Well Chemical Tracer (SWCT) Tests to Determine Residual Oil Saturation in Snorre Reservoir

“…This comparison actually reveals that the injection and production process is reversible, where the tracers travel the same stream line into the formation and out of the formation. • Gas lift: higher vapor pressure is a main source of non-ideal effect in some SWCT tests (Deans et al, 2007, DeZabala et al, 2011. In these tests, EtAc would be stripped from produced water by means of gas and NPA profile would be used instead of the EtAc profile to interpret the test and determine S or .…”

“…Based on the experience of other researches (Deans et al, 2007, Jerauld et al, 2010, de Zwart et al, 2011, DeZabala et al, 2011, in the present test, three-layer model is used in order to match the modelled tracer profiles to field data. Then, the fraction of total fluid injected and produced from each layer is set to match the modelled and measured EtAc profiles, and then, the S or for each layer is determined to match the modelled and measured EtOH profiles.…”