Single Well Chemical Tracer Tests Provide Rapid, In-Situ Saturation and Fractional Flow Measurements in the Aurora Oil Field, Alaska

Abstract

Abstract Single well chemical tracer tests were used to measure saturations and oil-water fractional flow accurately, quickly, and cost-effectively. The Aurora oil field in the Prudhoe Bay Unit, Alaska, was identified for fast-paced development leveraging existing facilities. A series of single well chemical tracer tests (SWTT) determined key volumetric and reservoir performance properties including: Initial oil saturation,Waterflood residual oil saturation,Miscible gas EOR residual oil saturation, andWaterflood water-oil fractional flow. The SWTT measurements of initial oil saturation, residual oil saturation to water, residual oil saturation to miscible gas, and oil-water fractional flow made during the initial field development in 2001 closely match current model parameters determined based upon 18 years of field history. Methods, Procedures, Process Given a fast development pace and relatively small field size, it was deemed impractical to collect low invasion core and perform expensive, complex, and time consuming special core analysis. Instead, a series of SWTT were performed on a single production well to determine key reservoir parameters within six months. This compares favorably to core acquisition and analysis which can take six times longer and cost ten times as much. Also, SWTT can be performed after a well is drilled and on production, so key tests can be performed without early, time-intensive special core analysis acquisition planning and rig time. Aurora production well S-104 was chosen as the key data collection well to describe Aurora field. The well was conventionally cored and had a full suite of open hole logs including nuclear magnetic resonance and focused micro resistivity. This well was an ideal candidate for a SWTT as it had a good quality cement job, no water or gas injection, a detailed near wellbore reservoir description, and the well would produce to surface with gas lift. The SWTTs were performed over a 30-foot perforated interval. A typical SWTT involves tracer injection, shut-in time, then production flowback. No downhole interventions were needed since SWTT are performed using readily transportable surface equipment like chemical injection pumps and well tracer sampling and measurement equipment including a gas chromatograph. Results, Observations, Conclusions Connate water saturation was lower than expected (13% ±2), increasing the estimated oil in place and calibrating the well log water saturation log model and reservoir model saturation-height function. Due to increased initial oil saturation proved by the SWTT, additional wells were justified in the southern portion of the field which added an estimated 2.5 million barrels of recovery. The waterflood residual oil saturation was higher than expected (30% ±2) indicating a more oil-wet system than previously assumed. The oil-water fractional flow data also indicated a more oil wet relative permeability curve than estimates from available analog curves. Finally, the miscible gas EOR test demonstrated miscibility and enhanced oil recovery in-situ by measuring very low residual oil saturation (4.5% ±2) to miscible injection gas. Novel/Additive Information This is the world's first successful SWTT water-oil fractional flow measurement. If data collection had included downhole pressure gauges, the test would have also measured relative permeability endpoints. The Aurora SWTT program provides an innovative solution to a classic challenge: Accurately determining key reservoir properties in a timely and cost-effective way. Reservoir simulation using SWTT results match 18 years of field performance, demonstrating the accuracy of SWTT measurements.