Perfluorocarbon tracers (PFTs), a class of six compounds, were used to help characterize the Shallow Oil Zone (SOZ) reservoir at the Naval Petroleum Reserve in California (NPRC) at Elk Hills. The SOZ reservoir is undergoing a pilot gas injection program to assess the technical feasibility and economic viability of injecting gas into the SOZ for improved oil recovery. PFTs were utilized in the pilot gas injection to qualitatively assess the extent of the pilot gas injection so as to determine the degree of gas containment within the SOZ reservoir.
Four different PFTs were used, two were injected over a days duration (approximately 12 grams of each) into a well undergoing 250 MCFPD of gas injection and the other two tracers injected over a days duration in another well undergoing 75 MCFPD of gas injection. Samples were collected from at least eighteen sampling wells surrounding the injection wells; collected at the wellhead in 1 liter sampling bombs and transferred at the NPRC site to smaller Capillary Adsorbent Tracer Samplers (CATS) which were then shipped to Brookhaven National Laboratory (BNL) for tracer analysis. Up to six tracers can be simultaneously analyzed within fourteen minutes down to a detection limit of several femto-liters of tracer per liter of reservoir gas.
PFTs have been detected in at least thirteen of the sampling wells over several months of sampling. Results will be discussed in qualitative terms of reservoir characterization since the design of the experiment does not allow quantitative certainty in reservoir interpretation. The tracer program has suggested changes in the geological interpretation of the reservoir since faulting affects the movement of the tracer tagged injection gas.
This work demonstrates that a relatively simple tracer program can be used to improve reservoir characterization.