The experience gained through the work carried out in oilfield 1 strengthened the capabilities of the tracer group at PINSTECH and resulted in better cooperation and enhanced acceptance of the technology by the end user. This provided an opportunity to extend radiotracer applications to another oilfield. Oilfield 2 is situated about 20 km away from oilfield 1. There are four production wells and two injection wells in this field. The pattern of wells is shown in Fig. 62. The distance between injection wells and production wells varies from 1125 to 3975 m and the depth of the wells varies from 4068 to 4267 m below mean sea level.

Objectives

The objectives of this study were to:

• Determine the breakthrough time between the injection and production wells;

• Assess the contribution of injected water in the production wells;

• Determine relative contribution of injected water and formation water to individual production wells;

• Investigate the presence of channels (if any) between the injection and production wells;

• Determine the mean residence time of floodwater in the reservoir;

• Determine the percentage radiotracer recovery and the swept volume by floodwater.

Tritium (as HTO) was injected in well 1 through a bypass loop and production wells 3, 4, 5 and 6 were monitored for tracer response. Tracer breakthrough was recorded in well 3 in 17 d. However, no tracer breakthrough was observed in production wells 4, 5 and 6 up to June 2008. Stable isotopes of water (2H and 18O) were also utilized to identify different water sources and their relative contributions to produced water. The data are shown in Figs 63 and 64.

The work related to oilfield 2 is in progress and the results obtained to date are as follows:

• The breakthrough time of well 3 is 17 d.

• The early breakthrough in well 3 indicates the channelling effect between injection well and production well 3.

• Mean residence time of tracer with respect to injection well 1 and production well 3 is 127 d.

• The volumetric response of tracer has determined that mean produced water volume from well 3 is 52 000 m3, which was achieved in 136 d after radiotracer injection and 301 d since water injection was started. These figures are in good agreement with a mean residence time of 121 d.

• The volumetric response of tracer has determined that mean injected water volume from injection well 1 is 200 490 m3, which was achieved in 131 d after radiotracer injection and 301 d since water injection was started. These figures are again in good agreement with a mean residence time of 127 d.

•
The maximum and mean velocities of injected water between injector well 1 and producer well 3 are 80.9 m/d and 110.8 m/d, respectively.

• About 53% of injected tracer has been recovered through producer well 3 within 427 d of radiotracer injection (up to 15 May 2008).

• Considering a mean produced water volume of 52 000 m3 and 53% recovery of radiotracer from well 3, the mean swept volume is determined as 27 560 m3. This is the average volume of reservoir swept by injected water which was produced by well 3.

• No radiotracer breakthrough was detected in wells 4, 5A and 6 up to 15 May 2008.

• Relative contributions of injected water and formation water to production well 3 are 77% and 23%, respectively.

I.3.3. Conclusions

The tracer test carried out in both oilfields provided excellent data, which can be used to validate modelling software. The highlighting point of these tracer tests was that the conjunctive use of the stable isotopes of water (2H and 18O) along with radiotracer provided very useful supplementary information, giving more credibility to tracer technology as applied to interwell communication studies. Therefore, stable isotopes can be successfully applied for interwell communication studies where there is reasonable difference in stable isotope indices of injection and formation waters. Further, stable isotopes are unique tools to identify different sources of groundwater.