Geochemistry: A Powerful tool for Reservoir Monitoring

In an offshore Abu Dhabi field, two main reservoirs are produced commingled. Monitoring the production contribution from each reservoir was previously achieved through either Production Logging or Selective Production Tests. This paper presents the successful introduction of Geochemistry as an alternative tool. This well known technique is based on the analysis of high resolution gas chromatograms of wellhead oil samples. With the help of a dedicated software pairs of peaks are selected, for which the height ratio is significantly different between oil samples of different reservoirs. The split in a mixed oil sample is then derived from the value of these peak ratios. Oil from each reservoir of the field was analyzed, and the two main reservoirs produced commingled were found to have a clearly different signature. A set of samples from commingled wells was then analyzed. The calculated oil splits were found in excellent agreement with values obtained from recent production logging data. The method was successfully extended to the case of wells producing three commingled reservoirs. It is now intended to routinely use this technique, with the following advantages: - Cost effectiveness - No well intervention or loss of production, - The only applicable method on wells where downhole access is impossible due to mechanical problems, - Valid information even if communication behind casing exists between the two reservoirs.