Core flushing

When drilling a new well it is important to identify the zones in which, eventually, water coming form the reservoir invades the core as well as to quantify the amount of this fluid that is added to the drilling fluid. Tracers are an excellent tool to accomplish this purpose.

Working method

While drilling, the selected tracer (tritiated water) is added to the drilling fluid. Its concentration is high enough to be easily detected by means of specialised instruments but low enough to make radiological hazards, negligible. As a matter of fact, the amount of tracer is so low that the tracer can be transported in an "excepted" package. That means that the usual regulations for radioactive materials are no applicable to such a package.

Samples from the drilling fluid are taken periodically at different known depths. Once the drilling operation has finished samples are send to the laboratory to be filtered, distilled and measured. Using this information a baseline of concentration as a function of the depth  can be obtained. Additionally, samples from the core are also taken and send to the same laboratory. However, in logging evaluation as well as in core analysis it is important to understand that drilling mud can have invaded the formation and replaced the natural fluids like water and oil; for this reason it is convenient and necessary  to quantify the volume of drill fluid added to the cores. 

If the concentration in the samples taken from the core is lower than the value given by the baseline, that means that an invasion of water took place with the consequent reduction of the initial concentration of tritium in water.

Experimental results

Next picture shows the baseline as well as the tracer concentration in the core samples as a function of the depth at which samples were taken for a typical case. The blue line measures the percentage of invasion water.

Experimental results

From the value of drilling fluid invading the formation the water saturation can be corrected. Water saturation is an important parameter in evaluation of oil reserves.

Evaluation of core flushing is other tracer application that NOLDOR S.R.L. proposes to the oil industry.

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