2.3.1. Fluid sampling in production wells

Sampling must be carried out according to the planned schedule and in most cases should start immediately after injection.

Sampling frequency will depend on the basic understanding of reservoir production dynamics. The decision to stop sampling is made jointly by the end — user and tracer team based upon the development of the tracer production curve and cannot be planned a priori. Generally, sample volumes of 0.5—1.0 L are preferred. This enables repeated analysis of the same sample if desirable. This possibility may be important for quality assurance of the analysis. In the case of water cut, preferably less than ~10% of the water sample is collected at the separator equipment point where the majority of the oil is removed.

In the case of too much oil, the water has to be recovered from a complex oil-water emulsion, and the quantity of water recovered may still be too little to obtain a high quality analysis with the best detection limit. For sampling in geothermal wells this problem does not exist.

A typical sampling schedule is introduced below. Tools for well head sampling include:

• Personal protective equipment (safety glasses, gloves, personal H2S

monitor and respirator (if required by local rules), etc.);

• Crescent wrenches;

• 20 L bucket for oil and water spills;

• 1 roll of electrical tape for sealing up the bottle cap;

• 1 L plastic wide mouth sample bottles;

• Absorbent pads (for oil spills);

• Rags (for water spill);

• Felt tipped permanent marker for writing on bottles;

• Sampling record book.

The sample is marked and identified by the well number, date of collection and, preferably, by the initials of the sampling engineer. The actual schedule of sampling and the method used is field dependent. Different methods may be considered:

• Manual sampling carried out by the operator;

• Automatic sampling at the well head;

• Continuous sampling;

• Downhole sampling.