Dynamic behavior of radio-labelled water tracer candidates for chalk reservoirs

A limiting factor to the success of interwell water tracer tests in carbonate reservoirs has been the lack of basic knowledge concerning the dynamic behavior of the tracer at reservoir conditions. The present paper, part of a larger comprehensive study of tracers at reservoir conditions, is a contribution to such necessary basic knowledge. Laboratory experiments have been carried out to examine the flooding behavior of seven radio-labelled potential water tracer candicates in chalk. The seven candidates are HTO, S 14CN −, 36Cl −, 125I −, 35SO 4 2−, H 14CO 3 − and 22Na +. For convenience, 131I − is used instead of 125I − in the laboratory experiments. Here, HTO is considered as an ideal water tracer against which the behavior of all other candidates are measured. The experiments are carried out in a small-core high-pressure rated flow rig at 90°C and 190 bar with purified seawater as the flowing (elution) liquid with a flow rate of 20 cm/d. The results are: HTO, S 14CN − and 36Cl − may be considered as near-ideal tracers for water flow in chalk, although the latter two show clear signs of ion exclusion. 131I − and 35SO 4 2− show a more complicated behavior with contribution both from ion exclusion, sorption/desorption and possibly also precipitation/dissolution chemical reactions. However, they may conditionally be applied as water tracers in chalk material. Radio-labelled bicarbonate, H 14CO 3 −, cannot be used as a water tracer in chalk possibly due to strong matrix interaction. 22Na + shows a slight sorption behavior probably governed exclusively by a reversible exchange with inactive Na + on clay crystals in the carbonate. Nevertheless, 22 may conditionally be regarded as a near-ideal, though slightly sorbing, tracer for water in chalk. The slight sorption may be exploited positively to measure the formation rock ion exchange capacity.