NEW EXPERIMENTAL EVIDENCE ON THE DOMINANT MECHANISM OF OIL RECOVERY BY LOW SALINITY WATER INJECTION IN CARBONATE ROCKS

摘要

In the past two decades,several investigations have shown an improvement in oil recovery by low salinity water injection(LSWI)in core flood experiments and some pilot tests have been performed. Despite numerous publications on this matter,the underlying mechanisms leading to improved oil recovery have not yet been confirmed. It has been recently reported that certain crude oil-brine interactions take place during LSWI that lead to formation of water in oil micro-dispersions. The role of this mechanism,which is linked to the natural surface active components of the oil,has already been demonstrated in improving oil recovery in clastic rocks. In this work,we will test whether micro-dispersion formation could also be applicable to carbonate reservoirs. This experimental investigation was performed on crude oil and rock samples obtained from a carbonate oil reservoir in the Middle East. Firstly,using an in-house methodology,a crude oil sample with high propensity to form micro-dispersion was identified. This crude oil(Crude-A)was then used in a series of oil/brine interaction tests involving formation brine and various diluted versions of it,to quantify the micro-dispersion formation capacity of the oil. Another sample from the same crude oil was then prepared by removing the compounds responsible for micro-dispersion formation(Crude-At). The two crude oil samples,one capable and the other incapable of forming micro-dispersion,were then characterised and prepared for core-scale experiments. We observed that the crude oil with high micro-dispersion resulted in additional oil recovery during tertiary LSWI compared to the crude oil with low micro-dispersion. Therefore,the presence or absence of the oil compounds behind micro-dispersion formation can make the system respond positively or negatively to LSWI. The effluents of the coreflood experiments were also analysed. Interestingly,identical behaviour in terms of both pH and ionic concentrations was obtained with the two different oil recovery profiles,indicating that the micro-dispersion formation propensity of the crude oil is the main factor determining the efficacy of LSWI in carbonate rocks. The results provide useful new insights into the role of crude-oil/brine interactions during LSWI in carbonate reservoirs and their impacts on the response of a reservoir to LSWI.