An undeveloped deepwater field, LC, in deepwater off the coast of Australia is a candidate for secondary waterflooding. But, will it be better to perform SeaWater Injection (SWI) and Produced Water Reinjection (PWRI), as has been done for all other water flooded oil fields in the same region, or to perform Low Salinity Water Injection (LSWI)? This major decision depends upon answering the questions: (1) Is LSWI likely to cost-effectively increase the oil recovery in LC? and (2) Will LSWI cause unacceptable risks to operating LC - for example by provoking formation damage and/or creating flow assurance problems? Previous investigations (Emadi and Sohrabi; 2012, 2013) revealed that interactions between crude oil and low salinity water that lead to spontaneous formation of micro-dispersions within the oil phase, can be an indication of improved oil recovery at core level.
The low salinity water injection (LSWI) study reported here had three major objectives: firstly to investigate the potential of this improved oil recovery (IOR) technique for the field LC using the reservoir rock and fluids, Secondly, to further validate our proposed mechanism (Sohrabi et al., 2015). Thirdly, to extend our previous investigation (Farzaneh et al., 2015) on the effect of total salinity and ionic composition in a reservoir rock with negligible clay content (such as that found in the field LC), as opposed to prior work on synthetic clay-free porous media. These objectives were achieved by a comprehensive set of experiments that systematically investigated the role of the rock/fluid and fluid/fluid interactions at different length-scales. The experiments included the following: micromodel tests, wettability analysis and adhesion measurements using contact angle, zeta-potential measurements (to investigate fluid/fluid interactions). Core floods were performed to confirm and quantify the IOR potential of LSWI for field LC. Field scale reservoir simulation studies that used the results of the four mentioned corefloods were performed to evaluate the expected IOR benefit for field LC of LSWI. Outcomes from these fundamental studies were used to tailor the brine composition for core flood activitiest to optimise the expected impact of LSWI on field LC. Our unique research approach allowed us to measure and understand the physical processes of low-salinity waterflood.
The objectives of the above mentioned experiments were successfully achieved. Some of the results were Notable: (1) the behaviour of natural surfactants in the oil of LC are influenced by both the ionic concentration and balance of the injection water, (2) when compared with SWI, LSWI recovers significantly more oil in corefloods, even though the clay fraction of the cores is lower than that which is often reported for cores that have reacted favourably to LSWI; and (3) removal of just the Ca2+Mg2+ divalent ions from the injection water unexpectedly increased the endpoint relative permeability of water.
The results of this extensive set of experiments present a case study for a real reservoir system, which includes a comprehensive set of data obtained by various methods at different scales and shed new insight into mechanisms of oil recovery by low salinity water injection. In addition, the oil in LC is biodegraded with an anomalously low asphaltene content. The dominant lithology is high permeability sandstone, which is mixed-wet in the oil zone. Field LC has a significant oil/water transition zone in which the wettability changes from being mixed-wet at the top to being water-wet at the bottom. It was important to use a simulator that can handle the effects of such wettability changes on the behaviour of LSWI.
The results of this extensive set of experiments present a case study for a real reservoir system, which includes a comprehensive set of data obtained by various methods at different scales and shed new insight into mechanisms of oil recovery by low salinity water injection.