If Shell is buying water rights they get the senority that goes with those rights. they do not go to the end of the line.

Absolutely. In fact the story in the Denver Post mentioned the seniority of some of the rights. But there is no way Shell can line up all of the senior rights in the area, which means that during low water years they will be subject to production curtailment. Furthermore, there is an absolute requirement to allow a certain amount of water to pass through the Colorado/Utah border, in essence a huge senior water right.

The way around this problem is to build reservoirs. Look for those proposals soon if Shell is serious, as reservoir permitting and build is a long process.