I love your smart decision there since you will start selling at the rate of $18 which sounds cool but I still don't get why it can be reduced to 0. Does this means that there can also be negative value (value below 0) during loss?

CFD means, contract for difference. I'm selling a contract, that contract does not take delivery. It's like a gambling between the broker and me. Selling a contract, will have to buy back later.

On spot market, the price is following the market consensus. It has nothing to do with my contract. This means, when you gamble, you look at real price. Because when oil fall below a reasonable value, nobody will ever trade their oil anymore. Gas station will close. The government will rather GIVE away petrol/gasoline for free rather than selling, because the operator who operate the gas station would cost more than the petrol itself. So the answer is, oil price will never go negative, the bi-product may be.