Even if the US doesn't need to import oil (and maybe it doesn't already) that doesn't mean oil companies are going to sell oil cheap just to be nice.
If the market produces enough supply to meet demand, prices will inevitably come down otherwise oil producers who are selling at higher prices will not make money or achieve any market share.
OPEC is the largest supplier of crude on the international market, and they cannot control prices or fluctuations because US producers are undercutting them and meeting market demand both domestically and internationally.
So they have tried to collude to restrict their supplies and keep prices higher as demand continues, but it is nearly impossible for them to do as more and more producers both domestic and Foreign replace the product they refuse to make available on the world market.
Now they are trying to flood the market with their supply to try and reduce profit margins for domestic or other international producers in order to corner market share, but our smaller more nimble technologically sophisticated domestic producers can provide supply faster, cheaper, and in greater quantity than OPEC so the Shieks are F'd either way!
Drill Baby drill!