Oilfield contractors hired to drill wells and fracture rock to raise crude and natural gas to the surface will have to lower prices by as much as 20 percent to help keep their cash-strapped customers working. Ultimately, that could carve out more than $3 billion from the 2015 earnings outlined by analysts for the world’s four biggest oil-service companies — Schlumberger Ltd. (SLB) , Halliburton Co. (HAL) , Baker Hughes Inc. and Weatherford International Plc. (WFT) The potential losses loom just as the service providers were looking ahead to higher rates after a glut in pressure-pumping gear dragged prices down in past years. Now, crude oil prices that have fallen more than 40 percent since June are squeezing them once again. As they look for ways to cut costs, oil producers will be pushing for discounts wherever they can find them. “They’re already going to confront significant cash-flow pressures […]