1. Experience doesn't transfer. The only transferable skills you will get is some management experience. Besides that, don't let yourself become deluded into thinking that the experience you gain as a wireline field engineer will gain you employment at a oil producer. Only 2 in 40 will achieve this.
2. Not only will the job be difficult but you will lead a miserable life, more than likely you will be sent to a small town where your only friends will be your co-workers, where your boss will always try to get more out of you, and when your on call you will be woken up constantly in the middle of the night, deal with condescending clients - the list goes on in on.
3. When the going gets tough, Schlumberger gets dirty, they won't just lay you off in many cases, they will make you work more for less days off, cut your pay, benefits and resources at work and make it miserable enough so that you will quit. It is happening now, many are wanting a lay off so they can get severance pay and qualify for unemployment but Schlumberger won't make it that easy.