First interview was a standard phone interview. The recruiter contacted me by email after finding my application through SLB's careers website for resume drops -- we set up a time and date and then he called me. Got questions like "Why SLB? Why field engineer? Describe a time when you've had to adapt to something really hard. Etc." Afterwards, got an email inviting me in for a second interview.
Second interview was same as others have described here. They flew me out to one of their sites, where I gave a presentation (topic assigned two days before, and working in a group with three other people) on an SLB segment. Then they had us do a team building activity ("build a thing in a limited amount of time with these random objects" kind of thing) and kept us up until past 2am with more presentations about the company.
Second day of second interview (the whole thing, plus travel time, eats up three days) they have you wake up around 6-7am for breakfast and take you out to a sample rig. You wander around, ask some questions, make sure your safety gear is on at all times, etc. Then they bring you back to the hotel and start exit interviews.
From what I could tell, the exit interviews were pretty brutal. Several people on my team actually came out crying, or very shaken at the least. Be prepared to have your conviction questioned. Be prepared to be asked extensively about WHY you want a certain segment, or to be out in the field at all. These guys REALLY want you to be sure that you know what you're getting into. The exit interview isn't for impressing the recruiters anymore -- it's for convincing them that you know exactly what you're getting into, and that you actually, actively want this job, more so than anything else, and that you aren't just trying to come in with half-assed convictions.
Then there's a dinner where you can chat with more field engineers. Very laid-back and fun, and nice after the grilling from the exit interview. You fly home the day after.
Ultimately, although I likely won't be joining them in the future, I feel it was a well-designed process. The recruiters were very friendly and accommodating, yet at the same time blunt and truthful about how hard the job would be. You think that, after reading a few things on the internet, including many of the reviews here, you KNOW you would be willing to do this job? The interview process is really designed to make you question that fully, and if you, like me, realize at the end that it's not for you, the process will have worked and they won't be unhappy at you for it.