Group discussion is one of the main processes for selection in Schlumberger.
They generally look for your confidence, ur presentation skills, your arguments on ur stand and ur ability to tackle the situation in short spam of time.
GD topic in Schlumberger are abrupt not like favourite GD topics on socio-economic and political issues.
So just be confident and reasonable during the GD u will definitely crack it.
Some of the GD topics in my campus were: Waka-Waka,I believe i can fly. just do it, on the rocks:P.........
In my grp they have given a picture of a crayon advertisement from the newspaper and told us to discuss on that......
Now , i think dat u got n idea of that they look for during GD , from a candidate.

They mostly select the students having good (around or above 8) GPA. But there is no such specific GPA criteria

Technical interview is the last and most important hurdle of Schlumberger selection process. Be ready for hardcore questions related to your subject.First they ask questions on major projects(internship projects etc.) which you have done.So prepare well with your project report before going for interview.Then they ask questions on your topic of interest in ur subject. So ,choose a topic and prepare very well in that.Interview can be of 50- 60 minutes.So , don't be nervous in between.
At last i just want to say that in technical interview , they don't expect from u that u will answer all the questions perfectly,but u should satisfy them with ur answers and ur knowledge of subject.

There is no such interview is there. Some common HR questions will be asked during the technical interview only, which does not matter much.Just answer the HR questions decently.

Please go through above mentioned sections , i think , i have given enough suggestions there.

In ma institute , I think around 150 people applied , but only 3 got selected at last. In ma branch only i got selected out of 34.
Although all the above mentioned criteria matter in selection process.But for me good project work(intern with Shell) and confidence were the key aspects.