Is it permissible to shorten and combine prayers in an oil field where there are more than thirty worshippers, and the group stays there for a period ranging between 15 and 25 days, noting that it is a closed area forbidden to the public, and the stay is without families?

As long as you reside at your workplace for four days or more, you are considered residents, and thus, you are not permitted to combine or shorten prayers.

If the company is not close to a place where Friday prayer is established, and there are no permanent residents in it for whom Friday prayer is obligatory, then you are not required to perform Friday prayer; instead, you should perform Dhuhr prayer. However, if it is close enough that you can hear the Adhan, or if there are permanent residents with you for whom Friday prayer is obligatory, then Friday prayer becomes obligatory for you.

For the validity of Friday prayer, it is a condition that it be established in collected buildings inhabited by those for whom Friday prayer is binding, whether in large cities or small villages.