There are actually liquid nitrogen services used in well servicing (I saw them but never inquired what they were for) and they were on the scale of a medium-sized service truck - so in theory, what you're proposing should be possible.
My guess is that in practice, it's not profitable given the current price of natural gas and the difficulties of keeping liquid nitrogen cold - the smaller the container, the shorter the time it can economically stay liquid. So you'd need either pretty bulky equipment that needs to be moved once the gas at a well runs out, or you'd have to spend an awful lot of the energy from the gas keeping your N2 cold during transport. Which even if it is profitable on paper, might not survive contact with the field - oil leases are not controlled environments, and Murphey's law is out in force out there in a way that can be easily underestimated.
My guess is that in practice, it's not profitable given the current price of natural gas and the difficulties of keeping liquid nitrogen cold - the smaller the container, the shorter the time it can economically stay liquid. So you'd need either pretty bulky equipment that needs to be moved once the gas at a well runs out, or you'd have to spend an awful lot of the energy from the gas keeping your N2 cold during transport. Which even if it is profitable on paper, might not survive contact with the field - oil leases are not controlled environments, and Murphey's law is out in force out there in a way that can be easily underestimated.