Ideas regarding the right pattern to use to facilitate any future transition to another DB system?

We are currently moving fast with MongoDB, but it is still not completely sure whether we will end up using it or another, such as SQL, from various reasons (cost, scalability, latency, environment, ...).

My question is what would be the best pattern to use to facilitate any future change. Is the Repository pattern the fittest to this task? any other idea?

We use C#, FWIW...