In the methods section one would use past tense due to what they have done was in the past. It has been debated whether one should use active or passive voice. The scientific journal Nature states ...

When you are writing, you can use words that show if the events have already happened, are happening now or will happen in the future. The past tense is used for things that have already happened.

However, changing the tense deliberately can have a positive impact on your reader. For example, if you were writing a persuasive speech you may swap from the past tense to the present tense to ...

using ‘have’ in place of had in that particular conditional clause would be totally wrong just as the choice of the tense in the main cause it is introducing should be consistently past ...