“My neighbor drilled a well by accident on my property. He now wants to enter the property to decommission the well. Is the well now mine and he can he trespass with equipment to do the work?”
Why does your neighbor want to decommission the well? If it’s solely for your benefit, then just saying “no, thanks” should be sufficient, since it would save him the time and expense.
If he’s got some reason of his own to do it, I would need to know what that is before I could answer your question. Ordinarily, one cannot trespass (cross onto the property of another without permission) unless there is some overriding reason to do so–such as to save life or property.