The answer depends almost entirely on when you ask. Ask after the deal closes and larger companies route it to legal or HR for permission, and they will almost always say no, because approving it is a risk with no upside for them.
Ask at the beginning, during negotiation, on every deal. Including the specific one worth copying: if we do a really good job for you, can I shoot a short two minute video of you talking about the problem we helped you solve. The problem, not you.
Do not wait until you have earned it. Earning it is what makes you comfortable asking, and by then the person who can approve it is no longer the person you are talking to.
Source: Referrals. Why, How and When?, episode 7 of the Oil & Gas Sales & Marketing Podcast.
The next step
If the way energy buyers actually buy is costing you deals, talk it through with ModalPoint, the go-to-market firm Matthew Bertram runs. A 30-minute discovery call: no scope, no commitment, and a mutual NDA before anything substantive. You get a reply within one business day, and the first call is with someone who works in the sector.