2 months or so ago I bought about 30 acres in the Brazos Valley, as part of the sale the sellers provided a brand new survey, as they didn't have one. There is about 1/2 acre (200x100) in the southwest corner of my property that is fenced into the neighbors place according to the survey. Also the fence goes onto the neighbors place in another spot but to a much lesser extent in another place.

The particular area is completely wooded with heavy underbrush and sits at the upper ledge of a creek bed with an about a 15 foot drop. Basically whoever built the fence followed the contour of the land rather than build the fence on the property line.

Nothing special or particularly useful about this small section other than it being the corner of the property. Previous owners didn't live here and just came out to hunt. The neighbor is full time, seems like a good guy and I have spoken with him several times.

No telling how long that fence has been there. Do I need to be concerned about adverse possession? Do I need to put up a new fence in that section? What does the OB recommend?

I dunno, I hear propane cannons work wonders!

I had the almost exact scenario on a piece of land.
Lots of fences were built because it was easier to build them where they could instead of where they belonged.
I considered these things when I made the decision to build the correct fence:
1.) I paid for the land in the purchase.
2.) I am paying taxes on the land
3.) I was told by my attorney that I could be held liable for someone getting hurt on the land.
4.) I paid for a survey to know where the correct boundaries are
5.) I didn't want to have it be a problem for my wife or family later.
Same as you, the property in question was heavily wooded and was going to take a lot of clearing and work to build the fence.
So I made the decision to ask my neighbor if he was interested in buying it from me. However, to make it legal it needed to be done right, which meant another survey to get a deed and title insurance and some closing cost.
I took that in consideration on the price.
Turned out he did not want to buy it ……so I wanted it legal and correct so I built the new fence as it should have been in the first place.
I didn't want some type of legal mess for me or the family later on…. that was my decision maker for me.
In our state adverse possession is pretty hard to do.
Don't remember about Texas as never had the need on our ranch properties there.
That's my nickel's worth.

Don't lose the surveyed corners. Drive a t-post or 2 inch pipe beside each one so they are not easily lost.
I'd be curious to see if the surveyor found the existing corners or if he "set" them.
Talk to the neighbor in person and go look at the corners. Does his survey agree?

Assuming no conflict with the neighbors, I'd put up a new fence on the property line after having the lines cleared and the surveyor stake every 200 ft in a straight line and stake the curves. On the corner across the county road, I'd put in a bigger pipe so it's not lost. Like 3 inch or bigger 10' pipe with 3' or more in the ground.

Had a similar problem recently and I put the new fence on the surveyed property line instead of the edge of a large drainage ditch where it had been all my life previously. One corner was across a bayou and I drove a 5" pipe to make sure we don't lose it but did not fence across the bayou to include a small corner. Luckily, my surveyor had found the old, large pipe corners called for in the legal description. The neighbors had just agreed to put the fence in the most convenient place 70 years ago.

I agree with Gunny. Talk to neighbor, but either buy it(resurvey to match fence) or fence to match survey. My parents' had an understanding with the person from whom they bought land in 1963. When he died in the 1980s it became a problem. And still is.

Tldr, tear it down.

I got a response back from my Realtor, who consulted an attorney. They suggest that if we don't want to re- fence the property right now, we could have an attorney draft a "boundary Line agreement" that both parties would sign saying both parties agree that the fence is not in the proper place and we agree that when when the fence does need to be replaced it can be moved to the right location in the future. Looking at the survey the fence does go on his side in a few places so maybe this would be mutually beneficial.

Seems like a short term fix, I'll have to invite the neighbors over for a beer and discuss.

This is way too logical of a response. Please delete the comment and remove user.

I've seen a lot of these that make it obvious what the situation is and that everyone acknowledges it. I've also seen a lot that were poorly drafted by an attorney that didn't understand the physical monuments they were referencing and made it completely unclear as to what the agreed boundary was.

May want to get it checked at least by the surveyor that did the recent work to make sure it makes sense.

Then get it properly recorded!

I have a "simplified" fence line on one side of my property where the neighbor in the past built a fence that goes off the line a bit in my favor to keep the fence line easier to build and maintain. My neighbor had the PL surveyed and marked, and we agreed any future fence replacement would attempt to stay more true to the PL. it's only 2-3 feet and one weird little corner about 15x15' so no big deal.