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How can I use the reservoirs pressure buildup for usefull interpretation and calculations? Any thoughts are welcome.

Plot the pressure vs horner time on semilog and then get a tangent. After the WBS the slope will give the permeability.

Also the skin can be calculated using the proper equations. Furthermore, to get the average pressure you can use other methods as MBH, Dietz, etc. Or, simply use a software as pansys and saphire

Hope this gives you stuff too look into.

Pressure buildup (or drawdown) tests are simply a method by which we place a step change into the production (ie shut-in and buildup, or producing & drawdown), and monitor how the formation responds

The shape and rate of the formation response gives us information on the formation such as (not exhaustive) permeability x height, skin and if long enough possibly boundary distance(s)

Think of three ropes attached to a wall - a thin one, larger one and really fat one - you close your eyes and give each a good shake, feeling how long it takes for the shake to hit the wall and rebound to your hand - from the time and intensity of the rebounded wave you would (even blindfolded) be able to infer something about the distance to the wall and the rope characteristics..... not exactly analogous to well testing, but similar enough to help you understand the principle I hope.

A pressure build up test is a test generally carried out on a producing wells. Wells that are still producing are shut in for a period of time to allow a pressure build up. During the process, down hole gauges are placed very close to the wellbore at different intervals to measure the reservoir response. At the end of the test, the guages are remove from the hole, downloaded and the data are retrieved and then analyse, using any of the available software, e.g. Saphire, Fekete or Pansys.
The analyse data is used to determine some reservoir parameters, such as Skin (which determines the extent of damage done to the reservoir). Other parameters are reservoir pressure, productivity index, drainage volume, drainage boundary etc. However, before you understand how to analyse, you need to understand the basic principle of well test. Your ability to recognise the flow regime is very important as it forms part of the basis to characterize your reservoir.