MANY FACTORS CAN INFLUENCE THE SHAPE OF A PRESSURE BUILD UP CURVE. AN UNUSUAL SHAPE MAY REQUIRE EXPLANATION TO HAVE A APPROPRIATE ANALYSIS, OR IT MAY PREVENT A PROPER ANALYSIS. IN ADDITION TO WELLBORE STORAGE EFFECTS, HYDRAULIC FRACTURES AND NATURAL FRACTURES CAN HAVE A MAJOR EFFECT ON BUILD UP CURVE SHAPE AND ANALYSIS. THE SHAPE OF THE PRESSURE BUILDUP CURVE ALSO CAN BE AFFECTED BY FLUID AND ROCK INTERFACES, WATER-OIL CONTACTS, LAYERING, AND LATERAL FLUID OR ROCK HETEROGENEITIES. WELLBORE STORAGE, WELLBORE DAMAGE OR IMPROVEMENT, AND GEOMETRY OF THE DRAINAGE AREA CAN ALSO AFFECT THE SHAPE OF A BUILDUP CURVE. ADDITION-ALLY, WELLS WITH A HIGH GAS-OIL RATIO CAN SHOW HUMPING DURING PRESSURE BUILDUP, IN SUCH CASES, THE BOTTOM-HOLE PRESSURE INCREASES TO A PEAK AND AFTER A PERIOD OF TIME DECREASES. IN THIS PAPER, A WELL IS SELECTED IN HAFT KEL OIL FIELD AND ITS BUILD UP TEST HAVE BEEN ANALYSED DURING 20 YEARS AND IN THREE PERIODS FROM 1990 TO 2010. THE CONDITIONS OF WELL IN EACH PERIOD HAS BEEN ANALYSED AND THE RESULTS SHOW THAT IN ADDITION TO WELLBORE EFFECTS, SOME RESERVOIR EFFECTS MAY INFLUENCE THE BUILDUP TEST IN THIS WELL.