The Role of Pressure Observation and Pressure Transient Analysis in Changing the Geological Concept of Mature Oil Field

Abstract

The field of interest was discovered in late eightees. The main productive zone 1 obtained oil. The reservoir 2 is uplift basement located in the center of the field, to which reservoir 1 is pinched out and confined to reservoir 2 from east and west. The reservoir 1 has a gas cap and production is characterized by high GOR with no signs of any decline despite the long production period. Therefore, all the available information of geology, production and pressure transient analysis were reviewed. There has been no production from the basement and just one well has been observing reservoir pressures for last ten years. These data were plotted bringing to unexpected result. The plot showed stable declining trend. So, there was a question – how does it happen when pressures are continuously decreasing while there is no production from the reservoir 2? Thus, gas sampling was performed on all three parts, pressure buildup and interference tests were conducted, geological data were reviewed and analyzed. After completion of the works, the following results were obtained. PVT analysis indicates similar properties of gas from all three wells. However, the most important fact is that the results of interference test proved obvious pressure communication between wells in the reservoir 1 and 2. In addition, analysis of geological data also does not exclude the possibility of the communication. As the result, a conclusion that the two reservoirs could be one reservoir is drawn up. It is assumed that the main channels for interconnection are fractures, since the formation 2 is metamorphic reservoir with fracture network. Reservoir pressure monitoring and pressure transient analysis became main factors and source of valuable information for changing the geological model of the field. Once again the importance of the pressure transient analysis is proved to be applicable not only by reservoir engineers but also by geologists. Eventually, the main results of the study influenced on the entire development strategy of the mature field.