Interpretation of Overpressured Zone in Tunu Field Using Eaton Formula and Sonic Log Data



As many as 34 wells have been drilled in Tunu Field, offshore Mahakam in East Kalimantan. Most of them have been drilled to a total depth of more than 3500m and many of them encountered the overpressured zone as confirmed by pressure data measurements. A Deq. pressure map using all available pressure data has been prepared. Unfortunately pressure data from tests are not always available in every well. This is due to the fact that the reservoirs at the deeper part of the well very often found tight or, eventhough the reservoirs have good porosity and permeability, they could not be correctly tested when they are covered by too heavy mud. The formation pressure interpretation could be developed from other data such as D’exponent during drilling or from wireline logs data at the end of drilling operations. Unfortunately the former has not always been seriously carried out as it requires several special conditions during drilling like the use of special drilling bit (rock bit), no drastic change on drilling parameter and the purity of shale lithology. In theory several wireline logs such as resistivity, density and sonic logs could be used to make formation pressure behaviour interpretation after drilling operations. However, in reality the use of resistivity log is not easy due to organic material content in the shale bed, while poor borehole conditions in many wells, particularly those drilled with water based mud badly affect the density log. Due to these facts, the available sonic log data of Tunu wells have been taken for the pressure behaviour interpretation. Its results are crosschecked with formation pressure data when this available. The Deq. maps of 1.1, 1.2, 1.3, and 1.4 of Tunu Field are developed from Sonic log data interpretation for this report.


Authors : M.D Saib & B.H Suwandi