Application of a New Method to Estimate Free Fluid Level and Recognition of Compositional Grading Using Wireline Formation Testing Data

Determination of fluid contacts in a hydrocarbon reservoir is extremely important in calculation of initial hydrocarbon in place and field development planning. The uncertainty in the present fluid type and fluid levels may have a significant impact on the reserves estimation and well completion strategies. Wireline Formation Testing is widely used to discover fluid contacts (or its generic term, Free Fluid Levels). Precise analysis of pressure data obtained from these tests is crucial in defining the type of fluid and fluid contacts. Although the traditional method of P-D Plot to determine a Free Fluid Level (FFL) is easy to implement, however it has the disadvantage of lack of information on uncertainty of the analysis. It is often difficult to identify and remove noisy data which may result in inaccurate estimation of contacts. A method has been mentioned in the literature by which Fluid Level is discovered using formation pressure data that are projected to a datum depth. With this method, it is very simple to find noisy data points which contribute to uncertainty in the FFL estimates. Another benefit of applying such method is to authenticate compositional grading presence in the reservoir. Also it can discriminate layers with different pressure behaviors in a multilayered reservoir. In this paper, several wireline formation testing data such as data from MDT and RFT tools -in different fields in Middle East- have been analyzed by previously mentioned method. A good agreement was observed between the results of this method and other data like petrophysical interpretation, geological evidences, DST results and finally PVT analysis. Also a correlation has been developed to confirm existence of compositional grading and a strategy has been proposed to calculate the rate of density change with depth in those reservoirs where variation of density is not extremely nonlinear.