Determination of Flow Units in Carbonate Reservoir With Multiscale Karst Morphology

Abstract

The Tahe reservoir, one of the largest scale carbonate reservoirs in western China, has very special cavities and fractures. The size of the cavities ranges from millimeter to meter scale, and the size of the fractures ranges from hundreds micrometers to millimeters scale. The length of some cavities can even reach kilometers. However, based on views of core testing results, there is insignificant flow in the rock matrix. This paper introduces a new and refined method to determine flow units in such Karst carbonate reservoirs. Based on fractal theory, fluid flow patterns can be divided into three types by using production data of the Tahe reservoir. Through porosity and permeability statistics of production layers on the established geological model, flow boundaries of different flow patterns were proposed. Flow units were classified in terms of the flow boundaries. As for refined flow units, subcategory flow units were determined by three graphical tools: the limit of dynamic synthesis coefficient (DSCL) method, modified flow coefficient (MS1 and MS2, which are derived by the Forchheimer equation) curve, and the stratigraphic modified Lorenz plot (SMLP). All the parameters of graphical tools help to reconcile geology to fluid flow by illustrating the important link between geology, petrophysics, and reservoir engineering. The use of this technique is illustrated with data from a specific block of the Tahe reservoir.