A oilfield, located in Iraq, is a large multi-layer oil field dominated by carbonate reservoirs of cretaceous Mishrif formation and Tertiary Asmari Formation. A oilfield is located in the transition zone between Zagros orogenic belt and the eastern edge of Arabian platform, mainly in the low-angle fold belt of Zagros structure. Due to the influence of the Zagros orogeny, the fault system of the oilfield is developed, especially the Asmari formation A. It is a complex carbonate reservoir composed of dolomite, limestone and sandstone. The plane and vertical heterogeneity are very strong. The development of micro-fractures is verified by core data, logging data and dynamic analysis. And the multiplicity of well test interpretation is obvious.
The core, thin section, imaging logging and well test data (Fig.1) of A oilfield all show that fractures are developed in Asmari formation, and the production performance shows that the existence of fractures improves the production of oil wells. However, due to the serious breakthrough of edge water and secondary bottom water, the water cut of the oilfield rises rapidly, leading to the difficulty in ensuring the development effect of the oilfield. Moreover, the fracture scale in the study area is too small to characterize the parameters, the plane distribution of fractures is different and its distribution law is difficult to predict accurately, which has also become the key to restrict the efficient development of oil fields. As carbonate reservoir has developed fractures and strong heterogeneity, its pore structure is more complex than sandstone reservoir, and the uncertainty of parameter interpretation is higher. Therefore, how to evaluate and obtain the reservoir parameters of A oilfield has always been a great concern.
In general, logging, core analysis and pressure transient analysis (PTA) are used to evaluate the reservoir parameters of carbonate rocks. However, core and logging analysis can be used to get static parameters in the range of centimeter to meter, while PTA can obtain static and dynamic parameters in the range of hundreds of meters to several kilometers, such as skin coefficient, boundary conditions, permeability and cross flow coefficient. Therefore, the PTA results are more practical.