Abstract
Carbonate reservoirs are widely spreaded in the world especially in China and middle east. Some of the carbonate formation is composed by small vuggs with radius of about or less than 1-meter, big caves with radius that can reach tens of meters, natural fractures and matrix. The current well testing models that assume dual or triple mediums cannot give reasonable interpretation such as the volume of caves and the connectivity between caves. Also, it’s hard to analyze the effect of caves’ morphology on flow and pressure dynamics.
We proposed a model for the well connected to one cave in the carbonate formation based on the three laws of mass conservation, momentum conservation and energy conservation mechanics. The concept of wave propagation is introduced and the model incorporating fluid flow and pressure wave propagation is built in the caves. Three key parameters including the wave coefficient, damping coefficient, and cave shape factor are definied to characterize caves. Parametric study of the three parameters are conducted to analyze the effect of the caves on flow dynamics and bottomhole pressure transient. For the cave connected to the wellbore, its volume can be obtained by history match of the log-log plot of the pressure and pressure derivative. The matrix is considered as storage medium and it provides fluid to the caves by crossflow. Field cases are conducted to validate the proposed method.
Results show there are four flow regimes in the log-log plot of the pressure and pressure derivative. The flow regimes include wellbore storage, transition flow from wellbore storage to cave storage, wave propagation and fluid flow in cave, formation flow. The four regimes can be used to characterize the features of fluid fow in carbonate reservoirs with caves. All of wave coefficient, damping coefficient, and cave shape factor present big influence on the bottom transient pressure and pressure derivative, especially in flow regime III which reflects the flow in caves. The volume of the caves connected to the wellbore for the field case is diagnosed. Both of the Log-log plots of the pressure and pressure derivative show good agreement with the pressure buildup data, therefore, the new method is validated and the results interpreted are accurate and confident.
