A new model and its application for predicting dynamic oil-water interface in fault-solution reservoirs based on material balance equation

Key words:  fault solution reservoir  material balance equation  oil-water interface  pressure solution  migration velocity

Abstract:

Fault-solution reservoirs were formed with complex fracture-cave structures, and it is difficult to determine its dynamic oil-water interface in the production process of the reservoirs. In this study, based on typical fracture-cave combination modes of isolated karst cave and multi-supplied karst cave, a basic solution of the bottom hole pressure and average pressure for the flow of micro-compressible fluid in the caves was derived, assuming that the flows in wellbore and karst caves are vertical and free. Considering the isolated karst cave as a single closed reservoir and the multi-supplied karst cave as multiple sets of reservoirs connected by fractures, a new real-time tracking and prediction model of the dynamic oil-water interface in the fault-solution reservoir was established by using the material balance equation, and a semi-analytical solution was obtained by using a successive iteration method. The analysis results show that this model is faster and simpler than the conventional numerical simulation method, and the prediction accuracy of the oil-water interface migration velocity is greater than 87％. For isolated karst caves, annual oil recovery rate of 2％-3％ can keep the rising rate of oil-water interface stable. With the increase of oil recovery rate, the rising velocity of the interface increases significantly in a nonlinear pattern. For multi-supply karst caves, the greater the conductivity of the fractures and the greater the number of connected karst caves, the greater the decreasing rate of oil-water interface rising. The new model in this paper can provide basic theory and method for water breakthrough warning and production optimization of the fault-solution reservoirs.