Research on fluid flow mechanism of fracture-cave carbonate reservoirs based on fluid mechanics

Abstract

ABSTRACT Facture-cave carbonate reservoir has not formed mature development theory and method up to now, and the development is world-class difficult problems, which are caused by that it is not clear to understand the particularity of reservoir characteristics and fluid flow mechanism. Because karst caves are the dominant reservoir space and production contributor, porous flow mechanics is not applicable. Reservoir body belongs to neither porous media nor continuum as a whole, hence fluid flow belongs to the research category of fluid mechanics. Contrary to widely adopted coupling method, this paper will focus on the fluid flow mechanism by only applying the theory of fluid mechanics and ignoring porous flow on the basis of analyzing the particularity of reservoir characteristics. Both capillary pores in carbonate matrix and slender dissolved cavities can be regarded as hydraulic long pipe, and fluid flow conforms to pipe flow law. Fluid flow in fracture belongs to parallel plate clearance differential pressure flow, which is similar to pipe flow. Fluid is static under gravity and fluid flow conforms to hydrostatics law in approximately equiaxed karst caves, and fluid flow out of karst cave is orifice outflow. Porous flow in reservoirs is unified with pipe flow. The essence of porous flow is the macro performance of pipe flow in countless capillary pores, which manifests as Darcy’s law under laminar condition. Porous flow in carbonate matrix can be completely ignored because it is extremely difficult to occur and the effect is very tiny. In dissolved-cavity reservoir body of millimeter scale, fluid flow is mainly slight pipe flow and gravity effect is significant, which leads to tiny production pressure differential and pressure drop funnel. In karst-cave reservoir body of meter scale, there is no fluid flow nor pressure drop in horizontal direction and the displacement of oil by water is piston-like displacement vertically upward in karst caves, hence pressure drop funnel does not exist at all. The production pressure differential generated by orifice outflow at the outlet of karst cave is very tiny and can be ignored. The new fluid flow mechanism based on fluid mechanics will provide theoretical basis for solving the difficult problems of developing fracture-cave carbonate reservoirs.