Method for Improving Volumetric Sweep Efficiency of Thick Massive Carbonate Reservoir Under Influence of Gravity Differentiation

There are thick massive carbonate reservoirs in the Middle East region with large reserves and high production, characterized by non-significant layering characteristics and large single-layer thickness. If vertical wells are used for commingled water injection development, the stability of the water flood front is significantly affected by gravity, which in turn affects the water flooding volumetric sweep efficiency. In order to clarify the influence law of gravity on this type of reservoir and improve the water flooding sweep efficiency, the mathematical analysis method was used to analyze the judgment of water flood front stability under gravity and the influence of reservoir thickness on water flood front stability. On this basis, the numerical simulation method was utilized to study the relationship between the water breakthrough time, the volumetric sweep efficiency at water breakthrough, the water recovery rate at water breakthrough and the perforation thickness under seven different conditions of gravity differentiation. The results show that under the influence of gravity, it is difficult to maintain stability in the water flood front of massive reservoirs with commingled water injection. The perforation thickness could be optimized in order to improve the volumetric sweep efficiency of water flooding. In homogeneous massive reservoirs, there is a negative correlation between the water breakthrough time and the perforation thickness. However, the relationship curve between volumetric sweep efficiency at water breakthrough, anhydrous recovery rate, and perforation thickness is related to the degree of gravity differentiation. When the degree of gravity differentiation is not significant, the relationship curve will show a change of first positive correlation and then negative correlation. When the degree of gravity differentiation is significant, there is a negative correlation between them. Therefore, there is an optimal perforation thickness affected by the degree of gravity differentiation. Based on the comparative analysis of volumetric sweep efficiency at water breakthrough under different gravity differentiation conditions, a graph was created to illustrate the relationship between the optimal perforation thickness and gravity differentiation. The research results can help increase the volumetric sweep efficiency and make the development design of massive carbonate reservoirs in the Middle East region more reasonable.