Research on influence of injection-production modes on longitudinal sweep characteristics of thick reservoirs

Abstract

The optimization of injection-production modes is of great significance in alleviating intra-layer contradiction and improving longitudinal sweep efficiency for thick reservoirs. The analytical flow model of both producer and injector partially penetrating the reservoir is absent currently， which fails to support the study of the sweep characteristics of thick reservoirs. Therefore， based on the conformal mapping method， this article transformed the section formation with partially penetrating producer and injector into a formation with a circular boundary and off-center fractured well， a formation with a unit circular boundary and two-branch fractured well， and a section formation with fully penetrating injector and partially penetrating producer， sequentially. The above transformations were repeated to transform the section formation with fully penetrating injector and partially penetrating producer into that with fully penetrating injector and producer. In consequence， the original two-dimensional flow process was transformed into a one-dimensional one， achieving analytical calculations of pressure and streamline distribution in thick reservoirs under different injection-production modes. The comparison with refined numerical simulation methods shows that the proposed method has high computational accuracy for different well spacing-thickness ratios. The influence of injection-production modes on longitudinal sweep characteristics of thick reservoirs was researched using the method presented in this article. The results show that for the bottom-injection-high-production mode， the top section of the injector acts as the main injection layer， and the top 1/3 of the injector controls more than 1/2 of the formation， resulting in more than 2/3 of the producer being affected. The main impact of water injection on the formation lies in the stage before the water breakthrough. The water breakthrough of bottom-injection-high-production mode is the latest and has the highest sweep efficiency， followed by bottom-injection-medium-production mode and medium-injection-high-production mode under the same penetrating thickness. The water breakthroughs of high-injection-high-production mode and bottom-injection-bottom-production mode are the earliest and have the lowest sweep efficiency. The larger penetrating thickness of the producer indicates a earlier water breakthrough. However， the sweep efficiency in water breakthrough increases. Therefore， the optimal penetrating thickness should be selected by comprehensively considering the factors of water breakthrough time and sweep efficiency.

Keywords

• thick reservoirs
• partially penetrating well
• production-injection mode
• conformal mapping
• longitudinal sweep