Gas Injection for Improving Oil Recovery in Highly Volatile Fractured Reservoirs with Thick Buried Hills in Bohai Sea

Abstract

The BZ oilfield in the Bohai Sea is a rare, highly volatile reservoir with fractures in the metamorphic rocks of buried hills. Clarifying the mechanism of gas injection for improving oil recovery and determining the optimal way to deploy injection-production well networks are critical issues that must be urgently addressed for efficient oilfield development. Experimental research on the mixed-phase displacement mechanism through gas injection into indoor formation fluids was conducted to guide the efficient development of gas injection in oil fields. We established a model of dual-medium reservoir composition and researched the deployment strategy for a three-dimensional well network for gas injection development. The coupling relationship between key influencing factors of the well network and fracture development was also quantitatively analyzed. The results show that the solubility of the associated gas and strong volatile oil system injected into the BZ oilfield is high. This high solubility demonstrates a mixed-phase displacement mechanism involving intermediate hydrocarbons, dissolution and condensation of medium components, and coexistence of extraction processes. Injecting gas and crude oil can achieve a favorable mixing effect when the local formation pressure is greater than 35.79 MPa. Associated gas reinjection is recommended to supplement energy for developing the highly volatile oil reservoirs in the fractured buried hills of the BZ oilfield. This recommendation involves fully utilizing the structural position and gravity-assisted oil displacement mechanism to deploy an injection-production well network. Gas injection points should be constructed at the top of high areas, and oil production points should be placed at the middle and lower parts of low areas. This approach forms a spatial three-dimensional well network. By adopting high inclination well development, the oil production well forms a 45° angle with the fracture direction, which increases the drainage area and enhances single-well production capacity. The optimal injection-production well spacing along the fracture direction is approximately 1 000 m, while the reasonable well spacing in the vertical fracture direction is approximately 800 m. The research results were applied to the development practice of the buried hills in the BZ oilfield, which achieved favorable development results. These outcomes provide a valuable reference for the formulation of development plans and efficient gas injection development in similar oil and gas fields in buried hills.

Keywords

Buried hills / Fractures / Highly volatile oil reservoirs / Gas injection development / Well network / Mixed-phase mechanism

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Harbin Engineering University and Springer-Verlag GmbH Germany, part of Springer Nature