Steamflooding To Enhance Recovery of a Waterflooded Light-Oil Reservoir

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 145005, ’A Case Study: A Successful Steamflooding Project To Enhance Oil Recovery of a Low-Permeability Light-Oil Waterflood Reservoir,’ by Wu Shuhong, SPE, PetroChina; Han Min, Greatwall Drilling Company; Ma Desheng and Wu Yongbin, PetroChina; Qian Yu and Yao Chunli, Daqing Oil Company; and Shen Dehuang, SPE, PetroChina, prepared for the 2011 SPE Enhanced Oil Recovery Conference, Kuala Lumpur, 19-21 July 2011. The paper has not been peer reviewed. Steamflooding can improve upon performance of a waterflood light-oil reservoir and enhance its oil recovery. In the Cy reservoir of the Daqing oil field, a steamflooding project is enhancing oil recovery after waterflooding. The mechanisms, strategies, barriers, and difficulties of reaching high recovery are discussed for steamflooding in a waterflooded light-oil low-permeability reservoir. A detailed reservoir-engineering study focused on the development method, the injection and production system, and the physical and numerical simulation. Introduction Steamflooding in heavy-oil sands is well documented as a mature technology, and while steam has been injected into light-oil low-permeability sands for almost as long, the mechanisms and effectiveness of this process are much less understood because of flow complexities in these sands and complexities of high-pressure steam injection. The full-length paper details the examination of thermal recovery in such a reservoir by use of physical and numerical simulations. Wettability alteration, interfacial tension, and threshold-pressure-gradient decline contribute to higher oil displacement and sweep efficiency. Vaporization, viscosity reduction, thermal expansion, and relative permeability variation account for more than three-fourths of the incremental recovery in steamflooding. The Cy reservoir is a low-permeability reservoir with a high wax content and oil viscosity ranging from 16 to 95 mPa·s. It underwent 10 years of waterflooding with only 10% of the original oil in place (OOIP) recovered from the reservoir. Challenges faced in this complex project were related to the heterogeneous nature of the reservoir, limited sand continuity, unfavorable mobility for the ongoing waterflooding, associated high thresh-old-pressure gradient, and poor injection response. In 2007, steamflooding was initiated to improve the performance and enhance oil recovery. The steamflooding project has shown promising results. The response to steam injection was quick and significant. Injectivity has doubled and productivity has almost tripled. Geological Characteristics The width of this massive sandstone is 1000 to 1500 m. The thickness is 2 to 4 m. It has low-permeability sandstone units interspersed with shales. The permeability range is 1 to 20 md, and porosity is 12 to 18%. Both the initial fractures and the hydraulic fractures develop in the west/east direction. Block C601 is 770 to 880 m deep with a reservoir temperature of 55°C and reservoir pressure of 8.4 MPa. The crude oil has high wax content and a high wax-precipitation point of 15.9 to 25.9% and 49 to 52°C, respectively. At reservoir temperature, the oil viscosity is 16 to 95 mPa·s, averaging 40 mPa·s.