Fast Predictive Models Developed for CO2 EOR and Storage in Mature Oil Fields

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 221978, “Development of Fast Predictive Models for CO2 Enhanced Oil Recovery and Storage in Mature Oil Fields,” by Yessica Peralta, Ajay Ganesh, and Gonzalo Zambrano, SPE, University of Alberta, et al. The paper has not been peer-reviewed. Reservoir modeling tools have played a significant role in designing subsurface fluid-injection methods such as CO2 enhanced oil recovery (EOR). However, these models are computationally expensive, requiring extensive geological and engineering data that often are not available in the early phases of carbon use and storage projects. This work presents the development of fast predictive models and optimization methodologies to evaluate CO2 EOR and storage operations quickly in mature oil fields. Model Description. The Weyburn oilfield is in southern Saskatchewan, Canada. Weyburn oil reserves are within a thin zone of fractured carbonates (maximum thickness of 30 m) deposited in a shallow carbonate shelf environment at a depth of 1350–1450 m. The reservoir consists of two main units, the upper Marly dolostone (thickness ranging from 0 to 10 m) and the lower vuggy limestone (thickness ranging from 0 to 20 m). Oil production began in 1956. CO2 miscible-flooding EOR was initiated in 2000, alternating with water in some wells to improve oil-recovery efficiency and to store CO2 for the long term. The Weyburn-Midale CO2 EOR model in this work is a subarea of the Phase 1A monitoring and storage project. It was developed by using a commercial compositional reservoir simulator. History matching was performed by using 216 well histories (producers and injectors) from April 1964 to the end of 2006. Numerical-Grid Construction. The total model dimensions are 7000, 7800, and 30 m, corresponding to the model width, length, and thickness, respectively. The number of gridblocks is 141×280×27, conforming to 1,065,960 total gridblocks. The reservoir thickness is approximately 30 m. The 27 vertical layers of the Weyburn-Midale CO2 EOR grid model are distributed as eight layers of marly dolostone (upper layers), four layers of vuggy intershoal, seven layers of vuggy shoal, and eight layers of vuggy lower shoal. Reservoir and Fluid Properties. The Weyburn-Midale reservoir is an anisotropic heterogeneous fractured reservoir. The marly unit is chalky intertidal dolostone with some interbeds of limestone with porosity ranging from 16 to 38%. The matrix permeability ranges from 1 to more than 100 md. The vuggy zone constitutes a heterogeneous subtidal limestone with varied diagenetic and depositional environments, resulting in porosity values from 3 to 18%. The matrix permeability varies from less than 0.01 to more than 500 md, where fractures control the direction and the magnitude of permeability anisotropy. The reservoir features an initial temperature of 63°C and a reference pressure of 15.2 MPa at a depth of 1440 m. The original oil and water in place are 40.16 MRm3 and 21.67 MRm3, respectively. The model’s total pore volume is 61.29 MRm3, with an initial fluids saturation of 65% oil and 35% water.