Gas Diffusion into Oil, Reservoir Baffling and Tar Mats Analyzed by Downhole Fluid Analysis, Pressure Transients, Core Extracts and DSTs
- Vladislav Achourov (Schlumberger) | Thomas Pfeiffer (Schlumberger) | Terje Kollien (Lundin) | Soraya S. Betancourt (Schlumberger) | Julian Y. Zuo (Schlumberger) | Rolando di Primio (Lundin) | Oliver C. Mullins (Schlumberger)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- August 2015
- Document Type
- Journal Paper
- 346 - 357
- 2015. Society of Petrophysicists & Well Log Analysts
- 0 in the last 30 days
- 192 since 2007
- Show more detail
In this reservoir study, two adjacent fault blocks have been subject to the same initial liquid and subsequent gas charges, yet fluid characteristics are different. Wells in each fault block have a gas-oil contact (GOC) and an oil-water contact (OWC), thus all depth-dependent in-reservoir fluid geodynamic processes are visible within each well. The two adjacent fault blocks are found to be at different stages of the same reservoir fluid geodynamic process yielding a ‘movie’ with two time frames.
Diffusion of gas, from the late gas charge, into the oil column causes a significant increase of solution gas initially at/near the GOC. This increase in solution gas causes the asphaltenes to migrate down in the oil column. Well 1 is in the middle of this process exhibiting huge disequilibrium gradients of gas-oil ratio (GOR), saturation pressure and asphaltenes. In Well 2, the diffusion of gas reached the base of the column expelling most of the asphaltenes out of the oil column creating a 10-m tar mat at the base of the column. In Well 2, the oil is nearly in thermodynamic equilibrium in contrast to large disequilibrium in Well 1. Asphaltene extracts of core plugs are consistent with these fluid profiles and reinforce conclusions. The disequilibrium oil column is associated with low vertical permeability as seen with pressure interference testing indicating multiple baffles. In drillstem tests (DSTs), the equilibrated oil column exhibited 10x greater production than the disequilibrium oil column. Equilibrated asphaltenes are associated with good production; here, disequilibrium asphaltene gradients and poor vertical permeability are associated with low production due to reservoir baffling.
|File Size||5 MB||Number of Pages||12|