Reservoir Simulation of Cyclic Steam Injection Including the Effects of Temperature Induced Wettability Alteration
- Berna Hascakir (Stanford University) | Anthony Robert Kovscek (Stanford University)
- Document ID
- Society of Petroleum Engineers
- SPE Western Regional Meeting, 27-29 May, Anaheim, California, USA
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 5.3.4 Reduction of Residual Oil Saturation, 5.4.1 Waterflooding, 5.6.1 Open hole/cased hole log analysis, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.5 Oil Sand, Oil Shale, Bitumen, 1.6.9 Coring, Fishing, 5.2 Reservoir Fluid Dynamics, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 5.4.6 Thermal Methods, 5.5 Reservoir Simulation
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Cyclic steam injection into representative, low-permeability, hydraulically fractured, diatomaceous, heavy-oil formations is investigated using reservoir simulation. Previous laboratory research has shown that diatomite reservoir rocks exhibit temperature-dependent wettability. Water-oil and liquid-gas relative permeability are varied here and a reservoir simulator used to evaluate the effects on oil recovery. Sensitivity studies are carried out for relative permeability curves compiled from literature for diatomaceous reservoirs. The various parameters included in the analysis are relative permeability end points and viscosity variations with increasing temperature, rock and fluid properties (such as thermal conductivity and heat capacity), injected steam temperature, pressure, and quality, bottomhole pressure for injectors and producers. The results indicate that the liquid-gas relative permeability affects process performance most sensitively because gas displacement determines the shape of the heated zone during cyclic steam injection. For the recovery of heavy oil from diatomaceous formations, realistic evolution of the liquid-gas relative permeability at steam temperatures increases the oil recovery in the range of 15 to 60% of the OOIP in comparison to cases with no evolution of wettability with temperature. Very short two week cycles (inject-soak-produce) make more oil and show greater sensitivity to the evolution of wettability with temperature.
Cyclic steam injection is a thermal recovery process that enhances recovery by reducing oil viscosity so that well productivity increases (Sarathi and Olsen, 1992). A cyclic steam injection process includes three stages: injection, soaking, and production. The cycle is repeated as long as oil production is profitable (Sarathi and Olsen, 1992). Cyclic steaming is used extensively in heavy-oil reservoirs, tar sands, to improve injectivity prior to steamflood, and in some cases to prior in-situ combustion operations (Sarathi and Olsen, 1992). Cyclic steaming as well as steamdrive has been applied successfully to low permeability reservoirs, such as diatomite, containing heavy and light oil (Kumar and Beatty, 1995; Kovscek et al., 1996a&b; Murer et al., 2000).
Diatoms are single-celled (unicellular) organisms that live as individuals or in groups called colonies. They exist in both salt and fresh waters (Antonides, 1998). When diatoms die, their silica shells accumulate on the floor of the body of water in which they lived. Thick layers of diatom shells have been fossilized in rock. Such layers, or beds, of diatoms are called diatomaceous earth, or diatomite (Diabira et al., 2001). Diatomite pore throats range from order 0.001 µm to order 10 µm (Jia et al., 2007). Accordingly, permeability is extremely low, ranging from 0.01 to 10 md (Kumar and Beatty, 1995; Jia et al., 2007). The rock is also a moderately soft and fragile mineral with porosity up to 70 percent. High porosity samples filled with air are light enough to float on water.
Although diatomite reservoirs are rich in oil resources (Ilderton et al., 1996), low permeability frustrates oil production. Because of the low matrix permeability, wells are hydraulically fractured. Waterflooding and cyclic steam injection are implemented successfully using hydraulic fractures (Kumar and Beatty, 1995; Kovscek et al., 1996a&b; Murer et al., 2000).
There are several parameters affecting the performance of cyclic steam injection such as reservoir geology, oil gravity, and initial conditions (Schembre et al., 2006 and Kumar and Do, 1990). There is still limited information even about the basic petrophysical properties of reservoir rocks at elevated temperature (Nakornthap and Evans, 1986). Relative permeability affects the performance most among the various input process modeling parameters (Stone, 1970 and Sato and Aziz, 1987). Oil-water and gas-liquid relative permeabilities are properties that are important and necessary in describing the mechanisms of thermal recovery (e.g., Dietrich, 1981, Davidson, 1969, Ayatollahi et al., 2005, and Hamouda and Karoussi, 2008).
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