Unconventional oil and gas resources such as shale gas, shale oil, CBM, tight gas and oil have attracted more and more attention worldwide in recent years. However, most of the formations of unconventional oil and gas are suffering from poor geological condition, thus the resources can not be developed without fracturing stimulation. Conventional hydraulic fracturing usually consumes a huge amount of water and also leads to the pollutions of surface water and even residential water. In addition, the formation damage caused by incomplete gel breaking, adsorption of polymers, clay expansion and water blocking are still not fully eliminated.
Thus, in this work, ultra-dry CO2 foam stabilized by graphene oxide (GO) were explored to get a fracturing fluid characterized by low water consumption, environmental friendliness, high efficiency and low formation damage. The foam quality of fracturing fluid in the study was higher than 90%, thus the water consumption of fracturing fluid was lower than 10% of total volume. The foam stability, rheology and dynamic filtration were studied by using a large-scale fracturing fluid test device.
The results showed that the stability and thermal adaptability of ultra-dry CO2 foam were enhanced by the addition of graphene oxide. The interfacial dilatational viscoelastic modulus of CO2/liquid was increased when the graphene oxide was used with saponin, implying that the bubble film interface became solid-like; The ultra-dry CO2 foam enhanced by the graphene oxide showed a shear thinning behavior. The effective viscosity of ultra-dry CO2 foam was increased by adding graphene oxide and its viscosity was higher than 50 mPa·s at a shear rate of 100s-1; Moreover, compared to pure surfactant foam, the filtration control performance of ultra-dry CO2 foam was also enhanced by graphene oxide. At a filtration pressure difference of 3.5MPa, the filtration coefficient of ultra-dry CO2 foam was decreased significantly by the addition of graphene oxide. Although the core damage caused by foam with graphene oxide was slightly higher than that of pure surfactant foam, the permeability damage was still below 10%, implying that the foam as a fracturing fluid is relatively clean to formation.
Ultra-dry CO2 foam fracturing fluid stabilized by graphene oxide provides a new high-performance fracturing system for unconventional oil and gas at water-deficient area. This study will be beneficial to fracturing applications characterized by low water consumption, environmental friendliness, high efficiency and low formation damage.
SAGD is a commercially proven technology for Athabasca reservoirs. It produces high oil rates and high ultimate recoveries. Injecting a solvent with the steam can reduce required steam rates and/or improve oil rates and recovery. Oil viscosity is reduced both thermally and by solvent dissolution.
A scaled model experiment was performed at the Alberta Research Council to examine the effectiveness of the Expanding Solvent-SAGD (ES-SAGD) process at a low pressure (1,500 kPa), which was still high enough to allow sufficient drive to transport the produced fluids to the surface at typical Athabasca formation depths. A lower SAGD pressure typically results in reduced oil production as a result of the correspondingly lower steam temperature. However, a lower operating pressure can also result in a reduced SOR because of lower steam density (lower mass of steam required to fill a specific volume of the steam chamber) and also a higher heat of vaporization, which allows more heat to be transferred to the reservoir when a given mass of steam is condensed. At lower pressures, the steam saturation temperature changes more quickly with saturation pressure than it does at higher pressures. Thus, at low pressure, a small reduction in pressure can lead to steam flashing in the producer and create unstable conditions.
This experiment was history matched and then a parametric investigation was performed based on field scale numerical simulations using CMG STARS. The same solvent (gas condensate) and solvent concentration (9.3 volume%) were used in the lab and field scale simulations. The condensate had many components, which were represented in the numerical simulations by four pseudo-solvent components.
2-D and 3-D field scale simulations examined the effect of: operating pressure, injection rate, sub-cool, oil and gas phase diffusion and dispersion, live oil versus dead oil performance, use of pressure drawdown when oil rates have declined, and compared low pressure ES-SAGD to low pressure SAGD.
The simulations indicated that the effects of production pressure, sub-cool, and solvent concentration must be considered simultaneously as they impact each other. At 1,500 kPaa production pressure and 10 °C sub-cool, co-injection of solvent with steam increased the average oil rate by 15% while reducing the SOR as compared to SAGD at the same operating pressure.
The ES-SAGD process was developed to improve the energy efficiency and oil drainage of SAGD (Nasr and Isaacs (2001) and Nasr et al. (2003). In this process, a small amount of solvent with a vaporization temperature closely matching that of steam is co-injected with steam, (Figure 1). As the solvent condenses with the steam along the boundary of the gas chamber, it dissolves in the bitumen thereby reducing its viscosity and increasing oil recovery rates. An experiment and simulations were performed to investigate ES-SAGD at a low operating pressure of 1,500 kPa.