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Multiphase system

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  • 1. H84MPS Multiphase System Department of Chemical and environmental Engineering University of Nottingham Impacts of Multiphase Flow on Hydraulic Fracturing Youhong Yao 4177603 26 March 2014
  • 2. 1. Introduction Hydraulic fracturing (“fracking”) is a stimulation technique used to exploiting and producing unconventional oil and gas, such as shale gas. In spite of the utilization of hydraulic fracturing could be traced back to the early 2000s, energy exploration and production companies commenced actively to integrate horizontal drilling with hydraulic fracturing to exploit shale gas until 2003 (Boudet et al 2014). In the last decade, due to the depletion of conventional energy resources and the constant increasing of energy demand of the world, driven by the population growth, there is a significant breakthrough has been made on the technique of hydraulic fracturing in order to exploit unconventional oil and gas to meet the energy demand of the world (Fink 2013). Due to the gas is trapped in pore spaces of the rock with limited permeability, so gas cannot continuously flow into a well, therefore, the gas flow need to be stimulated by widening fractures with the hydraulic fracturing stimulation method. The hydraulic fracturing process includes drilling horizontally through a rock layer and injecting a huge volume of high pressure fracturing fluid involves water, sand, and other chemicals that fractures the rock and thus increases the permeability so that facilitates the flow of oil and gas (Kotsakis 2012). Boudet et al. (2014) points out that the developments of hydraulic fracturing have made significant contributions to the exploitation and production of unconventional oil and gas and made wells more profitable because of increasing the productivity of wells. It is clear, therefore, that an advantage of hydraulic fracturing is economic benefit. Nevertheless, environmental and safety issues associated with hydraulic fracturing have become increasingly controversial, such as water contamination and earthquake.
  • 3. 2. Discussion and Justification This section presents multiphase issues with hydraulic fracturing and solutions for the issues, and then demonstrates economic effects, safety aspects and potentials of hydraulic fracturing in the future. 2.1 Multiphase issues with hydraulic fracturing Multiphase flow is simultaneous fluids flow of different phases such as gas, water and oil. Boudet et al. (2014) indicates that when multiphase flow of different phases occurs in hydraulic fracture, aspects such as relative permeability, capillary pressure, and wettability in the fracture and the rock could have effects on hydraulic fracturing cleanup process and the fracture fluid performance, and in the long term unconventional oil and gas exploitation and production will be dramatically affected by these factors. Figure 1 demonstrates the rate of gas production for single phase flow and multiphase flow. Obviously, gas production rates considerably decreased when multiphase flow occurs. Fig.1 Impact of single phase flow and multiphase flow on gas production rate Source: Boudet et al. (2014) With regard to wettability, in the area of unconventional oil and gas exploitation and production, it represents the relative affinity of a certain
  • 4. phase to contact reservoirs rock and the ability of a certain phase to maintain contact with reservoirs rock. If rock prefers water than for oil or gas, it could be defined to be water-wet rock; in this case, water layer will cover a majority part of rock surface in pores. Generally, wettability is affected by the minerals in the pores. According to Fahes and Firoozabadi (2007), wettability alteration has significant effects on increasing liquid mobility in reservoir rock. Due to the wettability characteristics of reservoir rock, a long time will be taken in the hydraulic fracturing cleanup process and the accumulation of gas or oil in the fracture will be reduced when pressure reduces below dew-point pressure, and lead to decrease of productivity. Multiphase flow through porous is an interactional result of capillary, viscous, and gravitational forces, and capillary force generally dominates in reservoir rock flow. Referring to capillary pressure, Boudet et al (2014) reveal that the pressure difference between the wetting phase and the non-wetting phase is capillary pressure (Pc). Capillary pressure equation for a gas-water system is defined as: Pc = Pg − Pw ……………………………………………… (1) Fig.2 Impact of wetting phase saturation on capillary pressure Source: Kotsakis (2012) Figure 2 demonstrates the impact of wetting phase saturation on capillary
  • 5. pressure; capillary pressure difference between the fracturing face and the reservoir will result in fracturing fluids to move to reservoir. Increasing the water phase saturation could be achieved by forcing the water flow into the fracture of rock, thus, the water pressure need to increase above the gas pressure. During the process of drainage, the water pressure will slightly decrease, while the gas will subtly imbibe. Obviously, a negative capillary pressure is needed for a higher water injection pressure than the gas pressure in order to displace gas (Kotsakis 2012). In terms of relative permeability, Boudet (2014) points out that permeability is the single phase flow conductivity of pores. Base on Darcy's law, the permeability K is defined as: K = qμL AΔP …….…………………………………………… (2) Where: μ= the viscosity of the fluid q= flow rate of the fluid L= length of porous medium A= cross section area porous medium ΔP= pressure difference across the length of the porous medium If multiphase fluids flow simultaneously through a porous medium, the relative permeability of individual could be defined. qi = ( Kkri μi )A Δpi Δx …………………………………………… (3) Where: qi= the flow rate of phase i kri= the relative permeability of phase i μi =the viscosity of phase i ΔPi= the pressure drop within the phase i Kkri μi = the mobility of the phase i Kkri=the total permeability of phase i Because the relative permeability of oil or gas is lower in reservoir rock, the accumulation of oil or gas in the fracture will be reduced when pressure reduces below dew-point pressure, and lead to the decrease of productivity.
  • 6. According to Ricardo (2011), when multiphase fluids flow is mobile within the fracture, pressure loss in a fracture may increase by an order of magnitude. In detail, relative permeability changes, alteration of fluid saturation, and phase interaction between fluids will make contributions to multiphase pressure losses. If the saturation of one phase increases, it results in flow area decrease of the other phase. In the hydraulic fracture process, initially, gas saturation in the reservoir rock is 100% before hydraulic fracturing process, due to the reservoir rock are being opened by hydraulic fracturing, the saturation of liquid increases to 30% from 0%, at the same time, the saturation of gas phase decrease to 70% from 100%. Eventually, the flow area of the gas phase has been reduced by 30% and result in decrease of gas production Ricardo (2011). Fig.3 Impact of fractional flow of liquid on the multiplier of pressure drop Source: Ricardo (2011) Relative permeability changes suggest that because of the saturation of one phase increases, the permeability decreases related to the other phase. For instance, in shale gas wells, if the water saturation increases in the fracture, the gas permeability in the fracture will decrease. The phase interaction between multiphase fluids shows differences of mobility. As two phase move through a porous media with significantly different velocities, one phase
  • 7. could interfere with the flow of the other. This phenomenon is particularly apparent in gas and water flow. Figure 3 illustrates the impact of multiphase flow on the multiplier of pressure drop, which demonstrates that a massive pressure drop resulted from a really small increase of water fraction. 2.2 Solutions to multiphase issues Due to the wettability characteristics and low permeability of reservoir rock, as well as pressure lose in fracture, the hydraulic fracturing cleanup process will take a long time and the accumulation of gas or oil in the fracture will be reduced when pressure reduces below dew-point pressure, and lead to decrease of productivity (Fahes and Firoozabadi 2007). Therefore, it is indispensable to seek solutions for multiphase issues. According to Li and Firoozabadi (2000), with method of chemical treatment of the rock to change the wettability to intermediate gas-wetting is approachable means for multiphase issue. An essential element in liquid accumulation resulted from lower liquid mobility due to extreme liquid wetting. Increasing liquid mobility is accomplished by means of the altering wettability of the rock from extreme liquid-wetting to intermediate gas-wetting by chemical solutions at higher temperature. As a result, the high saturation of the liquid could be prevented, and the high gas productivity could be achieved. In the study of Li and Firoozabadi (2000), steady relative permeability of untreated and treated Berea cores is measured at 24 ℃ to indicate the impact of chemical solution treatment on water-gas relative permeability. Figure 4 presents gas-water system relative permeability of treated Berea core with chemical solution is dramatically higher. Altering wettability to intermediate gas-wetting lead to increase of water mobility in certain water saturation, so gas mobility is increased by this means.
  • 8. Fig. 4 Treated and untreated Berea cores water-gas relative permeability Source: Li and Firoozabadi (2000) For multiphase flow issues of hydraulic fracturing, Ricardo (2011) reveals that using a viscous disproportionate permeability modifier (VDPM) is a reasonable approach to increase oil production. This polymer, VDPM is injected with fracturing fluid to reservoir rock could improve the irreducible water in pores, and then it is absorbed to pores surface and decrease the relative permeability of water without impacting the relative permeability of oil. Therefore, the rate of oil production is improved dramatically by this approach, as Figure 5 shows the productivity is substantial higher when VDPM is applied, compared with conventional technology of hydraulic fracturing. Fig.5 Impact of VDPM on oil production Source: Ricardo (2011)
  • 9. 2.3 Economic effects of hydraulic fracturing With regard to the economic impacts of hydraulic fracturing, generally, it could be more profitable because of the improvement of the productivity by hydraulic fracturing. As sdudy of Boudet et al (2014) indicates that the potential economic effects is a significant advantage of hydraulic fracturing technique development, such as creating job opportunities, increasing local revenues and individual incomes, as well as improving property values and public services. Nevertheless, a research about economic effects of the Marcellus shale in Pennsylvania showed that economic aspects were not as previous prediction. Shale gas exploration will cost more because the consumption of the hydraulic fracturing treatments is substantially high. 2.4 Safety aspects of hydraulic fracturing It is accepted that water in injection fracturing fluid is significant during hydraulic fracturing, 2–4 million gallons of water is needed per well, which contributes the depletion of ground water. Moreover, chemicals occupy 0.5-2.0% of fracturing fluid, for example, 4 million gallons fracturing system needs from 80 to 330 ton of chemicals, which could result in ground water contamination and public health issue. The Environment Protection Agency (2011) has been prompted to study the relation between ground water quality and hydraulic fracturing (Boudet et al 2014). Furthermore, Wang et al. (2014) indicates that the hydraulic fracturing technique might cause earthquake, due to a large volumes of fracturing fluid are injected to reservoir rock. In April and May 2011, two small earthquakes occurred near Blackpool because hydraulic fracturing was applied in Lancashire’s Bowland Shale reservoir. According to the study of Wang et al. (2014), the fracturing fluid injection alters the strains and stresses on the earth’s crust and relieves the effective stress, which could trigger earthquakes.
  • 10. 2.5 Potentials of hydraulic fracturing in the future Although potential economic effects of hydraulic fracturing are noticeable because of the improvement of the productivity of gas and oil, it is a controversial aspect for different study. In order to create potentials of hydraulic fracturing, further research should pay more attentions to optimization of economics and physics of hydraulic fracturing technique (Marongiu 2013). Additionally, Miller (2011) shows that future hydraulic fracturing relies on effectiveness of water treatment due to the injection of fracturing fluids in reservoir rock. 3. Summary and Conclusions In this paper, I explored multiphase flow issues associated with hydraulic fracturing from relative permeability, capillary pressure, wettability in the fracture and the reservoir rock 3 aspects, due to the multiphase flow occurs, which have substantial effects on hydraulic fracturing cleanup process and the fracture fluid performance, and in the long term unconventional oil and gas exploitation and production will be dramatically affected. Furthermore, I sought to find approachable solutions for multiphase issues, in the study of Li and Firoozabadi (2000), with method of chemical treatment of the rock to increase liquid mobility and alter the wettability to intermediate gas-wetting is approachable means for multiphase issue. Besides, Ricardo (2011) reveals that using VDPM to improve the irreducible water in pores of the rock and decrease the relative permeability of water without impacting the relative permeability of oil in order to reduce the impacts of multiphase flow. In terms of economic effects, it is a controversial aspect for different researchers. Additionally, considering safety aspects, water contamination and earthquake are two serious parts about hydraulic fracturing. With regard to potentials of hydraulic fracturing in future, optimization of economics and physics should be paid much attention. Finally, further study should take the public health and water contamination into consideration, this view is in line with Fink (2013).
  • 11. References Boudet, H., Clarke, C., Bugden, D., et al. (2014) ‘“Fracking” Controversy and Communication: Using National Survey Data to Understand Public Perceptions of Hydraulic Fracturing’ Journal of Energy Policy 65, 57-67 Fahes, M. and Firoozabadi, A. (2007) ‘Wettability Alteration to Intermediate Gas-Wetting in Gas-Condensate Reservoirs at High Temperatures’ SPE 3, 397-407 Fink, J.K. (2013) Hydraulic Fracturing Chemicals and Fluids Technology Oxford: Gulf Professional Publishing Kotsakis, A. (2012) ‘Regulation of the Technical, Environmental and Health Aspects of Current Exploratory Shale Gas Extraction’ RECIEL 21, 28-36 Li, K. and Firoozabadi, A. (2000) ‘Experimental Study of Wettability Alteration to Preferential Gas-Wetting in Porous Media and Its Effects’. SPEREE 3 (2), 139-149 Marongiu, M., Economides, M. and Holditch, S. (2013) ‘Economic and physical optimization of hydraulic fracturing’ Journal of Natural Gas Science and Engineering 14, 91-107 Miller, P. (2011) ‘Future of hydraulic fracturing depends on effective water treatment’ Journal of Hydrocarbon Processing 4, 18-22 Ricardo, P. (2011) ‘A New Approach to Hydraulic Fracturing Using Relative Permeability Modifiers Increases Oil Production While Holding Water in Check’ SPE 5, 16-17 Wang, Q., Chen, X., Rogers, H., et al. (2014) ‘Natural gas from shale formation – The evolution, evidences and Challenges of shale gas revolution in United States’ Journal of Renewable and Sustainable Energy Reviews 30, 1-28