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Abstract The key in unlocking unconventional reserves is to create massive fracture surface area. During the fracturing treatment, a huge volume of fracturing fluid is pumped to generate fractures and then followed with a large amount of proppant to provide enough conductivity for reservoir fluid to flow to the wellbore. The ultimate proppant distribution in the fracture system directly impacts well productivity and production decline rate. However, it is very challenging to predict how far proppants can go and where they will settle because of the complexity of the fracture system. Therefore, accurate modeling of proppant transport inside the fracture system is critical to enable stimulation optimization. Previous modeling and experimental studies were usually based on simple proppant settling velocity models and limited only to planar fracture cases. To accurately evaluate propped complex fracture systems, which are more common in unconventional reservoirs, advanced proppant transport models are required. In this paper, proppant transport in various fracture geometries is investigated using computational fluid dynamics (CFD) models, in which the interaction between proppant particles and the carrying fluid phase is fully coupled to track proppant movement in the fractures. The planar fracture case is first investigated using a CFD model and benchmarked with results from commercial software. The CFD models are then used to simulate the proppant transport in T-junction and crossing-junction scenarios, which are often seen in unconventional reservoir fracture systems. Parametrical studies are also conducted to better understand how proppant transport is affected by fracture fluid viscosity, proppant density, and fluid injection rates. The results from the proposed CFD models indicate that proppant transport within complex fracture geometries is significantly affected by fracture fluid dynamics and proppant properties. At fracture junctions, turbulent flow regime will develop, which helps proppant propagate to natural fractures. According to the parametrical studies, higher injection rate and light-weight proppant are beneficial in transporting proppant through fracture junctions to reach further in both hydraulic fractures and natural fractures. Proppant transport models developed in this work fully incorporate the interaction between proppant particles and carrying fluid dynamics. This study extends the current understanding of proppant movement in complex fracture geometries and helps optimize hydraulic fracturing design to improve unconventional well production performance.
Fernández, M. E. (Universidad Tecnológica Nacional) | Baldini, M. (Universidad Tecnológica Nacional) | Pugnaloni, L. A. (Universidad Tecnológica Nacional) | Sánchez, M. (YPF Tecnología SA) | Guzzetti, A. R. (YPF Tecnología SA) | Carlevaro, C. M. (Inst. Fisica de Liquidos y Sist. Biológicos)
An appropriate propped fracture is important for oil and gas production (especially in shale formations). The actual proppant placement during fracturing is generally unknown. Any fair prediction of the placement of the proppant may results in significant improvements for the fracture protocol design. We present experimental data on the transport and settling of particles dragged by water through a narrow wedge-shaped vertical fracture. We discuss some basic features of the dynamics of the settlement of the proppant dune and show results on the final placement for different pumping rates and particle sizes. Results are consistent with previous findings by others and confirm that some usual practices in the field are beneficial to maximize the propped volume and minimize arching.
In recent years, many oil companies have directed their efforts towards developing unconventional reservoirs. The challenges encountered to guarantee a profitable operation in these plays lead the industry to devote significant amounts resources to optimize processes such as hydraulic fracturing, a vital stimulation technique.
Hydraulic fracturing consist in the injection of fluids, along with proppants, into the formation aimed at creating and/or enhancing existing fractures to open high conductivity channels connecting the formation and the wellbore .
Proppants are granular materials that fill the fracture and support the closing pressure, keeping the fracture conductive during production. Although fracturing techniques have evolved, there is still opportunities to increase efficiency. Many of these opportunities are related with the way in which proppants are transported and deposited into the fracture.
Huang, Xu (Baker Hughes Incorporated) | Yuan, Peng (Baker Hughes Incorporated) | Zhang, Hao (Baker Hughes Incorporated) | Han, Jiahang (Baker Hughes Incorporated) | Mezzatesta, Alberto (Baker Hughes Incorporated) | Bao, Jie (Pacific Northwest National Laboratory)
Abstract During the fracturing treatment, fracturing fluid is pumped to generate fractures and then followed with a large amount of proppant to provide enough conductivity for reservoir fluid to flow to the wellbore. The ultimate proppant distribution in the fracture system directly impacts well productivity and production decline rate. However, it is very challenging to predict how far proppants can go and where they will settle because of the complexity of the fracture system. Previous modeling and experimental studies were usually based on simple proppant settling velocity models and limited only to planar fracture cases. In a recent numerical study, proppant transport in different complex fracture geometries was modeled. However, the fracture walls in the model were considered to be perfectly smooth. In this study, proppant transport in complex fracture geometries with different wall roughnesses was investigated using computational fluid dynamics (CFD) model, in which the interaction between proppant particles, the carrying fluid phase, and the rough fracture wall was fully coupled. A planar fracture case with smooth fracture wall was first investigated using a CFD model and benchmarked with results from commercial software. The CFD models were then used to simulate the proppant transport in T-junction and crossing-junction scenarios with different fracture wall roughnesses, which are often seen in unconventional reservoir fracture systems. The results from the CFD models indicate that proppant transport within complex fracture geometries is significantly affected by fracture wall roughness. Rough fracture wall can exert resistant drag force to proppant particles and carrying fluids and hence influence the proppant transport behavior and particle distribution. It is found rough fracture wall decreases both proppant horizontal transport speed and vertical settling speed which can lead to a better vertical coverage of proppant particles in the fracture. However, more pumping energy and time are required to transport the proppant particles to the same fracture length with rough fracture surfaces compared to smooth fracture surfaces. Studies on proppant density show light weight proppant has a better vertical distribution in fractures with rough walls due to more pronounced drag force effect. With high viscous carrying fluids, proppant in both smooth and rough fractures can transport further at the same transport time. Proppant transport models developed in this work fully incorporate the interaction between proppant particles, carrying fluid dynamics, and rough fracture surfaces. This study extends the current understanding of proppant distribution in complex fracture geometries and helps optimize hydraulic fracturing design to improve unconventional well production performance.
Proppant additives play an essential role in hydraulic fracturing as they provide support, which retains the fracture opening after the pumping is shut off. From a production point of view, a larger proppant size provides better permeability, while, at the same time, gravitational settling may cause significant distortion of the particle distribution inside the fracture for heavier particles. This study uses a recently developed model for proppant transport, that has been implemented for Khristianovich-Zheltov- Geertsma-De Klerk (KGD) and pseudo-3D (P3D) fracture geometries, to quantify the effect of particle settling. The proppant transport model is based on an empirical constitutive relation for the slurry that accounts for: i) a non-uniform particle distribution across the fracture width due to shear-induced migration, which distorts the parabolic velocity profile, ii) slip velocity in the direction of flow, which, in the limit of a jammed state, leads to Darcy’s law, and iii) gravitational settling. While the gravitational settling is the biggest concern when dealing with larger particle sizes, other effects may include earlier jamming due to proppant stalling in between the walls and higher permeability of the proppant plug, which promotes the fracture propagation in front of the jammed region.
Abstract Hydraulic fracturing has been successfully employed for unconventional oil and gas recovery for decades. The fracturing process is realized by injecting fluid, which contains the proppant materials used to keep the fracture open and productive, into a well at a high enough rate and pressure to crack open the formation. Hydraulic fracture-stimulated production plays an important role in unconventional hydrocarbon production. The distribution and transport of proppant significantly affect fracture conductivity and, in turn, the production rate and decline. It is widely recognized that the effective placement of proppant in a fracture has a dominant effect on a well's productivity, yet it is greatly underestimated owing to a lack of knowledge and practical means to deal with the transient proppant settling process inside the fracture. Existing hydraulic fracture models mostly simplify the proppant transport process or even totally neglect the effect. A common assumption is that the average proppant velocity is equal to the average carrier fluid velocity, and the settling velocity is calculated using Stokes' law, while some important forces exerting on proppant particles are not taken into account, which often leads to the overprediction of the effective fracture length by up to 300%. To effectively simulate the dynamic proppant settling inside the fracture requires the consideration of such factors, including the wall-effect lift force, drag, and leakage of the fluid into the formation. A numerical model has been developed to predict the transient transport and settling of proppants during hydraulic fracturing treatments and production to improve fracture conductivity. The model presented in this paper includes three stages. The initial stage simulates the homogenous phase behavior with a previously developed fracture injection and production model (FIPM) developed elsewhere. The FIPM can alternate between injection and production modes, with gas, oil, and water phases included, and gas-oil phase transition allowed. Therefore, leakoff and production into the formation are simulated based on the pressure and phase saturation fields, with gravitational, drag, and lift forces taken into account. During the final stage, a fracture-stimulated horizontal well in the Eagle Ford is used to validate the model, both for injection-induced water damage and production. During the initial proppant injection stage, the distribution of proppants can be considered to be homogenously dispersed in the injection fluid. Shortly after the injection started, because of geothermal effects, leakoff of injection fluid, and gravity, the proppant particles will settle, and the initial fracture conductivity profile is formed during this stage. This conductivity profile has a significant effect on the early-stage production rate. As production continues, the proppant particles will be shifted dynamically and will result in a change in the effective fracture length, width, and conductivity distribution over time. During the second stage, the leakoff and evaporation of the liquid phase is simulated as flow through porous media, taking the thermal gradient and evaporation latent heat into consideration. The settling and transport of transient solid proppant particles are modeled using the upstream scheme, accounting for the forces of gravity, drag, and wall-lift forces on the particles. Both injection and leakoff of fluids help determine the size and conductivity of fractures, as well as the water-envelope damage in the near-fracture region inside the formation. These can significantly impact the transient depletion of the reservoir, particularly during early production time. This effect is studied in terms of reservoir permeability, production rate, and phase distribution.