Optimization of hydraulic fracturing in tight gas reservoirs with alternative fluid
Due to the finite nature of petroleum resources and depletion of conventional reservoirs, the exploitation of unconventional resources has been a key to meeting world energy needs. Natural gas, a cleaner fossil fuel compared to oil and coal, has an increasing role in the energy mix. It is expected that the peak global natural gas production will remain between 3.7-6.1 trillion m3 per year between 2019 and 2060. Therefore, addressing the technical challenges posed by reservoir exploitation technologies in an environmentally responsible manner is critical for efficient energy production and energy secure of the world. Hydraulic stimulation is the first means of choice for improving productivity from tight gas reservoirs by creating highly conductive flow paths with the injection of high-pressure fluid. Millions of hydraulic fracture operations have been performed with water-based fluid which have caused environmental concerns such as water availability and ground water quality degradation due to surface handling and flowback water disposal. Moreover, in lower permeability gas reservoirs, the support agents (proppants) settle at the bottom of fracture due to delayed fracture closure, and slow flowback and water blockage cause hindrance to natural gas flow leading to inefficient fracturing operation. The solution is to replace existing water-based frac-fluids with alternative frac-fluids. Therefore, fracturing with light n-alkanes consisting of pentane, hexane, heptane, octane, nonane and decane is proposed. A single component fluid, or a mixture of components in different proportions can be designed according to reservoir conditions. The characteristics of hydrocarbon nature and significantly lower density and viscosity compared to water can help achieve the objectives of quick fracture closure for better proppant placement, expeditious flowback and reduced phase trapping. Thus, in addition to creating better conductivity fractures, environmental issues associated with water-based fluids can be minimized. To perform hydraulic fracturing with the alternative fluid, a multiphase multicomponent simulator for hydraulic fracturing and fluid flow in fracture and matrix based on popular thermo-hydro-mechanical (THM) FLAC3D-TOUGH2MP/TMVOC framework is utilized. After verification, the model is applied to case study of LPG application in a tight gas reservoir of Canada for hydraulic fracturing operation. Simulations with different frac-fluids such as water, propane and n-heptane have been performed and their fracturing performance including the proppant settling behavior have been compared. The application of proposed alternative fluid for hydraulic fracturing optimization of wellbore yx1 in a tight gas reservoir of Germany showed that fractures with better conductivity in consideration of boreholefracture connection can be created. Sensitivity analysis through numerous simulations for parameters such as injection rate, fluid viscosity and permeability were performed. The flowback analysis demonstrated that most of the injected alternative frac-fluid can be recovered within one week of fracturing which results in better clean up and early start of natural gas production. The fracture permeability and conductivity are dependent upon proppants. Conventionally, spherical proppants have been utilized, however, rod-shaped proppants offer higher porosity and permeability compared with spherical proppants. To analyze their performance, a production model for proppants with different shapes and sizes is implemented in FLAC3D-TMVOCMP framework. The simulations for production forecast with spherical and rod-shaped proppant fractures have been performed considering mechanical properties contrast between proppant and formation under the influence of closure stress with developed model. The application of model to wellbore yx1 showed that 7% more recovery can be achieved from aspect ratio 1 rod-shaped proppant compared to same diameter spherical proppant over a period of ten years. Based upon this work, proposed alternative fluid and rod-shaped proppants are recommended for efficient stimulation of tight gas reservoirs.
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