Numerical investigation of methane and formation fluid leakage along shale gas extraction wells: Application to the St-Lawrence Lowland basin

A. Nowamooz, J.M. Lemieux, J. Molson, R. Therrien

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Mineral and Gas II

ABSTRACT: Natural gas is currently the third-largest global energy source and its consumption is expected to increase substantially in the coming decades. Investment in natural gas continues to grow due to its availability and versatility, and because it is a cleaner energy source compared to coal and crude oil. The share of unconventional gas in total global gas production is projected to rise in the coming years. However, these projections are subject to significant uncertainty, particularly in regions where unconventional gas production is yet to occur or is in its infancy. Environmental concerns have also the potential to limit unconventional gas output. Under these conditions, future production is heavily dependent on response to environmental challenges, public acceptance and widespread access to expertise, technology and water. Shale gas represents almost half of the unconventional gas potential. One of the key technologies applied in shale gas exploration is known as hydraulic fracturing which involves pumping a mixture of water, sand and chemical additives at very high pressure into the target shale formation, creating small fissures through which gas can flow. To protect and isolate potable groundwater from hydraulic fracturing fluids in the wellbore, wells are generally lined with steel casing and cemented from the surface to below the level of drinking water supplies. In many cases, monitoring (during and after hydraulic fracturing) is also performed to check casing integrity and cement jobs. Despite these precautions, important risks of groundwater contamination have been identified from leakage of methane (CH4), brine and hydraulic fracturing fluids, if wells are not properly constructed and maintained. Moreover, it is generally admitted that the presence of cross-connections between nearby abandoned wells or boreholes and preferential pathways through the bulk media (either natural or fracking-induced) can also stimulate the transport of gas and contaminants from the fractured shale to shallow aquifers. To understand the role of these issues on the migration of gas and contaminants, a conceptual model of a shale gas extraction well is developed based on the predicted extraction scenarios for the St-Lawrence Lowlands, and using local geological and fluid properties. A parametric study is then conducted with a numerical model for two selected scenarios, well/formation integrity and preferential pathways through the bulk media, assuming isothermal multi-phase multi-component flow. The model is not calibrated, but the simulations are used to identify the possible time and space scales for gas and contaminant migration using a range of realistic geological and hydrogeological data from the St-Lawrence Lowlands. Numerical simulations and analysis of the various scenarios is currently underway. An example of scenario 1 is presented in Figure 1. Here, we studied the influence of hydraulic properties of cement between casing and rock of an abandoned well on the migration of gas from upper gas-bearing formations or from the target formation toward an unconfined shallow aquifer. The simulations are conducted using multi-phase multi-component simulator DuMux. Since the fluids develop radial symmetry around the well, only a 2D radial sector of a cylindrical domain is modeled. The domain consists of three homogeneous layers with a length of 1000 m and a height of 1000 m. The layers are Utica shale (target formation) with a permeability of 10-18 m2 and a porosity () of 3%, Lorraine shale (upper gas-bearing formation) with k= 10-20 m2 and 2% and a surficial unconsolidated aquifer with k = 10-12 m2 and . The top of the domain is the ground surface. Boundary conditions and initial values are presented in Figure 1. Only the mass transfer between cement and formations is considered. To take into account all key pathways for gas migration (through cement, fractures between cement/casing and fractures between cement/formation), several values of permeability are attributed to the cement. The total vertical flow rate (advective + diffusive fluxes) of methane gas within the cement just below the aquifer (at z = -55 m) is calculated. The results show clearly the influence of cement hydraulic properties on the leakage of the gas to the surface.

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