Papers by Mitchel Philip Tsar
SPE Europec/EAGE Annual Conference, 2012
ABSTRACT Tight gas reservoirs are mainly characterized by low matrix permeability and significant... more ABSTRACT Tight gas reservoirs are mainly characterized by low matrix permeability and significant damage. During drilling and fracturing of tight formations, wellbore liquid invades into tight formation and increases water saturation around wellbore and eventually reduces permeability near wellbore or adjacent to fracture wings. The damage to permeability caused by invasion of liquid into tight formation is controlled by capillary pressure and relative permeability curves. The phase trap damage is one of the main concerns in use of water based drilling or fracturing fluid, since due to high critical water saturation, strong capillary pressure, and sensitivity of tight sand to water. Therefore, use of oil based mud may be preferred in drilling or fracturing of tight formation. However invasion of oil filtrate into tight formation may result in a three-phase relative permeability curves in invaded zone in presence of reservoir gas and initial water, which may differently affect damage and productivity of tight gas reservoirs. This study evaluates phase trap damage in water-based in comparison with oil-based drilled or fractured tight gas reservoir. Reservoir simulation is used to study the effect of relative permeability curves on phase trap damage and well productivity, based on reservoir and core data from a West Australian tight gas reservoir. The results highlights benefits of using oil-based fluids in drilling and fracturing of tight gas reservoirs in term of improving well productivity.Introduction Tight gas reservoirs have production problems due to very low matrix permeability and different damage mechanisms during well drilling, completion, stimulation and production (Dusseault, 1993). The low permeability gas reservoirs can be subject to different damage mechanisms such as mechanical damage to formation rock, plugging of natural fractures by invasion of mud solid particles, permeability reduction around wellbore as a result of filtrate invasion, clay swelling, liquid phase trapping, etc. In terms of reservoir geometry, the tight sand formations are normally stacks of isolated lenses of sand bodies that are separated by shale layers (Holditch, 1979). During well drilling, completion, stimulation and fracturing in tight gas reservoirs, wellbore fluids invade the reservoir and may create a bank of fracturing agent around wellbore, causing significant reduction in well productivity. Liquid invasion increases water saturation from low initial water saturation to a higher value, and then as the near wellbore zone is cleaned up by gas production, the water saturation is reduced to critical water saturation. This process eventually results in permeability reduction in the invaded zone as shown in Figure 1. In high permeable zones with rapid filtrate loss into the reservoir, normally strong mud cake is built on wellbore wall, which stops further invasion of liquid into formation. However in tight zones, liquid invasion is experienced for a longer time due to weak mud cake on wellbore wall and also the strong capillary pressure suction effect. In addition, matrix porosity is low, i.e. there is small pore volume, and therefore invaded liquid travels deeper into tight rock matrix (Schlumberger formation testing, 2005).
Exponential decline curve analysis is widely used to estimate recoverable reserves due to its sim... more Exponential decline curve analysis is widely used to estimate recoverable reserves due to its simplicity. In most cases, however, an exponential model cannot provide a satisfactory match of overall production history. The generalised form of a hyperbolic decline model is more powerful in matching production history than the other decline models, but it is difficult to apply in practical production data analysis since it requires predicting two unknowns as decline constants. Although a hyperbolic model may provide a good fit to early-time production decline data; it may overestimate the late-time production, especially for hydraulic fractured wells in a tight-gas reservoir. In fact, the exponential decline model might be more reliable for forecasting the late-time production.
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Papers by Mitchel Philip Tsar