Stress Anisotropy, Long-Term Reservoir Flow Regimes and Production Performance in Tight Gas Reservoirs

Abstract

Tight gas reservoirs normally have production problems due to very low matrix permeability and different damage mechanisms during drilling, completion and stimulation. Tight reservoirs need advanced drilling and completion techniques to efficiently connect wellbore to the formation open natural fractures and produce gas at commercial rates. Stress regimes have significant influence on tight gas reservoirs production performance. The stress regimes cause wellbore instability issues while drilling, which can result in large wellbore breakouts. The stress regimes can also control the well long-term production performance, since they affect permeability anisotropy. The preferred horizontal flow direction is expected to be parallel to the maximum in situ horizontal stress. The production and welltest data in non-fractured as well as hydraulically fractured wells in tight reservoirs have indicated the presence of a long-term linear flow regime due to the well and reservoir geometry and also as a result of the permeability anisotropy. The stress anisotropy leads to different permeabilities in different directions, and the natural fractures that are aligned with maximum horizontal stress; they might have larger aperture and greater permeability. Due to the more severe stress anisotropy in tight formations, permeability in maximum stress direction might significantly be larger than permeability in the direction of minimum stress. This study represents evaluation of parameters that might control well productivity and long-term well production performance in tight gas reservoirs. Geomechanical modeling is performed in order to understand the effect of stress anisotropy on aperture evolution of natural fractures in different directions. Furthermore, single well reservoir simulation study is performed in order to generate pressure build-up data for a typical tight gas reservoir, in order to evaluate effect of reservoir geometry and permeability anisotropy on late time linear flow regime, and also assess the well production performance for different well and reservoir conditions.

Evaluating Factors Controlling Damage and Productivity in Tight Gas Reservoirs

“…Tight gas reservoirs are normally heterogeneous and anisotropic in nature, where permeability is a direction dependent property. The permeability anisotropy may be further enhanced by the pattern of earlier geological deformation and amplified by in-situ stresses (Dusseault, 1993), as the permeable conduits and natural fractures that are aligned with maximum horizontal stress (perpendicular to the minimum stress direction) may have larger aperture and greater permeability (Bahrami et al, 2010).…”

“…To achieve higher well productivity, open-hole completion in tight gas wells may be more effective than casedhole completion, because enlarged wellbore diameter due to wellbore breakouts can further reduce skin factor and significantly increase initial gas production rate (Bahrami et al, 2010). Open-hole perforation using deep penetrating charges run with shock absorbers may help in passing the damaged zone and effectively connect the wellbore to the virgin formation.…”

“…Controlling damage is important in tight gas reservoirs as the low deliverability and lack of connectivity between the sand bodies, make it challenging to produce gas at commercial rates Bahrami et al, 2010). This study evaluates phase trapping and liquid blocking damage effect on gas production for different cases of hydraulically fractured tight gas reservoirs.…”

“…Although one can estimate the average permeability value from welltest data in tight reservoirs using advanced welltest interpretation techniques (Bahrami et al 2010), estimating fracture storativity and interporosity flow coefficient from such welltest data might not be feasible since the dual-porosity response may not clearly be observed on pressure build-up diagnostic plots. The conventional approaches might fail to characterize fracture parameters in naturally fractured tight gas reservoirs, especially in complicated cases such as hydraulically fractured or horizontally drilled wells (Restrepo and Tiab 2009), and therefore application of welltest and image log data in the reservoirs may not be well understood and is proved to be difficult to interpret for meaningful results.…”

“…Wellbore instability while drilling due to stress regimes is also a common issue in tight sand formations, which can result in large wellbore breakouts (Dusseault, 1993). The severe stress anisotropy in tight sand reservoirs can result in higher permeability in the conduits perpendicular to maximum stress direction (Bahrami et al, 2010). In reservoir geometry, the tight sand formations are normally stacks of isolated lenses of sand bodies, vertically separated by shale layers.…”