Damage Mechanisms in Unconventional-Gas-Well Stimulation—A New Look at an Old Problem

Abstract

Summary After fracture stimulation of a tight gas well, production often rises slowly instead of showing an early transient. This indicates either severe reduction in fracture conductivity or reservoir damage. There is still no agreement in the industry about the most important damage mechanisms. Work performed by Pratts and Holditch in the 1970s showed that fracture-face damage from filtrate invasion is unimportant unless there is permeability damage in the invaded zone of at least 99%. New ideas have been proposed that may better explain the behavior commonly seen in actual tight gas well production data. These ideas include relative permeability with water and gas both immobile at a given saturation ("permeability jail"), small-scale reservoir heterogeneity, and stress-sensitive-matrix permeability at high drawdown. Experience from the field has been contradictory. Sometimes no water is produced back, but gas production does not appear to suffer. In other cases, the gas rate is significantly lower than expected, but significant fracture fluid is recovered. With the inherent coupling of fracture length and permeability in well-test interpretation and the practical impossibility of achieving radial flow in tight-gas reservoirs with large fracture lengths, it has been difficult to prove any theory about the cause of poor performance from tight gas fracture treatments. This paper shows simulation results of a number of damage effects on post-fracture production from an unconventional (0.001-md) gas well. Furthermore, realistic assumptions about proppant-pack cleanup show a connection not only between poor cleanup and short effective fracture length, but also reduction in contacted kh and connected reservoir volume. New reservoir-simulation results are presented that show a 50% reduction of production in the first years because of these effects in unconventional reservoirs.

Analyses of Processes, Mechanisms, and Preventive Measures of Shale-Gas Reservoir Fluid, Completion, and Formation Damage

“…The trends in production after hydraulic fracturing is strongly influenced by decrease of fracture conductivity and reservoir formation permeability by damage caused during fracturing and post-fracturing production impairment is influenced by various potential damage mechanisms involved in stimulation of unconventional-gas-wells (Shaoul et al 2011, Guo et al 2011, Bahrami et al 2012, and Li et al 2012. Jackson and Rai (2012) describe the sources of skin effects involved in hydraulically-fractured shale gas wells completion efficiency caused by various completion resistances.…”

“…Invading water-based fracturing-fluid remains inside the pore until production and therefore causes formation damage by clay swelling under prolonged exposure time, which is the actual formation skin effect (Shaoul et al 2011). Clay swelling by water imbibition can cause cracking and dissolution of shale (Wang et al, 2010).…”

“…The important factors affecting the production decrease after water invasion are initial water saturation, wettability, capillary pressure, and pressure drawdown; and entrapped water can be produced back during the cleanup period only if pressure drawdown can overcome capillary pressure (Shaoul et al 2011).…”

“…Their results indicate that none of the present trapping models is satisfactory in representing their data of the trapped-gas saturation vs. the initial-gas saturation. Gas can flow faster in high-permeability regions having low-capillary pressure than in low-permeability regions having high-capillary pressure; and high-effective stress-sensitivity of formation matrix to high drawdown can significantly reduce the pore-throat constrictions and therefore permeability, and consequently increase the capillary pressure (Bazin et al, 2010, Cluff et al, 2010, Shaoul et al, 2011.…”

“…The hydraulic fracturing water enters the formation by leakoff and water imbibition, and causes hydraulic damage by increasing the water saturation in the near-fracture formation; however, the high capillary pressure effects of narrow passages cause water to move into the reservoir formation to reduce the water content in the near-fracture surface; also, the relaxation effect in nonequilibrium water spreading is important (Mikhailov and Nikolaevskii, 2000, Shaoul et al, 2011, Civan, 2012. The high capillary pressure of small matrix pores tends to decrease the water saturation by spreading the water invasion region whereas the low matrix relative permeability tends to increase the water saturation by limiting the water invasion region, but the water imbibed into shale disappears also partly by vaporization under the effect of pore-proximity because the interface vanishes (Mahadevan et al, 2007, Alkouh andWattanbarger 2013).…”