Fracturing in Tight Gas Reservoirs: Application of Special-Core-Analysis Methods To Investigate Formation-Damage Mechanisms

Abstract

Summary Gas-well productivity in tight reservoirs is greatly impeded by fracturing-fluid interactions with the formation. New simulators introduce formation-damage mechanisms to calculate gas-well productivity. However, equations describing formation damage must be supported by experimental data obtained in conditions representative of fracturing operations. The purpose of this work is to derive absolute-permeability and multiphase-flow damages upon return gas permeability after core invasion by a fracturing fluid by methods used in the Special Core Analysis Laboratory (SCAL). The core permeability is in the microdarcy range with significant illitic content. Absolute-permeability damages caused by fracturing-fluid filtration and water sensitivity are measured. Water-saturation profiles recorded by X-ray in two-phase-flow experiments are interpreted. The methodology of interpretation provides the petrophysical data specific to the rock/fluid system: absolute permeability, relative permeability damage caused by hysteresis, and capillary pressure. In addition, simulations are presented for the evaluation of the effect of various operational parameters, such as pressure drawdown, on gas productivity. It is shown that permeability hysteresis is the determinant factor to explain low gas recoveries at short term. In the long term, the natural cleanup is very slow. The results, derived from a real rock/fluid system, are used to provide recommendations for improving backflow procedures. This methodology can be applied to any case of damage caused by the alteration of rock/fluid properties.

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

“…Unfortunately, the interfacial-tension lowering advantage of the surface-active additives of the fracturing fluids diminishes by their adsorption over the pore surface (Bazin et al, 2010).…”

“…Clay swelling by water imbibition can cause cracking and dissolution of shale (Wang et al, 2010). Bazin et al (2010) tests with clay-damaging water-based fluids indicate irreversible absolute permeability reduction by about a factor of four by the illite and montmorrilinite clays swelling and kaolinite clay migration (surface mobilization and subsequent pore-throat plugging) effects due to water-salinity gradients. Clay swelling by water imbibition can also reduce pore throats especially because it becomes more pronounced with time and additives used in fracturing fluids to prevent clay swelling lose their effectiveness because mixing with the resident water can dilute the fracturing fluids (Alkouh and Wattanbarger, 2013).…”

“…The alteration of the relative permeability and capillary pressure curves and formation of water blocks affect the kinetics of water removal; gas-phase entrapment occurring during fracturing fluid invasion (imbibition) causes the capillary pressure and relative permeability hysteresis; fracturing fluid cleanup requires a significantly long time because of the unfavorably low mobility of the gas phase ( ; initiation of gas flow requires a pressure drawdown sufficient to overcome the capillary-end effect; and the cleanup occurs faster and the gas return permeability is greater when the production drawdown pressure is higher (Bazin et al (2010).…”

“…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 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). Bazin et al (2010) investigated the formation-damage mechanisms involving the fracturing of tight gas reservoirs by means of special-core analysis and numerical simulation. They concluded that the absolute permeability damage by invasion of Wetting phase Wetting phase Nonwetting phase Nonwetting phase polymeric-fracturing fluids into tight reservoirs does not happen because the size of the polymer molecules is somewhat similar to the size of pore-throats in tight reservoirs.…”