Production impairment and purpose-built design of hydraulic fractures in gas-condensate reservoirs
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2000, Society of Petroleum Engineers Inc. Hydraulic fracturing has been the stimulation/completion method of choice for the vast majority of gas reservoirs throughout the world. There is no fundamental difference in the design of hydraulic fractures in reservoirs of any permeability. However, for low-permeability reservoirs, obtaining the indicated length of the hydraulic fracture has been the key element in the execution of the stimulation treatments; for high-permeability reservoirs increasing the fracture conductivity (width multiplied by fracture permeability) is important. For the latter, tip screen-out treatments have been developed. Damage to the fracture, which causes a reduction in the well performance, has manifested itself in two ways, 1) reduction of the proppant-pack permeability because of polymer residue, choking of the near-well region and over-displacement and 2) damage to the fracture face, i.e., reduction to the reservoir permeability normal to the fracture, because of polymer leakoff during the fracture execution. Both types of damage affect primarily higher-permeability formations, the first reducing the much-desired large fracture conductivity and, the second, providing an impediment to flow, which becomes important because of short fracture lengths. For long fractures and short penetrations of fracture-face damage the reduction to the well performance is insignificant. In gas-condensate reservoirs a situation emerges very frequently that is tantamount to fracture-face damage. Because of the pressure gradient that is created normal to the fracture, liquid condensate is formed which has a major impact on the reduction of the relative permeability-to-gas. Such a reduction depends on the phase behavior of the fluid and the penetration of liquid condensate which, in turn, depends on the pressure drawdown imposed on the well. These phenomena cause an apparent damage, which affects the performance of all fractured wells irrespective of the reservoir permeability (including very low-permeability values). Well testing of such wells would invariably calculate much shorter apparent lengths than actually placed. We are presenting here a model that predicts the fractured well performance in gas-condensate reservoirs, quantifying the effects of gas permeability reduction. Furthermore we present fracture treatment design for condensate reservoirs. The distinguishing feature primarily affects the required fracture length to offset the problems associated with the emergence of liquid condensate. Also, guidelines for the calculation of the appropriate pressure drawdown during production to optimize well performance are provided.
