Factors Affecting Water Blocking and Gas Flow From Hydraulically Fractured Gas Wells

This paper demonstrates the combined effects of formation permeability damage and relative permeability damage in the invaded zone of a hydraulically fractured gas well. Study results indicate that the damaged zone permeability must be reduced by several orders of magnitude and the capillary pressure altered before a serious water block to gas flow will occur. Introduction To optimize profit from low-permeability gas reservoirs, well stimulation normally is required. In a majority of the cases, a long hydraulic fracture will provide the most efficient and economic means of stimulation. To create long hydraulic fractures, a large volume of fluid, mixed with additives and granular propping agents, must be pumped into the formation. Such treatments can be expensive and require extensive engineering effort. The fluid injected during the fracturing treatment will leak off into the formation and will reduce the relative permeability to gas in the invaded region. Near the fracture, the permeability to gas will be reduced to zero. After injection has ceased, imbibition will begin to alter the fluid distribution, and when production begins, the fracturing fluid will flow from the formation into the fracture. As the fracturing fluid is produced, the relative permeability to gas in the invaded zone will increase and gas will begin to flow into the fracture. In some cases, the injected fracturing fluid may reduce the formation permeability in the invaded zone. Such damage can be caused by clay swelling, precipitation of solids, or migration of released fines. In reality, all fracturing fluids, no matter how expensive, do some damage to the reservoir adjacent to the fracture. Using clean fracturing fluids that are compatible with the formation rock and the reservoir fluid helps minimize such damage. However, in some reservoirs, the injected fracturing fluid does not readily clean up and several months of production may be required before the maximum gas flow rate is achieved. In the most extreme cases, a complete water block to gas flow can occur. To effectively design a fracture treatment for a water-sensitive reservoir, it is essential to understand the basic reservoir mechanisms that govern flow between the fracture and the formation. Gas and water relative permeability, relative permeability hysteresis, capillary pressure, and reservoir damage all tend to complicate the analysis of reservoir flow in fractured formations. By considering the combined effects of the above parameters, it is possible to analyze a given reservoir and determine the cause of slow cleanup and reduced gas productivity.