Case History of Hydraulic Fracture Performance in the Naturally Fractured Paludal Zone: The Transitory Effects of Damage

Abstract

ABSTRACT The pre- and post-stimulation reservoir performance of two naturally fractured Paludal sandstones in the Mesaverde Group were thoroughly tested and evaluated. The transitory nature and effects of natural fracture damage caused by two stimulation experiments are illustrated. These two sandstone reservoirs, located at the Department of Energy (DOE) Multiwell Experiment (MWX) site, designated Paludal sands 3 and 4, were initially characterized by integrating a comprehensive set of core, log, seismic, geological, well testing and production data. The data set indicated that the dominant production mechanism in Paludal sands 3 and 4 was a set of anisotropic natural fractures. The identification, quantification and incorporation of this anisotropic natural fracture system is critical to the subsequent post-fracture production and well test analysis. These reservoirs were modeled using a finite difference naturally fractured reservoir simulator. This simulator is capable of modeling transient matrix flow and provides the proper coupling of the propped hydraulic fracture with the anisotropic natural fracture system. The initial postfracture well tests and production data indicated that although these fracture treatments had been carefully designed and monitored, they did not properly achieve the desired enhancement. Subsequent reservoir modeling illustrated that these treatments had probably decreased the flow capacity of the natural fracture system near the hydraulic fracture, thus limiting the production capacity of the well. The natural fracture damage was assumed to be the result of residual treatment liquids and/or fracturing fluid blockage. The magnitude of these damage mechanisms should decrease with time. Paludal sands 3 and 4 were subsequently isolated for 18 months and then re-tested. The reentry testing and production indicated that the initial post-fracture damage was no longer present. This paper details the genesis of reservoir behavior from the initial pre-fracture well tests through the period of poor post-fracture production to the current period of enhanced reservoir performance.