This research presents an investigation of the sensitivity of fractured horizontal well performances to reservoir properties in ultra-tight, naturally fractured shale-gas reservoirs under depletion. The method of the research is analytical. Following a general review of the existing literature on well performances in unconventional tight reservoirs, the analytical trilinear flow model developed by Ozkan et al. (2011) is introduced. The original dimensionless trilinear flow solution in Laplace domain is converted to dimensional form to be used in productivity index calculations.;The productivity of fractured horizontal wells in shale-gas plays is discussed in terms of transient productivity index defined by Araya and Ozkan (2002). For the purposes of this research, the stimulated reservoir volume (hydraulically fracturing the horizontal well in order to create pathways from the induced fractures towards the existing naturally fractured reservoir, creating either a complex network of fractures through hydraulic fracturing, or rejuvenating the existing healed fractures to create an effective stimulation zone around the well) assumption is implemented. This assumption, commonly used for shale-gas wells, limits the drainage of the well to the stimulated volume around the well and strongly influences the average pressure to be used in productivity computations. Derivation of the average pressure relationship for the dual-porosity (naturally fractured) stimulated reservoir volume is presented and the expression for the average pressure is combined with the transient trilinear flow solution to obtain a relation for the transient productivity index. As a means of investigating the sensitivity of productivity to a selected set of reservoir properties, the derivatives of the productivity index with respect to these properties are derived.;The results of the research are presented in three forms: (i) productivity index versus time for the low, base, and high values of the selected property and the productivity index versus the change in property value at selected times, (ii) tornado charts indicating the sensitivity of the productivity index to the selected properties at different times, and (iii) change in productivity index (derivative of productivity index) with the variation of the selected properties. The results indicate that, unlike the common expectation, the productivity of the fractured horizontal wells in shale-gas plays is not very sensitive to the permeability of the natural fractures in the stimulated reservoir volume and/or the distance between hydraulic fractures. The permeability of the shale matrix and the density of the natural fractures in the stimulated reservoir volume, on the other hand, influence productivity by 600% and 300% respectively. The hydraulic fracture permeability and half-length appear to moderately influence productivity on a 5% to 20% range. These results should be useful to guide well completion in shale-gas plays, and the sensitivity analysis provided in this thesis should be useful for engineering analysis of projects.
展开▼
