Characterization of Well Performance in Unconventional Reservoirs using Production Data Diagnostics

Production analysis and forecast in unconventional reservoirs are challenging tasks due to high degree of uncertainty and non-uniqueness associated with well/reservoir properties. At this point the importance of diagnosis of production data to check the data consistency and to identify flow regimes has become significant. In addition, we believe that producing wells in various unconventional reservoirs exhibit unique production performance behavior due to geology, phase behavior, and completion practices. Therefore, the identification of the related performance behavior is crucial for development and forecast purposes. The primary objective of this work is to apply diagnostic methods to investigate and understand production performance characteristics of the wells producing in unconventional reservoir systems. For our purposes, we present examples from various shale gas plays. We propose the use of various forms of rate-time and rate-time-pressure plots for production data diagnostics. In particular, this work presents the utilization of the dimensionless βq,cp-derivative formulation (i.e., logarithmic derivative of the rate function with respect to logarithm of time) to identify production behavior characteristics. We also propose the use of several diagnostic plots to identify performance characteristics and data consistency. In addition the use of average rate functions are presented for better resolution. Introduction Unconventional reservoir systems can best be described as hydrocarbon accumulations which are difficult to be characterized and produced by conventional exploration and production technologies. Recently unconventional reservoir systems such as tight gas sands, shale gas, tight/shale oil, and coalbed methane reservoirs have become a significant source of hydrocarbon production and offer remarkable potential for reserves growth. Due to the low to ultra-low permeability of these reservoir systems, well stimulation operations (e.g., single or multi-stage hydraulic fracturing, etc.) are required to establish production from the formations at commercial rates. The advances in technology to produce and develop ultra-low permeability reservoirs such as shale gas reservoirs bring the difficulties and uncertainty associated with well performance. The uncertainty is mainly due to the lack of our complete understanding of the production mechanisms and behavior of these reservoir systems. And the difficulty is therefore associated with establishing the long term production decline in these reservoirs. In terms of production analysis in unconventional reservoirs the solutions based on the "linear flow" concept (Wattenbarger et al. 1998; El-Banbi and Wattenbarger 1998) are frequently used with the inclusion of effective fracture network length accounting for a single vertical fracture. Bello and Wattenbarger (2010) extend the previously mentioned linear flow solutions to account for the natural fracture network in shale gas reservoirs by proposing a linear dual porosity solution. For horizontal wells completed with multiple fractures in ultra-low permeability reservoirs, we suggest that multiplefractured horizontal well model should be used for analysis. The references on this particular well model in fact extend to late 1980s. van Kruysdijk and Dullaert (1989) provide an analytical solution based on "boundary-element method". van Kruysdijk and Dullaert show that at early time dominant flow is linear, perpendicular to the fracture face until pressure transients of the individual fractures begin to interfere leading to a compound linear flow regime at late times (linear flow is