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Abstract
This paper provides a look-back review of lessons learned from early exploration to the full-scale development phase of the Marcellus shale in Pennsylvania. Fracture stimulation of over 100 wells has resulted in an in-depth understanding of details needed to achieve optimal frac performance. Much of the necessary learning curve is derived from the empirical testing of theory and what many refer to as “trial and error.” The ability to evaluate and capture the best practices and develop them into a standing operating procedure (SOP) is one of the most important aspects in development of a new unconventional play. Lessons learned involving fracturing strategies and technologies to date have greatly narrowed the learning curve enabling more rapid advancement toward full-scale development.

Introduction
Shale reservoirs are characterized by extremely low permeability rock that has a number of unique attributes, including high organic content, high clay content, extremely fine grain size, plate-like microporosity, little to no macroporosity, and fickian vs. darcy flow through the rock matrix. This combination of traits has led to the evolution of hydraulic-fracture stimulation involving high rates, low-viscosities, and large volumes of proppant. Production from shale is dependent upon many variables including hydrocarbon content, total organic carbon, shale maturity, porosity, permeability, kerogen content, formation pressure, and net thickness. Improvements in drilling and completion techniques have improved gas recovery, namely landing a horizontal borehole strategically and creating a series of multiple staged hydraulic fractures. Even though horizontal drilling and fracturing have become the completion methods most commonly applied, a significant number of successful wells are being completed vertically in the Marcellus Shale.

The extremely low permeability of shale requires a complex fracture to create primary induced fractures, reactivate and/or intercept more naturally occurring fractures or parting planes and ultimately expose more surface area. Early increased production is dependent upon the number of natural fractures intercepted and long-term production is dependent upon the amount of surface area exposed in the fracture network. Total improved production is dependent upon complex fracture geometry, which is influenced by many factors: stress contrasts, fluid leakoff, natural fractures, layering, weak planes, brittleness, fracture height growth, differing critical stress, post-fracture retention of connectivity to the created frac network, and mechanical stratigraphy, which controls the frac network creation.

Large stimulation volumes of slickwater have been employed to create the extremely complex fracture fairway. High rate is needed to carry the large proppant volumes in a slickwater system and stimulation is achieved by bridging and diverting in induced fractures and natural fractures. The created fracture network is more productive than a dominant single fracture plane in a shale reservoir because more surface area is exposed for gas desorption and long-term natural gas production.
Although many lessons have been learned from previous successful shale plays, many new lessons and unique “tricks of the trade” have been developed or tailored specifically for the Marcellus. This is especially true of the fluid systems and geochemical environment that has driven a number of new developments and fluid innovations.