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Abstract
Chemical gas tracers have been used in gas flood projects to evaluate interwell communications, formation heterogeneity, channeling, and to calculate volumetric sweep efficiency. For the first time, these tracers are used to evaluate the extent of fracture communication with offset wells and also to evaluate zonal communication between four hydraulically fractured reservoirs. The four horizontal wells are drilled and completed in Berea, Chagrin, Lower Huron Siltstone, and Lower Huron reservoirs. Each well was individually fractured with nitrogen gas in nine stages. A different chemical gas tracer was injected in each of the horizontal wells during the fracture stages with the carrier gas. Upon the completion of each fracture treatment, the well was shut-in to frac the next well. Upon the completion of the last fracture treatment, the well was shut-in for the reservoir pressure to stabilize before all four wells were put back on production. Flowback gas samples were collected at the wellhead for tracer detection analysis and hence flowback analysis. In addition, produced gas samples were collected at the offset wells for gas tracer detection to evaluate the extent of the fractures, interwell communication and formation heterogeneity. The results of flowback, zonal communication and extent of fracturing are presented in detail.

Field Background
The Appalachian foreland basin is the end product of four primary events consisting of the Taconian, Salinic, Acadian and Alleghenian orogenies (Meckel, 1970; Ettensohn, 2004). Each event either generated new fracture and fault systems or reactivated existing systems. Uplift associated with each orogeny resulted in corresponding basin subsidence and pulses of clastic detritus (Walcott, 1970; Price, 1973). A portion of the clastic and organic deposition that occurred during the Acadian orogenic event represents the primary rock package targeted by the horizontal wells of this investigation.

In the study area the upper Devonian and lower Mississippian rock units were deposited over a time of shifting sedimentary environments that included low oxygen organic rich, turbidite, slope and shallow water high energy zones. The corresponding lithologies resulting from these environments represent approximately 1400 feet of stratigraphic section and include black shale (Huron Shale), siltstone (Lower Huron Siltstone), gray shale (Chagrin Shale) and sandstone (Berea) (Figure 1).

The study area, Mallory Quad field, is located on the nose of a southwest plunging anticline (Figure 2). Natural fracturing in the area is likely a result of a combination of tectonic loading, structural inflexions and catagenic processes (conversion of kerogen to hydrocarbon). However, catagenesis is likely the primary mechanism controlling the generation of natural fractures and the fracture orientation is largely linked to the stress regime at the time of fracturing. It is clear from geophysical logs, image logs, production logs and mud gas logs that fractures are present and that they do contribute to the gas production. Many of these fractures appear to cross lithologic boundaries and in some instances have been projected from one horizontal well to the underlying or overlying horizontal well.

Each of the described lithologic units has been targeted with a horizontal well (Figure 3). Each well was treated with one of four unique chemical tracers injected during stimulation operations. It is not yet conclusive, but possible that a portion of the chemical tracer migration pathways followed some of the natural fractures revealed by the log data. However, fractures induced through stimulation likely played a secondary role in generating conduits for tracer migration. At a minimum, the tracer data suggest a degree of communication between all of the rock units targeted by the horizontal wells.