--> Importance of Recognizing Open Fracture Networks When Estimating Shale Gas Reserves — A Geochemical and Microseismic Perspective

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Importance of Recognizing Open Fracture Networks When Estimating Shale Gas Reserves — A Geochemical and Microseismic Perspective

Abstract

It is important to be able to estimate the volume of shale drained that is associated with a borehole or with a hydraulic stage. The Stimulated Rock Volume (SRV) is commonly based on fitting the microseismic events within an envelope, however, it is semi-quantitative and rely on many assumptions that may not apply to every well hydraulically fracked. Abnormal absence of microseismic events in the vicinity of a horizontal borehole has been observed in various basins. In some cases, the lack of events is linear in nature and corresponds to a vertical fractured zone. In other cases it is more complex as seen in the Montney of the Altares Field; in this field three wells of a pad showed all of the events of some frac stages, 200 meters down from the borehole and within the Belloy, the underlying formation. The critical observation comes from the production log which clearly indicates that 92% of the production comes from the stages where no microseismic events have been recorded in the Montney. The solution to the problem came from the gas composition acquired using a chromatograph: the zones devoid of microseismic events clearly show a gas bimodality, one corresponding to the background gas, the other interpreted to be a dryer gas present in the open fracture system. Thus, geochemistry can give essential information related to the rock fabric; in this case, the presence of open fractures. Moreover, a new type of display involving produced gas geochemistry has brought a new way of looking at shale gas fields and to the rock volume drained. The new approach invokes simultaneously gas composition (C3+/C1+) against carbon isotopes of the ethane or propane if available or against iC4/nC4 in the absence of isotope data. The trends observed through time, for the same wells, are clearly linked to a stimulated rock volume that can be quantified after a simple normalization with respect to the volume produced. That new method can also clearly identify any well interference. This method has been tested successfully on other large shale data sets from various basins and should be kept in mind in field development planning and reserve estimates. Any frac design should take into account the existence of open fractures seen along a borehole through gas chromatography or mass spectroscopy. Plugs between frac stages should attempt to separate open fractured zones from non-fractured ones. It may be wise to first naturally deplete these intervals and fracture them later.