Controls On Gas Resource In Shales

Alton Brown

A gas resource (gas in place) must be present for a gas shale to be productive. Gas in place is controlled by rock volume, the storage mechanisms and their capacities, the porosity-permeability relationships of the reservoir, and an effective gas source. The only effective shale gas storage mechanisms are porosity (volumetric) storage and sorption storage. Porosity, gas saturation, and gas volumetric factor control volumetric storage. Porosity and saturation can be measured by wireline logs, and Bg can be predicted from reservoir pressure and temperature. Porosity storage increases almost linearly with pressure. Sorption gas storage  can be predicted from the TOC, reasonable estimates of sorption capacity of organic matter based on thermal maturity of the kerogen, reservoir temperature, reservoir pressure. Sorption storage also increases with pressure, but it forms a concave-downwards trend. As a result, sorption reservoirs have greater storage capacity at shallow depth whereas volumetric reservoirs store more gas at depth.. 

Gas shales have nanodarcy-scale permeability. A nanodarcy is to a millidarcy as a millidarcy is to 1000 Darcys. High water saturation reduces matrix gas permeability even further. It appears that water saturation separates potentially productive from non-productive deep gas shale intervals. Low gas shale permeability has two effects. First, gas shales must be self sourced, because there is insufficient capillary pressure to charge a gas shale from an external source.  Second, the low permeability creates drainage problems which makes a fracture system, either natural or artificial, necessary for production. Self-sourcing can be thermogenic or microbial. Thermogenic source is controlled by burial history and thermal maturity, whereas microbial gases are controlled by meteoric water influx into shales and potential microbial food sources in the shale. Some gas shales are so tight that they may not produce most of their gas even where fractured. In exceptionally tight gas shales, it is essential to have relatively closely spaced fractures to access the reservoir in reasonable production lifetimes (tens of years).     

Gas shales are not all alike. Based on the characteristics discussed here, three general types of gas shales can be identified. (1) conventionally fractured gas shales, where produced gas is from fracture porosity with little or no matrix support. (2) Shallow gas shales, where sorption is the predominant storage mechanism, and spaced, natural fractures are essential both for generating microbial methane and producing gas. High TOC content is especially important for this gas shale type.  (3) Deep gas shales, where volumetric storage is predominant, gas is predominantly thermogenic, and the reservoir must be fractured to achieve high production rates. Porosity, high pressure, and high gas saturation are most important factors controlling gas in place in deep gas shales.  

The complete the exploration checklist for the deep shale gas type, the susceptibility of the formation to fracture stimulation must be evaluated. Brittleness increases with the amount of  brittle minerals such as quartz, dolomite, and calcite. These minerals may also help preserve porosity or allow secondary porosity to form.