AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Overpressure in Shale Gas – When Geochemistry and Engineering Data Meet and Agree
(1) Talisman Energy Inc, Calgary, AB, Canada.
(2) Geomark, Houston, TX.
(3) Talisman Energy Inc, Pittsburg, PA.
A multidisciplinary study has shed a new light on the process of overpressure in shale gas reservoirs. The work consisted of the integration of a large geochemical data set with hydraulic frac and reservoir pressure data from the same wells. Results from this integrated approach suggest that distinct pressure domains exist and that their specific depths and attributes can be easily determined. Whereas various geochemical analyses give conflicting results at first glance, our study shows consistency in the results especially when engineering data is invoked in the analysis.
Diverging gas composition results are obtained when using varying sampling techniques. Geochemical compositions from gas chromatography differ from either geochemical compositions measured from isojars (cuttings) or from isotubes (free gas), the latter exhibiting the highest methane content. In stark contrast, both ethane and propane carbon isotopes give matching and consistent values at similar depths despite the sampling differences.
Three geochemical domains can be defined by their characteristic depth trends in ethane and propane carbon isotopes. A shallow domain is characterized by normally increasing isotope values (less negative) with depth. An intermediate domain is characterized by a reverse isotopic compositional trend. The deeper trend is again normal, however the values are much more negative than in the shallow domain; note that this deeper trend is much more linear when dealing with Ethane isotopes.
Using the geochemistry results as a starting point, frac gradients and reservoir pressure gradients are examined and re-analyzed. This integration revealed the existence of at least two pressure domains: a normally pressured domain and an over-pressured one. The traditional way of calculating a pressure gradient (reservoir or frac) is simply dividing the pressure value by the depth. Our data sets however indicate that individual gradients could be derived for each well or each area and that their intersection with the normal frac gradient is where geochemistry indicates the onset of overpressure.
Over-pressured systems can thus be estimated by geochemistry using any of the following parameters, either independently or in combination: gas composition, gas carbon isotope signatures and Rock-Eval data (Tmax). Each of these tools has its inherent strengths and limitations as do the various methods of collecting data, all of which will be reviewed.