Evaluation of Gas Shale Reservoirs

Rick Lewis
Schlumberger Oilfield Services, Oklahoma City, Oklahoma

The mature, organic-rich source rocks that have expelled much of the hydrocarbons that have been produced from conventional reservoirs in the United States now represent both developed reserves and potential resources. Shale gas has become an attractive target because they represent a huge resource (500 to 780 TCF) and as the higher prices for gas lessen the economic challenges of their development. Multiple operators are currently leasing and evaluating shale gas properties throughout the United States. If these formations can be economically developed, many thousands of wells will be drilled in this region during the next decade.
    Key reservoir parameters for shale gas deposits include: 1) thermal maturity, 2) adsorbed gas fraction, 3) reservoir thickness, 4) total organic carbon (TOC) content, and 5) free gas fraction within the pores and fractures. The adsorbed gas fraction resides on kerogen, insoluble organic matter, within the shales. Kerogen is characterized by very low density, low Pe, slow acoustic velocities and, typically, high gamma ray activity due to uranium concentration. Kerogen is generally quantified as TOC.
    A complete shale gas log evaluation requires calibration to core for thermal maturity and a metric to equate TOC to adsorbed gas. The best data for the latter is a Langmuir isotherm(s) that provides a characteristic gas pressure and volume measured at a specific temperature.
    A robust shale gas interpretation package has been developed utilizing the Platform Express and Elemental Capture Spectroscopy sonde (ECS) logging tools. The ECS is key to this package as it provides gamma-ray independent clay content plus a matrix density that compensates for the variable lithology typical of gas shale. ELANPlus is used to quantify kerogen, this value is converted to TOC based on thermal maturity, and the Langmuir isotherm(s) are used to calculate adsorbed gas. Porosity and gas saturations are calculated via ELANPlus, and the resulting free gas volume is converted to SCF/ton at reservoir conditions. Cumulative adsorbed and free gas is provided in BCF/section. This provides critical information as to the richness of the shale resources that is used to evaluate completion effectiveness.

AAPG Search and Discover Article #90065©2007 AAPG Southwest Section Meeting, Wichita Falls, Texas