Estimating Resources and Reserves in Unconventional Gas Reservoirs
DeGolyer and MacNaughton, Dallas, Texas, [email protected]
Unconventional gas reservoirs are characterized by extreme heterogeneity and multiple gas producing mechanisms. Factors controlling the distribution of gas resources include organic richness, maturity, gas content, and gas saturation state in addition to conventional elements such as porosity and water saturation. Understanding the core and log data needed to quantify these parameters, as well as their variability and underlying controls, are critical for making reasonable estimates of gas resources.
Factors controlling gas reserves include effective permeabilities, geomechanical properties, and drilling/completion techniques. Effective permeabilities are highly-variable depending upon matrix composition and natural fracture development. Geomechanical properties, including Poisson’s ratio and stress magnitudes/orientations, play a major role in stimulation effectiveness. Drilling and completion techniques, particularly longer laterals and more stimulation stages, have been critical in increasing initial gas production and estimated ultimate gas recoveries.
The choice of techniques to use for the estimation of resources and reserves depends upon reservoir complexity, data quality, and project maturity. The estimation of resources is commonly a probabilistic exercise that requires a range of key reservoir properties as input. The estimation of reserves requires economic gas production and uses a combination of various techniques including analogs, numerical simulation, rate-transient analyses, and decline curves.
The reserves booking process has become more challenging this year as the U.S. Securities and Exchange Commission (SEC) has modernized its rules to allow the reporting of probable and possible reserves, and to also allow the application of reliable technologies for increasing proven undeveloped reserves. Companies are trying to determine which technologies are most reliable for this process, how to best demonstrate their predictive capabilities, and how to translate this into more reserves.
AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009