Abstract

Gas-in-Place (GIP) analysis is generally performed for gas resource assessment (evaluation of exploration prospects), reservoir production modeling (development optimization and production forecasting), or geologic hazard evaluation (gas emissions during tunneling/mining). Four primary reservoir parameters are needed to calculate the GIP: reservoir or well drainage area; reservoir thickness; bulk density; and in-situ gas content. These variables are usually determined using data obtained from geophysical logs, well site testing, and laboratory analysis of drill cuttings or core.

Generally speaking, GIP assessment of conventional reservoir is a relatively straightforward volumetric calculation since the gas molecules are stored by compression within available free gas porosity and there is no significant gas molecule-reservoir rock interaction. On the other hand, unconventional reservoirs have three storage mechanisms; compression of gas within rock pores; absorption of gas by mobile hydrocarbons or brine; and adsorption of gas within micropores. These complex gas storage mechanisms are exacerbated further, due to the fact actual gas content is a function of geological factors which affect the retention of adsorbed phase gas within the reservoir. Thus, accurate in-situ gas content often cannot be calculated solely from knowledge of physical rock properties but instead must be directly measured from freshly cut rock samples. In addition to the complexities surrounding unconventional gas storage mechanisms and gas content determination, there are unique and difficult challenges when trying to ascertain deliverability and the design of stimulation and production strategies.

Critical data requirements, methodology, and analytical techniques used to assess unconventional reservoirs will be discussed. Additionally, recent advances in unconventional reservoir assessment technology and remaining exploration, production, and development challenges will be presented.