Unconventional natural gas produced from tight gas sands could significantly contribute to known gas reserves. Difficulties in reservoir characterization and estimation of production potential, however, has adversely influenced the economics of developing this resource. Conventional log analysis fails to adequately characterize tight gas sands.

The Gas Research Institute has funded a study to better understand the limitations of conventional log analysis when applied to tight gas sands. Results from the investigation clearly indicate a number of geologically dependent interpretation problems related to the complex depositional, diagenetic, and structural histories of tight gas sands. Porosity interpretation is complicated by variable matrix parameters and by incomplete fluid invasion into the formation. Complex lithologies preclude the use of a static grain density which is commonly assumed in the standard density-porosity equation. The low permeability of tight gas sands creates a variable invasion profile which has a dramatic effect on density and neutron tool response and on the interpretation of porosity and water saturation. Most equations and models assume filtrate invasion to at least the depth of investigation of the porosity tools; however, this may not always be true in tight gas sands.

Volume clay determination in tight gas sands is difficult. Conventional density/neutron techniques are limited because of the variable invasion profile. The gamma ray technique is highly dependent on depth of burial (clay type) and is not always reliable. Clays adversely affect porosity interpretation and resistivity tool response.

Formation water resistivities (Rw) are not always a constant parameter and may be highly variable. Rw is difficult to calculate because apparent water resistivity techniques are ineffective due to the absence of gas-water contacts in very tight gas sand reservoirs. Spontaneous potential (SP) interpretation techniques are ineffective in tight gas sands due to variable invasion profiles and the related failure of the electrochemical component of the SP to be developed to its full potential. Produced water samples can be contaminated by drilling fluid and frac fluid.

Water saturation (Sw) is difficult to determine in tight gas sands. Porosity, Rw, and clay corrections are critical parameters controlling accurate Sw calculations. Equations relating irreducible water saturation and porosity to permeability are not effective.

Cement quality, communication, stimulation effectiveness, matrix vs. natural fracture permeabilities, and commingled production can confuse the relationship between production and log interpretation.

All of these problems complicate log interpretation for tight gas sands. Complex lithologies, variable water resistivity, and incomplete fluid invasion are the primary factors which render most conventional log analysis techniques ineffective. This paper serves as a guide to identify critical parameters so that tight gas sand logging research may be more effectively directed.

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