Nuclear Logs Identify Gas-Bearing Sands
The wireline log measurement of formation resistivity has been used to identify oil and gas-bearing reservoirs since 1927. Resistivity and wireline log measurements of other formation properties are presently used in various conventional models and equations for the same purpose. Conventional resistivity-based model parameters and unknowns may include Archie cementation and water saturation exponents, formation water salinity and appropriate porosity and water saturation models (Sw) to calculate the amount of hydrocarbons present. Many of these parameters can be difficult to obtain in Rocky Mountain gas-bearing sandstone reservoirs that have changing and unknown formation water salinity. Which porosity model is valid, does porosity need to be corrected for clay content (Vclay), how is Vclay determined and which of many Sw models should be applied? An analysis of the gas effect on nuclear log measurements does not depend on these conventional resistivity-based models and parameters. It is an approach that can aid in gas-sand identification. This presentation discusses gas-bearing sand identification methods that utilize raw open and cased-hole nuclear log measurements. The techniques identify gas-bearing sand reservoirs when density-neutron gas effect is canceled out by clay effect and pinpoint the top of gas when water-sands immediately overlie a long stack of gas-sands. Fresh water-bearing sand with large mud log gas increases can be identified and not mistaken for pay. Examples from the Uinta, Piceance and Wind River basins are shown and discussed. These techniques allow additional information to be squeezed out of a standard log suite to aid testing and completion decisions. The methods are valid with similar nuclear log measurements in gas sands elsewhere. The methods can even work with poorly calibrated logs.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012