Gas Identifications in Tight Gas Reservoirs for Improved Well and Reservoir Management

Abstract

Identifications of gas in tight clastic reservoirs of Oman considered of high value as this would improve significantly well and reservoir management of a number of tight gas fields by finding more re-perforation opportunities, identifying water shut off opportunities and in-fill opportunities, and by optimizing gas production system. As a result, a study was conducted to see if PNC logs could be used for gas identifications in tight clastic reservoirs of Oman.Application of PNC over the tight gas environments pose two major challenges:1. Consistent environmental corrections of PNC logs in the gas filled boreholes and 2. Identification of water free gas bearing zones from water bearing and gas-water transition zonesThese limited the use of the PNC tools in the tight gas reservoirs in Oman until PNC logs were recently used for well and reservoir management of a number of major tight gas reservoirs in PDO. This paper presents an integrated collaborative approach between operator and service provider that enables such use.Unlike in the high porosity clean shale free lithologies i.e. clean sandstone or carbonate rocks, gas identifications in the tighter clastic rocks pose additional challenge as sigma and neutron porosity are very sensitive to shale content and sensitivity to gas is not as profound as in the higher porosities to offset for the shale effects. It is important to note that identifications of gas in the low porosity tight clastic gas environments rely more on the count rates and neutron porosity than on sigma alone.Deterministic evaluation of saturation from sigma alone may lead to incorrect results in low porosity clastic rocks.Precise borehole corrections were needed to account for the borehole signal in gas filled borehole. This paper describes the workflow employed to evaluate tight gas reservoirs of Oman in detail with application to real field examples and shows the value it added by better well and reservoir management.