One of the first sources for formation evaluation while drilling a well is the gas data provided by the mud logging services, that is used increasingly as preliminary real-time reservoir interpretation to identify gas-oil or oil-water contacts, reservoir entry points, lithological changes and other applications.
Gas ratio analysis and interpretation is a very valuable formation evaluation tool for geologists and petrophysicists to characterize the hydrocarbon fluid types and rock properties. Gas ratios when used in combination with wireline or logging while drilling tools can help resolve uncertainties that otherwise could only be resolved by testing a well. Jurassic deeper wells (>11000 ft) with a varying pressure regimes, very often the wireline logging or Logging while drilling (LWD) logs are cancelled due to several drilling related complications especially mud gain/loss situations. In these wells, Gas while drilling becomes a very unique and important tool that can provide significant insight into the reservoir properties. The gases liberated from the formation at the surface helps us determine the composition of the reservoir fluids and also provides information about the lithological changes, water saturation, and the mobility of the hydrocarbons contained into the rock. This broad spectrum of hydrocarbons assisted by the surface gas analyzer allows the separation of producible mobile heavy hydrocarbons from water wet zones. This ability to differentiate allows operators to geosteer a horizontal well in thin units of potential reservoirs by avoiding water bearing zones and staying in producible hydrocarbon zones. The end result is the ability to intersect the maximum amount of net pay in the target zones.
This paper highlights the case studies from deeper reservoirs of East Kuwait where a surface gas logging system employing a semi-permeable membrane extractor coupled with an advanced GC tracer chromatograph detector has been applied in some of wells drilled in Greater Burgan Magwa Marrat limestone reservoir. Advanced gas logging system was deployed in two vertical/deviated wells to characterize the gas ratios for the different units/zones of Marrat reservoir. The zonation from Gas ratio interpretation was validated with the available wireline logging data before using it for planning the horizontal well. The GWD and LWD were performed independently of each other to determine the effectiveness and reliability of the GWD method. GWD in conjunction with LWD was used for the first time in Kuwait to place a horizontal well in a thin (15-20 ft) layer of a deep Marrat reservoir. Use of realtime Gas ratio data and analysis, successfully helped to complete the well as planned high producer.