As natural gas resource development grows worldwide, operators need an improved method to effectively evaluate formation gas saturation through casing.

Over the years, the most versatile device for identifying and quantifying hydrocarbon saturation behind casing has been the pulsed neutron instrument. Historically, two types of measurements could be made from this tool. The first was the pulsed neutron capture (PNC) technique. This measurement offered relatively fast logging speeds and robust statistical precision in most environments and yielded a measurement, called Sigma (), of how fast neutrons were absorbed by the formation. In most reservoir rocks, this absorption is dominated by the amount of chlorine in the reservoir, with small effects from the rock matrix, shales and hydrocarbons. Since chlorine is typically found downhole in saline formation waters, variations in the measured value could be converted into a water saturation (Sw) measurement in cases of known, constant and sufficiently high formation water salinity. In low or unknown salinities, a second measurement, the carbon/oxygen log (C/O), could be used. This measurement looks at variations in the total amount of carbon and oxygen in the formation. Once changes in porosity and mineralogy are accounted for, changes in this C/O ratio indicate varying content of oil (carbon and hydrogen) vs. water (oxygen and hydrogen). This evaluation method allows measurements of Sw without prior knowledge of formation water salinity but may require extended logging periods to obtain reliable saturation answers.
A limitation of both of these measurements is that they required prior knowledge of the type of hydrocarbons being evaluated. In reservoirs containing only gas and saline water phases, the PNC technique can yield excellent answers. If the reservoir contains oil, water and gas in combination, PNC techniques are unable to quantitatively separate oil and gas saturations. Overlay techniques have been used to qualitatively identify gas under suitable conditions, but they are not capable of giving quantitative saturation estimates. Additionally, the high shale content found in many of today's tight gas plays gives a similar response to salt water, potentially masking the hydrocarbons.
The presence of gas dramatically reduces the total amount of carbon in the formation, limiting the dynamic range of the C/O measurement and therefore its accuracy. The C/O technique is also restricted to analysis of Phase 2 (water and total hydrocarbon) reservoirs.
Looking for gas in tough places
During 2005, Baker Atlas began offering its GasView service, a pulsed neutron gas analysis product, in selected areas around the world. Based on the upgraded Reservoir Production Monitoring (RPM) instrument, this service offers high-resolution formation gas saturation measurements through casing and is equally capable of being used in production monitoring applications as well as new well evaluations. This analysis technique is independent of the PNC or C/O saturation measurements, allowing Phase 3 analysis of reservoirs. The measurement is independent of formation water salinity, allowing accurate gas saturation measurements in a broad variety of reservoir conditions.
The GasView product combines three advancements to produce a quantitative gas saturation solution: improved instrumentation, a new characterization and advanced interpretive techniques.
• The service utilizes new instrumentation in the three-detector RPM-C tool to improve resolution by allowing gas detection ratios to be taken over longer spacing than conventional pulsed neutron instruments. These new measurements examine greater volumes, making them substantially more sensitive to gas-filled porosity, allowing the instrument to identify gas where conventional instruments cannot and to provide better resolution in gas saturation calculations.
• Characterization of the gas response of the measured curves allows more accurate quantification of gas saturation. A global gas response database has been constructed for standard openhole and cased hole completions; it quantifies the expected response of the gas measurements for varying combinations of mineralogy, fluid density, and gas pressure and density. More complicated reservoir and borehole conditions can also be individually characterized. This information is captured in the software's job planner, a tool that allows predicted forward modeling of the logging results and assists in optimizing logging and interpretation.
• Dynamic Gas Envelope (DGE) interpretation adds clarity by explicitly accounting for variations in porosity and mineralogy in the saturation result. While these effects must be removed from the saturation solution, they are included in the visual DGE representation for greater insight into the interpretive processing. The resulting picture of tool response to the formation and its contents - porosity and mineralogy, as well as gas - clearly shows the separate effects of these varables.
Applications
Over the last year, the GasView service has been used for a number of diverse applications.
One of the first applications developed for the service was the monitoring of injected gas into Middle Eastern carbonate reservoirs (Figure 1). Movement of the gas curve from the fluid line towards the gas line indicates increasing gas saturation. Computed saturations are shown in the left-hand track, while a display of total formation volume is shown on the right. Gas "flags" (red blocks) can also be set to delineate portions of the reservoir which meet specific porosity, mineralogy and saturation conditions specified by the operator, allowing quick assessment of the log interval.
Other applications of the service for reservoir monitoring include evaluation of gas cap expansion in Phase 3 reservoirs and the monitoring of multiple fronts in water alternating gas (WAG) injection programs. Recently, several North Sea operators have used the service to successfully identify unproduced condensate reserves in gas/condensate wells.
In North America, many of the applications of the GasView service involve evaluation of new wells. In these cases, the operator is typically looking for a cost-effective alternative to openhole logging. Using cased hole techniques for formation evaluation reduces the total time to drill and case the well, potentially saving significant operational expenses. In these applications, the service has proven its capability to accurately identify productive sands (Figure 2). In this analysis that shows three sections of a recently evaluated Wyoming well, expected gas response was forward modeled both for original gas pressure (black curve) as well as depleted gas pressure (red curve). This dual-pressure analysis technique has been successful in identifying reservoir intervals that contain gas but which are at unproductively low pressures due to offset production. In sand A of this example, the composite log curve (green) follows the modeled gas response expected from a reservoir at its original pressure, indicating a productive zone. In sand B, the measured gas response is significantly greater than what would be expected in virgin formation pressures and is in good agreement with the modeled response for a fully depleted zone. Such a zone, if completed, would likely produce little gas. Finally, the bottom sand C shows a productive sand in close proximity to a water interval. Such an interval, if produced, may produce excessive water, thus reducing the overall productivity and economic feasibility of the well.
With the launching of this new service, operators have a better method of identifying and measuring gas saturation behind casing. This advanced evaluation provides greater insight to the reservoir than previous methods and permits operators to more accurately define remaining reserves and build business cases. Its salinity independence enables a confident analysis that will be welcomed as the service is introduced to other parts of the world.

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