Wireline Logs

Well logging technology is highly advanced, owing to development during decades of utilization in oil and gas exploration and production. In recent years, many well logging tools have been applied to subsurface monitoring of CO₂ in fields where CO₂ storage and/or CO₂-enhanced oil recovery (EOR) operations are underway. Well logging consists of lowering instruments attached to an instrumented cable known as a wireline into a wellbore and as the cable is spooled, measurements made at the tool are transmitted to the surface where they are processed to yield data on the physical and chemical properties of a formations and their pore fluids.¹

Metal casing interferes with some wireline measurements, so in conventional operations wireline logging is carried out before casing is installed (i.e., in open hole conditions). Well logging performed after casing is installed is referred to as cased-hole logging.²

A number of standard well logging tools are used to characterize the lithology, mineralogy, porosity, fluid saturation, and structural complexity of formations at CO₂ storage fields prior to injection. Other logging tools are geared toward measuring and quantifying CO₂ in pore fluids during and after injection.³

Wireline Log Summary

Description: Mature technology in which tools lowered into wells on wireline cables (so that the tool is in communication with the surface) are slowly moved up the well collecting data designed to monitor the condition of the wellbore and changes in fluids in the near-wellbore environment. Examples of logs used in geologic storage monitoring include acoustic and resistivity.
Benefits: Commercial technology used to assess the condition of the well casing and cement and changes in near-wellbore fluid or formation composition. Under favorable conditions, log response may be highly sensitive to CO₂ outside the wellbore, and thus, there would be no need to perforate the well to detect CO₂.
Challenges: Area of investigation limited to near the wellbore. Sensitivity of tool to fluid change varies; only under optimum conditions are tools sensitive to dissolved CO₂ or changes in minerology. Working fluids in wells may affect log results. Logging requires wells that penetrate the interval of interest and mobilization costs may be substantive, limiting repeated surveys. If a well is perforated in an area charged with CO₂, the well requires pressure management. Both wireline and well casing may corrode, especially in the presence of CO₂, requiring management via metallurgy or corrosion inhibition.⁴

Application

Pulse neutron capture logs contains a source that emits neutrons into the formation. A detector measures decay times of gamma rays emitted by the capture of these neutrons by reservoir rock and its pore fluids; the data can be processed to estimate fluid saturations; it can also measure gamma rays emitted by inelastic neutron scattering to estimate carbon/ oxygen ratios.⁵

These logs have been used for assessing fluid saturation levels in oil and gas production in the past, however their ability to distinguish between oil, gas, and water in cased oilfield wells was not adequate for CO₂ monitoring. Recent development of new pulsed neutron measurements to provide enhanced sensitivity to CO₂, coupled with advancement in log analysis and modeling are being used to apply this existing technology directly to CO₂ plume monitoring and fluid saturation analysis during CO₂ storage or storage during CO₂-EOR.⁶⁷

Pulsed neutron capture logging has been used as part of monitoring of CO₂ injection and storage during enhanced oil recovery in depleted oil fields in Michigan.⁸⁹

Location carbon storage projects in Michigan where pulsed neutron capture logging has been used for CO₂ monitoring.¹⁰

Use of conventional logging tools in existing wells to evaluate fluid saturations was problematic because those tools perform only in non-cased boreholes. Enhanced oil recovery production involves cased-hole operations. As a result, pulsed neutron capture logging, which is capable of through-casing evaluation, was implemented to monitor the accumulation and migration of CO₂ in the Michigan fields. Baseline and repeat pulsed neutron capture logs recorded changes in the reservoir fluids as part of the monitoring and study effort for these sites. The tool proved a cost-efficient and effective technique for assessing fluids in cased-hole wells and aiding in monitoring the migration of injected fluids in the storage reservoirs.¹¹¹²

Part of a composite log presentation of pulsed neutron data analysis in a CO2-injection well. Computed saturation profiles are plotted against depth in the well during a baseline run in 2012 (track d) and then a repeat run in 2016 (track e) after CO₂ injection. Saturation displayed for water in blue, oil in green, and gas in red. Comparing the pre-injection baseline run on the left track with the post-injection run on the right track, the increase in gas saturation after injection is attributed to an increased presence of CO₂.¹³