Historically, peak oil production in the United States was at an all-time high in the early 1970's. The decline of oil production after this peak, initiated the interest in carbon dioxide enhanced oil recovery floods (CO₂ EOR) in 1972 to help extend the production life of some fields. With some exceptions, (Turkey, Abu Dhabi, Brazil, China, Malaysia, and the North Sea) the use of CO₂ for EOR has been minimal outside the US. The Permian Basin (PB) alone accounts for over 50% of CO₂ EOR worldwide projects (Melzer, 2012).

The PB encompasses over 300,000 acres of various reservoirs under CO₂ EOR floods. Through 2014, current oil production is in excess of 190,000 BOPD which represents 65% of CO₂ EOR production in the US. Production is projected to increase to 323,000 BOPD by 2020. Constant surveillance of the CO₂ migration during flooding is critical so that the injection pattern can be modified to optimize oil sweeping efficiency.

One common method for CO₂ monitoring is wireline well logging. Open-hole (OH) logs are normally acquired and then interpreted to evaluate the fluid saturation of the three phases^† (oil, water and CO₂). At times, due to operational conditions and efficiencies, OH logs are not available. This limits the capability for a reliable CO₂ monitoring for the surveillance teams. Cased-hole logging (CH) offers an operational advantage as well as a base line log for Time-Lapse monitoring.

Quantitative reservoir evaluation through casing has been accomplished for decades using a combination of neutron logging tools with different neutron sources and a variety of gamma-ray detector materials. The primary measurements of these tools are: Sigma (Σ), Thermal Neutron porosity (TPHI), Carbon-Oxygen ratio (C/O) and Gamma Ray Spectroscopy. A multimineral petrophysical analysis is then performed to provide quantitative volumetric results. One of the main challenges of this interpretation is the ability to differentiate gas filled porosity from very low porosity formations. Both measurements; Σ and TPHI respond similarly in these conditions. Unless porosity is known from other sources (i.e. OH logs), neither of these measurements alone can provide a reliable answer. A different petrophysical measurement would be desirable to allow for this differentiation.

This paper shows the application of a new pulsed neutron wireline service that measures a new formation property, fast neutron cross section (FNXS). This measurement makes possible differentiating gas-filled porosity and low porosity formations. FNXS is sensitive to the formation's atom density which is independent from Hydrogen Index and is sensitive to the gas-filled porosity. Other new features of the new tool is the ability to self-compensate for the influence of the borehole (like for instance gas filled boreholes) and the completion using a combination of near, far and deep detectors and various timing gates. These new measurement advancements allow the log analyst to solve the three phase problem via a multi-mineral solver when OH logs are not available.

This paper shows three CO₂ EOR case studies in the PB where CO₂ has been quantified for identifying good versus poor sweep efficiencies. The analysis shown here, helped the operator to design and optimize their completion strategy as well as having better understanding of the reservoir behavior under CO₂ flood.

^†Under reservoir temperature and pressure, oil and CO₂ may be miscible and in one phase. Saturation analysis is estimating volumetric as if oil and CO₂ were in two separate phases.