Evaluation of Pulsed Neutron Capture Results to Characterize Reservoir Fluids in Niagaran Pinnacle Reefs for MRCSP Large-Scale Test

Abstract

The Midwest Regional Carbon Sequestration Partnership (MRCSP¹) large-scale test involves assessment of CO₂ EOR and Storage in depleted oil fields in Michigan's Niagaran Pinnacle Reefs. A comprehensive field data collection effort is underway to support the characterization, monitoring, and modeling objectives of the program. The individual reefs/oil fields are in various stages of production from pre-EOR, to active EOR, to late-stage EOR. Much of the work has focused on a late-stage, depleted oil field known as Dover 33, which had undergone several years of EOR prior to the start of MRCSP injection in 2013. Opportunistic monitoring also is being conducted in several other active EOR fields. One of the monitoring technologies being tested is the Pulse Neutron Capture (PNC) wireline logging to help evaluate spatial and temporal distribution of various fluids in the wells. PNC logging is a wireline tool where the capture cross sectional area (sigma), a measurement of an element's ability to capture neutrons, is recorded in the near wellbore environment. Brine has a high sigma value, whereas natural gas and CO₂ have a low value. By repeating the measurement over time, the changes in the pore space fluid can be analyzed. Such logs have been utilized effectively for logging relative saturations of CO₂ and brine for CO₂ storage in saline reservoirs. However, their use for EOR sites that often have long well histories, variable pressure regimes, and complex fluid compositions (brine, oil, natural gas, CO₂ mixtures) is still under investigation. The MRCSP program offers a unique opportunity to test and validate PNC logging tools under a variety of field conditions and well configurations. In Dover 33, baseline PNC logs were collected in three wells: a vertical injection well, a high angle monitoring well and a lateral monitoring well. A year later, after approximately 168,000 tonnes of CO₂ were injected, the PNC logs were repeated in the two monitoring wells. PNC logs were run on three additional wells in other reefs. These include a baseline and repeat log in the Chester 2 reef (early stage EOR) and two initial logs in the Chester 5 (early stage EOR) and Charlton 30-31 reef (mid stage EOR). In this paper, we present results of these data sets and the resulting pore space fluid distribution. We also examine how the resulting curves relate to lithology, effective porosity, bulk mineralogy, injection pressure and fluid content. In addition, we present a work flow and data necessary to fully identify the reservoir fluid constituents. We also present the challenges in PNC log interpretation associated with combined pressure and fluid composition changes. ¹Funded by DOE-NETL under DE-FC26-0NT42589.

AAPG Datapages/Search and Discovery Article #90195 © 2014 Eastern Section Meeting, London, Ontario, Canada, September 27-30, 2014