In September of 2004 hurricane Ivan, a category 3 storm at the time, passed through the northern Gulf of Mexico. Severe wave action attributed to the storm triggered a subsea mudslide that toppled Taylor Energy Company’s (TEC) oil Saratoga production platform A at Mississippi Canyon block 20 (MC20). The superstructure, also known as the jacket, came to rest on the ocean floor approximately 210 m southeast of the original location. The collector bundle containing the original 28 well pipes was also dragged in the direction of the collapsed jacket, breaking off and being buried by deposited sediment at the northwest corner of the final resting place of the structure.
The MC20 site has since been associated with persistent plumes of oil and gas and surface oil slicks. These slicks are visible on the ocean surface from ships and by aerial and satellite remote sensing and have been used as a means of measuring the output of hydrocarbons from the site. However, it was determined that these estimates needed to be compared to collections and measurements from within the water column along with sediment collection for chemical analysis. Additionally, because vigorous and persistent gas plumes have been observed at the site, the flux of hydrocarbon gas at the surface and into the atmosphere was accessed. In response to a request by the Bureau of Safety and Environmental Enforcement (BSEE), the National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Coastal Ocean Science (NCCOS), in cooperation with NOAA’s Office of Response and Restoration (ORR) conducted a survey at the MC20 site.
NOAA’s NCCOS along with its partners at Florida State University (FSU), Florida International University (FIU), and TDI Brooks International, and with funding from BSEE conducted a series of integrated surveys from 1 to 7 September, 2018 at the former Taylor Energy MC20 site in the northern Gulf of Mexico. These surveys included surface and subsurface acoustic measurements, mid-water column oil, gas, and water collections, surface water sample collections, mid-water column video bubble collections, surface methane collections, and marine sediment collections. Ancillary physical water column data was also collected including ocean current profiles and conductivity, temperature, and depth.
Acoustic measurements of plumes under both relatively high and low current conditions, combined with oil droplet and gas bubble densities determined from deployment of a new device called the “bubblometer”, allowed for the calculation of flux rate estimates for oil and gas entering the water column at the northwest corner of the jacket. This acoustic flux rate is estimated by taking the measured backscatter volume of the combined oil and gas plumes and, using the measured rise rate of the plume from an ocean floor mounted Acoustic Doppler Current Profiler (ADCP) along with the measured American Petroleum Institute (API) gravity of the oil from laboratory chemistry analysis, calculating a range of modeled oil release required to account for the plume volumes observed. The range represents assumptions based on the ratio of oil to gas actually present within the plumes. We are confident in these range values based on measurements provided by the video bubble chamber and that methodology’s statistical characterization of the oil to gas ratio and bubble size distributions.
Video bubble chamber, or bubblometer, measurements from this survey also provide a related, but independent flux rate estimate from those calculated by acoustic measurements. The bubblometer chamber consists of a rectangular frame of known dimensions. One face of the chamber is open and mounted with lights and a high-definition camera. Video captured during this survey yielded approximately 800 individual sample images of oil and gas located in subsurface plumes and represents combinations of predominantly oil, predominantly gas, and varying combinations of the two. Gas and oil bubble volumes are estimated. Combining the volume estimates for each individual video “still” image sample with its three dimensional location within the plume measured by Ultra Short Base-Line (USBL) beacon locator along with the estimated plume size and density from acoustic backscatter and estimated oil rise rate from ADCP measurements allows for calculation of oil and gas flux.
The estimated oil flux rate calculated from the acoustic survey method is approximately 9 to 47 barrels of oil (bbl) per day. The estimated oil flux rate calculated from the bubblometer survey method is approximately 19 to 108 bbl/day. It is important to note that these are both estimates and while we are confident in the methods developed and calculations employed, these two ranges do not necessarily represent a final definitive government estimate of the flux of oil and gas being released at the MC20 site.
Our overall conclusion for the MC20 site is that there is ongoing release of reservoir oil and gas that enters the marine environment at the ocean floor and migrates up through the water column in a series of discrete and dense plumes. These plumes can separate into individual components or homogenize depending on the prevailing current conditions at the time the oil and gas enter the water column. Under relatively high current conditions, such as those observed on 2 September 2018, a portion of the predominantly oil plumes can become entrained by the currents and surface separately hundreds of meters away from the predominantly gas plumes. Certain portions of the oil partition out into the water column, either as microscopic droplets or through dissolution, or precipitate out onto the surrounding sediments where degradation continues. The oil that reaches the surface forms a surface sheen that continues to degrade through photolysis, evaporation, and further dissolution.
The detailed results of these combined surveys at the MC20 site are contained in this report which is organized into 8 chapters documenting in detail each correlated scientific focus area. Specifically, Chapter 1 presents background information about the study area and an overview of the project as a whole. Chapter 2 presents the physical water column characteristics measured at the site. Chapter 3 details the acoustic findings which resulted in multiple 3-dimensional models of the plumes under varying current profiles. Chapter 4 describes the physical characteristics of the bubbles of oil and gas observed using a unique tool called the bubblometer. Chapter 5 contains results for the surface methane measurements including spatial expression of measured values and an estimated methane flux rate. Chapter 6 presents the gas chemistry results from sediment and mid-water column captured gas samples. Chapter 7 documents the analytical chemistry results from oil, water, and sediment samples that were collected during the survey. And Chapter 8 presents calculated results from two different methods for measuring oil and gas flux estimates at the MC20 site.