Measuring Milestone's Carbon Impact


During 2020, Milestone commissioned a collaborative analysis (“the Analysis”) with SCS to develop calculations for estimating the gross amount of carbon sequestered by and the associated GHG emissions impact of our slurry injection process. For the Analysis, SCS reviewed the laboratory analysis reports of samples collected from our slurry injection waste stream and estimated the TPH content of typical barrels of slurry waste and water (as defined below) injected at our slurry facilities. Through a sequence of calculations, SCS derived the total CO2e per injected barrel and the gross and net emissions from medium and heavy hydrocarbon chain NMVOCs under two scenarios: slurry injection and land farming. We utilized the findings from the Analysis to estimate our total carbon sequestered through our slurry injection operations compared to the traditional industry method of land application. We present these estimates in Milestone’s Negative Carbon Footprint section of this report.

Sampling and Lab Testing

To promote homogeneity and quality control, samples were collected directly from the injection stream prior to subsurface injection. Up to three samples were collected from each Milestone slurry facility and submitted to a third-party environmental laboratory and analyzed for TPH via Texas Method 1005 (“TX1005”). This method examines the speciation of TPH into light (C6-C12), medium (C12-C28), and heavy (C28-C35) hydrocarbon chains.

Measuring TPH Carbon Content

From the lab analysis, SCS derived estimates of the carbon content (percent carbon by mass) and GHG equivalent (in CO2e) of NMVOCs in Milestone’s typical barrels of slurry waste and waters injected at our slurry facilities. First, SCS categorized the samples as “slurries” (i.e., drilling mud and other slurries) or “water” (i.e., produced saltwater or flowback). SCS calculated the average measures of the carbon content (percent of total sample mass) and light hydrocarbon chain mix (percent of total carbon content) by sample category. SCS estimates that our average blended injection stream is approximately 6% carbon (“C”) by mass in the form of petroleum hydrocarbons, implying 39 kg CO2e/bbl; 31% of this carbon content consists of light hydrocarbon chains.1,2

Emissions and Sequestration: Scenario Analysis

SCS then derived the emissions and sequestration effects of two scenarios, which illustrate the differences between handling slurry via traditional land application disposal and our state-of-the art slurry injection processes. For purposes of the analysis, SCS assumed 5% of solids volume is removed as non-injectable and sent to a landfill; the remaining 95% is sequestered via slurry injection.3

The various hydrocarbon chains of TPH react to the processes within these two scenarios differently. Light hydrocarbon chains volatilize into CO2 and non-CO2 GHG emissions after limited atmospheric exposure (i.e., weathering), while medium and heavy hydrocarbon chains degrade through aerobic anaerobic processes over time. SCS cited a recent academic article that estimates 70% of the remaining TPH (medium and heavy hydrocarbon chains) could reduce and become CO2 within approximately one year of landfarming activity.4

Key Findings

Based on the Analysis, SCS estimates that one barrel of slurry waste disposed of through land application generates approximately 75 kg CO2e of gross emissions in the first year or less from light-hydrocarbon chain volatilization and aerobic/anaerobic degradation of the remaining TPH. Under the slurry injection scenario, the same barrel of slurry waste would generate no further emissions once in the injection pipeline, resulting in sequestration of all 95 kg CO2e/bbl contained in each barrel of slurry waste on average.5

1 Based on (i) the carbon content and light carbon mix factors calculated by SCS, (ii) volume-to-mass conversion factors by waste stream per Milestone and AquaCalc, and (iii) an average mud mix of 40% during the period of the study.

2 Carbon mass converted to CO2e using a standard relative molecular weight ratio of 3.67 C/CO2e.

3 Both scenarios assume all water (“liquids”) volume is injected into Class II UIC wells.

4 SCS cited the recent academic article by Guarino et al. “Assessment of three approaches of bioremediation (Natural Attenuation, Landfarming and Bioaugmentation - Assisted Landfarming) for a petroleum hydrocarbons contaminated soil.” Chemosphere vol. 170 (2017): 10-16. doi:10.1016/j.chemosphere.2016.11.165, which estimates that 70% of the remaining TPH (medium and heavy chains) becomes CO2 through bioremediation in the first year.

5SCS estimates a total equivalent carbon content for solids of 25.8 kg C/bbl (95 kg CO2e/bbl).