Effects of Gas Flaring on Soil Fertility

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 93666, "Impact of Gas Flaring on Soil Fertility," by R.E. Akpojivi, SPE, and P.E. Akumagba, SPE, Petroleum Training Inst., Effurun, prepared for the 2005 SPE Middle East Oil & Gas Show and Conference, Bahrain, 12-15 March. Gas flaring is used in Nigeria to dispose of associated gas. Low flare stacks bring the flare close to nearby vegetation and soil. This study investigated the effect of oil-sector gas-flaring activity on soil-fertility parameters. Introduction Gas flaring is the burning of natural gas and other petroleum hydrocarbons at flare stacks in oil fields during operation. The casing-head gas produced as associated natural gas during oil production varies in composition from location to location; however, the basic components include methane, ethane, propane, iso-butane, n-butane, iso-pentane, n-pentane, n-hexane, CO2, H2S, He, and N2. Modern technology and commercial opportunities have permitted industrialized nations, which often flared natural gas during the early history of oil production, to now process it for commercial sale or reinjection into the reservoirs. However, several sub-Saharan African nations including Nigeria still use gas flaring as a method to dispose of associated gas during oil-sector operations, ostensibly because gas infrastructure is extremely low. For example, despite efforts to expand the gas-to-liquid industry in Nigeria, companies flared [667 Bscf of natural gas at 176 locations in 2002, according to the U.S. Energy Information Agency, Dept. of Energy]. This gas flaring represents [49%] of the total gas produced in Nigeria in 2002. Flaring of associated gas releases emissions rich in carbon, nitrogen, and sulfur oxides and soot. These acid gases are carried downward as acid deposition (wet and dry depositions) onto vegetation, soil, and water bodies in communities close to the flare sites. Also, intense heat and continuous illumination are associated with gas flaring. The low height of flare stacks ensures local pollution at ground level and nearby dry deposition, as well as close heating of surrounding vegetation and soil. Likely acid deposition and intense heat from gas flares are likely to harm the fertility of the surrounding soils. The obvious signs are the poor vegetation growth and scorched soils around gas-flare locations. Acid deposition decreases the soil pH. However, the effect of acid deposition on a particular soil ecosystem is influenced by such factors as acid sensitivity, neutralization capability, concentration and composition of acid reaction products, and the amount of acid added to the system. The basic cations (Ca2+, Mg2+, and K+) are replaced by hydrogen ions or soluble metals and are lost through leaching. Increased acidity reduces activity of soil microorganisms sensitive to low pH, thus decreasing decomposition of plant residues and the recycling of essential plant nutrients. The concentration of trace-metal ions in soil solution increases (including aluminium, copper, iron, zinc, boron, manganese, chromium, and nickel) to levels that may be phytotoxic. Phosphorus compounds in the soil become mostly aluminum and iron phosphates, resulting in a reduced availability of plant phosphorus. Plant uptake of molybdate is reduced. Nitrification by the main autotrophic genera involved (Nitrosononas and Nitrobacter) is inhibited; thus, NH4+ is the main form of nitrogen taken up by plants instead of NO3−. There is reduced symbiotic nitrogen fixation by legumes, except the Rhizobium strain, which is acid tolerant.