For about 25 years the oil industry has disposed of waste into the deep underground (drill cuttings, mud chemicals, sludge, produced water and sand, crude-contaminated soils, etc.). Small-scale annular injection projects are typical for disposing of the waste produced in single drilling campaigns, but large-scale projects are also ongoing using dedicated injection sites, where oil companies are able to dispose up to hundreds of thousands of tonnes of waste every year.
Most commonly, the technique consists of preparing a slurry comprising waste liquid and finely divided solid waste grains, and injecting it through wells into the deep underground. To ensure injectivity, the operations take place at pressures sufficient to induce hydraulic fractures in the target rock. Once injection ceases, pressures decline, fractures close, solid waste particles remain entrapped by stress, and the excess liquid dissipates through porous media flow.
There are two critical feasibility factors. The first is geological suitability: injection should take place in tectonically passive regions into sedimentary strata of high porosity and permeability. The presence of impermeable strata above the injection zone will protect shallow groundwater from contamination with the injected fluids. The second factor is that the injection process is executed with advanced monitoring techniques and periodic well assessment to provide accurate and updated operational control while establishing best operating parameters, minimizing environmental risk, and complying with regulatory needs. This paper discusses these aspects along with suggested guidelines in the context of several field case histories.
If such critical factors are resolved satisfactorily, wastes become isolated permanently with minimum risk to potable water sources, surface watercourses, or the sea. Furthermore, organic waste injection (e.g. human and animal biosolids, organic refuse) will also reduce CO2 emissions, and most of the carbon remains deep underground in solid form, the most secure approach to sequestration.
Deep waste injection is seldom used outside the oil industry; however, there are strong economic and environmental incentives for application to other waste streams. The most desirable stratigraphic conditions are typical of hydrocarbon-containing sedimentary basins where impermeable layers overlying porous and permeable strata have kept hydrocarbons in place for millions of years. Thus, primary candidate sites are likely to be old depleted reservoirs. Also, in a hydrocarbon province, stratigraphic data, operational experience, and technological expertise are available for its implementation.
This paper focuses on deep waste injection potential in new geographical areas, analyzing it as a disposal technique for different municipal and industrial waste streams. Although many of the technical and social issues are valid worldwide, the situation in Europe deserves particular attention:
• The environmental pressure on most European countries is large and increasing because of many densely-populated and heavily industrialized areas;
• Public awareness of the need to protect the environment has constantly been growing, driving the European Union toward approval of stricter regulations;
• Many geographical regions corresponding to the major sedimentary basins and flatlands in Europe are likely to be suitable;
• It is cost-effective compared to current disposal techniques used in Europe for most municipal and industrial waste streams;
• Despite no specific country-scale regulations to date, European Union law already includes deep underground injection as a permitted waste disposal method.