Lecture 8: Soil Fracturing

Transcription

1 ENGI 7718 Environmental Geotechniques ENGI 9621 Soil Remediation Engineering Lecture 8: Soil Fracturing Spring 2011 Faculty of Engineering & Applied Science 1

2 8.1 Introduction (1) Fracturing Fracturing creating fractures in dense soils and making existing fractures larger to enhance the mass transfer of contaminants The fractures increase the effective permeability and change paths of fluid flow, thus making in situ remediation more effective and economical Fracturing also reduces the number of extraction wells required, trimming labor and material costs Two types of fracturing Pneumatic fracturing + Hydraulic fracturing 2

3 (2) Pneumatic fracturing injects highly pressurized air or other gas into consolidated, contaminated sediments to extend existing fractures and to create a secondary network of fissures and channels accelerates the removal of contaminants by soil vapor extraction, bioventing, and enhanced in situ biodegradation (3) Hydraulic fracturing involves injecting a fluid, usually water, at modest rates and high pressures into the soil matrix to be fractured a slurry mixture of sand and biodegradable gel is then pumped at high pressure to create a distinct fracture as the gel degrades, it leaves a highly permeable sand-lined fracture with the sand acting as a propping agent preventing the fracture from collapsing 3

4 Two types of soil fracturing Source: Sharma and Reddy,

5 8.2 Applicability Fracturing is most appropriately applied to soils where the natural permeability is insufficient to allow adequate movement of fluids to achieve the remediation objectives in the desired time frame. silty clay/clayey silt sandy silt/silty sand clayey sand sandstone siltstone limestone shale 5

6 Fracturing techniques are equally applicable to both vadose zone (unsaturated) soils and saturated zone soils to improve the flow of air and water, respectively fracture formation in the range of from 20 to 35 ft or more is possible for near-surface soils Fracturing, by itself, is not a remediation technique has to be combined with other technologies to facilitate the reduction of contaminant mass and concentration e.g. in situ biodegradation (by enhancing the delivery of oxygen and nutrients into inaccessible locations) in situ air sparging (by creating fractured pathways to collect the injected air laden with contaminants) 6

7 8.3 Description of the process The selection between hydraulic (water-based) and pneumatic (air-based) fracturing are based on the following considerations: soil structure and stress fields the need to deliver solid compounds into the fractures target depth desired areal influence contractor availability acceptability of fluid injection by regulatory agencies 7

8 Hydraulic Soil Fracturing Effective in soils and rock Long term permeability enhancement Specialized equipment and fluid chemistry expertise required Low leak-off prevents spreading of subsurface contaminants Pneumatic Soil Fracturing Primarily effective in rock Short term permeability enhancement in unconsolidated sediments Less equipment and expertise required Injected air can potentially spread soil vapour phase contaminants Fracture clogging by fines is minimized Fractures are unsupported; because frac sand is designed to act as a migration of fines quickly clogs geotechnical filter while maintaining enhanced fractures permeability Greater range of adaptability with remediation technologies (e.g. SVE, Bioremediation) Not readily adaptable to many remediation technologies 8

9 (1) Hydraulic fracturing injecting a fluid into a borehole at a constant rate until the pressure exceeds a critical value and a fracture is nucleated The most widely used fracture fluid for environmental application the continuous mix grade of guar gum The injection pressure required to create hydraulic fractures is remarkably modest (less than 100 psi) Injection pressure as a function of time during hydraulic fracturing Source: Suthersan,

10 Method for creating hydraulic fractures in soil Source: Suthersan,

11 Source: Slack, 1998 Hydraulic Fractures 11

12 (2) Pneumatic fracturing advancing a borehole to the desired depth of exploration and withdrawing the auger positioning the injector at the desired fracture elevation sealing off a discrete 1 or 2 ft interval by inflating the flexible packers on the injector with nitrogen gas applying pressurized air for approximately 30 s repositioning the injector to the next elevation and repeating the procedure a typical fracture cycle approximately 15 min a production rate with one rig 15 to 20 fractures per day 12

13 Schematic of pneumatic fracturing process Source: Suthersan, 1997 Injection rates of up to 1000 scfm sufficient to create satisfactory fracture networks in low permeability formations 13

14 8.4 Limitations The technology should not be used in areas of high seismic activity Fractures will close in non-clayey soils Investigation of possible underground utilities, structures, or trapped free product is required The potential exists to open new pathways for the unwanted spread of contaminants (e.g., dense nonaqueous phase liquids) 8.5 Cost Pneumatic fracturing $8 to $12 per ton Hydaulic fracturing 160 to $180 per ton for remediation in a 1-year treatment and $100 to $120 per ton in a 3-year remediation 14