ABSTRACT:
A semi-analytical hydraulic fracture model for injection of a compressible substance (biosolids) is developed. The approach is a modification of the Perkins-Kern-Nordgren (PKN) hydraulic fracture model modified with a pseudo-dynamic leak-off function that describes the deposition (filtration) and plugging effect of biosolids on the fracture wall in a porous medium. The pseudo-dynamic leak-off function is given in terms of the net pressure and the resistance of the filter cake to flow. In contrast to the conventional PKN leak-off model, the simulation results show that the induced fracture volume is much larger because of the impaired leak-off and because of the volumetric effects and consolidation of the biosolids in the fracture. Solids contents and biosolids compaction behavior have significant impacts on the geometry of fracture (width, length, volume). The model was developed to help guide large-scale injection of municipal and animal biosolids as an environmentally more secure method of treatment than surface approaches.
1 INTRODUCTION
Since about 1988, non-hazardous solid waste materials such as drill cuttings, drilling mud, produced sand, saltwater, emulsions and precipitates have been successfully re-injected into the deep subsurface using oilfield hydraulic fracturing technology (Veil & Dusseault 2003, Bruno et al. 1995, Dusseault et al. 1994, Dusseault 1995). With success in oilfield applications, deep underground injection is proposed as an alternative technology for treatment of degradable biosolids, particularly sludges from animal husbandry and wastewater treatment. Slurry consisting of 5-20% by volume of biosolids and other fine-grained solid wastes can be injected down-hole above the formation hydraulic fracture pressure into appropriate sandstone formations. Injection invariably attracts concern about possible negative impacts; however, experience with millions of tonnes of non-hazardous oil field wastes in many geological environments above the fracture pressure give confidence that it can be used safely for biosolids disposal (Tsang & John 2005, Reed et al. 2001).
