Hydraulic fracturing stimulation techniques and formation damage mechanisms—Implications from laboratory testing of tight sandstone–proppant systems

Abstract

Reservoir formation damage may seriously affect the productivity of a reservoir during various phases of fluid recovery from the subsurface. Hydraulic fracturing technology is one tool to overcome inflow impairments due to formation damage and to increase the productivity of reservoirs. However, the increase in productivity by hydraulic fracturing operations can be limited by permeability alterations adjacent to the newly created fracture face. Such an impairment of the inflow to the fracture is commonly referred to as fracture face skin (FFS). Here, we focus on mechanically induced fracture face skin, which may result from stress-induced mechanical interactions between proppants and reservoir rock during production. In order to achieve sustainable, long-term productivity from a reservoir, it is indispensable to understand the hydraulic and mechanical interactions in rock–proppant systems. We performed permeability measurements on tight sandstones with propped fractures under stress using two different flow cells, allowing to localise and quantify the mechanical damage at the fracture face. The laboratory experiments revealed a permeability reduction of this rock–proppant system down to 77% of initial rock permeability at 50 MPa differential stress leading to a permeability reduction in the fracture face skin zone up to a factor of 6. Considerable mechanical damage at the rock–proppant interface was already observed for stresses of about 5 MPa. Microstructure analysis identified quartz grain crushing, fines production, and pore space blocking at the fracture face causing the observed mechanically induced FFS. At higher stresses, damage and embedment of the ceramic proppants further reduces the fracture permeability. Therefore, even low differential stresses, which are expected under in-situ conditions, may considerably affect the productivity of hydraulic proppant fracturing stimulation campaigns, in particular in unconventional reservoirs where the fracture face is considerably larger compared to conventional hydraulic stimulations.