Mechanical and Hydraulic Aspects of Rock-Proppant Systems : Laboratory Experiments and Modelling Approaches

Reservoir formation damage is an issue that can affect the productivity of a reservoir during various phases of fluid recovery from the subsurface, including drilling, production, and workover operations. Hydraulic fracturing technology is one tool to overcome inflow impairments due to formation damage, as well as increase the productivity of reservoirs. To this end, fluid is pumped into the reservoir and a fracture propagates as long as the pressure at the fracture tip is sufficiently high. In general, the availability of the newly created fracture area is limited for production, if no proppants (sieved sand or ceramic spheres) are placed in the fracture to keep it open. Indeed, hydraulic fracturing operations can themselves be a source of reservoir formation damage. Permeability alterations adjacent to the newly created fracture face can impair the inflow into the fracture. Such impairment is commonly referred to as fracture face skin (FFS). In context of this work, a new damage mechanism is proposed: the mechanically induced fracture face skin. This new mechanism results from mechanical interactions between the proppants and the reservoir rock, due to the increasing stress on the rock-proppant system during production. Proppant embedment into the fracture face and proppant crushing leads to fines production and can impair the fracture performance. In order to achieve sustainable, longterm productivity from a reservoir, it is indispensable to understand the hydraulic and mechanical interactions in the rock-proppant system. In order to analyse the hydraulic and mechanical interactions, laboratory experiments using three different flow cells were conducted. These flow cells were used to localise and quantify the mechanical damage at the fracture face, as well as to investigate the long-term stability of a rock-proppant system under in-situ reservoir conditions. The laboratory experiments identified a permeability reduction at the fracture face up to 90 %. The mechanical damage at the rock-proppant interface begins immediately with loading the rock-proppant system and for fracture closure stresses below 35 MPa, the damage is localised at the fracture face. Optical investigations identified quartz grain crushing, fines production and pore blocking at the fracture face, which are responsible for the observed mechanically induced FFS. For higher stresses, the damage moves into the proppant pack and can considerably affect the proppant pack permeability. Numerical and analytical modelling of the rock-proppant system identified highly inhomogeneous stress distributions in the granular system of quartz grains and proppants. High tensile stress concentrations beneath the area of contact between quartz grains and proppants are observed even at small external stress applied to the rock-proppant system. These high stress concentrations are responsible for the early onset of damage at the fracture face and illustrate the types of proppant failure. The fracture patterns observed in the rock-proppant system were successfully matched with the stress pattern from the numerical and analytical models. The modelling approaches highlight mitigation strategies to attenuate proppant embedment effects, to reduce fines production and to avoid a mechanically induced impairment of fracture performance. In particular, a coat of resin that covers the proppant will help to distribute the stresses at the contacts and suppress the high tensile stress concentrations. The long-term stability of rock-proppant systems was investigated by means of long-term experiments up to 40 days at in-situ conditions. The experiments highlight that the permeability of rock-proppant systems is affected by electro-chemical and thermal interactions, as well as by mechanical effects. After a sufficient duration of experiment, the permeability stabilises at a constant level, which is minor influenced by the mechanically induced FFS. In summary, the mechanically induced FFS exists and has an influence on the fracture inflow performance whereas the long-term productivity of a reservoir is not impaired Scientific Technical Report STR 11/09 DOI: 10.2312/GFZ.b103-11098 Deutsches GeoForschungsZentrum GFZ