Traditionally, in-situ closure stress on proppant is calculated from minimum horizontal stress minus flowing bottomhole pressure (FBHP, or estimated FBHP). This scenario yields maximum closure stress occurred in the fracture next to the wellbore, and the least closure stresses on proppant are at the tips of the fracture. During fracturing-treatment-design phase, proppant is then selected based on its tested and published strength to withstand this calculated maximum closure stress, hoping that proppant does not get crushed severely or completely.
Results of post-fracture analyses show that most fractures have much lower fracture conductivities and much shorter fracture half-lengths than that of the design values. (Analysis methods and short post-fracture producing time prior to a welltest can also contribute to calculation of lower fracture conductivity and shorter fracture half-length.) There are many factors, such as fracturing fluid's gel residue, formation fines dislodged during fracturing process and fines from proppants being crushed which migrated and plugged part of the fracture during production, actual fracture height being greater than the simulated fracture height, fracture propagated out of pay-zone and not all of proppants stayed in the pay-zone, actual leakoff being higher than the value used for the simulation, etc., that could affect the effective fracture conductivity and fracture half-length.
However, one of the most important factors that may have been overlooked, prior to selecting the proppant for the fracture treatment, is how the in-situ closure stress on proppant has been calculated, i.e., the equation itself.
This paper shows insight of why traditional calculation of in-situ closure stress on proppant may be erroneous, and also shows insight of new value of in-situ closure stress on proppant that should be used for fracture design in the future.