Subsurface and Operational Challenges in a Successful Microfrac Operation and Remedial Solutions – Observations from Recent Experiences in Tight Unconventional Reservoirs

Abstract

In this study, we discuss the subsurface and operational challenges and effective remedial solutions for the wireline straddle packer microfrac operations to achieve successful breakdown and conclusive closure pressure, based on our recent experiences from the recently concluded microfrac campaign in the challenging Triassic and Cambrian unconventional reservoirs of Northern Africa. We share our experiences from the deeper clastic and carbonate targets which are tight, extremely strong, and exhibit a very high breakdown pressure gradient of up to 1.16 psi/ft. In case of breakdown pressure elevated by external factors (effect of mud filtrates etc.), the pump-out method facilitated the formation breakdown. The majority of the microfrac tests in these tight formations showed the proven benefit of multiple rapid loading and unloading cycles by injection (hole pressurization) and pump-out (hole depressurization) in inducing near-wellbore fatigue to effectively weaken the formation before achieving successful breakdown. The effect of near-wellbore poroelasticity was also observed, resulting in increasing fracture reopening and closure pressures in the consecutive microfrac test cycles; the pump-out technique was also effective in eliminating such effects. The tight unconventional reservoirs exhibited a low leak-off coefficient resulting in poor natural fall-off pressure decline response resulting in an overestimation of closure gradient. Therefore, controlled flowback by a small volume pump was adopted to achieve induced closure and conclusive Shmin estimates.