Our goal in this paper is to quantify the impact of proppant settling in the wellbore on the efficiency and distribution of proppant into perforation clusters. This phenomenon has been experimentally studied on the lab scale in the past but no field-scale simulations have been performed to quantify the amount of proppant settled in the wellbore and its impact on proppant placement.
Critical velocity correlations are used to estimate the velocity at which proppant settling can occur and incorporates them into a numerical simulator for simulating proppant transport in multiple perforation clusters. The proppant distribution in each perforation cluster and the amount of proppant settling in individual wellbore sections is computed. Comparisons made with cases where no settling is considered clearly show that the total proppant mass entering individual perforation clusters is impacted when proppant settling is considered. A parametric study is then conducted to determine which parameters (number of perforation clusters, flow rate, proppant size etc.) have the largest impact on proppant placement.
Our results show that proppant settling occurs primarily at the toe-side of the stage where the wellbore fluid velocity drops below the critical velocity. Wellbore settling occurs mainly near the last cluster as the fluid leaks off into all the heel-side perforations. Proppant wellbore settling cannot continue indefinitely because as proppant deposits in a wellbore section, the flow area is reduced and this in turn increases the flow velocity to above the resuspension velocity. The effect of proppant settling becomes more pronounced as the number of clusters in the stage increases. It is most pronounced in refracturing scenarios where a large number of perforations are open to flow. The settling progresses towards the upstream clusters as the toe side clusters are plugged and the critical velocity in that region drops below a threshold value. The injection rate, fluid viscosity, proppant diameter, proppant density and fluid density also have a significant impact on the total amount of proppant that settles.
This paper quantifies proppant placement in perforation clusters and proppant settling in wellbore sections during a fracturing stage. It provides a quantitative estimate and an explanation for the combined effects of proppant inertia, perforation bridging and proppant wellbore settling. The results suggest many ways to minimize the impact of proppant settling in the wellbore.