Effect of Overdisplacement of Proppant in Hydraulic Fracturing Treatments on the Productivity of Shale Gas Reservoirs

Abstract

Overdisplacement of proppant in hydraulic fracturing treatments is an operational consequence dependent on the type of completion. It is described as the injection of clean fluid to remove residual proppant from the wellbore lateral and/or to transport plugging elements after the pumping of the designed fracturing treatment is finished. This thesis investigates the effect of overdisplacement of proppant on the productivity of Shale gas reservoirs stimulated with transverse fractures. In tight gas reservoirs, where permeabilities are higher if compared with Shale gas reservoirs, overdisplacement is considered detrimental for the productivity, but studies of this effect are scarce in the literature neither for tight nor Shale gas reservoirs. In the case of Shale gas applications, where many treatments are required in the same well, there is a large economic benefit due to time savings when treatments can be overdisplaced. In Shale gas wells, where the gas flow rates per propped fracture are low, it has been proposed that overdisplacing the treatments does not impair productivity. The effect of overdisplacement is strongly related to formation strength (influencing fracture opening) and completion type, where for the latter, the perforate and plug systems, the treatments are normally overdisplaced. For modern ball drop systems, it is claimed that overdisplacement is not required, and it has been suggested that this can potentially improve the production compared to wells where the treatments have been overdisplaced. This work presents an analytical model that quantifies what is the erosion effect of overdisplacement on the proppant bank. The model is based on the physics that describe the slurry transport in pipes. A rock mechanics approach was adopted to calculate the geometry of the fracture after overdisplacement assuming the formation of an arch at the top of the proppant bank. Productivity calculations were done based on analytical solutions for gas wells including the effect of reducing fracture connectivity to the perforations due to overdisplacement. The results showed that even in low permeability formations, the reduction of the proppant bank height due to overdisplacement will impair fracture productivity. The height of the open fracture and the permeability of the different fracture zones, specially the closed fracture, will define the impact of overdisplacing the treatment. An analysis about the impact of changing overdisplacement operational parameters, such as viscosity, proppant size and concentration, showed that significant erosion rates occurred when using low viscosity fluids and larger proppant sizes. The results presented in this work calculated the impact of overdisplacement for a set of specific treatment and reservoir parameters and constitute a first step in the modeling of overdisplacement effect. Accurate prediction of the overdisplacement effect may certainly lead to benefits on production and costs reduction for future treatments. Further modeling of proppant distribution and fracture mechanics in Shale reservoirs will be the next step to have a better understanding of the effect and relevance of overdisplacement in fracturing treatments.