As more wells are drilled and completed in deeper reservoirs, various methods are being applied to overcome choking effects in propped fractures and enhance well productivity. Choking effects can result from permeability damage caused by fracturing gel residues, low proppant concentrations, proppant crushing from high closure stresses, or embedment/intrusion of formation materials into the proppant pack. This paper describes the laboratory testing of a new well stimulation method that can use low-quality sand for generating stable, highly conductive channels within a propped fracture to help maximize and maintain production of hydrocarbon from the formation reservoir to the wellbore.
Mini-pillars of various particulate materials were formed by coating them with a tackifying agent or a curable resin and placing them in molds to be cured before testing. These mini-pillars were installed in conductivity cells using various layout configurations to determine the effect of closure stresses on pillar height, diameter expansion, conductivity measurements, and choice of particulate materials. Laboratory experiments were performed to evaluate the formation and stability of mini-pillars and proppant-free channels surrounding the pillars.
The obtained results in this study indicate that flow capacity of conductive channels prepared from proppant pillars with low-quality sands was comparable to those prepared using high-quality sand or high-strength manufactured proppant. Proppant crushing was not observed to be a concern when applying fine particulates during this fracturing process because flow capacity of proppant-free channels between aggregate masses dominates flow through the propped fracture, making the formation of proppant pillars with high-quality sand or high-strength proppant unnecessary. As long as proppant pillars are held in place without being previously dispersed or broken up to ensure the integrity of proppant-free channels, low-quality sand or particulates can be a practical and economical source of solids material for preparing these proppant aggregates.
For conventional hydraulic fracturing treatments performed in low-permeability reservoirs, improved fracture cleanup and longer effective fractures have a direct result on increased fracture conductivity (Soliman and Hunt 1985). The effective fracture length can be defined as the length of the created fracture that actually cleans up and contributes to production. Increasing the effective fracture length actually improves reservoir exposure, resulting in a higher effective drainage radius.
The following factors can contribute to the damage mechanisms of fracture conductivity, and consequently, decrease well production (McDaniel 1989; Mahadevan and Sharma 2003; Weaver et al. 2007):
Incomplete removal of gel residue
Proppant stress cycling
Fines infiltration, fines invasion, fines migration
The application of proppant-free channel fracturing treatments can overcome the choking effects that are often encountered in conventional propped fractures. Flow of produced fluids is established in the proppant-free channels and not through the proppant pack because the path of least resistance is now through the proppant-free channels.