(398b) Fracturing Fluid Retention and its Effect on Fluid Flow in Microfractures of Tight Oil Reservoirs

Polyacrylamide-based friction reducer, serving as one of the primary additives in fracturing fluid for tight reservoirs, could reduce the fluid friction associated with high pump rates in pipelines and wellbore. However, low recovery of fracturing fluid results in much chemical residual in microfractures, which is closely related to relative permeability near fracture, flowback of fracturing fluid, production rate, etc. Our work is to investigate the effect of fracturing fluid residual on oil and water flux.

2. Methods, Procedures, Process

This study used tight sandstone core samples (φ1 in.×1 ft.) to fabricate fracture models. Each core sample was cut in half from the center, and stainless steel sheets with different thicknesses were inserted between the two halves to model fractures. The sandstone cores were used in the experiments because they could better model the adsorption-entanglement effect of polymer on fracture surfaces. Oil or brine was injected at various velocities before and after tight sandstone fracture models were fluxed with friction reducer solution, and residual resistance factors to oil and brine were specified with different fracture widths and friction reducer concentrations.

3. Results, Observations, Conclusions

Polymer or polymer gel has the function to preferentially reduce water relative permeability in conventional reservoirs. The similar finding was reached responsible for fracturing fluid residual in microfractures of tight reservoirs. In the first cycle of oil/brine injection experiments after friction reducer solution flux, residual resistance factors to oil (F_rr,oil) and brine (F_rr,brine) exhibited power-law characteristics through their shear rates. This phenomenon occurred because oil/brine gradually flushed more and more polymer out of fracture models. In addition, higher oil/brine shear rates squeezed polymer coating on fracture surfaces more, creating larger channels through which oil/brine can pass. The result that F_rr,brine was larger than F_rr,oil revealed chemical residual could selectively reduce the permeability to water more than to oil in microfractures. The supplementary of fracturing fluid filtration experiments confirmed adsorption-entanglement and blocking are the basic mechanisms by which chemical residual on fracture surfaces can modify flow characteristics. At the same shear rates, smaller fractures presented higher residual resistance factors. The reason was chemical residual on fracture surfaces occupied a larger portion of cross-sectional area in smaller microfractures. Besides, chemical residual grew with an increase in friction reducer concentration, resulting in higher flow resistance to oil and brine.

4. Novel/Additive Information

Low recovery of fracturing fluid leaves much chemical residual in microfractures, which would definitely affect oil and water production performance. This study discussed the mechanisms responsible for disproportionate permeability reduction in the fractured tight sandstones, and attempted to introduce the fundamental of conformance control into hydraulic fracturing. The results could provide an insight into the development of fracturing fluid for tight reservoirs and flowback performance after the fractures were generated.