Summary
Foam fracturing treatments pumped down 3.5 to 4.5 in. tubing at up to 80 bbl/min have presented problems in control and execution. Wellhead pressures used to predict net entry pressure can give erratic results in the absence of good knowledge of bottom-hole pressure. Since nitrogen ( N2) delivery rates are pressure sensitive, errors in bottomhole pressure estimation control problems with both rate and foam quality. Perforation erosion, if unaccounted for, can signal a need for increase in rate to maintain bottomhole pressure.
We used a laboratory study of foam in laminar and turbulent flow and a series of field experiments with bottomhole gauges to calibrate foam friction pressure in tubulars at very high rates. To measure viscosity of foam fluid with N2 at 1,000 psi, we used a small-scale, flow loop viscometer that could be applied in both laminar and turbulent flow. Foam parameters included foam qualities from 25 to 80% N2, and guar concentrations from 0 to 40 lbm/Mgal. The effect of surfactant concentrations from 0.2 to 1.0% were also studied.
Field foam treatments pumped at high foam rates were equipped with bottomhole real-time transmitting gauges. Step-rate tests were performed on both neat foam and sand-laden foam to calculate friction pressures. Actual friction pressures from these treatments were compared with pressures based on laboratory data. The laboratory correlation can predict tubular friction accurately for foam fluids up to very high foam rates.
In several of the treatments, small perforations were used in an attempt to provide limited entry stimulation. With the bottomhole-gauge data, we were able to assess the effects of perforation erosion due to changes in sand concentration.
