Cleanup of internal filter cake during flowback
The flow initiation pressure (FIP) is used as an estimate of the differential pressure (between the reservoir and the well) required to initiate production. The standard practice to measure FIP uses a constant flowback rate. This method is shown to be inadequate to measure the FIP. An improved flowback method, which uses a series of constant differential pressures, is used instead to measure the FIP. This method closely represents the constant drawdown experienced between the reservoir and the wellbore. In addition the permeability during flowback is measured at increasing differential pressures, resulting in a spectrum of return permeability values. Two types of drilling fluids (sized calcium carbonate and bentonite) are used for conducting the filtration and flowback experiments for porous media ranging in permeability from 4 to 1500 md. Both single-phase and two-phase experiments are conducted in lab-simulated open-hole and perforated completions to better understand the factors affecting the FIP and the return permeability spectra. vii We observe small values for FIP in all the experiments (considerably smaller than those measured using the constant flowback method). The values of FIP yield pressure gradients that are achievable in vertical wells but may not be easily achieved in horizontal wells. The FIP and the return permeability spectra are controlled by the cleanup of the internal filter cake. A Bingham fluid in a network of pores is used to model the cleanup of the internal filter cake during flowback. The results indicate that very large pressure gradients are required during flowback to cleanup the entire internal filter cake. However, a pressure gradient of 10 psi / inch is found to yield a skin factor < 1 for most open-hole completions. For perforated completions, pressure gradients up to 20 psi / inch and flow rates up to 0.3 bbl/day/perf yield skin factors < 2.