Unconventional oil well development is enabled by long horizontal lateral and multistage hydraulic fracturing. Well production tends to peak during the first months, then decline rapidly and produce a more stable flow rate during a longer period, called the trilinear flow regime. During this period, well production is unstable, showing severe slugging behavior. This behavior might be worsened due to certain trajectory effects, for example, toe-up, where gas lift usage can amplify the instability if it is not controlled properly. This study experimentally and numerically addresses the effect of two factors on flow stability: 1) the trajectory and its related inclination angle and 2) total gas-to-liquid ratio (GLR).
First, experimental runs were conducted at the University of North Dakota Department of Petroleum Engineering UTP (undulated two-phase) flow loop for different trajectory configurations and gas lift injection rates in a range that preserved flow similarity to full-scale conditions. Then, recorded pressures, flow rates, and hold-up signals were processed for flow instability and slug characteristic calculations. The laboratory-scale numerical models were run through a transient multiphase flow simulator and history-matched to reproduce the observed data and investigate inaccessible flow parameters.
The experimental and numerical results show that the toe-up configuration promotes more severe slugging behavior in most of the cases. In contrast, the gas lift was shown to reduce lateral section flow instabilities. However, its effect can only be seen at high kinetic energy (flow rates and/or superficial velocities) because of the transition from gravity-dominated to friction-dominated flow regimes and vice versa. Slugs accelerate and get longer through the lateral section length before decelerating and shortening because of gravity and liquid fallback in the vertical section. The highest instability occurs in the lateral to vertical bend.
From the experimental and numerical findings and given the amplitude of pressure and flow rate fluctuations, it is expected that the completion and hydraulic fractures experience high pressure fluctuations which might impair reservoir inflow productivity.
