Foam Hydraulics Modeling for Sand Cleanout in Low Bottom Hole Pressure Wells

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Abstract As foam is significantly less dense than water, and it can have a rheology capable of holding sand, it is an attractive fluid for sand cleanout in completions of low bottom hole pressure (i.e. underbalanced) wells. However, predicting the behavior of foam can be technically challenging, and mobilizing equipment to perform foam operations can be expensive. The primary objective of this work is to demonstrate how a foam hydraulics model can be used to determine the feasibility of performing a foam operation for a given well and to maximize sand cleanout rate in wells where feasible. Foam is a compressible fluid characterized by its quality (ratio of gas volume to total (gas+liquid) foam volume) and works as a stable fluid within a certain range of quality. Foam’s quality depends on its pressure. As pressure increases, foam’s quality decreases due to compression of gas in the foam. In this work, we make simplifying assumptions to develop a one-dimensional steady-state hydraulics model for foam flow through coiled tubing and then back up the annulus of deviated wells that contain a sand bed. The hydraulics model for foam captures foam’s compressibility, non-Newtonian rheology, non-linear hydrostatic pressure profile, friction pressure including slip velocity, the effect the geothermal gradient has on the foam, and the effect suspended sand has on the foam. We obtain results for the foam pressure and quality at various locations along the coiled tubing and wellbore annulus, as well as the time it takes to circulate foam through the coiled tubing and annulus. In more detail, we obtain results for the hydrostatic pressure and frictional pressure drop for different sections of the coiled tubing and annulus. The results show that an increase in foam quality (i.e. increased ratio of gas to liquid) results in a decrease in hydrostatic pressure (lighter foam) but an increase in the frictional pressure drop (more viscous foam), following experimental evidence. The bottom hole pressure of foam is a function of the hydrostatic pressure and frictional pressure drop in the annulus. Thus, as the foam quality increases, the bottom hole pressure can increase, decrease, or stay the same. The results bring out the non-intuitive behavior of foam and demonstrate how challenging it can be to control a foam hydraulics system. We compare our model predictions for the total pressure drop across the coiled tubing and annulus to field data and obtain reasonable agreement. Finally, we describe how our model can be applied to achieve the primary objectives of this work (described above) through two examples. The examples demonstrate how a foam hydraulics model can be used to support and guide operational decisions.