Solids Cleanout Analysis Reduces Screen Out Risk

Abstract

Abstract A mechanical packer and abrasive perforator, conveyed by Coiled tubing(CT), grants great flexibility to the design of an annular fracturing treatment. On the downside the solids from a previous fracture treatment, or abrasive perforation cut, can be picked up by the subsequent pad fluid leading to a screen out. This is complicated by the fact that tolerance to solids varies between formations. To avoid this, solids transport during abrasive perforating, fracturing, displacement, and cleaning must be considered. Unique to this analysis is the ability to model the distribution of solids at any point in time, and the subsequent pick up of solids by the clean pad fluid. This leads to an unprecedented job optimization opportunity for reservoirs that can tolerate some solids in the initial pad stage, and risk mitigation for those which can’t. The model considers a transient mass balance for the solids bed and slurry. Non-uniform distribution of solids in the slurry is considered, which means the solids are not transported with the average slurry velocity. A drift flux model relates solids velocity in the slurry to the average slurry velocity. The difference between these is described by two terms. The first term accounts for variation in the cross sectional profiles of liquid velocity and solids concentration. This considers turbulent diffusion, settling velocity, and a liquid velocity profile based on Prandtl mixing length theory. The second term accounts for the interaction of gravity and buoyancy. Experiments to measure cleaning time involved a transparent pipe with a smaller pipe located eccentrically inside and a known amount of solids. Water is then injected through the smaller pipe and circulated in the annulus to clean the solids. The time required to remove all solids from the annulus is measured for a number of deviation angles and flow rates. Simulation results are compared to experimental data at all deviation angles. A field case is presented for a well that initially screened out during the pad after abrasive perforating. Finally a sample simulation details the solids transport for the entire process and a cleanout that was required following a screen out in the fracturing stage. This highlights how the model can be used to predict the axial distribution of solids in coiled tubing and wellbore at any time, for uphill and downhill flows, at all deviation angles.