Abstract
Optimum placement of injected fluids during scale inhibitor or acid stimulation treatments is often difficult to achieve in long horizontal or deviated wells. Crossflow effects that displace the injected fluids away from the target zone can exacerbate the problem, even when coiled tubing is used to deliver the chemicals to the desired location. To overcome these problems, diversion techniques, using waxes or polymer cross-linked gels, can lead to improved placement with little detrimental impact on post treatment production.
Placement of the diverter itself is not always simple, particularly for gels where the rheology is controlled by temperature and shear rates. Modelling techniques are being developed to assist engineers to design the various stages of these diverter treatments. The model described here was developed, using a commercial reservoir simulation code, to simulate the placement of polymer gels in high crossflow wells. The model parameters were based on a field inhibitor squeeze treatment performed on a 2,000 ft highly deviated well that (cross) flowed 3,500 bbls/day during shut-in. The effect of the crossflow during the various injection and shut-in stages of the squeeze treatment would be to displace the inhibitor away from the water producing zones, hence leaving these sections of the well unprotected. Accurate placement of the gel would prevent loss of inhibitor to the thief zones, ensuring protection for the intervals vulnerable to scale, and thus extending the time before the well would need to be re-squeezed.
The initial gel placement model demonstrated the interaction between viscosity, friction and transmissibility profile along the well at various injection and crossflow rates. A threshold viscosity was identified above which the injectivity profile was dominated by connection transmissibility factors rather than the friction and crossflow forces. The model was then developed to incorporate the effect on the gel viscosity of varying shear stresses along the wellbore and in the formation.
Development of a model that can predict gel placement enables optimisation of the diverter stage of a treatment. It also leads to more accurate modelling of subsequent chemical injection (eg. inhibitor or acid), and hence the benefit to squeeze lifetime resulting from the placement strategy can be assessed.
