Paper: Prediction of The Location and Movement of Fluid Interfaces In A Fracture

Recent developments in stimulation practice have resulted in two or more fluid systems being used continuously during a treatment. Fracture acidizing, for one, has involved the use of a highly efficient pad fluid preceding the acid into the fracture. In the use of the crosslinked gels, it is common practice to precede the gel-sand slurry into the fracture with a pad fluid of an entirely different character. Where these techniques are used, it is useful to know the rate at which one fluid is displacing another and the location of the interface between these two fluids. The importance of this information is as follows: 1. In fracture acidizing, the location of the fluid interface indicates the farthest point that the acid has reached into the fracture. 2. In using the crosslinked gels with proppants, the location of the interface pinpoints the leading edge of sand-laden fluid and hence, propped fracture length. It can also provide valuable information concerning sand scheduling. 3. Where overflush volumes are used behind high strength acid treatments, this information is valuable in sizing the volume to be used as overflush. The gross, averaging techniques used in the past have not allowed the location of the interface between two fluids or stages of the same fluid. In order to develop a method to accurately predict the interface location it was necessary to go back to the basic fluid mechanics of fracturing. In this analysis a greatly simplified expression was developed which predicts fracture area within 10% (or less) of the Howard, Fast, and Carter equation." This simple equation has greatly simplified fracturing and fluid displacement analysis without introducing significant errors in accuracy.