Minifracs are routinely pumped before fracturing treatments to determine the in-situ fracturing parameters such as fracture closure pressure and fluid leak-off coefficient. These parameters are essential for design adjustment, pressure match, and fracture dimension estimate. The method most commonly used for determining closure pressure is pressure decline analysis during the shut-in period, using G-function plot, square-root time plot or other plots. Unfortunately, the shut-in decline data is often difficult to analyze and can yield inconsistent and erroneous closure pressures.

A new injection test for closure pressure determination, called "equilibrium test", is described in this paper. In the test, a fluid is first injected at a normal fracturing rate for a period of time to create a hydraulic fracture, and then pump rate is dropped to a small rate and held constant. The treating pressure will initially decline as in the conventional shut-in decline, but will gradually level out and start increasing, when equilibrium between the small injection rate and the leak-off from the fracture is reached. The pressure response can be easily analyzed to obtain an estimate of closure pressure. Fluid efficiency can also be estimated from the decline slope. A detailed discussion of the method, analytical calculations for determining the closure pressure and several field cases are presented in this paper.

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