Abstract

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.

Introduction

A " minifrac" is routinely pumped prior to a main proppant fracturing treatment to determine the in-situ fracturing parameters such as fluid efficiency and leak-off coefficient, which are used to calibrate and optimize the main treatment design. A prerequisite and the key to a reliable estimate of fluid leak-off properties is the correct determination of fracture closure pressure. Erroneous closure pressure leads to incorrect fracture closure time and net pressure, and consequently an incorrect fluid leak-off coefficient. As a result, any design adjustment to the main fracture treatment based on this erroneous fluid loss estimate could result in a non-optimal treatment or premature job screenout. Furthermore, a correctly determined closure pressure is essential in generating a correct net pressure plot that is routinely monitored during a fracture treatment to diagnose fracture propagation behavior. The engineers on-site often rely on the net pressure plot to determine if a screenout is imminent and make real-time decisions on whether and when to cut the job short to avoid a severe screenout and having to cleanout a large amount of proppant left in the wellbore. Net pressure match using a fracture model is also a common practice that helps the engineers to estimate fracture dimensions and adjust the design to improve future treatments.

There are several methods that can be used to determine closure pressure. These are injection tests that involve injecting a fluid into the formation of interest and analyzing the pressure response during the injection and the shut-in period. The most commonly used methods in a hydraulic fracturing job are shut-in decline analysis (minifrac and/or a separate pump-in) and step rate test. Another method that has relatively limited usage is pump-in flowback test. A brief discussion of these test methods is given in the following. More detailed information can be found in References 1–2.

Step Rate Test.

In a step rate test, a low viscosity fluid is injected into the formation and pump rate is increased in steps until fracture extension occurs (see Fig. 1a). The stabilized pressure corresponding to each rate step is then plotted against the rate. Ideally, the data points will fall on two intersecting straight lines, as illustrated in Figure 1b. The slope of the first straight line corresponds to the pressure response of matrix leak-off at low pump rates, and the slope of the second straight line, usually smaller than the first slope, corresponds to fracture extension at the higher pump rates. The intersection point of the two slopes is the fracture extension pressure. The extension pressure corresponds to the pressure in a propagating fracture and is greater than (hence an upper bound of) the closure pressure. Estimate of closure pressure is sometimes obtained as the intersection of the extension slope line with the y-axis (at zero pump rate)3.

Step Rate Test.

In a step rate test, a low viscosity fluid is injected into the formation and pump rate is increased in steps until fracture extension occurs (see Fig. 1a). The stabilized pressure corresponding to each rate step is then plotted against the rate. Ideally, the data points will fall on two intersecting straight lines, as illustrated in Figure 1b. The slope of the first straight line corresponds to the pressure response of matrix leak-off at low pump rates, and the slope of the second straight line, usually smaller than the first slope, corresponds to fracture extension at the higher pump rates. The intersection point of the two slopes is the fracture extension pressure. The extension pressure corresponds to the pressure in a propagating fracture and is greater than (hence an upper bound of) the closure pressure. Estimate of closure pressure is sometimes obtained as the intersection of the extension slope line with the y-axis (at zero pump rate)3.

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