Constant rate injection of a fluid into a well for microfrac and step rate testing is accomplished through the use of an injection system. The system provides accurate flow rate control (within 2% of the desired rate) during injection, which results in superior quality of data such as the fracture extension pressure, etc. for formation analysis. The injection system pressure, etc. for formation analysis. The injection system eliminates discontinuities in flow rate which may be present when using an engine-driven pump with a manual transmission. The inherent flow rate fluctuations present in the positive displacement pump flow rate are also minimized by positive displacement pump flow rate are also minimized by the injection system. The system permits use of a large fracturing pump for low flow rate injections, thereby increasing the rangeability of the service equipment, which results in lowering the costs.
Laboratory test data and numerically generated results of a computer model are presented in this paper, along with case histories of two jobs.
Positive displacement pumps are commonly used to pump fluids during various phases of a well stimulation. Flow pump fluids during various phases of a well stimulation. Flow rate requirements for these pumps typically range from a few barrels/minute to several hundred barrels/minute. The treating pressures have a range of values from 1,000 to 15,000 psi (6.9 pressures have a range of values from 1,000 to 15,000 psi (6.9 to 103.4 MPa). With the advent of formation testing methods such as microfracturing and step-rate testing, these flow rate ranges have become even wider. The following describes the microfrac and step-rate tests emphasizing the low flow rate requirements of these processes.
The microfrac test is employed to determine the minimum in situ principal stress in a formation, which is used to determine the fracture orientation for a fracturing treatment. The test is performed by creating a small fracture using low constant injection rates of 0.5 to 25 gal/min (32E-6 to 16E-4 m3/s) for a short period of time (3 to 10 minutes). The minimum principal stress is determined from the analysis of the pressure decline after shut-in. This minimum principal stress may in low leak off formation be considered equal to the fracture closure pressure in the design of a fracturing treatment. Special flowback tests may also be performed to determine the fracture closure pressure.
For formations that have extremely high fluid loss, analysis of the pressure decline for the determination of the minimum principal stress is often futile. Besides using a fluid system with appropriate fluid loss additives, the test procedure and analysis must be modified. Because of this problem, the fracture reopening pressure is sometimes used as minimum prinicipal stress. The fracture reopening pressure is by prinicipal stress. The fracture reopening pressure is by definition equal to or slightly higher than the fracture closure pressure. The fracture reopening pressure is usually difficult pressure. The fracture reopening pressure is usually difficult to determine, especially if a relatively high injection rate is used. Fracture reopening pressure is observed on the pressure vs. time plot shown in Fig. 1. It is characterized by a decrease from the constant rate of pressure increase during the early injection time period of a test. To accurately and consistently observe the fracture reopening pressure, the appropriate constant injection rates must be maintainable. If fluid loss is "extremely high", low rates may not open the fracture.