Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Reexamination Certificate
2000-10-26
2004-04-20
Moskowitz, Nelson (Department: 3663)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
C181S103000, C175S050000, C166S250100
Reexamination Certificate
active
06724687
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to creating and operating oil, gas or geothermal wells and more particularly, but not by way of limitation, to fracturing subterranean formations and determining characterizing information about the fractures, such as for use in monitoring or controlling the fracturing process or in performing subsequent fracturing jobs. This more generally includes determining characteristics of subterranean structures by obtaining and evaluating signals created in the well in response to one or more excitation events. As a specific, but non-limiting example, the present invention can be used to determine geometry (e.g., length, width and height) and events during the creation of fractures in oil or gas-bearing formations.
Characterizing a well during operations relating to creating or operating the well can provide various information about what is downhole in the well or adjacent subterranean formations. This information may be used in performing the operation(s) on the respective well, or it may be useful in planning or conducting operations on other wells. Such information includes, for example, structural information (e.g., what objects are downhole, locations of what is downhole, and events that occur downhole) and parametric information (e.g., pressure, temperature and flow rate).
For example, knowledge of fracture dimensions may permit wells to be drilled in optimal locations to take advantage of non-uniform drainage or injection patterns that hydraulic fractures may produce. In this way it may be possible to extract more of the resources in a field using a smaller number of wells than would be possible if fracture geometry were not known. Furthermore, information about the rate of hydraulic fracture growth can be used in improving the design and production of the fractures, thereby resulting in economic savings to the individuals and organizations who use hydraulic fractures in their operations.
Well characterization encompasses a wide range of technologies. One is well logging prior to installing casing. Sonar, with piezoelectric pressure signal generators operating in the audible frequency range, may be used. Sonar technology is expensive, time consuming, and relies on extensive software to interpret the reflected wave pattern.
After casing is cemented in place, well characterization typically includes techniques based on pressure/time transient analysis. In these, steady state is established, such as by making the well produce, capping it off, or by pumping fluid into the well; and then, for example, a well outlet valve at the surface is manually opened or closed at a normal speed. This starts a gradual change in well pressure, slow enough that it can be read from gauges in intervals of seconds to an hour or more. The reason for the pressure transient slowness is that the Darcy Law for fluid seepage governs it. Pressure/time data and their derivatives are graphed on semi-log and log-log coordinates. The uniqueness of these slopes provides sufficient information to estimate well productivity, formation permeability, and reservoir geometry. These tests are performed without pulsatile flow present; therefore, the data have a high signal to noise ratio.
During well servicing such as in a fracturing process, pumps requiring thousands of horsepower are in operation. Pumping rate and treating pressure are operational constraints for a number of reasons. Injecting at too high a rate and thus pressure has the potential for fracturing out of the productive zone. The rate may also be limited because some fluids degrade under high shear rate. Another reason to limit the injection pressure may be tubular structure or available pump horsepower. However, high pumping rate is desirable to achieve high fluid efficiency, defined as the ratio of fracture volume created to the fluid volume pumped.
To collect well-defined pressure/time data during pumping, one must work with strong pressure signals. At high pumping rates, velocities may reach up to 40 feet/second in the flow passages. Transient fluid flow changes make a significant impact on the local friction pressure drop. Fracturing jobs often start with a “mini-frac” test. To do this, the pump speed is suddenly reduced (e.g., from 15 to 10 barrels/minute). The result is a sinusoidal pressure transient from which fluid efficiency, near well damage, and minimum in situ stress can be calculated.
Fracture size is another desirable characteristic to know. This has previously been obtained using conventional hydraulic impedance testing. In conventional hydraulic impedance testing, a relatively short duration pulse is produced at the surface and then the reflected signal is observed for one peak indicating the mouth of the fracture and another, smaller peak indicating the tip of the fracture. The time between the peaks is indicative of the fracture length and with an assumed volume and fracture profile, either the height or width can be determined. A shortcoming of this technique is that it is usually done in a static fluid condition due to large amounts of noise from pumps hiding the smaller reflected peak. The time frame for the pulse is typically longer than the travel time for the wave into and out of the fracture (especially at the start of the fracture stimulation process when the fracture is relatively short), which further smears, degrades or masks the signal of interest.
Other fracturing characteristics that are desirable to know and have been determinable include breakdown pressure when the fracture begins, screenout when proppant in the fracturing fluid reaches the tip of the fracture and plugs it off, and fracture closure pressure that exists after the fracture has partially closed when the fracturing pressure is released. These have been interpolated from various pressure versus time curves. For example, screenout has been deemed to exist at the beginning of a segment having a 1:1 ratio (slope of 1) in a curve representing the square root of pressure versus the square root of time; and fracture closure pressure has been interpolated from a pressure versus square root of time plot by drawing two tangential lines to the curve and at their point of intersection taking that pressure as the fracture closure pressure.
There is the need to obtain such information as mentioned above more directly if possible rather than having to infer it as in the current state of the art.
SUMMARY OF THE INVENTION
The present invention overcomes the above-noted and other shortcomings of the prior art, and meets the aforementioned needs, by providing a novel and improved well characterizing method and system and a novel and improved fracturing method.
The present invention uses an excitation event that creates a responsive signal having lower and higher frequency components, which higher frequency component provides information about one or more characteristics of the well. For example, the present invention can be used to obtain or measure signatures of hydraulic fractures from pressure signals. In a particular application, the invention uses a dynamic pressure response during a fracture stimulation job, which response undergoes signal decomposition by the use of wavelet processing to measure the response. In one use, a pulse whose rise or fall time is shorter than the travel time in the fracture excites the fracture at its natural, or resonant, frequency. For example, a flow rate change in the form of a step function or square wave, with rise and fall times measured in milliseconds, induces the type of flow transient needed to excite a natural acoustic frequency inside the fracture. The wave reflected back through the casing perforations contains the acoustic signature of the fracture. This results in a higher frequency wave being modulated on the wellbore's resonant lower frequency wave. The higher frequency wave provides additional information (e.g., fracture geometry, such as fracture length) about the well. Non-limiting examples of other determinable information pertaining to a subterranean fra
Loth John L.
Stephenson Stanley V.
Gilbert, III E. Harry
Halliburton Energy Service,s Inc.
Kent Robert A.
Moskowitz Nelson
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