Monitoring - Step Rate Testing

Step Rate Testing

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Tony Settari ASettari@TaurusRS.com Taurus Reservoir Solutions

Abstract

A step rate injectivity test is normally used to estimate the transition from matrix (or pseudo-matrix - a fracture may already be present but the bulk of the flow is radial) flow to fracture-dominated injection, according to a change in slope of a plot of pressure versus rate. Step rate testing allows for determination of when a new hydraulic fracture occurs and/or when a pre-existing fracture opens/propagates.

It can be run after a conventional falloff or a final falloff segment can be used in the test. Repeated falloff testing can also be used to assess if a reservoir has been altered by thermal changes, in in-situ stresses or changes in kh associated with thermal effects.

Key Issues

Testing Procedures Precautions Data Analysis

Key Issue 1: Testing Procedures

Recommendation 1.1:	Use bottomhole pressure!
Under any circumstances, have an approximation of the conversion between surface and bottomhole treating pressure. Use the Wellbore Hydraulics routine in the PWRI Toolbox. If reasonable, obtain bottomhole pressure-temperature measurements.
Recommendation 1.2:	Use uniform rates and time steps!
It has been recommended that each time step is one hour long to ensure that the radius of investigation is large enough. Obviously, this may not always be practical. Either shorter times are called for because of economic or operational limitations or substantially longer times are required to allow for thermal stabilization. Ideally, in relatively low permeability formations (k < 5 md), each injection should last for one hour. 30-minute injection times are adequate for formations with permeability exceeding 10 md. Regardless of the length of time for each stage, it is essential to keep the time steps of equal duration.
Recommendation 1.3:	Have an adequate number of measurements above and below the apparent fracturing pressure.
Attempt to obtain at least three readings above and three readings below the "parting" pressure. Fracture gradient can depend on the average reservoir pressure.
Recommendation 1.4:	Consider doing a stepdown and/or falloff measurement with the step-rate measurements.
After the rate increasing segment, two approaches are possible. The first is to back down on the rates to assess if the cement sheath has been damaged. Alternatively, some specialists prefer a long falloff after the last rate because it can be analyzed to get fracture properties (dimensions). A good compromise, if possible, may be to combine both techniques - step up in rates, followed by a step down cycle (the stepdown procedure may provide indications of whether or not there has been fracture growth out-of-zone, etc.), followed by stepping back up and a falloff. This may be the premium procedure and modifications may be required because of operational constraints and available time.

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Key Issue 2: Precautions

Recommendation 2.1:	The signature on the pressure-rate curve can be anomalous if the fluid loss changes. Consider this in interpreting "anomalous" data.
If step rate data from different tests are being compared, recognize that the slopes can be different if the water quality is different. This is shown in Figure 1. Figure 1. A schematic variation (conceptual data) of how pressure signatures can vary for different water qualities (after Murray). The slopes, after fracturing occurs, are dependent on the fluid loss. With decreasing water quality, there is reduced fluid loss, but there is additional fracture growth.

Recommendation 2.2:	Consider the impact of vertical fracture growth.
As a note of caution, "standard behavior" is contingent on a fracture that is contained within one zone. If the fracture grows out-of-zone dramatically, the excess pressure may decrease because of the growth more than the change in pressure due to friction in the fracture. A negative slope may appear in the post-fracturing regime. Figure 2. A schematic (conceptual data) indicating how pressure and rate may be affected by out-of-zone growth.
Recommendation 2.3:	Pre-existing fractures can complicate interpretation.
If there is a pre-existing fracture, it will always have conductivity, even at pressures below the reopening pressure. This is particularly true if it is self-propped (jammed open) or if we are testing a converted production well that has been conventionally stimulated. Inflection may not be seen or there may be a slight curvature followed by a straight line linear or bilinear flow regime. Examples of behavior for a pre-existing, closed and a pre-existing, self-propped fracture are shown in Figures 3 and 4. Figure 3. An idealization of a closed (but still conductive) pre-existing hydraulic fracture. Figure 4. An idealization (conceptual data) of a self-propped, pre-existing hydraulic fracture..
Recommendation 2.4:	Don't ignore changing temperature and pressure conditions.
Depending on the volumes injected, thermal effects can come into play, either due to viscosity changes or in-situ stress changes. In conjunction with this, it is extremely important to incorporate any changes in reservoir pressure if you are comparing SRT data taken at different times in the injection life cycle. In comparing consecutive step-rate test programs, be certain that you are aware of any stress field alterations that have occurred due to poroelastic and/or thermoelastic effects. Figure 5. These data, from a well-controlled and monitored pilot in the Shell Eider field, show the variation of measured stress levels with temperature. The lines are all approximately parallel, indicating consistency in the quality of the injected water (or insensitivity).
Recommendation 2.5:	Use plots of daily rate and time.
Be sure to calculate the rate based on the percentage of the day that pumping occurred, if this is possible or necessary. Figure 6. Long-term variation of wellhead pressure and injection rate, showing the influence of temperature and quality.

Recommendation 2.6:

Adequately account for the well history that has occurred before the test has been run.

The injection and shut-in history before the test can have a substantial impact on the results when you use superposition technniques to handle multi-rate situations.
When first asked about how you might account for varying rates before the step rate test, you might be a lttle too casual. You might say "Ahh, we just inject at a very low background rate and we don't worry about it." Or you might say "I account for the variable rate by including it with one rate and an equivalent time." Maybe, but:
- Do not ignore rate fluctuations before the test. You can see the affect by referring to SPE 16798, by Singh et al. Include pre-test rate history in your superpositions.
- At a minimum, use a three times duration and ignore the rest (incorporate previous injection history that is three times the length of the step rate test period).
- Even better, iterate and see what effect it has on the analysis.

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Key Issue 3: Data Analysis

Recommendation 3.1:	Use the following philosophy.
The simplest, and least desirable, method of analysis is to plot surface pressures at the end of each step versus injection rate. It is preferable to use measured or inferred bottomhole pressure data. It is also desirable to consider this as a multi-rate test and to process the data accordingly. There is a Multi-Rate Tool in the PWRI Toolbox that can assist in doing this. Record data continuously at a reasonably high frequency. Include a long falloff after the last stage.

Recommendation 3.2:	On certain key wells, it may be appropriate to use graphical methods, multi-rate analyses, and simulation..
The first step is to look at basic radial flow relationships and see if radial flow at the low rates makes sense. The first step is graphical. The second is to match the low and high rate ends with analytical models to look at the two limiting situations, with and without a fracture. If there is a deviation from low rate prediction and the actual data (in the right direction) it indicates a good test. The final step, if warranted, is numerical analysis to make all parts fit together.

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Last Modified: 06/12/2002