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9. The Step Rate Test

There are two types of step rate test, the step up test and the step down test. One is used fordetermining fracture extension pressure, whilst the other is used for determining near

wellbore friction. Both tests can be extremely useful when designing the treatment. Wheneverpossible, bottom hole pressure data should be used, as this is more accurate than calculatedBHTPs.

9.1 The Step Up Test

The step up test is used to determine the fracture extension pressure, Pext. This is usually 100to 300 psi higher than the fracture closure pressure, Pclosure , which is a very important factorin fracture design. Usually the results of the step up test will be used to determine an upperboundary forPclosure and to give the expected BHTP.

To carry out the step rate test, it is usually to use either KCl water or linear gel. However, if

this test is to be combined with the minifrac (see Section 10), then the actual frac fluid shouldbe used.

The test itself consists of pumping fluid into the formation at various rates. These rates startoff slowly and gradually increase. For example, these could be the pump rates for a typicaltest; 0.25 bpm, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, 5.0 and 10 bpm. The first step is usually thelowest rate that the pumps can manage. It is important to get as many stages at low rate aspossible. At each stage, first achieve the rate, then wait for the pressure to stabilize and finallyrecord the exact pressure and rate. Then move on to the next stage.

What is important with this test is to get stabilized pressure. It is not that important to get theexact rates. Often, pump operators will fiddle with the rate for 30 seconds or so in order to getexactly 0.75 bpm. This is not necessary. Get approximately the correct rate and then leave it,

so that the pressure can stabilize and be recorded.

Once the test has been carried out, a plot of pressure against rate can be made, as illustratedin Figure 9.1a:-

Figure 9.1a The Step Up Test

Rate

Pressure

Pext

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The change in gradient of the slope shown in Figure 9.1a marks the change from Darcy radialflow (lower rates) to Darcy linear flow at higher rates. This is the point at which our fracture iscreated and hence this is our extension pressure.

When carrying out a step up test it is important that no artificially induced fracture exists prior

to the test. Thus, if any pumping above the frac gradient has already been carried out,sufficient time should be taken for the fracture to heal up before commencing the step ratetest. On very hard rocks, this could be several days.

9.2 The Step Down Test

This test is used to determine the nature of any near wellbore friction that may exist, i.e. tosee if it is perforation or tortuosity dominated. As the name suggets, the step down test is theopposite of the step rate test. Instead of starting at low rates and increasing, the rates arestarted high and decreased.

When performing the step down test, it is important that the fracture is open the whole time,otherwise the test is invalid. Therefore, this test is often carried out after a step up test. It isnot uncommon to step up then step down right after.

Figure 9.2a The Step Down Test

Figure 9.2a shows the relationships between pressure and rate for the step down test. Thedifferent shapes of the curves indicate how the near wellbore friction is dominated by theperforations, by the tortuosity or by a combination of the two.

For perforation friction:-

Pnwb Q2..................................................................................... (9.1)

whereas for tortuosity:-

Pnwb Q

.................................................................................... (9.2)

Rate

Pressure

Tortuosity Dominated

Perforation

Dominated

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Of course, usually the near wellbore friction is a combination of perforation friction andtortuosity. Modern fracture simulators, such as FracPro and MFrac, allow data from the stepdown test to be inputted directly into the model. As perforation data has already been inputted(number of shots and shot diameter), the model can then derive the friction due to thetortuosity.

References

Gidley , J.L., et al.: Recent Advances in Hydraulic Fracturing, Monograph Series Vol 12, SPE,Richardson, Texas (1989).

Economides, M.J., and Nolte, K.G.: Reservoir Stimulation, Schlumberger EducationalServices, 1987.

Cleary, M.P.:, Johnson, D.E., Kogsbll, H-H., Owens, K.A.: Perry, K.F., de Pater, C.J.,Stachel, A., Schmidt, H., and Tambini, M.: Field Implementation of Proppant Slugs to AvoidPremature Screen-Out of Hydraulic Fractures with Adequate Proppant Concentration, paperSPE 25892, presented at the SPE Ricky Mountain Regional/Low Permeability ReservoirsSymposium, Denver CO., April 1993.