Repeat Formation Tester

The Repeat Formation Tester has the following characteristics:

1. An almost unlimited number of pressure tests can be made during one run in the well.

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2. Two fluid samples (from separate depths) or a segregated sample can be taken each run.

3. Pressure measurement incorporates a strain gauge transducer or a quartz crystal with a direct digital read-out at the surface. Accuracy of this gauge practically eliminates the need to run Ameradas in combination for absolute pressure confirmation.

An adaptor allows Conversion of the open hole RFT to a cased hole RFT.

Tool Size

Minimum tool size can be reduced to 5.2" OD. Except in very unusual cir- cumstances, no attempt should be made to run it into a hole less than 6¹/2"

nominal OD. The basic tool will cope with holes up to 9^3/4" OD, but spacers may be added to increase the maximum to 14^3/4" OD, although in this case the closed tool size is 10.2" (see Figure 3-1 for tool specification). Schlumberger has also a few slim hole tools (RFT-N) with an OD of 3³/₈".

Tool Description and Operation (Open Hole)

When the tool is set, a rubber pad moves forward against the borehole wall. A probe pushes forward into the formation, and the piston inside the probe retracts, allowing formation fluid to enter the filter (see Figure 3-2). Fluid flows initially into two pre-test chambers: from the appearance of the pres- sure build-up formation permeability can be assessed. This can be used to determine whether the interval is suitable for sampling, and whether a good seal has been obtained. Since only a small amount of fluid has been with- drawn from the formation the final build-up pressure gives an accurate value of formation pressure.

Operation (Cased Hole)

The cased-hole RFT pre-test capability permits the operator to ascertain that a good seal against borhole fluids has been achieved prior to taking a sam- ple/pressure measurement. However, seal against borehole fluid before firing does not guarantee that seal will be maintained after firing. Moreover, it cannot be ascertained before sampling if the tool's ‘probe' is not plugged prior to sampling as is the case in openhole. Also, the depth of penetration of the charges (maximum 5.15"), presently limits the tool's application in washed out zones with thick cement sheath.

Sampling Techniques

Two sample chambers may be run with the RFT. In vertical holes, one may be 23 dm³, the other 3.8 dm³. In deviated holes the largest chamber is 10 dm³. A different sample of formation fluid can be collected in each chamber, or alternatively a segregated sample can be taken. The tool is operated at

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sampling depth and the pre-test completed. The larger sample chamber is filled first, then the smaller chamber is filled at the same depth. The contents of the smaller chamber form the segregated sample to be used for analysis.

See Figure 3-3 for an indication of possible arrangements for sample cham- bers and Amerada adaptor.

Multiple Pressure Testing

1. Discuss RFT pressure testing operation with reservoir engineer/petro- physicist beforehand so that reason for taking the measurements is fully understood.

2. Use gamma ray for depth control. Check CCL if job is in casing in order to ensure that depth chosen does not correspond to a collar.

3. Calibration and Tool Check

(i) A shop calibration not more than two months old forms part of the survey. This should include pressures up to 345 bar (5,000 psi) taken at ambient and 3 different temperatures between 77ºC and 121ºC, plus a zero pressure ‘offset' reading taken at a low temperature.

(ii) At the wellsite, the electronics is calibrated with ‘zero' and ‘calibrate' (9,995 psi) values, and the ‘Offset' calibrated rheostat is set at the zero

(iii) Once the surface equipment has been set up, no alterations to panel settings should be made during a sequence of pressure tests.

(iv) It is advisable to carry out a dry test in the casing to check if the tool packers are all right.

4. Measurements should always be taken from bottom to top to minimise depth errors and the chances of the tool becoming stuck. This requires that the tool be shop calibrated with decreasing rather than increasing pressures.

5. Switching tool to calibrate position removes power and the sensor cools.

Once calibrated, switch back to ‘measure', wait 10 minutes for temper- ature stabilisation, leave surface panel settings for the sequence of tests.

6. Plot stabilised mud and formation pressure measurements against depth (ahbdf and TVDSS). Check that mud pressures lie on a straight line, and check the gradient against reported mud weight. Ensure that formation pressures are less than mud pressure. Compare pressures from other wells in same reservoir (see Figure 3-4).

7. On completion of measurements, select three arbitrary depths and repeat

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pressure measurements. Repeat pressures should match original readings within 340 mbar (5 psi).

8. The results of the testing have to be forwarded to the Head Office by telex and by normal mail. Examples of reports as to be sent by normal mail are shown on the following pages.

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Repeat Formation Test Report (1) Pre-test results

WELL:

DATE:

1) Runs and tests should be numbered sequentially, i.e.

Run 1 Test 1- 1.1 Run 1 Test 2- 1.2 Run 2 Test 1 -2.1 etc.

2) Pressures should be corrected for temperature and pressure using the shop cal- ibration for the particular tool used (strain gauge only).

3) High-good-moderate-poor-tight (see Figure 3-4).

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Repeat Formation Test Report (2)