GUIDELINES
Exploration/Appraisal wells
It is recommended (EP89-1500) that formation strength tests are carried out for all casing shoes below which drilling will be carried out. This includes the conductor string if a BOP is planned to be used.
Development wells
Formation strength tests are justified in the majority of cases below all casing shoes. Tests may be omitted, however, if no hydrocarbon bearing or over-pressured forma- tions are to be penetrated in the following hole section.
Important notes
1) All formation strength tests should be carried out with the lowest drilling fluid densi- ty necessary for primary well control of the formations exposed during the test. The drilling fluid density should only be increased for the rest of the section after the test is complete.
2) In situations when good zonal isolation behind the casing is critical to the well’s success in both short and long term it is recommended to carry out the formation strength test using a retrievable packer, to avoid the creation of micro-annuli. 3) When testing below intermediate casing strings, the annulus outside the casing
being tested should be left open and observed for returns. Do not neglect to close the side outlet valves following the test.
4) Information obtained from formation strength tests is dependant on the inclination of the hole. Data from vertical holes is not generally applicable to deviated ones. 5) Breakdown tests (e.g. minifrac tests) can be carried out when abandoning a well to
FORMATION STRENGTH TESTS
PROCEDURES
The cement filled pocket is drilled out along with a minimum of about 20 ft (6m) of new formation. The test is conducted using a low volume high pressure pump (i.e. the cementing pump) and calibrated pressure gauges over a variety of ranges. The drilling fluid system pumps and gauges are not sufficiently accurate enough to perform the operation. The procedure is:
• Circulate and condition the drilling fluid to a consistent density. If high gel strengths are present it may be necessary to reduce them to ensure good pressure transmission. Conditioning can be done with the bit at the shoe to avoid wash outs.
• Pull back the bit into the casing shoe.
• Ensure the well is full. Close in the well using the BOP pipe rams around the drill pipe. • Open the annulus between the current and the previous casing string and monitor for flow. • Slowly pump down the drill string until surface pressure approaches ca. 100 psi (700 kPa). • Carefully measure tank levels etc.
• Pump uniform increments of volume – 0·1 to 0·25 bbl (0·016 to 0·4 m3) then stop and
wait two minutes for pressures to stabilise. For each increment the following are noted: - cumulative volume pumped
- pressure immediately after pumping ceases (final pumping pressure) - static pressure after two minutes (final static pressure)
Some operators prefer to apply the continuous pumping method whereby pumping is per- formed at a selected slow rate with simultaneous monitoring and plotting of pressures and volumes. This method is used wehn testing consolidated formations, during which the two minute pressure stabilisation period does not markedly influence the final test result.
• Plot the cumulative volume pumped against both the dynamic and static pressures on a graph.
• Continue to pump incremental volumes until one of the following occurs: - a pre-determined limit pressure has been reached
- the static pressure line deviates from a straight line (i.e. a linear relationship between pressure and volume pumped).
Note that the difference in elevation between the derrick floor and the cementing unit should be taken into account when determining the limit pressure and using the result of the test.
If the pump pressure suddenly drops, stop pumping but leave the well closed in. This indicates a leak in the system, cement failure or formation breakdown. Record the pres- sures every minute until they stabilise. If the drop in pressure is related to formation breakdown, this data can be used to derive the minimum in-situ stress.
• Bleed off pressure at surface and monitor the returns. Determine how much fluid has been lost to the formation.
• Top up and close the annulus
It should be noted that the objective of a Formation Strength Test is not to break down the formation and generate/propagate a fracture. The point at which the pressure/ volume plot deviates from a linear relationship is called the leak off point. It should be taken as the last measured point on the straight line; no extrapolation should normally be performed that would yield an increased formation strength.
The leak off point is sometimes also called the formation intake point.
A formation strength test that is terminated when a leak off point is identified is called a Leak Off Test. A formation strength test that is terminated when a pre-determined pressure is reached is called a Limit Test.
FORMATION STRENGTH TESTS
ILLUSTRATIONS
Desired test pressure
Cumulative volume Cumulative volume Cumulative volume Cumulative volume Pressure Pressure Pressure Pressure Formation intake pressure Formation intake pressure Formation intake pressure
Unconsolidated plastic formations Consolidated permeable formations
Consolidated formations, low or zero permeability
Consolidated formations
“Limit test”
Final pump pressure after each increment Final pump pressure after waiting period