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If it is confirmed that it is not possible to free the string by working the pipe and spotting oil or acid pills, the string shall be backed-off in order to allow proceeding with a different method, such as running jars or wash pipes, or abandon the hole and side-track.

There are two methods for estimating the depth at which a string is stuck:

• Applying tension and measuring the pipe stretch.

• Locating the tow point with a free-point indicating tool.

13.3.1. Measuring The Pipe Stretch

A reasonable estimate of the depth at which the pipe is stuck can be obtained by calculation using Hooke's Law. Applying two different tensile loads (T₁< T₂) to the drilling string, two magnitudes of stretch (S₁< S₂) are measured.

Calculating: the differential stretch (E = S₂

-

S₁), differential pull (P = T₂

-

T₁) and applying Hooke’s Law, it is possible to determine the depth of free point (L) using the following formula. Wdp = Plain end pipe weight in kg/m E = Differential stretch in mm P = Differential pull in kN

P Wdp = Plain end pipe weight in lbs/ft E = Differential stretch in ins P = Differential pull in lbs

The value obtained is less reliable as deviation increases due to down hole friction.

Another minor inaccuracy is introduced by neglecting the changing cross section of the string at the tool joints.

13.3.2. Location By Free Point Indicating Tool

A Free Point survey shall be run to select the back-off point.

Free Point Indicators are essentially accurate strain gauges which measure molecular rearrangement between drag springs, setting dogs or electromagnets.

The tool is run on a logging cable through which measurements of torque and stretch are sent to surface read-out instruments.

The Free Point Indicator is lowered to various depths and, at each depth, tension and torque are applied to the string at the surface. The strain gauge indicates whether the pipe reacts at that depth to the applied tension and applied torque.

The read-out of the instrument is given in percentage i.e. 100% represents entirely free pipe.

Pipe which appears to be free in tension does not always react to applied torque. There is a greater chance of succeeding with the back-off if the pipe is free under both tension and torque.

Separate slim acoustic logs are designed to indicate intervals of stuck, partially stuck or free pipe, which may exist below the upper stuck point.

Interpretation of free point data is very subjective and susceptible to operator skill, hole condition, etc.

13.3.3. Back-Off Procedure

Drill pipe or drill collars can be unscrewed downhole by exploding a charge inside a selected tool joint connection, close to the stuck point.

Requisites for a successful back-off are the following:

• There must be sufficient minimum inside diameter.

• The charge must be accurately placed across the connection

• There must be sufficient string shot strength.

• Neutral or slightly positive tension is applied at the back-off point.

• Sufficient left hand torque must be applied at the back off point.

As a general rule, the first attempt to back-off should be made at the first connection above the free point. If there is a failure, the second attempt should be performed on the first stand above the free point. Subsequent attempts should be made moving upward one stand at a time.

13.4. FISHING

13.4.1. Inventory Of Fishing Tools

The following tools shall be always available on the rig for the various hole sizes drilled:

• Fishing jars to match the drill collars in use.

• Bumper subs to match the drill collars in use.

• Overshot and oversize guides with grapples, baskets and extension subs, to catch all diameters of tools in hole.

• Taper taps for drill pipe body and tool joints.

• Junk baskets or Globe-type baskets.

• Reverse circulation junk baskets.

• Junk subs.

• Fishing magnets.

• Milling tools.

• Re-dressing tools for 5" and 3¹/₂" sheared DP.

• Impression blocks.

• Fishing tools to catch electrical log tools (supplied by electrical log contractor) and relevant crossover.

• Safety joints.

13.4.2. Preparation

Before fishing operations the following preparations shall be carried out:

1) Apply the greatest accuracy to all measurements.

2) Draw a complete sketch of the equipment to be run, specifying lengths, inside and outside diameters and a description of each tool.

3) Make sure that the Contractor's personnel directly involved in operations is fully acquainted and familiar with equipment to be used and its limitations.

4) The fishing equipment should arrive to the rig fully inspected. Further inspection and maintenance shall be carried out on the rig if in prolonged use.

5) Keep mud properties in good conditions at all times.

6) Keep rig the equipment in good working conditions at all times.

13.4.3. Fishing Assembly

The standard fishing assembly consists of the following:

• Fishing tool + Jar and Bumper Sub + Drill Collars + Heavy Weight Drill Pipe + Drill Pipe.

• Use as many drill collars as is in the fish. If the required number of drill collars is not available on the rig, use a jar accelerator.

• A Safety Joint should not be run. Since the Safety Joint will not transmit left hand torque, it would not be possible to back-off below it using a string shot.

• However, a Safety Joint could be run between the catching tool and the jar when a non releasing tool such as taper tap is being employed.

• Avoid any restrictions in the bore of tools, run above the catching tool, which would prevent the use of a cutting tool or the back-off shot within the fish.

• Where losses are expected the use of a Circulation Sub in the fishing assembly should be considered.

13.5. FISHING PROCEDURES

In document Vanesa Parada Alfaya (página 140-148)