To combat stuck pipe, BP Exploration Inc. has developed a training program and constructed several flow diagrams for drillers to refer to. Four of these flow charts are given in this first of a two-part series.
Fig. 1 traces reasons for increased torque during rotary drilling.
Fig. 2 charts causes for the drag trend increasing when moving the drillstring from static. Reasons for increasing resistance and drag while tripping in and out are covered in Figs. 3 and 4.
Flow charts for reaming, circulating, and running casing will appear in the concluding article of this series.
BP's training program addresses the basic causes for stuck pipe, surface indications, and actions that can be taken.
CAUSES
Causes are broken down into differential sticking and the following eight mechanical problems:
- Formation-related sticking in unconsolidated, mobile (flowing salts or plastic shales), fractured/faulted, geopressured, and reactive formations
- Key seating
- Undergauge hole
- Well bore geometry such as deviation and ledges
- Poor hole cleaning
- Junk
- Collapsed casing
- Cement related such as cement blocks and green cement.
The force to overcome differential sticking is:
Force = DELTA P x A x C
where:
DELTA P = Differential pressure between formation and mud column, psi
A = Simplified contact area, sq in.
C = Friction coefficient that depends on the nature of the permeable zone and the type of mud. The coefficient ranges from about 0.15 for oil-based mud to 0.35 for a spongy filter cake not in good shape.
In geopressured formations, the pore pressure exceeds the mud hydrostatic pressure, and brittle shales can cave into the well bore to stick pipe.
Reactive formations, such as gumbo shale containing large amounts of bentonite, hydrate and swell. Severe bit balling can occur. Although most likely in surface hole, these types of formations have been encountered at 10,000-11,000 ft in the Flexure trend of the Gulf of Mexico.
An unconsolidated formation can collapse into the well bore forming a bridge to prevent pipe movement. Examples of these types of formations are loosely compacted sand and gravels such as the producing formations in the Flexure trend of the Gulf of Mexico and the gravel and boulders in the upper portion of wells in Alaska.
Mobile formations exist in a plastic condition.
When the hole is drilled, mobile formations are unrestrained and flow into the well bore. Wet or plastic shales and salt can flow. On the surface, plastic shales can look brittle; but downhole they are not and can stick the drillstring.
Pieces of formation can break off and jam the drillstring when drilling in natural fissured formations or faulted zones. These pieces can vary from small up to boulder size.
Pipe rotation usually initiates key seating. After the key seat is mature, drill collars or tool joints can become stuck when tripping out. Key seats can form in the open hole and at the casing shoe. On the weight indicator, a key seat is a sudden increase, not gradual.
Well bore geometry with sharp doglegs can stick the drillstring. Also, ledging, hard formations interbedded with soft, can stick pipe.
A new bit can jam in an undergauge hole. Undergauged holes can be caused by drilling long sections of abrasive formations that can wear out the bit's gauge protection.
One of the most prevalent reasons for sticking pipe is inadequate hole cleaning. The overloading of the hole with cuttings can be caused by uncontrolled penetration rates, incorrect mud properties, insufficient circulation time, or insufficient annular velocity.
With the small clearance between the bit and casing, even small junk such as a cutter from a polycrystalline-diamond-compact (PDC) bit can stick the drillstring. For example, compare the OD of an 8 1/2-in. stabilizer with the 8.525 in. drift diameter of 9 5/8-in. 47 lb/ft casing.
Jamming of the drillstring can occur from cement blocks falling into the well bore from casing ratholes or cement plugs. Green cement is dangerous because it is not easily detected when running in hole, but can flash set when pressure is applied.
Reactive formations such as salt can collapse casing that is too light or old. Casing can also collapse if it is landed in too much tension. This effectively reduces its collapse rating.
JAMMED STRINGS
The main surface indications for stuck pipe are the inability to circulate, move string freely either up or down, and rotate string. Observing whether the problem is getting worse, staying the same, or getting better with time is also important.
If the string does jam, the necessary corrective action depends on the reason for sticking.
For differential sticking, the string should be slumped and rotated, the hydrostatic head needs to be reduced, and spot and releasing agents can be tried.
With a key seat, the string should be worked down and rotated, but for an under-gauged hole the string should be worked up.
If well bore geometry is the cause, work the string up if running into the hole, but down if pulling out of the hole.
In case of poor hole cleaning, work the string down to establish or improve circulation.
If junk is causing the sticking, the string needs to be worked up and down.
When encountering green cement, work the string up. Acid can be pumped if available. If cement blocks are causing the interference, the string should be worked up and down. Also acid can be used.
With collapsed casing, the string should be worked down.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.