STUCK PIPE-CONCLUSION TEAM EFFORT ESSENTIAL IN COMBATING STUCK PIPE

Four additional flow diagrams, developed by BP Exploration Inc., are presented here to help the drilling team understand the likely symptoms that may lead to stuck pipe.

This is the concluding article of a two-part series that began in OGJ on Apr. 1, p. 61.

Fig. 1 traces possible causes for increased torque or required increases in weight while reaming in. Fig. 2 charts causes for torque and drag increasing while reaming out of hole.

Reasons for increased drag and resistance while circulating out are diagramed in Fig. 3. Fig. 4 gives probable causes for increases in downward resistance while running casing or after a connection.

BP's training course, for which these flow diagrams were developed, emphasizes the need for a team effort to combat stuck pipe. The driller, mud logger, and mud engineer are the three people most likely to notice the signs of impending stuck pipe first. Their observations should be communicated to the drilling supervisor, contractor supervisor, or drilling engineer so that the entire drilling team can develop the best solution.

DRILLER

From a typical drilling console, the driller can observe fluctuations in the drillstring weight, mud-pump pressure and strokes, tong torque, and rotary speed and torque.

Depending on the cause for the tight hole, the indicators will be affected differently. For instance, if mobile formations are encountered, gradual increases will be observed in the drag and resistance on connections, the torque, and the rate of penetration trend. The pump pressure will increase.

Formations over the shale shaker will change, and connection time will increase because of the need to ream.

In reactive formations, drag and resistance on connections and pump pressure will increase. A gradual increase will be observed in the torque, and a gradual decrease will occur in the rate of penetration.

Large quantities of gumbo will be observed over the shale shaker, and connection time will increase because of reaming.

MUD LOGGER

The mud logger is vital in analyzing the types of formations being drilled.

Indicators that he can observe are:

Cavings
d-exponent
Shale density
Gas in mud
Flow line temperature
Shale swelling test
Shale factor
Time scale.

When drilling reactive formations (swelling shales or gumbo), large quantities of cavings will occur. Caving morphology will be masked because of the swelling. These cavings can accumulate to form mud rings.

The shale swelling test is a good indicator of reactive formations.

Also, the shale factor will have a high value. The tight hole problem is time dependent and is usually delayed in reactive formations.

In geopressured formations (brittle shales), the cavings will appear as a large quantity of splinters or blocky with striations. The d-exponent trend will have a marked decrease. Shale density will also decrease.

The shale swelling test may provide some indication of swelling in geopressured formations.

The shale factor will have higher than average expected values. A tight hole may occur at any time in geopressured formations.

Cavings are not present in fractured and faulted formations, but the gas in mud may increase if hydrocarbons are present.

A tight hole tends to occur immediately after a fractured and faulted formation is drilled.

Salt and plastic clays are two mobile formations. A small quantity of cavings may be present in some plastic clays. The shale swelling test may show swelling, and the shale factor will have a relatively high value if plastic clays are penetrated.

Tight hole development may be a slow process in mobile formations.

Cavings are not present when drilling unconsolidated formations. If the unconsolidated formation is sandstone, the d-exponent will follow the same trend as for sand formations. The gas-in-mud indicator may increase if the formation is hydrocarbon bearing. A tight hole tends to happen immediately after the unconsolidated formation is drilled.

MUD ENGINEER

The mud engineer is responsible for controlling the mud specifications, recognizing problems, and suggesting remedies.

As a rule of thumb, the more inhibited the mud, the more stable the hole. Not including oil-based mud, increasing the salinity will increase the inhibition of a mud.

A listing of muds in order of increasing inhibition is as follows:

Freshwater, bentonite mud
Freshwater, lignosulfonate mud
Seawater mud
Calcium-based mud
Potassium mud
Magnesium mud
Magnesium/potassium mud
Polymer mud
Oil-based mud.

Potential problems may take place by either increasing or decreasing a property of the mud. For example, raising the mud density will increase the overbalance in the hole and may cause differential sticking and lost circulation.

Lowering the mud density will decrease the overbalance but may cause an influx from mobile, geopressured, or unconsolidated formations.

Increasing the rheology of a mud (viscosity and gel strength) can cause higher surge/swab pressures that can result in formation instability, influx, or lost circulation. But decreasing mud rheology can reduce hole cleaning by diminishing the cutting suspension and carrying capacity.

High fluid loss is always undesirable. Differential sticking can result from increased filter cake thickness caused by high fluid loss. Also, formation instability can arise because of excessive filtrate entering the formation.