Methods to Regain Circulation

Although the only "sure cure" for loss of circulation is to set casing, other measures frequently are successful. The first step to be taken when loss of circulation occurs is to make sure the drill string is in a safe zone. Then critically examine the operating practices and the mud properties at the time the loss was noted. Correct any deficiencies. Sometimes the waiting period and the needed modifications are sufficient to restore circulation. If, however, the hole still cannot be filled with water, determine the level at which the water stands.

Consider the possibility of loss at the casing seat. Proceed with such remedial measures as appear to be applicable under the conditions imposed by the drilling equipment in use and the formations exposed in the hole. Set up a definite sequence of methods to be tried. A mixture of sizes and types of lost-circulation materials usually is more effective than a single, ungraded material. Do not continue to pump away successive large volumes of mud of the same composition. Drilling blind (i.e., pumping enough drilling fluid to cool the bit and lift the cuttings into the zone of loss) in some instances is the least expensive procedure.

Treatments to regain circulation usually are applied batchwise. "Soft plugs" do not require long periods of waiting, as does cement, nor do they cause undesirable changes in the properties of the mud used in drilling. Bridging and plugging agents should be mixed with bentonite-water mud to a consistency as thick as can be poured or pumped into the hole.

The "gunk plug" is an adaptation of the gunk squeeze method used in oil-well drilling to plug fractures. The gunk plug forms when water comes in contact with bentonite that has been placed in the hole as a concentrated suspension in diesel oil. Water causes the bentonite to swell to form a putty-like mass. When the gunk plug is placed through the drill pipe, care must be taken to have enough diesel oil precede and follow the bentonite suspension to serve as a barrier to prevent any mixing with water before the suspension has been displaced from the pipe.
A rapidly filtering, high-solids slurry often is effective in sealing vugular and fractured formations by forming a bridging mass of particles. Diatomaceous earth, or diatomaceous earth and asbestos, is mixed in the proportions of one pound of solids per gallon of water and placed in the zone of loss. Fine fractures are bridged by the cake formed.

Unstable Hole

"Unstable hole" and "caving hole" are terms applied to a number of conditions involving such problems as hole enlargement, tight hole, fill on bottom after trips, and solids build-up in the mud. Frequently these difficulties are associated with shales. Terms such as heaving shale, swelling shale, caving shale, gumbo, mud-making shale, and sloughing shale are applied to such troublesome shales.

Causes of Caving Hole

Caving hole usually is caused by the combined effects of wetting by water and structural instability. After a water-sensitive material becomes wetted, mechanical breakdown usually follows.

Some instances of caving hole can be avoided by following good drilling practices. Erosion and breakdown of weak formations frequently are caused by: high velocity of mud in the annulus, pressure surges resulting from rapidly raising or lowering the drill string, fast rotation, vibration of the string (sometimes caused by bent pipe), and failure to fill the hole as the pipe is withdrawn. Such mechanical causes are within the control of the driller.

Excessive pump pressure breaks down weakly-cemented formations. Mud properties can affect pump pressure and often can be altered to reduce it. The viscosity and weight should be held at the lowest value that will satisfy the requirements for hole cleaning and control of fluids in the formation. Oil can be added to reduce the density. Certain organic polymers act as "friction reducers" for water and thus lower the pump pressure.

Holes drilled into folded and faulted zones may be structurally unstable. The first step in control is to stop any seepage losses by adding finely divided bridging materials, and to reduce filtration and thereby minimize wetting along fracture planes. Sometimes an increase in mud weight will aid in the stabilization of tectonically weakened formations.

Although almost any finely divided and poorly consolidated material may disintegrate in water, clays usually are responsible for caving by wetting effects. Because all clays take up water or hydrate, when a weakly consolidated formation is bonded by clay, wetting can cause the entire mass to disintegrate. Bentonitic clays can swell and bind the tools or block the hole after the pipe has been withdrawn. Clay sticking to the drill string can reduce the size of the annulus and cause a rise in pump pressure that leads to fracturing and loss of circulation.

Drilling Caving Ground

Mechanical breakdown of weak formations often can be avoided, as has been pointed out above. To supplement good drilling practices, maintenance of the lowest practical density and viscosity will minimize pressure effects. Lubrication of fracture planes by invasion of water or mud filtrate can cause caving. Often reduction of filtration solves this problem.

Commonly, caving is caused by the action of water on some constituent of the formation. Hence, control is based on limiting the effects of wetting. Usual methods are the use of (1) film-forming, protective colloidal materials, (2) solutions of salts, and (3) oil muds. The inhibitive effect of salts on clays is often combined with that of protective colloids or oil muds. Salts inhibit the swelling of clays, but only rarely do salt solutions alone assure the stability of clays.

Wetting effects can be greatly reduced by the protective or coating action of certain organic polymers. Such polymers in water form very viscous films on clay surfaces and thus oppose the entry of water into the clay.

Salts (such as calcium chloride) reduce the swelling of clays and may be useful.

Stuck Pipe

Fishing to recover stuck drill pipe is an expensive and unproductive (no hole being made) operation. As with loss of circulation, the driller can do much to avoid the problem of stuck pipe. In many instances of stuck pipe, the difficulty arises while circulation is stopped.

Causes of Stuck Pipe

Unstable hole is usually blamed for stuck pipe. Junk in the hole, balled-up bit, hole collapse because of loss of circulation, settling and packing of cuttings of heavy minerals, and thick filter cake are other causes. Differential-pressure is a major cause of stuck pipe in oil-well drilling.

Characteristics of differential-pressure sticking are (1) bit off bottom and pipe immovable, (2) permeable formations exposed in the hole, (3) circulating rate and pressure normal after drill string sticks.

The mechanism of wall-sticking of the drill rod is as follows. If the drill pipe stops moving while it is touching a permeable formation, the area of filter cake between the pipe and the hole wall is isolated from the rest of the drilling fluid. With the pump shut off, filtration continues because of the difference in pressure between the mud and the fluid in the porous rock. The pressure which had been carried by the isolated filter cake is transmitted to an equivalent area of the drill pipe. As filtration proceeds, more and more area of the drill pipe is pressed against the filter cake and stuck to it, isolated.

The ratio of pipe-to-hole diameters affects the area of pipe isolated from the hydrostatic pressure and also the relative increase in isolated area as filtration continues.

The rate of filter cake buildup is an important factor. The amount of friction between the pipe and the filter cake significantly affects the effort to release the stuck pipe.

How to Minimize Sticking

From the above discussion of the mechanism of wall-sticking, certain practices in mud control obviously help avoid stuck pipe. These are (1) minimum mud weight for least pressure differential and to assure low solids content for thin wall-cake, (2) low filtration rate for slowest buildup of cake when circulation is stopped, and (3) minimum friction between the wall-cake and the pipe. Least friction will be assured by keeping the mud free of -,and by adding a lubricating agent.

Release of Stuck Pipe

The force required to free the stuck pipe depends on (1) the area of contact, (2) the magnitude of the pressure differential, and (3) the friction between the rod and the mud cake. The usual treatment to release wall-stuck pipe is to place oil in the hole. Oil, because it is lighter than water or mud, reduces the differential pressure. Displacement of mud by oil also stops the buildup of filter cake. The ability of the oil to penetrate between the rod and the filter cake is enhanced by adding a surfactant to promote oil-wetting of the steel. As soon as the pipe is found to be stuck, displace the mud from the annulus with diesel oil containing one-half gallon of lubricant per 10 gallons of the oil. Continue to work the pipe and pump a few gallons of oil at intervals.