Chapter 21 - Lost Circulation

Chapter 21

Lost Circulation What is Lost Circulation? Lost circulation—the significant and continuing loss of whole mud or cement slurry to a formation—is one of the most common and troublesome of downhole problems. It has been a hindrance to drilling, completion and workover operations ever since rotary rigs first came into use, and it continues to have a profound negative impact on well economics. Estimates of what lost circulation problems cost the drilling industry worldwide, directly and indirectly, run into the hundreds of millions of dollars annually. Although drilling ahead and primary cementing are of particular concern, lost circulation can occur during any well procedure that involves pumping fluid down the hole. Indications of lost circulation may range from a gradual drop in pit level to a partial or complete loss of returns. In extreme cases, the fluid level in the annulus may drop rapidly, sometimes by hundreds of feet. Lost circulation invariably results in higher costs for materials, services and additional rig time. Depending on the timing and severity of its occurrence, it can lead to the loss of formation evaluation data, because the information normally obtained from mud returns and drilled cuttings is no longer available. It can also result in reduced well productivity if the loss zone is also a potential pay interval. If the wellbore fluid level drops far enough and fast enough, it can allow fluid to enter the wellbore from a higher-pressured formation. And when this influx or kick does occur, it makes well control all the more difficult because of the inability to circulate kill fluid.

Occurrences For lost circulation to occur, there must be: (1) a formation with flow channels that allow passage of whole fluid from the wellbore, and (2) an overbalance, or positive pressure differential between the wellbore and the formation. Both of these conditions must be present, although one or the other of them may predominate. A very small overbalance may be sufficient to drive fluid into a highly porous and permeable rock, for example, while even a relatively non-porous, impermeable rock can accept amounts of fluid if the overbalance is high enough to induce hydraulic fracturing. Some types of rocks, because of their high primary porosity and permeability, are practically designed to cause lost circulation problems. Unconsolidated formations, gravel beds, loose conglomerates and shallow or highly depleted sandstones have long been recognized as having natural lost circulation tendencies. They most often cause gradual drops in pit levels, although continued drilling time and additional exposure to the wellbore may result in partial or complete mud losses (Fig. 21-1). Secondary porosity and permeability—as occurs in naturally fractured sandstones, shales and carbonates—are also conducive to lost circulation. Natural fractures may be either horizontal or vertical, depending on a rock’s depth, mechanical characteristics and stresss environment. In a horizontal fracture network, lost circulation may first show itself as a gradual lowering of the pit level, with a complete loss of returns occurring as additional fractures are encountered. Vertical 21.1

fractures, on the other hand, will take progressively increasing amounts of mud as drilling progresses and more of it is exposed (Fig. 21-2).

Fig. 21-1 Lost Circulation in a Porous and Permeable Zone

Fig. 21-2 Loss of Returns in a Naturally Fractured Formation If lost returns occur in an area where offset wells have not experienced lost circulation, then the problem is likely the result of induced fracturing during well operations, rather than the presence of a natural fracture network (Fig. 21-3). Most induced fractures are related in some way to the drilling fluid or cementing programs, although sometimes the well architecture may itself be a contributing factor, such as when a surface or intermediate casing string is set too high. Mechanical failures, such as leaks in a shallow casing string, can also result in lost circulation (Fig. 21-4).

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Fig. 21-3 Lost Circulation due to Induced Fracturing

Fig. 21-4 Lost Circulation due to Casing Leak The most severe lost circulation problems occur in cavernous or extremely vugular formations (Fig. 21-5). These are typically limestones that have been leached by water. The void spaces in these formations can be large enough that when they are encountered, the drill string may actually drop up to several feet preceding a sudden, complete loss of returns. Rough drilling may occur just before a bit encounters a cavernous zone.

Fig. 21-5 Lost Circulation due to Caverns or Vugs

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Prevention To prevent or at least minimize problems due to lost circulation, it is necessary to address the conditions that cause it to occur, either by sealing off the problem formation, or by reducing the wellbore pressure differential. Prevention of lost circulation starts with the mud system. The best way to seal off a potential loss interval is to keep filtrate losses to a workable minimum and maintain a thin, firm, impermeable filter cake along the borehole wall. In an area where porous, permeable zones are a known problem and a low-weight, low-solids mud is being used, it may be a good idea to pre-treat the mud with solid lost circulation material (LCM). This material should be fine enough to pass through the shale shaker with the other mud components, and sized so as to plug small openings in the formation. If mud losses are fracturerelated, however, such pre-treatment will not be effective, especially in weighted mud systems. The mud weight schedule is perhaps the single most important factor in preventing lost circulation. The closer the hydrostatic pressure of the mud column gets to the formation fracture pressure, the more likely lost circulation becomes. Local drilling conditions and well parameters will determine how much of an overbalance is required to optimize drilling performance, control formation pressures and allow for abnormal or unexpected conditions. If the well cannot be safely drilled using a conventional mud system, then a rig equipped for underbalanced drilling might be an option to consider. Even when the mud weight is well below that required to fracture the formation, lost circulation can still result from a high Equivalent Circulating Density (ECD) caused by excessive pump pressure and poor hydraulics practices. The mud’s rheological properties (viscosity, yield point and gel strength) should be specified to maintain the desired cuttings transport and suspension qualities, but at the same time should. allow the well to be circulated at an optimal pump pressure. High surge pressure is a major contributor to lost circulation. Surge effects can be minimized by avoiding excessive speed when tripping in the hole, not spudding through bridges or other restrictions, breaking circulation gradually and, again, maintaining circulating at the minimum pump rate needed to ensure adequate hole cleaning. Selection of casing depths is crucial to preventing lost circulation, and is closely related to the design of the mud program. In many wells, it is necessary to set one or more strings of intermediate casing to protect low-pressured zones from the higher mud weights required for deeper intervals. In selecting these casing points, the well planner should ensure that they are not themselves located in potential loss zones.

Diagnosis There are a number of methods for combating lost circulation, each of is effective when properly applied. Selecting the best method for a particular situation involves three diagnostic steps: (1) determining at what depth the loss is occurring, (2) describing the type of loss zone and (3) evaluating the severity of the loss.

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Intuitively, one might expect lost circulation to occur at or near the bottom of a well, where the Equivalent Circulating Density is at its highest. It is far more common, however, for the loss zone to be farther up the hole-typically near the casing shoe—where fractures may have been opened, re-sealed and then re-opened as the well is drilled deeper with increasing mud weights. Techniques for finding lost circulation zones commonly involve the use of production logging devices, including spinner, temperature and radioactive tracer tools. Once the loss zone is located, it may be described in terms of lithology and the type of loss occurring. If there is a slow but steady decrease in pit level, for example, and mud logs or other data indicate that the loss zone is a sandstone, then high permeability and porosity are likely the causes of the problem. If the loss of returns is sudden, on the other hand, induced fracturing is the most likely cause. The severity of the problem is expressed in terms of the amount of mud lost and the static fluid level drop. Seepage involves a gradual lowering of the pit level (generally from one to 10 bbl/hr). Losses ranging from 10 to 50 bbl/hr are considered partial. Complete losses involve fluid level drops ranging from 200 to 500 feet, while severe complete losses describe drops of more than 500 feet where there is evidence of vugs or caverns. In the worst case of lost circulation, an underground blowout, the loss zone is not only taking drilling mud, but also formation fluid from a higher-pressured interval. Remedial Measures Techniques for controlling lost circulation are designed to seal off the loss interval. They may entail (1) allowing the formation to heal itself by removing the conditions that caused the lost circulation; (2) using Lost Circulation Material (LCM) or drilled solids to bridge off the interval; (3) spotting a high-viscosity plug across the interval; (4) squeezing the interval with cement; (5) setting pipe across the interval; or (6) abandoning or sidetracking the interval. Depending on the location, type and severity of the problem, remedial measures may employ a combination of these techniques. No one method is applicable to all types of lost circulation.

Removing the Conditions that Cause Lost Circulation When lost circulation results from induced fracturing, a pause in operations or a change in drilling practices may help to eliminate the original cause of the fracture. In some cases, stopping circulation and allowing solids to build up against the borehole wall may heal an induced fracture. One such procedure involves pulling the pipe into protective casing or a secure portion of open hole, shutting down the mud pumps for a minimum of 6-8 hours, and in the meantime attempting to fill the hole with water, and then gradually resuming circulation in stages. Lowering the mud weight is an effective way of reducing the hydrostatic pressure of the mud column and thus the pressure differential into the formation. This is only feasible, of course, if there is no danger of a kick. Another step would be to adjust the mud viscosity and gel strength based on hole conditions: either increasing the viscosity and gel strength to help slow the flow of mud into permeable zones, or decreasing them in order to reduce the pump pressure required for circulation, thereby lowering the equivalent circulating density and reducing losses from induced fractures. Decreasing the pump rate can likewise lower the circulating pressure.

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Lost Circulation Materials (LCM) In porous, permeable zones, lost circulation most commonly results from inadequate bridging agents or wall-building characteristics. Fixing the problem is thus a matter of adding solids to seal off the interval. In some cases, simply drilling ahead and allowing the formation cuttings to bridge the loss zone may be enough to restore circulation. In others, it will be necessary to add solid LCM to the mud system in concentrations not exceeding 10 - 20 lbm/bbl [28.5 - 57 kg/m3]. Table 21-1 lists general classifications of solid lost circulation materials and gives some examples for each category. Solid lost circulation materials are most effective when different textures and sizes are used in combination. These materials are only suitable for plugging porous, permeable formations— perhaps because of their generic name, operators have often wrongly used them as an all-purpose remedy for all types of lost circulation.

Slurries and Plugs An alternative to adding LCM on a system-wide basis (and an effective technique for unconsolidated permeable zones) is to spot a high-filter-loss slurry, consisting of water mixed with conditioning additives and bridging agents, directly in the lost circulation zone. To spot the slurry, a drill bit (without jet nozzles) is run to the top of the loss zone, and the slurry is displaced to the end of the drillpipe. The slurry is then squeezed into the formation by closing the blowout preventers and pumping at low pressure. As water is squeezed out of the slurry, the bridging agents form a seal across the interval. Table 21-1: Common Solid Lost Circulation Materials Classification Fibrous materials

Flake (lamellar)

Granular materials

Examples Wood fiber (shredded wood, sawdust), paper pulp, glass fiber, cotton fiber, animal hair, leather fiber, straw and shredded tires

Cellophane, mica (fine and coarse), plastic laminate, wood chips. Nut shells (fine, medium, coarse and very coarse), ground plastics, seed grains, coarsely ground rock materials (e.g., bentonite, asphalt, limestone).

Description Relatively little rigidity. Can be forced into large openings, where they bridge over and form a mat or base that acts to seal off the formation when solids from the drilling fluid deposit on it. If the openings are too small for the fibers to enter, a bulky, easily removable external cake may form on the walls of the hole. Not recommended for oil-base muds. Not normally used in cement because they tend to plug surface and downhole cementing equipment. Also may contain organic chemicals that can seriously retard cementthickening time Sealing action similar to that of fibrous materials. Cellophane products are not recommended for use in oil-base muds.

Tend to form a bridge just inside the opening of the pore. Must contain particles that approximate the size of the opening, as well as a gradation of smaller particles to form a seal. Granular materials may be used in oil-base muds.

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Combination

Selected blends of fibrous, flake materials and grain LCM

Blended products containing cellophane flakes are not recommended for use in oil-base muds.

In naturally fractured formations, a plug may be pumped across the loss interval. These plugs are designed to be pumpable at the surface, and then to develop shear strength when placed downhole. The two main types of plugs are soft plugs, also known as reinforcing plugs, viscous pills or gunk squeezes, and hard (cement) plugs. Soft plugs typically consist of a bentonite/diesel oil base (for water-base muds) and a water base (for oil muds), with additives such as LCM, cement and polymers added for special applications. These plugs develop a viscous, gel-like consistency, and offer the advantage of being deformable under pressure surges, making them less likely to break down. Hard plugs can also be used to seal off natural fractures. They have high compressive strength and enough flexibility to allow for good control of their flow and setting properties. They have a greater tendency than soft plugs, however, to break down under pressure surges, and can be harder to drill out. In soft formations, a hard plug may act as a whipstock and cause the bit to sidetrack. Many of the concerns about the use of cement as a lost circulation material have been alleviated in recent years through the development of light weight slurries, crosslinked cements and other advances, and these same advances have aided in preventing lost circulation during primary cement jobs. Cavernous formations present a special challenge. If the void spaces are small enough, and if the formation can withstand the pressure surges inherent in drilling it may be possible to seal the caverns. Otherwise, the operator might attempt to drill blind (i.e., with no returns) or underbalanced (using air, foam, mist or aerated mud as a drilling fluid), and then cement casing across the loss interval.

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References 1. Ivan, Catalin, Bruton, James and Bloys, Ben: “Lost Circulation can be Managed Better than Ever.” World Oil. Gulf Publishing Company, Houston, TX USA (June 2003). 2. Romero, S.N., Monroy, R.R., Johnson, C., Cardenas, F. and Abraham, G.A.T.: “Preventing Lost Circulation Using Lightweight Slurries with Reticular Systems: Depleted Reservoirs in Southern Mexico.” Paper SPE 92187, presented at the 2004 SPE International Petroleum Conference in Mexico held in Puebla, Mexico, 8-9 November 2004. 3. Adams, N. and Ogle, K. “Drilling Problems and Drilling Optimization.” International Petroleum Industry Multimedia System (IPIMS). IHRDC, Boston, MA, USA. 4. Moore, Preston L.: Drilling Practices Manual, Second Edition. PennWell Publishing Company, Tulsa, OK USA (1986). 5. Mata, F. and Veiga, M.: “Crosslinked Cements Solve Lost Circulation Problems.” SPE 90496, presented at the SPE Annual Technical Conference and Exhibition held in Houston, TX USA, 26-29 September 2004.

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