In many areas considerable thicknesses of rock salt must be penetrated. Solution and erosion of these beds can cause excessive hole enlargement which in turn may be a source of future trouble and expense.
The principal means of avoiding these problems is to prepare a salt saturated mud system prior to drilling the salt, thus avoiding the dissolving effect.
Some areas are characterized by shale sections containing bentonite or other hydratable clays which continually adsorb water, swell, and slough into the hole. Such beds are referred to as heaving shales and constitute a severe drilling hazard when encountered. Pipe sticking, excessive solid buildup in the mud, and hole bridging are typical resultant problems. Various treatments are sometimes successful, such as
Depending on the severity of the occurrence, any of the above may be satisfactory. The last resort is changing to oil base or water-in-oil emulsion mud.
A blowout occurs when encountered formation pressures exceed the mud column pressure which allows the formation fluids to blow out of the hole. This is the most spectacular, expensive, and highly feared hazard of drilling. Needless to say, proper mud density is the principal factor in avoiding this problem; however, borehole pressure reductions below mud column pressures are in many instances caused by too rapid withdrawal of the drill string. This is known as pipe pulling suction (swabbing) and has become recognized as a large factor promoting blowouts. This is particularly true in areas where a very delicate overbalance of formation pressure is necessary. The magnitude of the pulling suction depends on speed of pipe withdrawal, hole-pipe clearance, and mud viscosity and gel strength. This is a further argument for keeping mud viscosity at a minimum.
Lost circulation is defined as the loss of substantial quantities of whole mud to an encountered formation. This is evidenced by the complete or partial loss of returns (returning annular mud flow). The annular mud level may drop out of sight and stabilize at a pressure in equilibrium with formation pressure. Lost circulation occurs when formation permeability is sufficiently great to accept whole mud; the voids are too large to be plugged by the solids (clay, cuttings, etc.) in the mud. A further obvious requirement is that the mud column pressure must exceed the formation pressure. Some of the undesirable effects of lost circulation are the following:
Bugbee26 has shown that permeabilities in excess of 300 darcys are necessary for the loss of mud solids to permeable sands. This conclusion was based on a relationship between pore size and permeability (recall from Chapter 2 that dimensionally, permeability is an area). The only clastic rocks which can exhibit such permeabilities are very coarse gravels. These beds are rarely encountered in drilling and they are probably a minor contributor to lost circulation in general.
Vugular limestones having finger size (and larger) holes, however, are a common source of trouble in many areas (West Texas, Mid-Continent).
Faulted, jointed, and fissured zones may occur in any rock type and are probably the most common source of lost circulation. In these zones, the fractures are held closed by the forces existing in the earth at that depth. However, when the mud column pressure exceeds the stress holding a fracture closed, it opens, accepts the mud, and loss of circulation occurs. Further, rocks which are not naturally fractured may be broken down (fractured) by excessive bore hole pressures. High mud density contributes to this; numerous writers have shown, however, that pressure surges caused by rapid pipe running are a major cause of induced fractures.26-28 This is the exact opposite of the pulling suction which was mentioned as a blowout cause.
The hazards created by cavernous or open fissured formations need no explanation, as it is obvioq^ that they may constitute the most severe case of lost circulation. Natural caverns and open fissures several feet thick have been drilled into. In Illinois, old mine tunnels are occasionally encountered which furnish a considerable problem. Severe cases in this category cannot be successfully plugged with bridging materials and cement. Consequently, drilling without returns (blind drilling) is continued for some distance below the cavernous section. Casing is then set through the zone and cemented; after this, normal drilling is resumed.
The alteration of mud properties, primarily density, may help or even cure some lost circulation problems. This approach includes the use of air, gas, and aerated mud in those cases where they are applicable. Fluid flows from the formation (including blowouts) may eliminate this course of action. Sometimes a waiting period allowing the formation to adjust to the new pressure conditions may alleviate the situation.29 The spotting of plugs containing mud, cement, and lost circulation materials opposite and into the problem zone is a common cure.30 Spotting involves pumping the material down the drill pipe with a sufficient (calculated) volume of displacing fluid behind it to position the plug at the desired level in the hole. This requires a knowledge of the zone's location, as it is not necessarily at the bottom of the hole. The location of the loss zone may be determined with various downhole surveying devices.31
Lost circulation materials are commonly circulated in the mud system both as a cure and a continuous preventive. These materials are undesirable from a mud property and pumping equipment standpoint, but are tolerated as a necessary evil. While nearly everything has been used for this purpose, the more common materials are listed in the following classifications:
Fibrous and lamellated materials are most effective in coarsely permeable rocks where the voids are relatively small. Larger openings require use of a granular material having sufficient strength to form a bridge across the void. These effects are well illustrated by the work of Howard and Scott.82 In their experiments, mud containing various concentrations and types of lost circulation materials was circulated through different sizes of slots or simulated fractures. The slots were considered sealed when the plugging materials withstood 1000 psi differential across the fracture. The main conclusions of this study were:
The materials evaluated and the maximum size fractures sealed by each are shown in Figure 6.18.
Fractures cause a major portion of severe lost circulation problems and since, in general, their sealing requires a granular material having sufficient strength to bridge the opening, let us consider the mechanics of bridging. An early worker on this subject was Coberly,33 who was primarily concerned with determining the proper liner slot size necessary to prevent the entry of unconsolidated sand into the bore hole of producing oil wells. For the time being we will skip Coberly's problem and look only at the results of his work as pertaining to lost circulation. His problem, then, was to determine the size slot necessary to exclude certain sized sand (a granular, bridging material). This is the exact reverse of studying what granular particle sizes will bridge a given slot, and the results should be equivalent.
From numerous bridging tests both with steel balls and actual oil sands, Coberly reached the following conclusions which have long been verified by practice:
Applying these principles to lost circulation additives, one would expect the action of bridging materials to follow the same laws. Application of these principles also requires that the granular materials have sufficient strength to stand the pressure differential across the bridge. This lack of strength explains the failure of many materials to seal large slots.
Properly designed bridging materials then should seal fractures wider than themselves. The bridge is initiated when two of the larger particles start into the fracture at the same time and then lodge against each other.
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