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Rotary Drilling Problems

R. S. Cartwright of L.W. Prunty Drilling Company, Ponca City, OK - (AIME Tulsa Meeting, October, 1928)

This paper covers a number of issues facing rotary drillers, including:
 
Rotary Drilling Controls
     The Halliburton Control
     The Hild Differential Drive
     Elimination of Drilling Controls Not Justified
Mud-laden Fluid
     Test for Mud
     Effect of Heavy Mud
     Removal of Cuttings
     Use of Clay Substitutes
     Mud for Deep Drilling
Distribution of Time in Rotary Drilling
     A Study of Four Oklahoma Wells
     Shutdown Time and Important Item
Relation of Weight on Rock Bit to Cutting Speed
     Torsional Stregth of Drill Pipe
     Angle of the Hole
     Recommended Weights on Bits
     Correct Weight for Disk Bit Operation
     Weight Indicators
     Weights Handled
     Use of Recording Gage
Discussions
 
The section on "Mud Laden Fluids" is reproduced in whole in this article. It is the first detailed description of the state of the art of drilling fluids I have seen.  If you would like to read the whole paper, please send me your email address requesting the 1928 Rotary Drilling Problems paper.
 

Summary of Mud-Laden Fluids

Cartwright points out that "the average rotary operator does not give much serious consideration of the condition of his mud."  He does make a plea for developing standards for the muds used.
 
Cartwright seems to use the term heavy or heavier mud to refer to an increased amount of solids in the mud.  A heavy mud, then, is a more viscous mud.  He also mentions increasing the density of the mud to control pressures using either barite or iron oxide.
 
The only test procedure he discusses is using a "centrifugal oil-testing machine."  He measured the amounts of cuttings, free water, cuttings and weight per gallon.  He metions that the amount of cuttings, including sand, should be minimized.  He mentions that cuttings increase the wear on slush pumps and can, when the flow stops, contribute to settling around the bit sticking the drill string.  He recommends a mud weight of 10 lb/gal for general drilling with free water not to exceed 2%, clay residue 19% and the cuttings 1%.
 
He recommends against using crude or fuel oil in the mud.  He does, however, recommend circulating oil at 25 vol % may free stuck pipe.
 
If a suitable clay is not available, he recommends using cement, lime or plaster.
 
In the Mud for Deep Secion he does not mention mud types or formulations but discusses pump output in gal/min, but mixes the terms volume and gal per min. which is confusing.    He states that most rig slush pumps operate at 70% or less efficiency.

The following is a copy of the full Mud-Laden Fluid section.

 

MUD-LADEN FLUID

 The primary purposes of mud-laden fluid, or mud as it is commonly termed, in its application to rotary drilling, are, first, to carry the cuttings from the hole, permitting them. to settle out in the circulating pits, and, second, to wall up the hole, once it is drilled, and seal off minor oil, gas or water sands. It  serves a further incidental purpose in acting as a cooling medium on the cutting tool, and to a limited extent as a lubricant in the hole. To accomplish these ends effectively it must be of approximately the correct density and viscosity, and must have the property of penetrating sands encountered. It  must also permit of easy circulation by the slush pumps.

The average rotary operator does not give much serious consideration to the condition of his mud. Generally it is either good or bad according to the individual driller's opinion and experience. Differences of as much as 3 lb. per gal. in mud in use on wells drilling in the same field, under practically identical conditions, have been observed. On one well a difference of as much as 11/2  lb. per gal. during a single period of 12 hr. has been checked. It  is not illogical to assume that certain standards could be worked out for the usual drilling conditions, and that greater drilling efficiency would result if  the correct type of mud were maintained, rather than depending on the hit-or-miss methods sometimes followed.

Mud is a colloidal suspension of finely divided clay in water, the clay being derived from the formations encountered in the hole and the mixture resulting from the cutting and churning action of the cutting tool with water flowing at high velocity from the slush pumps. When drilling in a clay-bearing formation there is a tendency to "make" mud-to form a heavier colloid-while in other formations, particularly sand, there is the opposite tendency, that is, for the mud to thin. Normally, there is an excess of mud-forming material drilled, such excess clay settling out in the circulating system, and making it possible, by mixing or thinning with water, to maintain a fluid of fairly identical characteristics.

Test for Mud

 As a quick and effective method of testing various samples of mud the writer employs an ordinary centrifugal oil-testing machine. When rotated at a high velocity for a known time the samples separate into their constituent materials, and by using graduated tubes it is readily possible to determine the exact amounts of cuttings, free water, etc., in each. Table 2 shows the results of such a test on four typical muds, taken from four different deep wells in the Mid-Continent district.

The several samples differ greatly, although the four wells from which they were taken were drilling under about the same conditions, and the muds were considered suitable by each operator.

It is obvious that the amount of free water should be maintained at a minimum, and that an excess indicates an unstable colloid. The clay residue is largely dependent on the weight of the mud per gallon and varies with it, but should, if the proper weight is maintained, be kept at the lowest possible point. With it, as with free water, the amount deposited is in direct ratio to the stability of the colloid. Cuttings, by which is meant foreign nonmud-forming constituents such as sand and shale, should also be limited as much as possible. Not only do they cause excessive wear in the slush pumps, but in the event of failure in the circulating system they are liable to settle around the bit, preventing either rotation or hoisting.

The most practical test for everyday purposes is a careful check of the weight per gallon. If the operator will do this, he will find it easy to maintain whatever standard his experiments prove most desirable. For all general purposes the writer has found that a mud weighing about 10 lb. per gal. is best. In such a mud the free water should not exceed 2 per cent., the clay residue 19 per cent., and the cuttings 1 per cent.

Effect of Heavy Mud

A mud which is too heavy builds up on the walls of the hole and hinders getting back onto bottom after having pulled out. The depositing action on the walls is particularly noticeable in sand formations, and while it is desirable that a sand should be sealed off, mud much heavier than 10 lb. per gal. has a tendency to so reduce the diameter of the hole that it is difficult to get a full gage cutting tool through, thus delaying drilling operations and often requiring some time for reaming the hole back to full gage. If the colloid is especially stable, it is, of course, possible to use a heavier mud without much settling, but such is not usually the case, and it has been the writer's observation that where mud as heavy as ll lb. per gal. has been regularly used, considerable time has been required to get back on bottom after a trip out of the hole.

Mud much below the standard of weight favored by the writer, is not sufficiently heavy to carry the cuttings from the hole, thus impeding the progress of drilling. Nor will it properly wall up the hole, making it liable to caving, with consequent loss of drilling speed and danger of sticking.

Removal of Cuttings

The most effective method which I have found for settling out cuttings is the use of a long ditch or conductor, through which the mud must pass as it is discharged from the hole. It has also heen found that passing it through a settling pit, where it is forced to spread out over a wide area, is also effective for this purpose and leaves the circulating pits free of foreign matter. If the weight of the mud will not be too seriously reduced it is also advisable to inject a small stream of water into it as it is discharged from the hole, in order to facilitate removal of the cuttings.

There is a general opinion among some drillers that the deeper a well is drilled the heavier the mud should be. This theory cannot be sound, for mud performs the same functions at 5000 ft. that it does at 2000 ft., and any increase in weight will result in the conditions already referred to.  This is assuming, of course, that heavy gas pressures will not be encountered.  If such is the case the mud must be as heavy as is practical to handle, or rather, sufficient to maintain hydrostatic pressure at the bottom of the hole to more than equal the sand pressure.

It is common practice with some operators, after reaching considerable depths to use some crude or fuel oil in the mud, their purpose being to provide a lubricant and a solvent for shale cuttings which might settle around the cutting tool if circulation were suspended. The writer has found that the addition of oil for this purpose is not desirable, as its actual effect is to coat the finely divided cuttings, as in the flotation method of ore separation, making it almost impossible to get them to settle out.  There is the further objection that a mud mixed with oil is difficult to handle in the slush pumps, as it has a solvent action on the balata valve disks commonly used in mud pumps. He suggests that the concentration of oil be not permitted to reach more than 2 per cent. by volume.

If the cutting tool is stuck, and can neither be hoisted nor rotated due to cuttings settled around it, it is often possible to free it by circulating oil.  On the few occasions that it has been possible for the writer to check the concentration of oil required for this purpose, it has been found that not more than 25 per cent. by volume is necessary, although in extreme cases pure oil and not an emulsion with mud is required.

Use of Clay Substitutes

When there is no reserve supply it is necessary to use some substitute for clay in mixing mud.  Ordinarily, cement, lime or plaster, which are easily procured, is used. They are not very satisfactory because, in spite of thorough mixing, they rapidly settle out.  Barium sulfate and iron oxide form true colloids, and either, if available, is much to be preferred as a mud-making substitute. For adding weight to mud, iron oxide is particularly desirable as it can be readily used to make a mud weighing from 15 to 16 lb. per gal. When the addition of such material is necessary, it is common practice simply to dump it in the circulating pits. By using a small tank conveniently located on the derrick floor, and connecting it directly to the slush pump suction pipe one can obtain exactly the mixture desired without sustaining any loss in getting it into circulation.

Mud for Deep Drilling

Due to the limitations of existing equipment, it has not been possible to make extensive experiments with reference to the proper volume of mud for deep drilling. The writer has found, however, that a volume of from 400 to 450 gal. per min. with a pressure of about 500 lb. per sq. in. is desirable for a hole of 11 or 12 in. dia. For maximum drilling speed the bit must be kept clean by washing, and all cuttings washed from the hole ahead of it.  In hard formations, where drilling speed is reduced, it is entirely possible and desirable in order to save fuel to reduce the volume, but it may be generally concluded that up to a volume of 450 gal. per min., cutting speed per unit of time is in direct ratio to fluid volume.  For holes smaller than 11 or 12 in. dia. it is probable that the volume should be calculated on the basis of the area of the hole for maximum efficiency. It  is considered entirely possible, in a small diameter hole, to " wash" the hole, that is, to circulate a sufficient quantity of fluid under high pressure, particularly in soft formations, to cause side wall caving, and it is suggested that this point must be kept in mind in all volume calculations.

A check of the fluid efficiency of slush pumps might prove interesting to many operators. Only by exercising considerable care has it been possible to maintain an efficiency of 70 per cent., which is probably considerably higher than the general field average. If  the recommended volume of at least 400 gal. per min. is to be maintained most of the slush pumps now in use must operate at not less than 65 per cent. efficiency.  It  will undoubtedly be found that efforts toward that end will be amply repaid in increased cutting speed.

In this connection, it may be mentioned that the development of large power-driven slush pumps is highly desirable. A number of manufacturers are now working on this problem, but it is certain that if other forms of power than steam are to be applied to rotary operation larger and heavier pumps for power drive than have hitherto been available must be supplied to meet the competition of steam equipment.

-- End --

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