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Rotary Drilling Problems
R. S. Cartwright of L.W. Prunty Drilling Company, Ponca City, OK - (AIME Tulsa Meeting, October, 1928)
Summary of Mud-Laden Fluids
The following is a copy of the full Mud-Laden Fluid section.
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
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
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
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
Mud for Deep Drilling
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.
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