Rapid Growth of the Mud Industry

After the California Division of Oil and Gas Operations was established in 1915, the employment of engineers in the oil industry began to increase. Usually, the engineers were concerned only with production and little attention was given to drilling.20 Several discussions on the use of mud were reported, however, in "Summary of Operations California Oi! Fields"2 J and some of these were reprinted in oil industry publications.22

The mechanism of wall building in the drill hole was studied to some extent23 and the observation was made by Kerwan24 that mud solids do not penetrate sandstone even under a pressure of more than 2000 psi (140 kg/cm-) but that filtering action takes place, forming a sheath on the surface as shown in Figure 2-4.

Wall Cake Drilling Fluid
Figure 2-4. Laboratory test showing filter cake formation on sandstone. A = Bottom of nipple. B = Mud sheath on wall of hole. C = Extent to which water squeezed from mud fluid penetrated the sandstone. (From Kirwan?* Oil Weekly,)

The presence of this "wall" in an oil-bearing sand might either prevent oil shows while drilling or create difficulties in bringing in a well. Thus a negative requirement of the drilling fluid—that its use should not interfere with the collection of information while drilling or with final completion -was recognized.

Problems and methods of treating muds in the field began to receive some attention. Nevertheless, in most areas, "The average rotary operator did not give much serious consideration to the condition of his mud. Generally, it was either good or bad according to the individual driller's opinion and experience."25 Based on practices of the mining industry in ore beneliciation central mud reclamation plants were installed in some areas of concentrated drilling activity.26,27

In the late 1920s, several of the major oil companies initiated research into drilling and production practices. Drilling mud was recognized as a colloidal suspension and was accepted as a subject for investigation, primarily by chemists. Sellers of clays emphasized the colloidal character of their products. "" The development of gel structure when the flow of clay suspension stopped was accentuated as a desirable feature. The superior mud-making qualities of Wyoming bentonite were generally recognized.

From the ceramic industry, mud chemists adopted the methods of testing clay suspensions with such instruments as the MacMichael and the Stormer viscometers {see Figure 2-5). However, thinning agents, such as caustic soda and sodium silicate, useful in the ceramic industry, were generally not effective in muds. Alkaline solutions of various natural tannins were suitable,29 as were the pyrophosphates and polyphosphates.30

H.N. Marsh31 proposed a simple, rugged funnel as a means of measuring the apparent viscosity of muds in the field. H.N. Herrick32 emphasized the distinct difference between the flow behavior of muds and liquids. He pointed out that the apparent viscosity as measured with the Stormer viscometer could not be used to estimate the pressure required to pump mud through pipes. Measurements made with a pressure efflux viscometer, however, could be used.

The introduction of new and greatly improved drilling equipment in the early 1930s u stimulated the investigation of the role mud plays in drilling performance. Through local and national meetings, both the Division of Production of the American Petroleum Institute and the Petroleum Division of the American Institute of Mining and Metallurgical Engineers afforded outlets for papers on drilling mud. The Institution of Petroleum Technologists, Trinidad Branch, provided a forum for the discussion of developments in mud practices in that area. The Oil Weekly, The Oil and Gas Journal, and The Petroleum Engineer printed many of the papers presented at API and AIME meetings as well as numerous contributed articles on muds.

At the first World Petroleum Congress in 1933, five papers were presented on mud. More papers were published on drilling mud between 1930 and 1934

Stormer Viscometer Dimensions

mud to over an inch with ordinary field muds. S. Gill3' cited tests on Gulf Coast sand cores thai showed little penetration of mud or filter cake and concluded that "properly proportioned muds" would do little damage unless the lnfiltered water affected the flow of oil. C.P. Parsons3* reported penetration of mud up to an inch in unconsolidated sand and traces of mud to a depth of several inches.

Mr. Rubel35 initiated a study by Union Oil Company of mud cake formation and of filtration into sand cores. Jones and Babson3'J reported that tests were conducted at pressures up to 4,000 psi (280 kg/cm3) and temperatures up to HS^F (135JC). Obvious differences were noted in the filtration characteristics of several field muds. The immediate application was not directed to avoidance of productivity impairment, however, but to hole stability. Tests showed that when problems existed with caving formations, mud cakes were thick, and excessive quantities of water were lost. When these muds were replaced by muds that produced thin filter cakes and allowed small losses of water, the difficulties either disappeared or were reduced greatly. The laboratory filtration equipment, however, was not suitable for use in the field. Phil H. Jones40 later described a simple, rugged

Figure 2-6, Static filtration tester, (From Jones.*0 Courtesy of API.)
Filter Press Design Drilling Fluid Cap

Figure 2-7. Static-performance tester. (From Jones.40 Courtesy of API.

1 Air Inlet Pipe

2 Cap

3 Reservoir

4 Screw Press -5 Filter Paper

6 100 Mesh Screen

7 Perforated Plate

8 Bottom Plate

9 Graduated Cylinder

Figure 2-7. Static-performance tester. (From Jones.40 Courtesy of API.

device that was practical for field use. This instrument (shown in Figure 2-6 and 27), with minor modifications, continues to be the routine filter press for the evaluation of mud performance at the well site.

The filter press (or "wall-building tester" as it was called) provides a useful tool, helping the mud engineer relate the mud's physical properties to specific hole problems. The subject of filtration is dealt with in Chapter 6.

In spite of the general recognition of the role mud weight plays in the control of pressure, blowouts still occurred too frequently. In the Gulf Coast of Texas, the Conroe field (known from bottom-hole pressure measurements to have had normal pressure) was the scene of several disastrous blowouts. Humble Oil and Refining Company (now Exxon) engineers investigated the problem and noted a direct association between blowouts and withdrawal of pipe from the hole, even though mud weight was more than adequate to overcome bottom-hole pressure. In a field study, a subsurface pressure gauge was either attached to the bottom of the drill pipe or left in the hole while the pipe was withdrawn. G.E. Cannon41 reported that the pressure reduction, or "swabbing action," was sufficient to cause a blowout with muds that developed high gel strength on standing. The swabbing effect was not dependent on the viscosity of the mud (as measured in the Stormer viscometer at 600 r.p.m.) nor on the density, but on the strength of the gel that developed on standing. The size of the pipe-hole annulus and the length of the pipe were also significant factors.

The solution to the problem was to use low-gel-strength mud- not to raise mud weight. Measurement of gel strength at the well was considered necessary for safe control of mud properties.

Several investigators had emphasized the importance of gel-forming colloidal clay in mud performance (For examples, see references42-43 44). Development of gel structure at low clay concentration was an indication of colloidity. Until the filter press became available as a field instrument, however, there was no practical method of estimating colloidity. Because filtration properties are dependent on the nature and amount of colloidal material present in the suspension, the filter press provides an overall evaluation of the colloidal fraction and, as such, has been of major importance in the development of drilling fluids technology.

With the introduction of various devices for the measurement of mud properties, and with variations in procedures as well, the need for uniformity in instruments and methods was recognized. The Houston Chapter of the American Petroleum Institute Division of Production in 1936 began a study to formulate uniform practices.4s This report led to the adoption of Recommended Practice on Standard Field Procedure for Testing Drilling Fluids, as A.P.I. Code 29. Through the years, additions and modifications have been made under the Committee on Standardization of Drilling Fluid Materials. The publication has become A.P.I. RP 13B,46 which is reprinted at intervals and the number of pages has grown from six in 1938 to thirty-three in 1978.

The need for better supervision of the mud at the rig was realized as mud-related drilling problems were recognized. The American Association of Oil Weil Drilling Contractors asked the Petroleum Extension Service of the University of Texas for help in crew training. Around 1945, under the supervision of John Woodruff, courses began for field men in active drilling areas. A training manual, Principles of Drilling Mud Control. appeared in 1946 and was revised and expanded as the program continued. The API Southwestern District Study Committee on Drilling Fluids, under the guidance of J.M. Bugbee, assumed responsibility for preparation of the manual in 1950 and edited the 8th through the 10th editions (1951 1955). The 11th edition (1962) was edited by H.W. Perkins and the 12th edition {J 969 i by W F. Rogers. Principles of Drilling Mud Control has served as the basic manual for the training of thousands of men directly concerned with the practical application of drilling fluids.

Development of Mud Types or Systems

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