Drilling With a One-Step Solids-Control Technique
- A. Hayatdavoudi (U. of Southwestern Louisiana)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1989
- Document Type
- Journal Paper
- 31 - 40
- 1989. Society of Petroleum Engineers
- 4.2 Pipelines, Flowlines and Risers, 2.4.3 Sand/Solids Control, 1.11.4 Solids Control, 4.3.4 Scale, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.10 Drilling Equipment, 1.11 Drilling Fluids and Materials, 1.2.3 Rock properties, 7.2.2 Risk Management Systems, 5.5.2 Core Analysis, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 1.6 Drilling Operations
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The objectives of this paper are to report a new philosophy in solids control and field practice, referred to as fluid conditioning, to make a comparative evaluation of surface solids-control equipment, and to show details of a comprehensive mineralogical and physicochemical analysis of removed solids. For brevity, only a few field results are emphasized. For example, in a weighted mud system of 15.2 lbm/gal [18.2 x 10(2) kg/cm3], the adjustable concentric device of fluid conditioner rejects a greater percentage of 0.5-to 1.0- m solids than a centrifuge. In an unweighted mud system of 9.5 lbm/gal [11.4 x 10(2) kg/m3], fluid contaminants such as calcite, dolomite, and salt are rejected through underflow. Another field example based on microscopic and scanning electron microscopic (SEM) size analysis showed that in a 12-lbm/gal [14.4 x 10(2)-kg/m3] mud system, it is necessary to increase the operating pressure by a few psi to reduce the amount of 6- m material. Our research in the area of particle size analysis of small, oilfield particles - e.g., smectite, sepiolite, Wyoming bentonite, and barite from bags of dust collectors-reveals that a quick-fix Fraunhofer diffraction analysis in the range of 10 m and less is totally invalid in quantitative works. Finally, through the analysis of fluid rejects, we concluded that after the rig shaker (assuming that the rig shaker performs well), the one-step solids-control fluid conditioning can deliver desirable rheological properties and a reasonable size range for the suspended material.
The importance of solids control in any drilling operation became evident when well costs and drilling problems could be correlated to drilling fluid properties, size, and concentration of suspended solids. Problems are manifested through reduced hole size in highly permeable or highly plastic and overstressed zones, decreasing equipment and bit life, slow penetration rate, and partial increase in system pressure losses because of viscous effects. Through downhole solids control and the application of vortex drilling, the problems of reduced bit life and slow penetration have been addressed successfully. However, a simple and economical surface solids-control method to complement the downhole systems becomes necessary when we desire to reduce viscosity, yield point, gel strength, total solids, concentration of drilled solids, and low micron-size material. This aim will be achieved when we are able to partially reject the undesirable materials from the drilling fluid system.
Drilling Fluid Quality
Exploring the effects of the size of particles in suspension and of additives or contaminants on rheological properties is important to understand the flow behavior of drilling fluid because fluid quality depends on particle size to some extent. If a material is added to drilling fluid to give the fluid a specific property, we refer to the material as an additive; however, if we drill into a formation that adds material to the fluid system without control or without any specific purpose, we refer to that material as a contaminant, regardless of material size, solubility, or insolubility.
With regard to particles, size is defined in many different ways. Among these are dA, the projected area diameter; dp, the projected perimeter diameter; dV, the volume diameter; ds, the surface diameter or the diameter of a sphere having the same surface area as the particle, dss, the specific surface diameter; and df, the freefalling diameter. From these, it is obvious that some confusion will arise if, for example, we equate dA measured by one instrument with df measured by another. To make matters worse, there are no clear standard size classifications and nomenclature in the oil field. For example, API Bulletin 13B classifies particles larger than 74 m as sand, particles 74 to 2 m as silt, and particles smaller than 2 m as colloids. This is further complicated by such classifications of API Bulletin 13C. For example, a colloidal particle is 2 to 0 m (what is meant by "0" size is not clear), an ultrafine particle is 44 to 2 m (this includes practically all barite material), a fine particle is 74 to 44 m, a medium particle is 250 to 74 m, an intermediate particle is 2,000 to 250 m, and a coarse particle is larger than 2,000 m. Both classifications disagree with the classic geological scale. In this scale, colloidal clays are 4 m, silt particles are 62.5 to 4 m, and sand particles are 2,000 to 62.5 m. We used the classic geological scale and diameter as equal to spherical diameter. See Appendix A for details of size analysis. In general, the quality of drilling-fluid filtration property depends on particle size, particle shape, and filter-cake packing arrangements. Smectite (gel or bentonite) is from 4 m to submicron size; it is flat to platy in lower temperatures and it packs well. When smectite - 4 to 6% by volume - is added to water, fluid loss decreases. This is shown in Fig. 1.
Field personnel usually evaluate viscosity, yield point, and low gravity content of a mud with the Fann VG meter, retort data, and mud weight. A number of graphs and charts - such as those in Figs. 2 through 7 - are designed to make the correlation of mud properties a simple task for field personnel. However, the search for simplicity has caused many confusing criteria and conclusions with regard to the performance of any given solids-control equipment. The causes for the confusion are rooted in (1) neglect of the retort accuracy, (2) neglect of the corrections for chloride, oil, and the chemical contents of the mud, (3) use of a variety of graphs prepared by different laboratories and research organizations, (4) neglect of the examination of barite, gel, and shales or other solids for mineral contents and the specific gravities of each mineral constituent, (5) neglecting to deaerate the mud before testing, and (6) disagreement about a standard for the "range" of "desirable" mud properties. The causes of the rise in viscosity in the field are often attributed only to an increase in the fines and low-gravity solids in the mud, as shown in Fig. 8. This conclusion ignores other causes of the increase in viscosity - such as an addition of caustic soda (Fig. 9), contaminants such as salt intrusion and sloughing, especially in combination with kicks (Fig. 10), and an increase in the calcium content caused by drilling into anhydrite, calcite, or formations containing other forms of soluble calcium (Fig. 11). There are many sources of fines that could increase the viscosity. For example, formations containing quartz and anhydrite react with the high pH of burned mud. Our experience and observation led us to believe that a pH 11 would be detrimental to the mud system. This, of course, causes further sloughing of the wellbore and the appearance of very fine, etched quartz grains in the mud system. The fines that may be found in a mud system could also come from unclean and used muds and poor-quality barite. It has been observed that when the volume of impurities in barite, such as iron oxide or quartz with a hardness of 5 to 6, increases, the percent of barite fine size increases. This may be because of the grinding of hard material against softer material (such as barite with a hardness of 2 to 3) at high shear rates of bit nozzle.
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