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.
Solids Size
API Bulletin 13B5 classifies particles larger than 74 μm as sand, particles 74 to 2 μm as silt, and particles smaller than 2 μm as colloids. This is further complicated by such classifications of API Bulletin 13C.5 For example, a colloidal particle is 2 to 0 μm (what is meant by “0” size is not clear), an ultrafine particle is 44 to 2 μm (this includes practically all barite material), a fine particle is 74 to 44 μm, a medium particle is 250 to 74 μm, an intermediate particle is 2,000 to 250 μm, and a coarse particle is larger than 2,000 μm. Both classifications disagree with the classic geological scale.6 In this scale, colloidal clays are :5 4 μm, silt particles are 62.5 to 4 μm, and sand particles are 2,000 to 62.5 μm. 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 /-tm 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.
Mud Rheology
Field personnel usually evaluate viscosity, yield point, and low gravity content of a mud with the Fann VG meter, retort data, and mud weight. 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.
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. Therefore, to understand the nature of the fines and the increase in viscosity, we recommend thorough petrological, mineralogical, X-ray, and SEM studies of (1) barite or any other weight material, (2) gel, (3) fonnation cuttings, and (4) solids-control-equipment reject, feed, overflow, underflow, and suction-pit mud, with microscopic size analysis at each mud-up stage of drilling operations. This paper presents several analyses of this type.
New Technique and Apparatus for Surface Solids Control
We define fluid conditioning as a technique in solids control that mechanically discards some controlled amount of drilled solids in each circulation while conditioning the fluid so that it would retain its desirable rheological properties.
The word “conditioning” means that the feed-mud rheological properties to fluid conditioner (FC) or the return-mud rheological properties, including their size distribution, approach that of the suction-pit mud after several circulations. This says that we have conditioned the mud to fit our standard if, for example, the return viscosity and weight (assuming that no kicks, addition of material, or aeration is taking place) is very close to that of the suction pit.
To conform to the above definition and to achieve the proper rheological goal mechanically, we should first ensure that the mud system used is as chemically balanced as possible with the formation, and should design an instrument that in one step separates and discards controlled amounts of small and large particles. This is shown in Fig. 1. The frequency distribution of the solid line represents the untreated mud before FC installation.
When FC is put into operation, the large-particle end of the distribution curve, the area enclosed by AFE with dashed lines, begins to shrink with circulation time.
In this case, Point 4 gradually moves toward Point 1. In the meantime, the small-particle end of the distribution curve, represented by Area BCD, behaves similarly-i.e., Point 1 moves toward Point 4. As a result, the remainder of the fluid that was represented by Area AEOCB (solid line) with a median at Point M (ideally between 8 and 15 μm) will gradually approach Area AFNCB, the “conditioned” state. The value of Point M depends on the system operating pressure and feed particle distribution.
Mechanics and Operating Modes of Fluid Conditioner
Mechanics of FC Cone. A schematic of the Fe system is shown in Fig. 2. The feed, underflow, and overflow parts operate as a standard cone except that feed diameter is adjustable. The vortex finder can be manipulated to give a higher gravity force in the body section of the cone.
The body side arm scalps large particles when near the wall and reduces the thickness of a viscous, heavy layer traveling down to the cone section, thus giving the large cones a better chance to separate particles. This is a form of mechanical dilution that lessens the need for addition of dilution water to the Fe. Furthermore, when the body side arm is rotated or pushed in and out, it regulates the flow rate through the underflow section.
The side arm is located inside a space in the body section and extends to a certain depth controlled by a free vortex volume rotating and moving upward to the vortex-finder overflow port.
The vortex-finder side arm operates in a way mechanically similar to the body side arm in collecting stray large particles moving upward to the overflow port. The vortex-finder side arm scalps most of the escaped large particles from the cone-wall section.
The flows from the body side arm and vortex-finder side arm are combined and directed to the feed sump to be circulated again. Upon slight circulation in the system, the size distribution in these two parts reaches equilibrium and converges to the same distribution.
This is because the separation of particles within the system follows a statistical probability path, particularly in cases of fast drilling of surface to intermediate holes. The underflow may be run over a shaker or discarded. The small-particle port (concentric) rejects smaller particles. The concentric can discard undesirable material up to 10 gal/min [0.00063 m3/s] per cone per internal circulation.
Therefore, with a two-cone Fe, 20 gal/min [0.00126 m3/s] may be expected at high rig pump rates and 60 gal/min [0.00378 m3/s] at low rig pump rates. The latter is because fluid is processed about three times (internal circulation) before it is picked up by the rig pump.
The higher the feed pressure, the smaller the particle size in this part of the Fe and the larger the particle size in the underflow discharge. The flow rate through this port can be adjusted to a desirable rate of 1 or more gal/min [6.3 x 10^-5 m3 / s]. The overflow of FC is returned to the suction pit.
Solids Control Modes of Operation
The FC system generally can be operated at pressure ranges of 40 to 150 psi [276 to 1034 kPa], at a flowline temperature of IS0°F [S2,2°C], and at flow rates of 500 to 1,200 gal/min [0.032 to 0.076 m3/s]. Special-purpose FC’s for hightemperature operations in gasohol plants have operated at high
volumes and temperatures greater than 200°F [93.3°C]. The system may be used in several ways, depending on what is needed most. Some suggestions follow.
Unweighted Mud
- Blank screen if some wet discharge can be tolerated.
- Run screen if wet discharge cannot be tolerated. In this case, combine cone overflow with shaker underflow and return combined mud to suction pit.
- Discard the wet discharge from the small concentric particle port. The rate of discharge may be adjusted from one to several gallons per minute. In general, the higher the feed pressure and flow rates, the smaller the particle size in concentric stream.
- The screen may be changed, depending on sand content of suction-pit mud, Usually, overflow sand content varies from zero to a trace to less than ½ % .
Weighted System
- When operating at 100 to 150 psi [690 to 1034 kPa], use the small-particle port (concentric) to discard the low-micron-size particles at a desirable rate, Adjust the rate to control fluid viscosity or low-gravity solids content of the mud. The concentric on a single cone discharges 1 to 10 gal/min [0.000063 to 0.00063 m3/s].
- If drilling is slow, no shaker is needed; most of the barite or other weight materials may be returned to the suction pit. This operation is similar to running a centrifuge, except that the FC processes the mud volume much more than the standard oilfield centrifuge, In this case, depending on the rate of material discarded from the system, the barite recovery will be equal to or better than current system recoveries.
- If the drilling rate is medium to high and retention of highgravity solids outweighs discarding low-gravity solids, shale shakers may be used with 60- to 160-mesh screens, if they are kept slightly wet; a shaker that is too dry may lose a larger amount of weight material.
- Use of the coarser weight material may require coarser screens. Also, if oil muds are used, the coarser screens may be required if weight material retention is desired.
Water Mud Recycling System
Environmental concerns and savings in wastewater haulage cost may be addressed through recycling of water. When operating pressure for the FC at low mud weight is above 120 psi [S27.4 kPa], rig wash water, rain water, etc., may be collected in a 50-bbl [S-m3] sump dug near the FC pump, An auxiliary 2-in. [0.050S-m] valve with hose and strainer should be installed near the suction end of the pump. When the sump is filled with dirty waste water, the hose with strainer head is lowered in the sump and the dirty water is picked up by opening the valve at the suction end of the pump. In this manner, the waste water is processed through the FC. Discarded material is combined with
the rig shaker overflow and stored for the subsequent haulage, In general, the total liquid discharge from FC is about 25% of discarded water from desander,. desilter, and mud cleaner, depending on the cone sizes, number of cones, and flow rates used in this operation.
It should be noted that when the fluid is well conditioned, the concentric flow rate should be reduced; otherwise, the mud will be stripped out of ” gel” and barite settling will result.
