basic drilling fluid course part II (English)

Good, my steemit people I was away for a while, but here I come back with the second part of my basic course of drilling fluids, where I am talking about some of the properties of the fluid, I hope it will be of pleasure and understanding.

Properties of drilling fluids
The properties of a fluid are pre-established values that are adjusted in the field according to the behavior of the perforation. It is the responsibility of the specialist to take a sample of the mud at the entrance and exit of the well to compare values and proceed to make the necessary adjustments if the situation warrants it. The properties are depending on the type of fluid and can be physical or chemical.

Physical properties

Density.- Density, commonly called weight, means mass per unit volume and is measured with a mud balance (Fig.1) of sufficient precision to obtain measurements with a margin of error of 0.1 lb / gal.

The density can be recorded as weight, in lb / gal or lb / pc, as a pressure gradient, in lb / in2 x every 1000 feet or as specific gravity. Gravity is dimensionless and is related to the weight of a given volume of any substance compared to the weight of an equal volume of water at the reference temperature, ie:

The density of mud is one of the most important properties to maintain during the drilling of a well. Through it the pore pressure is controlled and the attacks are avoided, that is to say the incorporation of an uncontrollable flow of fluid from the formation inside the well. Its correct value should always be maintained at the entrance of the well and not at the exit, even if the mud is being cut by gas. This situation does not necessarily mean a problem of attack and it is a mistake to try to maintain the density at the exit, especially when drilling gas carrier formations. The density is obtained mainly with Calcium Carbonate and Barite, the first has a specific gravity of 2.7 and the second of 4.2 With the Calcium Carbonate density of Drill-In fluids can be carried up to 2 lb / gal above its original weight and with Barite densities are reached up to 20 lb / gal. There are conventional and pressurized scales used in the field to measure the density of drilling fluids and cement slurries, as shown below. The use of a pressurized balance allows obtaining better results in sludge cut by gas.

Fig. 2 Conventional balance Fig.3 Pressurized balance
The pressurized balance allows to measure more accurately the density of a sludge containing trapped air or gas. With this type of scale densities are measured between 6.0 and 22.5 lb / gal and with the conventional between 6.5 and 23.0 lb / gal. Both scales must be calibrated frequently with fresh water at room temperature. In case of not obtaining readings of 1 g / cc or 8.33 lb / gal. Lead pellets should be added or removed, as the case may be. If it is not possible to achieve the calibration as established by API, the scale must be discarded. A momentary solution that works until a new balance is acquired, is to add or subtract the difference obtained with the water; For example, if the density of water is greater than 8.33 lb / gal, the difference is subtracted from the density of the mud and if it is less, it is added. It is not advisable to hit the balance cup with the lid to get the air out of the mud, because it can be damaged and its furring function can be affected. The exact capacity of the scales varies according to the manufacturer and can be between 140 and 190 cc.

Viscosity.- Is the resistance to the flow of a fluid and is described as the ratio of the shear stress to the cutting rate. The following terms are used in the field to describe the viscosity of a fluid:
• Funnel viscosity
• Apparent viscosity
• Effective viscosity
• Plastic viscosity
• Viscosity at low cutting rate

Viscosity funnel.- This viscosity is used as a reference parameter to detect the relative changes in the properties of the fluid. It lacks a scientific basis and does not provide enough information to determine the rheology or flow characteristics of a fluid, but it does allow detecting to a certain extent the degree of contamination of dispersed, non-inhibited fluids, lignosulfonate type. These fluids are characterized by a state of flocculation when affected by any type of contaminant and when this happens it considerably increases its funnel viscosity. The funnel viscosity is measured with the Marsh funnel (Fig. 4) and the result is recorded in seconds per quart.

                                           Fig. 4 Marsh funnel with cup

Water is a Newtonian fluid whose funnel viscosity is 26 ± 0.5 sec. at 70º F (21º C). For non-Newtonians, there is no particular value in terms of its funnel viscosity; however, a practical field guide that has given good results in clay base fluids, is to maintain that viscosity at a value equivalent to four times the fluid density (lb / gal).

Apparent viscosity.-It is the viscosity corresponding to half of the reading obtained at 600 RPM in a direct reading rotary viscometer (Fig. 5). It is a function of the plastic viscosity and the yield point and is related to the maximum concentration of clay solids that accepts a mixture of water and bentonite without reaching the state of flocculation. It is used in the field to determine the performance of a clay. In a Newtonian fluid the apparent viscosity is numerically equal to the plastic viscosity.

Effective viscosity.- It is the real viscosity of a fluid under specific conditions of cutting speed, pressure and temperature. This viscosity, at any cutting speed, is determined based on the following formula:

                                                                           300 x Reading the viscometer
                                              VE (cP) = --------------------------------------------
                                                                          RPM

Plastic viscosity.- This viscosity is a measure of the internal resistance to fluid flow, attributable to the quantity, type and size of the solids present in a fluid. It is calculated with two readings of the rotary viscometer, that is:
VP (cP) = Read @ 600 PRM-Read @ 300 RPM

            Fig. 5 Direct reading rotary viscometer with heating container (thermo-cup)

An increase in plastic viscosity can mean an increase in solids, a reduction in the size of the debris, a change in the shape of the solid particles or a combination of these effects. However, in most cases the increase is due to the increase in solids. This results in an increase in the total surface area of the exposed solids and consequently, a reduction in the penetration rate by increasing the mechanical friction between solids.
The plastic viscosity also depends on the viscosity of the liquid phase of the fluid. In fact, by decreasing the viscosity of the water by increasing the temperature, the plastic viscosity decreases proportionally.

The oil emulsified in the water-based sludge behaves as a suspended solid and influences the increase in the plastic viscosity of these fluids. In the reverse emulsions this viscosity depends on the oil / water ratio, so that the higher the ratio, the lower the plastic viscosity. In addition, the primary emulsifiers used in the formulation of these emulsions have a remarkable impact on said viscosity.

Freshwater fluids have lower plastic viscosity than salines with similar densities.

Long chain polymers have a greater influence on the increase in plastic viscosity than short chain polymers. The increase becomes more evident just after the initial mix. This property is stabilized after several circulations, so it is recommended not to measure it when mixing the polymer.

The plastic viscosity is a good approximation of the viscosity through the jets of the wick. As this viscosity approaches that of the Newtonian fluids, the advance of the perforation will be greater.
In summary, the low plastic viscosity allows:
• Drill faster.
• Reduce pressure changes.
• Increase the power supplied to the wick.
• Improve the fluid flow in the ring.
• Reduce the use and wear of the equipment The following field rule allows to estimate quite accurately the value of the plastic viscosity of particularly densified fluids.

                                                               VP (cP) = (W-4) 3

Transfer point.- This property is a measure of the attraction force between particles under dynamic or flow conditions. It constitutes the force required to initiate the flow of a non-Newtonian fluid. It is independent of time and is generally associated with the Bingham model. In a reogram of shear stress versus shear rate, the yield point is the value of the shear stress corresponding to a shear rate of zero sec.-1.
The transfer point is related to the transport capacity of the fluid and to the characteristics of dilution by shear, it depends mainly on the volumetric concentration of the reactive solids. In this sense, it increases when the concentration of this type of solid increases and decreases as the forces of attraction are reduced by chemical treatment.

Chemical thinners neutralize the forces of attraction between particles and consequently decrease the yield point when it increases by excess reactive solids, while the dealers reduce it when it is affected by any contaminant. The free water contaminates the oil-based sludge and consequently increases its yield point. In this case, the yield point drops when completely emulsifying the water in the oil. The high yield points in the reverse emulsions decrease when the oil / ag ratio is increased. The yield point of the scattered mud is approximately equal to the weight of the mud and that of the non-dispersed mud is slightly higher.
This rheological property is calculated from the data of the standard field viscometer (Fig. 5), as follows:

                          PC (lb / 100 ft2) = 2 x Reading @ 300 RPM-Reading @ 600 RPM
                          PC (lb / 100 ft2) = Reading @ 300 RPM - Plastic Viscosity

Resistance or gel effort. - Property that measures the forces of attraction between particles under static conditions. It is related to the capacity of suspension that acquires the fluid when the circulation stops; At the same time, this capacity also depends on the viscosity at low cutting rate (LSRV) and the thixotropy of the fluid.
The gel strength can be measured at any desired time and corresponds to the maximum reading of the dial obtained at 3 RPM in a standard viscometer. It is commonly measured after intervals of 10 seconds, 10 minutes and 30 minutes and is recorded in lb / 100 ft2.
The gel can be progressive or flat. The first indicates accumulation of solids and occurs when the difference between the value of the initial and final gel is wide. On the other hand, they are flat when the values are high and almost equal, in this case the fluid presents a flocculation appearance. Benthic systems develop progressive gels as a function of time and viscoelastic flat gels.

                                               ---- Progressive ______ Plans
                                                                         Fig. 6

In either case, whether progressive or flat, the gel should be kept at values that are low enough, as for:
• Facilitate the settlement of undesirable solids in the sand trap.
• Decrease the swabbing effect when removing tubing.
• Allow the release of gas.
• Achieve the proper functioning of the drill pumps.
The treatment applied to the gels stress is the same as that used for the transfer point, because both properties are measures of the attractive forces, with the difference that the gel stress is referred to the static conditions and the yielding point. the dynamic conditions.