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STABILIZATION OF NONDISPERSIVE WASHING SYSTEMS WITH HIGH COLLOIDAL BENTONITE FROM DEPOSIT «ZYRYANSKOE»

Flocculant (copolymers of vinyl acetate and maleic anhydride, partially hydrolyzed polyacrylamide HPAM, copolymers of sodium acrylate and methacrylate, polyoxyethylene etc.) prevents dispersion of drilled solids particles and facilitates their removal from the mud at the purification system, and regulates the structural and mechanical properties of the mud. The stabilizer (sodium polyacrylate) «encapsulates» the dispersed particles of bentonite, reducing the filtration rate of the mud, without causing changes of its viscosity. In addition, sodium polyacrylate protects bentonite phase of the mud from flocculant that is present in the system.

Nondispersive mud is considered to be of high quality, if the ratio of cuttings particles in the solid phase to an equivalent bentonite content in it is approximately 1:1. The clay content of the solid phase in the system should not exceed 4% of the volume.

During deepening the borehole a high molecular flocculant (HPAM) minimizes the increasing the concentration of the colloidal clay phase entering the drilling mud [1,2]. As a result, the concentration of colloidal clay fraction (particle size less than 1 micron) becomes dramatically low. Apart from a flocculating action in the polyacrylamide system, drilling mud depletion with colloidal clay fraction occurs as a result of primary coagulation on ingress of saline formation water, acid gases (carbon dioxide) or by mud pollution with drilled solids particles (sludging).

At the same time, to maintain the required level of structural characteristics and effective retention capacity of drilling mud, concentration of the colloidal phase in nondispersive mud should be at least 25 - 30 kg / m3. And over 70% of particulate matter (layered plates) should have a size of less than 1 micron, and the normal distribution of surface electric charge should be as follows: «faces» are negatively charged and the «edges» are positively charged. In this case a «healthy» coagulation structure of drilling mud of type «face-edge» («house of cards») is achieved, which establishes impenetrable dense filter cake on the walls of the borehole and is characterized by an optimal efferent and gelling characteristics.

By reducing the concentration of the colloidal phase to less than 20 kg / m3 the structure of nondispersive mud is destroyed (GS value tends to zero). To maintain the required structural characteristics in this case rinsing liquid should be processed with xanthan biopolymer (2 kg / m3), which increases the cost of the drilling mud. Besides biopolymer rapidly loses efficiency as a result of several factors: biodegradability, mechanical destruction of molecules, adsorption on the drilled solids particles and removing reagent from the mud.

It was found that the maintenance of the necessary structural and mechanical characteristics of nondispersing mud is effectively achieved by processing the circulating drilling mud with 4-6% bentonite slurry prepared from high colloidal bentonite. After the addition of bentonite slurry the concentration of micron colloidal phase in nondispersive mud should not be reduced less than 25 - 30 kg / m3.

It is optimal to use bentonite enriched with montmorillonite from deposit Zyryanskoe with the following mineral composition as the high colloidal bentonite:

Table 1. Mineral composition of bentonite from depozit Zyryanskoe (vol. %)

Mineral Vol. %
Montmorillonite 74 (60 - 87)
Hydromica 15 (10-30)
Quartz-siliceous 5,1 (2-8)
Calcite (nodules) 2,8 (1,5-6)
Limonite (nodules) 2,8 (2,0-4,6)
Heavy minerals 0,3 (0,2-0,5)

Such efficiency of simple processings with highly colloidal bentonite suspension is due to the peculiarities of interaction of negatively charged particles in a highly dispersed bentonite mud.

According to the Derjagin-Landau equation [3.4] at the long distances between colloidal bentonite particles attraction prevails (secondary potential minimum (potential well). At the medium distances between the colloidal clay particles repulsion dominates (electrostatic coagulation barrier). At the close distances at maximum convergence of colloidal particles attraction prevails - the primary potential well.

The possibility of coagulation is determined by the height of the barrier and the depth of the potential wells (Figure 1).

Figure 1

In the clay suspension there is a long-range interaction between two colloidal particles at a distance of about 100 nm, corresponding to the secondary potential well. At the same time there is an act of slow coagulation, and other particles are attached to these particles at long distances, which leads to the emergence of a structured sol having a gel strength (tensile strength of the structure). Thus, the magnitude of the Gel Strength (analogue Gels (10”/10’)) indirectly characterizes the energy of the Van der Waals attraction of the particles (the depth of the secondary potential well), and the suspension thixotropy (thixotropic gel hardening coefficient) - the speed of a slow coagulation in the secondary potential well.

Processing a mud with the fine bentonite suspension increases the concentration of negatively charged colloidal particles, which leads to an increase in energy of repulsion between the particles, reducing the depth of the secondary potential well and decreasing the GS of the suspension (Fig. 2).

Figure 2

Thus, the concept of processing nondispersive mud with high colloidal bentonite from depozit Zyryanskoe with high natural content of montmorillonite (at least 75 - 80%), provides additional system stabilization and optimize rheological flow curve and maintains optimal structural and mechanical characteristics of the mud in conditions increasing the concentration of the cuttings in the drilling fluid. The ability of the particles of high colloidal bentonite to form on the walls of the well thin and impermeable filter cake reduces the consumption of polymer fluid loss agent, and supports the stability of the borehole walls.

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