The vast majority of drilling fluids, whether water-based or oil-based, often have compatibility issues with cement slurry. In cementing construction, if the cement slurry and drilling fluid come into direct contact, serious chemical interference will occur, which can shorten the thickening time of the cement slurry, increase the consistency of the slurry, cause cementing accidents and finally lead to cementing failure. To ensure the safety of cementing construction and improve the quality of cementing, a special working fluid needs to be added between the drilling fluid and cement slurry to isolate, buffer, flush, and dilute the drilling fluid. This working fluid is called the preflush of cement slurry, and can be divided into flushing fluids and spacer according to its function.

Classification and Principle of Action of Preflush

Initially, clean water was used as the preflush, but its isolation effect and suspension capacity were poor, which could not meet the requirements for isolation. With the changes in usage conditions, increased cementing requirements, and the emergence of various complex wells, higher requirements have been put forward for the isolation fluid itself. Starting from the 1970s, real flushing and spacer fluids were gradually developed.

1. Classification and Principle of Action of Flushing Agents

Classification：mainly including surfactant based chemical flushing fluids, solid phase flushing fluids, and non clay solid phase flushing fluid systems.

Working principle：

(1) . Physical Scour Theory.

It uses the turbulent flushing effect of flushing fluid and hydraulic mechanical action to flush the wellbore wall, remove the virtual filter cake and oil stains on the wellbore wall, in order to improve the bonding strength of the first and second interfaces.

(2) .Chemical Flushing Theory.

It is achieved by adding one or more chemical reagents to the flushing solution formula to effectively remove false filter cakes and oil stains on the wellbore, improve the wellbore, enhance the water wetting environment, and enhance the bonding strength of the first and second interfaces.

(3) .Polymer Adsorption Bonding and Dialysis Bonding Theory.

By utilizing the wetting and washing properties of non ionic surfactants to flush the wellbore wall, and then utilizing the adsorption properties of low degree polymers, competitive adsorption occurs in easily collapsed and water sensitive formations. High polymer polymers are preferentially adsorbed on the wellbore wall to protect easily collapsed and water sensitive formations.

2. Classification and Working Principle of Spacer Agents

Classification：At present, the spacer agent used domestically and internationally are mainly flushing type spacer, emulsified spacer, viscous spacer, temperature resistant, salt resistant, calcium resistant spacer, and other functional spacer.

Working principle：

(1) . Flushing Isolation Theory.

By adding diluents, surfactants, solid inert particles or granular porous materials, etc., the viscosity and shear force of the drilling fluid in the contact section are reduced using diluents. The surface activity of surfactants has wetting, emulsifying, and penetrating effects on the wellbore wall, and in combination with the adsorption of polymers and the turbulent erosion of solid inert particles, the filter cake can be easily removed from the casing wall and wellbore wall.

(2) . Planar Displacement Theory.

Adding a viscosity increasing agent to the spacer can increase the viscosity shear value, flatten the distribution of flow velocity profile, and facilitate the displacement of drilling fluid; The wall shear stress generated by high viscosity shear value can effectively remove the virtual filter cake and residual drilling fluid in the well, and improve the interface bonding quality.

(3) . Turbulent Diffusion Theory.

Spacer with good flow performance can achieve turbulent flow at low displacement, utilizing its strong lateral disturbance and diffusion to mix drilling fluid with it, and then expel the drilling fluid retention zone.

(4) . Density Difference Principle.

The density of the spacer is required to be between the drilling fluid and cement slurry. During the displacement process, due to the density difference, the buoyancy effect generated greatly reduces the mixing between the slurry and promotes the improvement of displacement efficiency.

3. Principle of Improving Drilling Fluid Performance with Preflush

1) . The flushing fluid effectively improves the rheological properties of drilling fluid and enhances the efficiency of cementing displacement. The rheological properties of the drilling fluid before cementing seriously affect the displacement efficiency of cementing injection. The thixotropy of drilling fluid has a significant impact on the efficiency of cement slurry displacement, especially at low speeds. With the increase of annular return velocity, the drilling fluid structure is damaged, the static shear force is weakened, and the retained drilling fluid attached to the wellbore is easily displaced and taken away.

Flushing fluid is a Newtonian fluid with a density close to water, low viscosity, and turbulent flow in the well. It is mixed when in contact with the drilling fluid, greatly reducing the viscosity and shear force of the drilling fluid, improving the flow performance of the drilling fluid, enhancing the turbulent flow ability of the drilling fluid, and thus improving the displacement efficiency without damaging the stability of the drilling fluid.

2) . The physical and chemical effects on the surface of the preflush improve and increase the interface bonding quality. The chemical preflush system with the addition of surfactants can cause wetting reversal on the surface of the casing, and the wettability changes from oil to water, which is conducive to removing the oil film on the surface of the casing that affects the interface bonding quality; Adding a diluent to the preflush can dilute the drilling fluid and penetrate into the interior of the virtual filter cake, breaking down the structure of the filter cake and improving the interface bonding quality.

3) . The physical, chemical and hydrodynamic effects of the pre liquid can improve the effective removal of interface retention; Under high temperature conditions underground, the drilling fluid trapped locally in the annular wellbore gap, irregular wellbore diameter, or eccentric casing will solidify and adhere to the filter cake on the wellbore wall, forming a virtual filter cake. The drilling fluid is circulated again, and this residue is difficult to detach. After the cement slurry solidifies, it shrinks and dehydrates, forming micro cracks in the cement annulus, leaving a hidden danger of oil, gas, and water channeling in the annulus, which seriously affects the lifespan of oil and gas wells. The spacer with adjustable rheological properties can effectively remove residual drilling fluid and virtual filter cake by adjusting the viscosity shear value to increase wall shear stress, achieving the goal of improving the interface bonding quality.

Below pic shows the flushing and spacer additives of our company. For detailed information of each product, you can visit our official website www.dicoenergy.com.

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