Abstract: Synthetic drilling fluid is a synthetic or modified organics as continuous phase, saline as dispersed phase, which composed of emulsifier, flow-type regulator and soon. It is a kind of water-insoluble synthetic oil-based drilling fluid and has the operating performance of oil-based drilling fluids. Synthetic drilling fluids as the alternatives of oil-base drilling fluid has been developed in the early 1990s.Furthermore, after nearly 20 years of research and application, the current has already progressed into the second generation of synthetic drilling fluids. This article summarizes the composition and characteristics of synthetic drilling fluid, analyzed and predicted the advantages and disadvantage of synthetic drilling fluid and future development prospects combined with the domestic and foreign research present situation. This provides a theoretical guidance for oil field workers learn drilling fluid.

With the gradual prominent energy issues and environmental issues, as well as the rapid development of drilling technology, people have higher and higher requirements for energy mining especially for petroleum. Traditional water-based drilling fluid scope is narrow, in today's increasingly complicated downhole conditions, more and more unable to meet the demand. However, oil-based drilling fluid has a greater contamination for the environment. So it is difficult to promote the use which forced constrains of relevant environmental protection regulations. Therefore, as an alternative system of both the emergence of synthetic drilling fluid retention in both environmental characteristics of water-based drilling fluids, they also inherited a very good performance characteristics of oil-based drilling fluids, become a maritime complex formation and other sensitive land essential for regional drilling fluid system. The synthetic drilling fluid had substance ability in the field application process, a series of advantages such as cuttings carrying capacity, good lubricity, and inhibiting shale. It can also solve the problem of environment pollution, logging and testing data.

1. Composition characteristics of synthetic drilling fluid

Synthetic drilling fluids are generally made from synthetic or modified organic continuous liquid phase, dispersed phase, dispersed solid phase as a base fluid, by adding emulsifiers, fluid loss control agents, stabilizers, modifiers and increase the flow-type agent to synthesis, and is a water-insoluble synthetic oil-based drilling fluid, so it has the characteristics of oil-based drilling fluids. Among them, the continuous liquid phase are mainly esters, ethers, poly α- olefins, linear paraffin, linear alpha olefins and heterogeneous olefins, and so on. Dispersion liquid is generally caci2 saturated brine; dispersive solid phase is generally organic soil.

Synthetic drilling fluids due to the integration of the environmental characteristics of water-based drilling fluids, and the performance characteristics of oil-based drilling fluids, while it has a lubricating effect which can effectively improve the penetration rate, a class of drilling fluid with rapid development in recent years. Compared with conventional drilling, it has the following characteristics:

(1) Better inhibition performance and stronger anti-pollution suppression capability. Synthetic drilling fluids due to its filtrate does not contain an aqueous phase; it has the ability to inhibit clay, which can effectively reduce the wellbore instability, but also can effectively avoid hydration dispersing, so as to achieve a stronger anti-pollution ability.

(2) It has excellent rheological properties and thermal stability. In general, the viscosity of the synthetic drilling fluid is 2-4 times higher than oil-based drilling fluid. But under the condition of high temperature, the viscosity is small and the next time without thermal degradation, thermal stability excellent performance.

(3) It has good lubrication properties. Synthetic drilling fluids are commonly composed of extremely strong substance (in particular the second generation synthetic base fluid), having excellent lubricating performance. The lubricating properties can fully meet the drilling requirements, can be used as the interface of lubricant.

(4) Strong environmental compatibility. Synthetic drilling fluids is due to the small toxicity, biodegradable, environmentally friendly and popular, its vapor does not contain aromatic compounds, virtually non-toxic to mammals, completely meet the international emission standards and applicable to all over the world. 

(5) Comprehensive use of low cost. Although the price of the synthetic-based drilling fluid is higher than oil-based drilling fluids, considering the use of synthetic-based drilling fluid can effectively improve the drilling rate, drilling stability, saving the cost of processing drilling cuttings and control the cost of environmental pollution, as well as for multiple re-used features, and its cost comprehensive use is lower than the oil-based drilling fluids and water-based drilling fluids.

2. The classification of Synthetic drilling fluid and current situation research

2.1 The first generation of synthetic drilling fluids

The first generation of synthetic drilling fluid systems are mainly have ester-based drilling fluid, ether-based drilling fluids, α- olefin-based drilling fluid and shrinkage aldehyde based synthetic drilling fluids. Among them, the ester-based drilling fluids through organic vegetable fatty acids in the presence of water and an alcohol and an acid catalyst synthesis, was the first successfully developed and put into use synthetic based drilling fluids; ether-based drilling fluids through an alcohol-based solution condensation and oxidation in the system, is a general term r1-o-r2-type compound with an ester group has a similar physical properties, does not contain any aromatic substances. Using variant diethyl ether base drilling fluid than using a single ether base drilling fluid is more prone to microbial degradation, environmental performance is better; polymerization α- olefin base drilling fluid prepared from the polymerization of ethylene, a higher degree of polymerization, the molecular chain end retains a double bond, easy to produce microbial degradation. Base fluid does not contain aromatic and cyclic hydrocarbon compounds, non-toxic and readily biodegradable; acetal-based drilling fluid base fluid produced by the condensation of aldehydes, which has a advantage of kinematic viscosity and flash point lower than the ester, ether base fluid, However, the relatively cost is high and the actual use is few.

2.2 The second generation of synthetic drilling fluids

The second-generation synthetic drilling fluids are mainly have cable type α- olefin drilling fluid, the olefin-based drilling fluids, linear alkyl benzene based drilling fluids and linear paraffin-based drilling fluid. Among them, the linear α- olefin drilling is currently the most widely used, its more other synthetic-based drilling fluids, the unique base fluid has the advantages of low pour point, less residue on the cuttings and the low unit cost; internal olefin-based drilling fluids and linear α- olefins drilling fluids have similar structure, with low viscosity and low-cost advantages, but high toxicity; linear alkyl benzene base drilling fluid has the similar chemical properties with formaldehyde. It is low kinematic viscosity and low cost, but it contains aromatic hydrocarbons and other toxic substances used in practice less; linear paraffin-based drilling fluids are most representative of the second generation of synthetic drilling fluids, which itself has a relatively low cost, stable performance, degradation rate is moderate and this series advantages all highlighting the excellent performance of the second generation of drilling fluids. The Toxic of linear paraffin-based drilling fluid is slightly higher than the ester group, poly α- olefins, applicable to all types of environment is complex, higher environmental requirements drilling area.

3. The development of synthetic drilling fluids

Synthetic drilling fluids appeared as substitute for oil-based drilling fluids and gradually developed, and its excellent features are ideal drilling fluid in the drilling industry at the moment, and its substitute for the water and oil based systems where the complex environment with higher requirement for drilling fluids is with high practical value. In today's increasingly serious environmental pollution, people's environmental awareness are unprecedented strong, and synthetic-based drilling fluids is bound to be welcomed by the majority of the drilling companies by small contamination (or zero contamination), and it has good prospect. But with the widespread use of synthetic-based drilling fluids in offshore oil fields, there have been some new problems, such as rheology regulation problem as a prevalent problem for synthetic base drilling fluid, including hole cleaning, barite sag and high circulating pressure loss. The main reason is that the temperature significantly affect the rheological properties, with low temperature viscosity of the drilling fluid is too high, and the viscosity of drilling fluids is too low at high temperatures, especially the dynamic shear force is too low, not sufficient to carry the solid phase. Meanwhile, research applications show that synthetic-based drilling fluids have certain threshold limit values as emulsion stability at high temperature, oil ratio and drilling fluid density, to make these fluids become entirely acceptable substitute as oil-based drilling fluids, it needs to solve many problems.

Regarding to the above problems, further research on synthetic-based drilling fluid system should make efforts in seeking and improving base fluid of synthetic-based drilling fluid, emulsifiers and flow-type regulators, to further improve the high-pressure and high-temperature stability of the system, further improve the rheology of system and to improve its density threshold limits and reduce costs while meeting the  environmental requirements. Meanwhile, the recycle and reuse should be carried out for the synthetic-based drilling fluids, to further improve the effectiveness, thereby to achieve lower costs and protect the environment.

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