Drilling Fluid System

According to the requirements of scientific drilling, drilling fluid system should have performances of low viscosity, low gel, low solid content, low filter loss, and with good lubrication property.

Selection of Drilling Fluid System

Along with the development of petroleum drilling technology, drilling fluid technology has made considerable progress. According to dispersed medium in drilling fluid, drilling fluid can be divided into oil base, water base and gas base drilling fluid. In oil base drilling fluid there are a lot of organic hydrocarbon substances, which may seriously interfere borehole log, being unsuitable for scientific drilling. Gas or foam base drilling fluid is mainly applied to drill the strata with low formation pressure, and rotary blowout preventer needs to be installed at well mouth for safe drilling which may cause many troubles to drilling construction. Moreover, as down hole power is employed, drilling fluid is needed to work as the transmission and working medium for down hole power engine, but because of the compressibility of the gas base drilling fluid the power cannot be effectively transmitted to ensure a normal running of the down hole power drilling tool. Therefore, gas base drilling fluid is unsuitable for scientific drilling either. Only water base drilling fluid is the best choice of CCSD-1 Well and it is the most common type of drilling fluid in oil drilling and in geological exploration drilling. With good dispersion property to drilling materials, the properties of water base drilling fluid, especially rheological and weighted bearing capacity can be easily regulated; besides, the influence of drilling fluid to borehole log can be reduced as much as possible by optimizing mud making materials and mud conditioners. So, water base drilling fluid can fully satisfy the requirements of scientific drilling.

Organic drilling fluid treating chemicals are the most common in drilling, and the organic polymers for drilling fluid are different substances composed of repeated or homologous cells or monomers. Organic colloid materials can be applied to drilling fluid for declining filter loss, stabilizing clay soil, flocculating cuttings, increasing carrying capacity and can work as emulsifier and lubricant. With a variety of polymers, low solids non-dispersed mud system (LSND) can be widely used.

According to fabrication methods, organic polymer can be classified into three kinds, namely: natural polymers which can be used with simple treatment, such as amylum and guar; semi-artificial polymers such as sodium carboxymethyl cellulose (CMC) and derivant of amylum and balata; the most popular petroleum chemical derivants such as polyacrylamide (PAM) and polyacrylate are full-artificial polymers.

Based upon the types of monomer, polymer can be classified into single, double or copolymer with manifold monomers. According to the structure, copolymer can be of linear style or branched chain style, but cross-linking can take place in both styles by connecting of covalent bonds. Different compositions of a variety of monomers produce polymers with different structures, so, with different performances when working as drilling fluid treating agents. And this provides broad space for developing drilling fluid treating agents with different drilling purposes.

LBM-SD Composite Drilling Fluid Material

LBM-SD composite drilling fluid material was developed and manufactured by Beijing Institute of Exploration Engineering according to the 5000 m continuous core drilling situation of CCSD-1 Well, to satisfy the requirements of single property of drilling fluid, long term in drilling construction and high quality. LBM-SD is mixed and epurated with organic copolymer (LPA) and artificial sodium bentonite. According to the requirement of drilling operation, a satisfactory performance of drilling fluid can be obtained without adding other treating agents except water with a certain ratio. Thus, field mud making technology can be simplified and drilling fluid performance can be easily controlled and regulated.

LBM-SD is a kind of composite mud powder with low viscosity, low gel strength and low fluid loss, with strong capacities of salt resistance, calcium-magnesia resistance and temperature resistance. LBM-SD is a kind of selective flocculating and non-beneficiated mud making material. The main component ofLBM-SD is LPA, which is a kind of liquid low polymer copolymerized at low temperature with acrylic acid, acrylonitrile and acrylamide, mainly contains groups such as –CONH2, –COONa, –CN and (–COO)2Ca, etc.

Drilling Fluid Mechanism and Composition of LPA Polymer

One of the most obvious innovations of modern drilling fluid technology is the introduction of superpolymer flocculant and the development of low solids non-dispersed and clayfree superpolymer mud system. Superpolymer flocculant can make the cuttings and inferior soil in drilling fluid in a nondispersed flocculating state, which then can be cleaned by solids control equipment, and thus the problems of cuttings dispersing and accumulation exist in dispersed drilling fluid system can be well solved. The polymer in drilling fluid not only mainly works for flocculating, but also for restraining, lubricating for drag reduction, cross-linking for sealing and dilution shearing. One superpolymer molecule is adsorbed on several grains and as a bridge connecting each grain, then the adsorbed grains was agglomerated and coagulanted by curling of superpolymer.

With different molecular weights and degrees of hydrolysis, polymer can be divided into beneficiated and non-beneficiated selective flocculants according to its functions. Both the flocculants can flocculate the cuttings and inferior soil in drilling fluid but do not flocculate the colloidal grains of bentonite. The difference of the two flocculants is whether they can improve the viscosity of drilling fluid. The sketch of beneficiated and non-beneficiated active mechanism of polymer is shown in Fig. 1. Non-beneficiated selective flocculation is employed for diamond drilling while beneficiated selective flocculation is employed for oil drilling. LPA in LBM-SD composite mud material developed especially for CCSD-1 Well belongs to non-beneficiated selective flocculant.