| The acidizing technology, especially the fracture acidizing deeply, is one of the most important treatment to stimulate the reservoirs. The working fluid used in the fracture acidizing technology is of significant importance, which is comprised of gelled acid, emulsified acid, foamed acid and so on at the present time. Although these acids play a important role in the process of fracture acidizing, they are not so good, especially in the situation of high temperature. So this paper concentrated on the crosslinked acid which can stand up to high temperature.The gelling agent which can viscosify the strong hydrochloric acid(20%) effectively at high temperature(120℃) and crosslinking agent are the key components. Based on researching literature, we developed a copolymer as the gelling agent in the way of water solution polymerization, using AM and AMPS as the monomers and employing a novel initiator system which is described in detail in the corresponding literature. The conditions of the polymerization are also optimized using the intrinsic viscosity and the convention ratio of the monomers as the criteria, these polymerization conditions are the best one:the value of pH is 6 to 7, the concentration of monomers is 30%, the weight ratio of AM and AMPS is 3:2, the concentration of the initiator is 0.5‰, the reaction temperature is 15℃and the reaction time is 7 hours. The structure of the polymer is characterized by the infrared spectrophotometry(IR), the spectra shows that the product of the polymerization is indeed the copolymer of AM and AMPS.The crosslinking agent is another important component of the crosslinked acid. There are many amide groups on the macromolecular, so we screen among the crosslinking agent which are commonly used to crosslink the amide group, then an appropriate crosslinking agent named YQ-2 is synthesized which can work in the 20% hydrochloric acid at the temperature of 120℃. The laboratory experiment shows that YQ-2 can crosslink the gelling agent effectively to form crosslinked acid.The factors that influence the properties of the crosslinked acid were investigated, using the crosslinking time and stability at 120℃as the criteria. The results show that on the premise of the heat stability of the gelling agent, increasing the weight ratio of AM and AMPS can decrease the crosslinking time and improve the heat stability of the crosslinked acid; increasing the amount of the gelling agent and crosslinking agent YQ-2. on the basis of these results, the type and amount of gelling agent as well as the amount of crosslinking agent YQ-2 which is defined as the volume ratio of YQ-2 and acid are determined, the former is 2wt% P1 and the latter is 1:5.Evaluation of the additives used in the crosslinked acid is carried out to select the best ones, then the compatibility of these additives with each other and the gelling agent as well as the crosslinking agent is evaluated to determine the additives which are best suited ones to be used in the composition, they are:clay stabilizer named FP-1, corrosion inhibitor named BM-1, ferric ion stabilizer isoascorbic acid, cleanup additive named CF-5A. After that, the formula of crosslinked acid is determined:20% hydrochloric+2%P1+YQ-2 (1:5)+2%FP-1+2%BM-1+1.5% isoascorbic acid+1%CF-5A。Subsequently, the property of the crosslinked acid is evaluated. Results of these experiments show that the crosslinked acid can suspend the proppant effectively, the rate of reaction between acid and rock is dramatically slow down, the corrosion of the crosslinked acid is slight and it can break thoroughly to flow back readily. It is proven that the cosslinked acid we developed has the potential to be a good working fluid in the process of the fracture acidizing deeply. |
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