Yufei Gong

Abstract：This paper will present the development novel temperature control viscosity acid（TCA）technique that uses for acid fracturing in Ordovician formation in TZ region of TARIM，China. The Ordovician formation in TZ field is extreme heterogeneous carbonate with naturally fractures，that locates in deep underground of 5000-6300 m，the reservoir temperature is around 120～145℃，and the reservoir close-fracture pressure the above 90MPa. During the past decade，many acid fracturing techniques（such as gelled acid technique and emulsified acid technique）have been applied，but the results were not satisfactory. Under these difficult circumstances，in order to gain perfect acid fracturing effectiveness，we have succeeded in developing TCA acid fracturing technique.

Keywords：TCA Acid Fracturing Technology Ordovician Reservoir Temperature

1 The Study and Preparation of TCA

Novel type of temperature controlling viscosity acid system is developed according to the above discuss. The gelling agent should be of stability and easy to resolve when it is indifferent acids. Besides，it can be reactivated to make it viscosity grow up when needed.

According to the capability of the TCA gelling agent，the molecular structure of the gelling agent can be a comb type multi-cation polymer. The novel TCA gelling agent is prepared under stringent condition to make sure that the molecular weight is less than 800 thousands. Figure 1a shows the molecular structure in which the end radicals on the backbone and branched chain deactivated the molecule，so the molecular chain no longer grows. When the temperature is over 70℃，the gelling agent-polymer in the acid are reactivated and the polymer continues to grow or connect with other polymer molecules. As a result，the molecular weight and the visrnsity of the acid system grow rapidly if the acid system has also a activator which make the activation energyreduce once more. The polymer is going to decompose when it is under the temperature of over 100℃ for 1 to 4 hours. The Figure 1 shows the process of viscosity growing and the molecule decomposing.

2 The Speed Retarding Capability of the TCA in High Temperature

After the TCA becoming a viscous fluid，it has the capability of retarding the sped of the acid-rock reaction. The acid-rock reaction inclouds the following steps：（1） transfers to the rock surface；（2）H* reacts with the rock；（3）The reaction products departs from the rock surface to the spent acid. When the acid enters into the carbonate reservoir，the temperature of the acid system and the viscosity of the system begin to grow up. Because of the high viscosity of the system，the diffusing rate of the H* to the rock surface and that of the reaction products to the acid fluid are reduced，therefore the acid-rock reaction has been restrained.

The dissolution rates of Ordovician rock samples from Lunnan in Tarim Basin in hydrochloric acid，gelling acid and TCA acid are determined at the temperatures of 90℃ and 120℃ using a rotating disk instrument.

Former studies show that pressure has little effects on the reaction of hydrochloric acid with carbonate rock when the reaction pressure is over 7MPa. According as the diameter of the core samples is 2.54 cm and the discharge capacity is 5m3/min，the rotate speed of the rotating disk instrument. A spent acid prepared by 20% hydrochloric acid reacting with rock powder in order to eliminate the effects of homoirons. The results are showed in Figure 3. Figure 3 shows that the HCI solution reacts most quickly with the rock，and then gelling acid and TCA，respecetively. The reaction speed of 20% HCl with carbonate reservoir rock is，which of 20% HCl gelling acid is，that of 20% HCl TAC is . The acid-rock reaction speed of 20% HCl is 6.26 times of that of TCA，the acid-rock reaction speed of gelling acid is 2.38 times of that of TCA.

References

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（作者單位：Southwest Petroleum University）