ABSTRACT:

The mud shale of Jidike formation in Keshen block, Kuqa depression, Tarim Basin is well developed. In the process of drilling, the problems of downhole block falling and stuck are serious. Through research, the inhibition performance of the drilling fluid used in this block is insufficient, and the hydration of mud shale leads to the decrease of the strength of the well rock, which is the main reason for the frequent occurrence of wellbore instability. Based on the theory of rock mechanics, according to the instability characteristics of the target well section, this paper selects the applicable collapse pressure prediction model, takes the cohesion of rock as the evaluation index of drilling fluid anti-sloughing performance, and constructs the relationship chart between collapse pressure and cohesion. According to the field operation requirements that the hole enlargement rate is controlled to be less than or equal to 15% and the drilling fluid density is 1.9 g/cm3, the anti-sloughing performance requirements of drilling fluid in mudstone section of Jidike formation are determined as follows: the rock cohesion is greater than or equal to 8.4 MPa. With this performance requirement as the target, the inhibition performance of existing drilling fluid is optimized by using the experimental device and continuous scratch tester that can simulate the deep well conditions and according to the experimental evaluation process of inhibition index. Finally, the dosage of anti-sloughing agent KCL is 2%, and the dosage of organic amine is 0.5%. The optimized drilling fluid can meet the anti-sloughing performance requirements of stable wellbore in Jidike formation.

1. Geological Characteristics and Engineering Difficulties
1.1. Geological Characteristics Of Jidike Formation

Jidike formation in Keshen block of Tarim Basin is buried deep (4600–6500 m). The top part of the upper member is mainly composed of mudstone and silty mudstone, with local blue gray mudstone marker layer; the lower thick layer, super thick layer mudstone, silty mudstone with siltstone and argillaceous siltstone; the middle part is mainly composed of gravelly fine sandstone, gravelly medium sandstone, fine sandstone and siltstone, with local silty mudstone and argillaceous siltstone; the lower part is mainly composed of a large set of mudstone, in which the upper part is brown red mudstone intercalated with medium thick medium sandstone and fine sandstone with gravel, the lower part is brown, brown mudstone, silty mudstone intercalated with thin and medium thick siltstone, and the bottom part is a set of fine sandstone and siltstone with gravel. XRD (X-ray diffraction) was used to analyze the mineral composition of rock samples from Jidike formation. The results are shown in Table 1. It can be seen from table 1 that the clay content of mudstone in Jidike formation is 15–26%, and the average content of illite in clay minerals is up to 10.6%, which shows that it is easy to hydrate and disperse, and the borehole wall is broken.

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