Abstract：
During the drilling of deep wells and ultra-deep wells in Sichuan and Xinjiang, hard and brittle formations are prone
to continuous crumbling and falling blocks, resulting in frequent drill jams. Based on the traditional well destabilization theory,
these complex problems were mainly tackled by gradually increasing the drilling fluid density in the field, but the results were
not satisfactory. Some key oil and gas wells with a depth of more than 7,000 m, after increasing the drilling fluid density, the
well wall crumbling and falling blocks not only did not slow down, but instantly collapsed burying the drilling tools. Many wells
can only be drilled sideways, and there are even some boreholes scrapped. Due to the lack of theoretical support, the problem of
crumbling and falling blocks in hard and brittle formations has been unable to be effectively prevented and controlled, and has
become one of the key drilling problems that restrict the drilling of deep and ultra-deep wells. Based on the similarity between
the phenomenon of crumbling and falling blocks in hard and brittle formations and rock burst in tunneling and other projects, this
paper focuses on the hard and brittle formations such as limestone, dolomite, and shale, where well wall crumbling and falling
blocks are more frequent. In this paper, rock mechanics experiments such as rock compression and tensile strength are carried out
systematically. Through the comparative analysis of the shape of falling blocks during the drilling process and the rock failure
shape in rock mechanics experiments, the related phenomena are explained by the rock burst theory, and the rock burst behavior
is classified. Based on rock burst theory, rock burst is divided into three categories: weak, medium, and strong. And a compre
hensive analysis was conducted using the strength brittleness index
R
and the elastic energy index
W
. The calculation models of
the strength brittleness index
R
and the elastic energy index
W
based on uniaxial compressive rock strength are established, and
the interval values of
R
and
W
for rock burst identification are also determined. A set of comprehensive identification methods for
rock burst tendency and classification of surrounding rocks in hard and brittle formations during the drilling of deep wells is pro
posed. This paper shows that the order of rock burst propensity of the three studied lithologies is: dolomite > limestone > shale,
which is consistent with the severity and phenomenon of crumbling blocks in the well wall, verifying the reasonableness of this
paper on rock burst propensity of hard and brittle formations and the graded comprehensive discrimination method, providing a
solid background for the study of theoretical methods of well wall instability prevention and control based on the classification of
hard and brittle formations.
