While drilling into
the igneous rock formations of the Shunbei
area, problems such as loss of well circulation and borehole collapse
occur frequently, seriously hindering the efficient development of
oil and gas resources. Aiming to solve this problem, the physicochemical
and mechanical properties of igneous rocks are studied through a series
of laboratory tests to determine the main factors influencing formation
collapse and instability. In addition to the laboratory results, the
weak plane effect of fracture mechanics, the seepage effect of drilling
fluid, and the hydration effect of the drilling fluid on the borehole
wall stability of igneous rock formations are evaluated and analyzed
by establishing a mathematical model. The results show that the microfractures
in the igneous rock are relatively developed and can be divided into
unfilled fractures and calcite-filled fractures. The mechanical strength
of the matrix igneous rock is higher than that of the rock samples
with microfractures. The compressive strength of calcite-filled and
unfilled fracture samples is 1/3–1/4 that of matrix igneous
rocks, and immersion in the drilling fluid has little influence on
the mechanical strength of igneous rocks. The fracture weak plane
effect has the greatest influence on wellbore stability. With the
increase of the number of fractures at different angles, the collapse
pressure equivalent density of the formation subject to the mechanical
weak plane effect increases by 21% compared with that of the homogeneous
formation without fractures. The seepage effect of the drilling fluid
on borehole stability is secondary, and the equivalent density of
the formation collapse pressure increases by 11%. Because of the low
content of clay minerals, the hydration effect of the drilling fluid
on borehole stability was minimal and the equivalent density of collapse
pressure increased by only 2%. During the drilling process, considering
the weak plane effect of the microfractures and the seepage effect
of the drilling fluid, the drilling fluid density should be controlled
at about 1.82 g/cm
3
. The effective plugging ability and
rheological properties of the drilling fluid should be improved for
the formation with microfractures. The research results can provide
a theoretical basis for the safe and efficient development of igneous
reservoirs.