Experimental study and evaluation for borehole stability of fractured limestone formation

Abstract

With the increase of drilling depth, the stability of the borehole in deep fractured formation has received more and more attention. The exploration and development of fractured formations is difficult, because complex conditions such as borehole collapse, circulation loss, pipe-sticking occur frequently, causing huge economic losses and affecting the later drilling operations. Therefore, it is very important to study the stability of fractured formation. Observations of the microstructural characteristics and mechanics performance testing reveal the influence of fracture on the mechanical properties of limestone and enable the establishment of a model of collapse pressure as a function of fracture strike (the angle between the fracture and the maximum horizontal principal stress). The results showed that the fractures have great influence on the mechanical strength of rock and that the compressive strength of rock containing fractures is ∼30% lower than that of bedrock on average. The influence of different fracture strikes on the stability of the borehole varies, with a maximum effect occurring for fracture orientation of 40°–60°. The model of fractured formation collapse pressure is consistent with the results of mechanical experiments. This research results provide a guiding value for stability studies of fractured formations.