Study on the Dynamic Change Law of Collapse Pressure in Mudstone Strata-Taking Block D as an Example
Authors: Zhang J. C. , Li J. , Li Z. H. , Lian W. , Hu P. J. , Huang H. L. 
Published: September 2023

ABSTRACT
The mechanical-chemical coupling reaction occurs when drilling fluid intrudes into mudstone, which leads to the increase of collapse pressure. In this paper, a dynamic calculation model of collapse pressure with drilling fluid intrusion time is established according to the Mohr-Coulomb criterion. Then the pattern of drilling fluid intrusion into the core of Block D and the change of rock mechanical characteristics after intrusion are analyzed by experiments. Finally, the dynamic collapse pressure of well X in Block D was predicted by combining the reservoir section parameters. The results show that: (1) drilling fluid intrusion will increase the water content of the perimeter rocks, reduce the uniaxial compressive strength, elastic modulus, cohesion, and internal friction angle, and increase the Poisson's ratio, and the longer the intrusion time, the slower the increase of water content and the smaller the change of rock mechanical parameters per unit time; (2) drilling fluid intrusion will cause the collapse pressure of the perimeter rocks to increase, and the longer the intrusion time, the slower the growth; (3) the collapse of the reservoir section of well X pressure equivalent density increased to 1.20g/cm3 in 21.8 days, which is basically consistent with the drilling reality.

INTRODUCTION
When the hydrostatic column pressure in the wellbore is not sufficient to balance the collapse pressure around the well, the rock around the well will be sheared and damaged, which is one of the root causes of well wall instability (Westergaard, 1940; Cheatham,1984; Veeken et al, 1989; Yew et al,1990; Rahimi and Nygaard,2015). For well wall instability, the mechanical causes were mainly studied in the beginning (Aadnoy and Chenevert, 1987; Fuh et al, 1988; McLean and Addis,1990). It was not until around the 1990s that Yew et al(1990) first combined mechanical and chemical effects to quantify mud shale stability, which centered on linking the total water content of the shale to its mechanical performance. Later, Mian C et al (1995) conducted a similar study on shales from Daqing oil field in 1995 using the same method and concluded that the characteristic performance of rock mechanical parameters was also related to the water content inside the rock. On this basis, two mainstream directions of well wall instability research have been formed: mechanistic mechanism research (Ewy,1999; Moos et al,2003; Gholami et al;2015) and coupled mechanic-chemical interaction research (Hale et al,1993; Zeynali, 2012; Akhtarmanesh et al,2013). The study of the combined effect of coupled mechanics-chemistry on well wall stability is focused on rocks with high mud content (clay content), such as shale, mudstone and mud shale. Due to the intrusion of drilling fluid, illite, illite and montmorillonite mixed layers contained in the clay will absorb water and swell, resulting in certain changes in rock structure and physicochemical characteristics (Muller, 1969;Tan et al,1996; Ghasemi and Sharifi, 2021;Fanhui et al, 2021). As the drilling fluid intrusion time increases, such rocks mechanically exhibit a gradual decrease in strength and hardness, which eventually leads to an increase in the collapse pressure of the rocks on the well wall, and the borehole collapses when the collapse pressure exceeds the hydrostatic column pressure in the wellbore (Tan et al, 1997; Loret et al, 2002; He et al, 2016; Jia et al, 2019).