Study on Wellbore Stability and Instability Mechanism in Piedmont Structures

Abstract

Piedmont tectonic belts are rich of oil and gas resources, however the intense tectonic stress and broken formation may cause great drilling problems in piedmont structures such as borehole collapse, lost circulation and gas cutting. Through analysis of in situ stress properties, bedding structure and mechanical characteristics, wellbore instability mechanism was expounded from rock mechanics, chemistry of drilling fluid and drilling technology. The high tectonic stress, formation strength decreasing and fluid pressure rising after mud filtrate seepage are main reasons for borehole collapse. The methods of calculating collapse and fracture pressure and determining drilling safety density window were put forward based on mechanical analysis. In order to reduce drilling problems in piedmont structures, some countermeasures should be taken from optimizing well track and casing program, using proper mud density, improving inhibitive and sealing ability of drilling fluid. Good sealing ability can reduce seepage and cut off pressure transmission, enhancing the effective support force. This is the key technology of maintaining wellbore stability in hard brittle shale in piedmont structures.

Synthesis and Plugging Performance of Nano-Micron Polymeric Gel Microsphere Plugging Agents for Oil-Based Drilling Fluids

“…However, as shale gas recovery gradually shifted from shallow to deep layers, complex geology was more often encountered during drilling, and it was still challenging to deal with wellbore instability with OBFs [ 8 ]. The invasion of OBF filtrate could cause formation instability, and pressure transfer via micro-fractures may result in wellbore instability [ 9 , 10 , 11 , 12 , 13 ]. Given the abovementioned factors, one of the keys to improving wellbore stability is to strengthen the plugging performance of OBFs and reduce filtrate invasion into formations [ 14 ].…”

Preparation of the Tetrameric Poly(VS-St-BMA-BA) Nano-Plugging Agent and Its Plugging Mechanism in Water-Based Drilling Fluids

“…However, shale formations are mostly developed with natural pores and fractures, 4 during the development of shale gas, there is still drilling fluid filtrate infiltrating into shale pores and fractures, 5 , 6 which weakens the structural force of the wellbore and causes the instability problem; 7 − 9 therefore, in order to improve the wellbore stability, it is necessary to strengthen the ability of water-based drilling fluid to plug microfractures. 10 − 12 Adding effective nano-plugging materials to drilling fluids is the key to solving the problem of shale wellbore instability. 1 , 13 There is a wide range of existing plugging agents, including ultra-fine calcium carbonate, fibers, asphalts, polymers, gel particles, and so forth.…”

“…My country is rich in shale gas resources, with resource reserves as high as 134 trillion cubic meters, Sichuan is also a major province of shale gas, with resources accounting for 20.46% of the national total, and the shale gas resources in the Sichuan Basin have become the main development area of national energy development. − With the increasing demand for shale oil and gas resources, the rapid development of the deep drilling fluid technology has provided strong support for the exploration and development of deep shale oil and gas. However, shale formations are mostly developed with natural pores and fractures, during the development of shale gas, there is still drilling fluid filtrate infiltrating into shale pores and fractures, , which weakens the structural force of the wellbore and causes the instability problem; − therefore, in order to improve the wellbore stability, it is necessary to strengthen the ability of water-based drilling fluid to plug microfractures. − Adding effective nano-plugging materials to drilling fluids is the key to solving the problem of shale wellbore instability. , There is a wide range of existing plugging agents, including ultra-fine calcium carbonate, fibers, asphalts, polymers, gel particles, and so forth. However, these plugging agents are too large to effectively plug nanoscale pores .…”

Synthesis of the Oil-Based Nanoblocker Poly(MMA–BMA–BA–St) and the Study of the Blocking Mechanism

“… 9 , 10 As the complexity of drilling encountered formations increases, oil-based drilling fluids can still lead to severe well wall collapse for hard, brittle, and fractured formations with developed laminae and microfractures, 11 − 14 despite their natural inhibition, due to the fact that the intrusion of filtrate can easily make the formation unstable, in addition to the transmission of hydraulic pressure through microfractures, which can also lead to well wall destabilization. 15 − 19 Therefore, it is the focus of scholars’ research to strengthen the oil-based drilling fluid to seal the nanoporous seam, improve the stability of the well wall, and maintain the stable performance of the drilling fluid. 20 , 21 …”

“…With the depletion of conventional natural gas, shale gas, as an important unconventional gas, is receiving increasing attention. , Shale gas exploration and development, mainly in the Sichuan basin, will be the main force for future shale gas production growth in China. − The shale reservoir is characterized by low permeability and strong adsorption, and microfractures are developed, the formation is easily fractured, and the clay mineral content is high and water-sensitive. , Oil-based drilling fluids are natural inhibitors. Oil-based drilling fluids are highly resistant to contamination and have good lubricity, so they are commonly used for drilling long horizontal section shale gas wells. , As the complexity of drilling encountered formations increases, oil-based drilling fluids can still lead to severe well wall collapse for hard, brittle, and fractured formations with developed laminae and microfractures, − despite their natural inhibition, due to the fact that the intrusion of filtrate can easily make the formation unstable, in addition to the transmission of hydraulic pressure through microfractures, which can also lead to well wall destabilization. − Therefore, it is the focus of scholars’ research to strengthen the oil-based drilling fluid to seal the nanoporous seam, improve the stability of the well wall, and maintain the stable performance of the drilling fluid. , …”