Abstract
Finding out the reasons for wellbore instability in the Longmaxi Formation and Wufeng Formation and putting forward drilling fluid technical countermeasures to strengthen and stabilize the wellbore are very crucial to horizontal drilling. Based on X-ray diffraction, electron microscope scanning, linear swelling experiment, and hot-rolling dispersion experiment, the physicochemical mechanism of wellbore instability in complex strata was revealed, and thus, the coordinated wellbore stability method can be put forward, which is “strengthening plugging of micropores, inhibiting filtrate invasion, and retarding pressure transmission.” Using a sand bed filtration tester, high-temperature and high-pressure plugging simulation experimental device, and microporous membrane and other experimental devices, the oil-based drilling fluid treatment agent was researched and selected, and a set of an enhanced plugging drilling fluid system suitable for shale gas horizontal well was constructed. Its temperature resistance is 135°C and it has preferable contamination resistibility (10% NaCl, 1% CaCl2, and 8% poor clay). The bearing capacity of a 400 μm fracture is 5 MPa, and the filtration loss of 0.22 μm and 0.45 μm microporous membranes is zero. Compared with previous field drilling fluids, the constructed oil-based drilling fluid system has a greatly improved plugging ability and excellent performance in other aspects.
Highlights
At present, China’s shale gas exploration and development is at a rapid development stage, and great breakthroughs have been made in the marine shale of the Ordovician Wufeng Formation-Silurian Longmaxi Formation in Sichuan Basin [1]
By studying the wellbore instability mechanism of the Longmaxi Formation and Wufeng Formation, this paper selected the oil-based drilling fluid treatment agent and targeted to optimize the micro-nanoplugging agent and further constructed a drilling fluid system with strong plugging and high lubricating oil base to reduce the incidence of complex accidents in the horizontal section of the Longmaxi Formation
The Longmaxi Formation and Wufeng Formation are hard and brittle shales mainly composed of illite, with developed microfractures and strong hydration of some formations
Summary
China’s shale gas exploration and development is at a rapid development stage, and great breakthroughs have been made in the marine shale of the Ordovician Wufeng Formation-Silurian Longmaxi Formation in Sichuan Basin [1]. In the drilling process of shale gas horizontal wells in the Changning block, affected by the hydration expansion of shale and the development of rock microcracks, the wall spalling is serious, resulting in complex accidents such as sticking and lost circulation, which brings great economic losses to drilling operations [2, 3]. By studying the wellbore instability mechanism of the Longmaxi Formation and Wufeng Formation, this paper selected the oil-based drilling fluid treatment agent and targeted to optimize the micro-nanoplugging agent and further constructed a drilling fluid system with strong plugging and high lubricating oil base to reduce the incidence of complex accidents in the horizontal section of the Longmaxi Formation
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