Study on Cement Sheath Integrity of Shale Oil Horizontal Wells in Northeast China

Abstract

ABSTRACT: Shale oil in northeast China is about to enter the stage of large-scale hydraulic fracturing. Cement sheath integrity must be ensured for horizontal shale oil wells. In this paper, the mechanical parameters of cement sheath are measured. A numerical model of casing - cement sheath - formation assembly is established to analyze the stress law of cement sheath. The integrity of the cement sheath is analyzed. The results show that the elastic modulus of cement is 6 GPa and the Poisson’s ratio is 0.17. The compressive strength and tensile strength of cement after cyclic loading are 90MPa and 7.5mpa ~ 11.25mpa respectively. The maximum radial stress of cement sheath increases with the increase of fracturing pressure, the decrease of displacement, and the increase of elastic modulus. The maximum tangential stress of cement sheath increases with the increase of fracturing pressure and elastic modulus of cement. The tensile strength of cement sheath cannot meet hydraulic fracturing requirements. The elastic modulus of cement should be reduced to 4 GPa or the tensile strength should be increased to be higher than 15 MPa. This study would guide the shale oil wells in northeast China. 1. INTRODUCTION As hydraulic fracturing technology matures and international oil prices recover, more and more unconventional oil and gas resources are being developed, such as shale gas, conglomerate reservoirs, and shale oil. In particular, shale oil resources are becoming a new direction of exploration and development of unconventional oil and gas resources in China. Shale oil has been developed scaled in Jimsar block, in Inner Mongolia, and Tianjin district. At the same time, shale oil resources have been successfully explored in northeast China and will soon enter the stage of large-scale development. Based on the development experience of shale gas and other unconventional oil and gas resources over the past few years, casing deformation and sustained casing pressure are the most common problems in horizontal wells during multistage fracturing (Adams, 2017; Atkinson, 2020; Chaojie Zhao, 2020; Xi Yan, 2021.). Well integrity issues have a significant impact on the safety and production of horizontal wells. Fig. 1 shows the casing deformation logging results of Wel-Wei204H39-6 in the Weiyuan block and Well-JHW033 in Jimsar Block, Xinjiang respectively. Logging results showed casing deformation in both wells. One of the wells is a shale gas horizontal well and the other is a shale oil horizontal well. Thus, shale oil wells face the same well integrity issues as shale gas wells.