Abstract
Wellbore instability is a frequent problem of shale drilling. Accurate calculation of surge-swab pressures in tripping processes is essential for wellbore pressure management to maintain wellbore stability. However, cutting plugs formed in shale horizontal wells have not been considered in previous surge-swab pressure models. In this paper, a surge-swab pressure model considering the effect of cutting plugs is established for both open pipe string and closed pipe string conditions; In this model, the osmotic pressure of a cutting plug is analyzed. The reduction of cutting plug porosity due to shale hydration expansion and dispersion is considered, ultimately resulting in an impermeable cutting plug. A case study is conducted to analyze swab pressures in a tripping out process. The results show that, in a closed pipe condition, the cutting plug significantly increases the swab pressures below it, which increase with the decrease of cutting plug porosity and the increase of cutting plug length. Under the give condition, the swab pressure at the bottom of the well increases from 3.60 MPa to 8.82 MPa due to the cutting plug, increasing by 244.9%. In an open pipe string condition, the cutting plug affects the flow rate in the pipes and the annulus, resulting in a higher swab pressure above the cutting plug compared to a no-cutting plug annulus. The difference increases with the decrease of the porosity and the increase of the length and the measured depth of the cutting plug. Consequently, the extra surge-swab pressures caused by cutting plugs could result in wellbore pressures out of safety mud density window, whereas are ignored by previous models. The model proposes a more accurate wellbore pressure prediction and guarantees the wellbore stability in shale drilling.
Highlights
As a fossil resource preserved in nanoscale pores in organicrich shale, shale oil is an essential part of the world energy market [1]
Due to the special trajectory of horizontal wells and the unique characteristics of shale formation, problems caused by wellbore instability are frequent in drilling engineering
The cutting plug is a cutting particle cylinder in the annulus between the drilling string and the wellbore formed in three conditions (Figure 1): (1) the cuttings accumulated at a borehole-enlarged section slide down due to the disturbance of the tripping pipe string, (2) the cuttings in the vertical and small-inclination sections settle down and accumulate in the large-inclination and horizontal sections when circulation is stopped, and (3) the cuttings at the lower side of the wellbore as a cutting bed are pushed and accumulated by the large diameter tools in repetitive reaming or tripping processes like the bulldozer work
Summary
As a fossil resource preserved in nanoscale pores in organicrich shale, shale oil is an essential part of the world energy market [1]. Most of the wellbore instability problems occur in tripping in/out processes, due to surge-swab pressures caused by pipe string movements. Fontenot and Clark [19] established a comprehensive steady surge-swab pressure model considering the drilling fluid property variation with depth. Krishna et al [26] proposed explicit flow velocity equations for steady-state surge-swab pressure models of yield power law fluid to achieve more convenient calculation. Different from steady-state models, dynamic models take account of the elasticity of the pipes and the wellbore, the inertia and compressibility of drilling fluid, and the variation of tripping velocity [10]. In shale formation drilling, the effect of an influential phenomenon on the surge-swab pressure has not been considered in existing studies: the effect of a cutting plug. A case study is conducted to give a deeper insight
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