Abstract
Gravity displacement often occurs when drilling a vertical fractured formation, causing a downhole complexity with risk of blowout and reservoir damage, well control difficulty, drilling cycle prolongation, and increased costs. Based on an experimental device created for simulating the gravity displacement, various factors affecting the displacement quantity were quantitatively evaluated by simulating the fracture width, asphalt viscosity, drilling fluid density, and viscosity under different working conditions, and a liquid–liquid displacement law was obtained. Using the theories of rock mechanics, fluid mechanics, and seepage mechanics, based on conformal mapping, as well as a fracture-pore double substrate fluid flow model, we established a steady-state mathematical model of fractured formation liquid–liquid gravity displacement by optimizing the shape factors and using a combination of gravity displacement experiments to verify the feasibility of the mathematical model. We analyzed the influence of drilling fluid density, fracture height and length, and asphalt viscosity on displacement rate, and obtained the corresponding laws. The results show that when the oil–fluid interface is stable, the fracture width is the most important factor affecting the gravity displacement, and plugging is the most effective means of managing gravity displacement.
Highlights
The Kazhdumi Formation in the Yadavaran Oilfield of Iran is mainly composed of dark asphalt shale and gray limestone, in which asphalt heavy oil is extensively developed (Du et al, 2016; Rafieepour and Jalalifar, 2014)
The formation has a fracture path that allows drilling fluid to flow from the wellbore into the formation, which is one of the conditions for gravity displacement (Shu et al, 2011)
The following assumptions were initially made to transform the model : ‹ Based on the semi-circular physical simplification model, it is considered that the drilling fluid in the wellbore only invades the formation on the right side of the y axis in Figure 8 and replaces the fluid in the corresponding formation
Summary
The Kazhdumi Formation in the Yadavaran Oilfield of Iran is mainly composed of dark asphalt shale and gray limestone, in which asphalt heavy oil is extensively developed (Du et al, 2016; Rafieepour and Jalalifar, 2014). Keywords Gravity displacement, fracture formation, drilling, mathematical model, experimental study
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