Abstract

Oil and gas wells are generally drilled with the intention of forming a filter cake on the wellbore walls to primarily reduce the large losses of drilling fluid into the surrounding formation. Unfortunately, formation conditions are frequently encountered that may result in unacceptable losses of drilling fluid into the surrounding formation despite the type of drilling fluid employed and filter cake created. It is extremely important to optimize filter cake thickness as very thick filter cake can cause stuck pipe and other drilling problems. The focus of this research is to use a computational fluid dynamics (CFD) technique to numerically simulate filter cake formation on the vertical wellbore wall at high-pressure (25,500 psi or 175.8 MPa) and temperature (170°C) conditions. Here, the drilling fluids were treated as a two-phase system of solid particulates suspended in a non-Newtonian fluid. Drilling process simulations were performed for drilling fluid with two particle sizes, 45- and 7- µm, under extreme drilling conditions of high pressure and temperature. The comparison of both scenarios clearly shows that the drilling fluid with larger particles (45 µm) forms thicker filter cake compared to drilling fluids with smaller particles (7 µm). We have further used FLUENT CFD code to successfully simulate filter cake formation on the wellbore wall at moderate pressure (2,000 psi or 13.8 MPa) and temperature (30°C) conditions with drilling fluid of 45 µm particles. The results for axisymmetric and planar wellbore show that the cake formed during extreme drilling processes is thicker than that formed for shallow drilling processes. Filter cake formed on the vertical wellbore wall is nonuniform for both extreme and shallow drilling process.

Key words: Filter cake, two-phase flow, computational fluid dynamics (CFD), deep drilling.