Experimental and Numerical Study of Drilling Fluid Removal from a Horizontal Wellbore

Abstract

Abstract After a well has been drilled, the drilling fluid should be removed and replaced with either cement and/or completion fluids. For effective zonal isolation and optimum hydrocarbon production during the life of the well, the entire drilling fluid should be removed from the annulus. Cement and completion fluids are sensitive to drilling fluid contamination, and even a thin layer of oil-based drilling fluid could prevent the cement from bonding to the formation and the casing. In addition, for optimum hydrocarbon production, the cement sheath must be able to withstand the stresses throughout the life of the well.1 Several factors affect the success of drilling fluid removal from horizontal annuli. Under static conditions, drilling fluid usually forms a gel structure. Under positive differential pressure, the drilling fluid loses filtrate and forms a filter cake on the formation face. Sometimes the filter cake is mushy and difficult to remove, but the formation fluid can easily flow through it. The successful removal of the gel and mushy filter cake depends on the structures that form and how these structures behave under flow conditions. In addition, casing centralization affects the fluid flow profile in the annulus, which affects gel and filter-cake removal. To investigate the mechanism of drilling fluid removal, we conducted both numerical and experimental studies. In the experimental studies, chemical flushes and spacers were tested for their effectiveness in removing drilling fluid. The experiments showed that annulus cleaning begins around the inner pipe and progresses outward at increasing fluid flow rates. Analytical fluid flow models and full 3D multiphase numerical models allowed us to estimate flow profiles and the success of removing drilling fluid under downhole conditions. The large-scale experiments and analytical/numerical modeling have led to a better understanding of the factors controlling drilling fluid removal from horizontal wellbores. Introduction Many studies have been published that discuss the efficiency of drilling fluid removal.2–11 As this research indicates, drilling fluid that gels excessively and/or has lost filtrate to the formation can be difficult to remove. For a successful cement job, both the gelled drilling fluid and mushy filter cake should be removed before the cement is pumped.3 Because the use of laboratory-prepared drilling fluid has limited validity, circulated oil-based drilling fluids were used in this study. The use of oil-based drilling fluid enabled us to study the removal of gelled drilling fluid and filter cake as well as the use of water-wetting designed spacers. Experimental Setup Fig. 1 (Page 6) shows a schematic cross section of the experimental cell. The setup had a 3-in. OD inner pipe and a 5-in. ID outer pipe. The inner pipe was placed at a standoff ranging from 100% (fully centralized) to 0% (lying on the low side of the hole). The outer pipe was equipped with a heating jacket to maintain static temperature. Fluid inlet and outlet lines to the cell consisted of 3-in. hoses with a smooth transition to the annular section. The cell had a straight 8.5-ft long annular section. A foot-long core section was mounted flush with the outer pipe surface. Pressure was applied to the cell, and filtrate was collected, building up a filter cake on the core's surface. Fig. 1 (Page 6) also shows thermocouple locations (T1 through T3) and pressure measurement locations. The pressure transducers, P1 and P2, measured pressure drop, while P3 measured absolute pressure. Flow rate was measured by a 3-in. magnetic flowmeter and a 1-in. mass flowmeter. Toward the outflow end of the annular cell, two temperature-compensated conductivity probes were mounted flush with the outer pipe. The oil-based drilling fluids gave a zero conductivity reading, contrary to the water-based spacer. As a result, the removal of oil-based drilling residue from the outer pipe surface could be observed from its onset. In addition to the components already described, a rheology flow loop was used to characterize both drilling fluid and spacer rheology.