Erodability of Partially Dehydrated Gelled Drilling Fluid and Filter Cake

Abstract

SPE MembersAbstract. Partially dehydrated-gelled drilling fluid and filter cake must be displaced from the wellbore annulus to achieve a successful primary cement job. A term 'Erodability of drilling fluid' is defined in this paper and was calculated from measured parameters in a large-scale test model. Differential pressures were measured in the annular space between the casing and a man-made, permeable formation and also inside the casing. Three different drilling fluids used in typical field operations were tested. Each drilling fluid was tested over a period of four days. Experimental data on erodability of drilling fluid filter cake is presented as a function of time, flow rate, and aging. A mechanism is proposed for the erosion of partially dehydrated-gelled drilling fluid and filter cake. Recommendations are given to improve the removal of partially dehydrated-gelled drilling fluid and filter cake and thus achieve a successful primary cement job in field operations.Introduction. The main purpose of a primary cement job is to seal the annulus and thereby provide zonal isolation from formation fluids. The potential outcome of a primary cement job is mainly affected by the condition of the wellbore when the cement is pumped downhole. Figure 1 shows the condition of a typical wellbore at the end of drilling and after a shutdown period to log and run the casing. The figure shows filter cake next to the formation wall, followed by partially dehydrated-gelled drilling fluid and moderately-gelled drilling fluid. We define moderately-gelled drilling fluid as the fluid that has developed gel strength in the absence of shear and refer to it as MG drilling fluid. Partially dehydrated gelled drilling fluid is the fluid which, in addition to developing gel strength in the absence of shear, has also lost a portion of its water (fluid); we refer to it as PDG drilling fluid. If cement is pumped into a wellbore as shown in Figure 1, chances are that it will channel through the MG and PDG drilling fluid and produce a poor cement job.To obtain a successful primary cement job, the wellbore must be properly conditioned by breaking the gel strength of the MG and the PDG drilling fluid and by removing as much as possible of the filter cake before pumping the cement. Beirute et al. have discussed the effect of mechanical scrapers and flushes to improve the circulatable hole. Smith et al. have reported that under similar operating conditions, the degree of hole conditioning achieved is a function of the drilling fluid used. From the work done by these authors it can be concluded that to obtain a successful primary cement job, the job must be designed specifically for the particular wellbore and drilling fluid. This can only be done if the erosion mechanism of PDG drilling fluid and filter cake is understood.Deposition and erosion of PDG fluid and filter cake has been a subject of study in the areas of ultra-filtration of proteins and juices through membranes. Porter proposed a convective transport of fluid toward the membrane for deposition, and concentration induced diffusive transport of particles away from PDG fluid and filter cake for erosion. Davis et al., and Leighton et al. have discussed a shear induced hydrodynamic diffusion mechanism for migration of particles away from the filter cake and PDG fluid as the fluid is sheared. Fordham et al. studied the filtration of bentanite drilling fluids and could not verify the convection-diffusion mechanism for bentonite systems.P. 219^