Excessive cement filtrate loss is known to cause formation fluid influx and migration through the setting cement. The filter cake deposited by the drilling fluid controls or limits the cement slurry's filtrate loss. The effectiveness of a particular filter cake to limit cement slurry's filtrate loss depends on its permeability. A series of dynamic fluid loss (DFL) tests were performed on a 2-in diameter by 2.5-in long permeable man made cores during which filtrate volume was measured as a function of time for a constant shear stress. Two drilling fluid types, one with high fluid loss and the other with low fluid loss were used for the DFL tests. An equation was developed to determine filter cake permeability based on filtrate volume, shear stress, plastic viscosity and yield point of the fluid.
In the DFL tests, the low and high fluid loss drilling fluids stabilized in flow rate and thickness in less than 15 minutes. The final permeability of the filter cake stabilized at 0.5 and 2.0 nano-darcys for the low and high fluid loss drilling fluids respectively. Since the high fluid loss drilling fluid produces a filter cake that has four times the permeability of the low fluid loss drilling fluid, the latter fluid should be used. That is, the drilling fluid must be conditioned to have low fluid loss during cementing. Thickness measurements of the filter cake as a function of time allow the calculation of the permeability of the filter cake which reduces the cement slurry's filtrate.
The drilling fluid consists of a mixture of solids, liquids (water or oil), and chemicals, with the liquid being the continuous phase. The solids may be active solids such as bentonite and polymers or inactive such as barite. To stabilize the wellbore, the drilling fluid attempts to seal the borehole by the solid and polymer bridging on the formation face. The deposition of the solids occurs only if a pressure differential is established away from the wellbore. Since the solids do not readily enter the formation pore spaces, a layer of high density cake deposits on the borehole wall. The thickness of the cake increases until the cake's permeability approaches zero. This can occur under dynamic or static fluid conditions. The solids and fluid loss polymers control the final thickness of the cake development.
There are situations where the filter cake is not completely removed during the cementing process1–5. For example, when cementing some casing strings, especially the surface strings that protect the potable water sands, shear rates are low, filter cake build-up is the greatest, and drilling conditions limit mechanical removal of filter cake. Therefore, the annulus is partially filled with cement and filter cake. Initially, zonal isolation is achieved due to the cement and filter cake possessing near zero permeability1. However with time, the polymers and chemicals in the filter cake degrade allowing the permeability of the filter cake to increase which compromises the annular seal. This leads to further degradation of the filter cake, primarily from gas influx into the permeability of the filter cake.