Drilling fluids filter cake

J. P. Plank and F. A. Gossen. Visualization of fluid-loss polymers in drilling mud filter cakes. In Proceedings Volume, pages 165-176.64th Annu SPE Tech Conf (San Antonio, TX, 10/8-10/11), 1989. 

If the drill string becomes differentially stuck, mechanical methods or spotting fluids can be appHed, or the hydrostatic pressure can be reduced (147). In general, penetration of water- or oil-based spotting fluids into the interface between the filter cake and the pipe accompanied by dehydration and cracking results in reduction of differential pressure across the drill string (147,148). Spotting fluids are usually positioned in the open hole to completely cover the problem area. 

Polyethercyclicpolyols possess enhanced molecular properties and characteristics and permit the preparation of enhanced drilling fluids that inhibit the formation of gas hydrates prevent shale dispersion and reduce the swelling of the formation to enhance wellbore stability, reduce fluid loss, and reduce filter-cake thickness. Drilling muds incorporating the polyethercyclicpolyols are substitutes for oil-based muds in many applications [195-197,1906,1907]. Polyethercyclicpolyols are prepared by thermally condensing a polyol, for example, glycerol to oligomers and cyclic ethers. 

Polyacrylates are often added to drilling fluids to increase viscosity and limit formation damage. The filter-cake is critical in preventing reservoir invasion by mud filtrate. Polymer invasion of the reservoir has been shown to have a great impact on permeability reduction [98]. The invasion of filtrate and solids in drilling in fluid can cause serious reservoir damage. 

One of the basic mechanisms in fluid loss prevention is shown in Figure 2-1. The fluid contains suspended particles. These particles move with the lateral flow out of the drill hole into the porous formation. The porous formation acts like a sieve for the suspended particles. The particles therefore will be captured near the surface and accumulated as a filter-cake. 

Predictions on the effectiveness of a fluid loss additive formulation can be made on a laboratory scale by characterizing the properties of the filter-cake formed by appropriate experiments. Most of the fluids containing fluid loss additives are thixotropic. Therefore the apparent viscosity will change when a shear stress in a vertical direction is applied, as is very normal in a circulating drilling fluid. For this reason, the results from static filtering experiments are expected to be different in comparison with dynamic experiments. 

Clays or shales have the ability to absorb water, thus causing the instability of wells either because of the swelling of some mineral species or because the supporting pressure is suppressed by modification of the pore pressure. The response of a shale to a water-based fluid depends on its initial water activity and on the composition of the fluid. The behavior of shales can be classified into either deformation mechanisms or transport mechanisms [1765]. Optimization of mud salinity, density, and filter-cake properties is important in achieving optimal shale stability and drilling efficiency with water-based mud. 

The drill-in fluids are typically composed of either starch or cellulose polymers, xanthan polymer, and sized calcium carbonate or salt particulates. Insufficient degradation of the filter-cakes resulting from even these clean drill-in fluids can significantly impede the flow capacity at the wellbore wall. Partially dehydrated, gelled drilling fluid and filter-cake must be displaced from the wellbore annulus to achieve a successful primary cement job. 

To remove filter-cake, a physical method can be applied wherein a fluid is oscillated in the annulus prior to cementing [948,949]. The direction of flow of the fluid in the annulus is changed at least twice. The oscillatory flow of the fluid removes the drilling mud and the filter-cake from the annulus. After this oscillatory flow treatment, the cement slurry is pumped into the annulus. 

J. W. Dobson and T. C. Mondshine. Well drilling and servicing fluids which deposit an easily removable filter cake. Patent EP 672740, 1995. 

B. L. Todd, B. R. Reddy, J. V. Fisk, Jr., and J. D. Kercheville. Well drilling and servicing fluids and methods of removing filter cake deposited thereby. Patent US 6422314,2002. 

J. Weaver, K. M. Ravi, L. S. Eoff, R. Gdanski, and J. M. Wilson. Drilling fluid and filter cake removal methods and compositions. Patent US 5501276, 1996. 

C. G. Zhang, M. B. Sun, W. G. Hou, and D. Sun. Study on function mechanism of filtration reducer the influence of fluid loss additive on electrical charge density of filter cake fines. Drilling Fluid Completion Fluid, 12(4) 1-5,1995. 

Figure 33 shows the time dependence of the volume of static filtrate collected from a typical water-based drilling fluid. The initial volume collected at time f = 0 is termed the spurt loss and represents the uncontrolled flow of filtrate and fine solids into the filter medium the spurt loss continues until a filter cake forms on the surface of the filter medium. 

Figure 37 shows a schematic of the dynamic filtration of a drilling fluid. The drilling fluid is filtered across the filter medium at the pressure difference APfd while being subjected to flow (assumed laminar in the following discussion), which gives rise to a shear stress rc at the cake-fluid interface where the shear rate is yc. 

Figure 42. Dependence of (a) limiting dynamic filtration rate (Q/d) and (b) inverse of filter cake thickness (l/hc) on shear rate of drilling fluid. (Muds 2 and 3 from reference 139 with Fordham and Ladva s data from reference 135.) Lines show least squares fit to data from muds 2 and 3.

The dynamic filtration theory of Outmans (127) requires experimental terms such as particle-particle stresses, particle friction factors, and thickness of a shear zone within the filter cake that would be difficult to determine. However, the qualitative picture of dynamic filtration presented by Outmans, namely, irreversible adhesion of solid particles up to a certain thickness that is determined by the shear stress (or shear rate) at the surface of the cake, accords with the experiments of Fordham and co-workers (129,135). Once a filter cake has formed under dynamic conditions, it is difficult to remove it by subsequent changes in yc or vm. Figure 44 shows the effect of changes in the flow rate on cumulative filtrate volume. The limiting filtration rate obtained when the initial flow rate of the drilling fluid was 1.8 m3/h remained unaltered when the flow rate of the drilling fluid was increased to 7.0 m3/h in a step-... 

Figure 51. SEM micrographs of filter cake formed under static conditions from simple bentonite drilling fluid formulated with (a) water and (b) 0.18 M CaCl2. (Reproduced with permission from reference 145. Copyright 1988 Society of Petroleum Engineers.)...

The imbalance in the chemical potential of the water in the shale and drilling fluid results in a tendency for water to enter the shale. Equation 110 is applicable to both water- and oil-based drilling fluids. When fij = fjL sh equation 110 gives the well-known expression for the swelling pressure (Psh — P) between the shale and the drilling fluid. The permeability of shales is very low and the rate of filtration into the shale will be below the critical filtration rate (140) and no filter cake will form on the surface of the shale. 

The s mthesized organo-clays as a fluid loss control additive performed well in oil phase. The organo-clays modified with 2.0 cation exchange capacity cetyltrimethylammonium bromide and 2.0 cation exchange capacity sorbitan monooleate yielded 100% colloid fraction in colloid fraction tests, showed low filtration loss of 5.7 ml, and left a filter cake approximately 68 /rm thick in American Petroleum Institute filtration tests (31). This indicates that the synthesized organo-clays can be potentially used as fluid loss control additive in oil-drilling excavation (30). 

A drilling fluid is composed of a carrier fluid and solids (clay or polymer). The carrier fluid carries the solids down the borehole where they block off the pore spaces on the borehole wall. The blockage is referred to as a filter or mud cake. The ideal mud cake will form quickly during construction of the wellbore and prevent intrusion of drilling fluid into the formation. At times additives such as detergents are added to the drilling fluids to counteract some of the formation characteristics such as swelling and stickiness. 

Fluid loss controlling agents filtrate or fluid loss reducers serve to decrease fluid loss. The drilling mud fluid loss property is a measure of the liquid phase tendency to pass through the filter cake. Examples of this type include names such as polyacrylates, starch, and carboxymethyl starch. 

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