Formation of internal cake due to the invasion of reservoir rock by drilling, completion, and fracturing fluids is known to be one of the major causes of productivity reduction. Although the causes of internal cake formation are relatively well identified, the actual physical mechanisms involved in internal cake formation are yet to be understood. In particular, a quantitative means of relating physical properties of fluids to the degree of productivity reduction are not well established. A detailed investigation on the role of the rheology of viscoelastic fluids on the formation of "internal filter cakes" is, therefore, needed.

The extensional viscosity for pure liquids without any structure is 3 times the shear viscosity. For these liquids, contribution of the extensional viscosity to the pressure loss is, therefore, constant and often neglected. However, for viscoelastic fluids, such as polymer based drilling and completion fluids, the extensional viscosity may be several orders of magnitude larger than the shear viscosity. This will lead to a significant increase in pressure loss, which is very often attributed to the development of internal cake.

A series of core flow experiments have been conducted to investigate the possible relationship among the polymer concentration, fluid extensional viscosity, shear viscosity, filtration loss characteristics, and pressure drop across the core samples and hence, any change in the original rock permeability.

Results of extensional and shear viscosity measurements, API filtration loss tests, and formation damage tests conducted by using two different Partially Hydrolized Polyacrylamide (PHPA) solutions (1.5 and 3.0 lb/bbl) are shown in this paper.

Comparison of the PHPA test results with the previously published Xanthan Gum (XG) test results are also provided.


The rate of fluid filtration into the reservoir rock (leak-off rate) is one of the most critical parameters that need to be controlled carefully during drilling, completion and stimulation operations. A strict control of fluid filtration characteristics is required to limit borehole instability, excessive torque and drag, pressure differential sticking and formation damage.1 The problem becomes even more critical when drilling/completing horizontal wells with water based fluids where the fluid remain in contact with the pay zone for a long period.

Fluid loss control is generally achieved by; increasing the viscosity of the fluid2–3 and developing internal/external filter cake using fluid loss control additives4. If the viscosifiers and fluid loss control additives are not selected properly, both mechanisms may lead to significant reduction of permeability.1,5

One of the most common features associated with the flow of viscoelastic fluids in packed beds is that, much higher pressure losses are observed than that can be attributed to the shear flow of these fluids in the porous media. Such excessive pressure losses for viscoelastic fluids are well documented in the literature for a variety of aqueous polymer solutions and polymer melts. It has been recognized that the excess pressure drop may be due to the substantial extensional component present in this flow configuration.5–1 Durst et al.8, for example, suggested that as much as 75 % of the pressure losses in the formation may result from extensional viscosity.

Jones and Walters9 developed a macroscopic media model where they studied the flow of xanthan gum and polyacrylamide solutions. They have reported that xanthan gum solutions has extensional thinning characteristics while Polyacrylamide solutions show extensional thickening behavior.

Naverrete et al14, suggested that for the xanthan gum solutions the flow was dominated by the shear viscosity, whereas for the PHPA solutions the flow was dominated by the extensional viscosity, especially, at high-enough extensional rates.

A decade ago, Svendsen et al.15 has shown that extensional viscosity can be successfully used to establish an internal filter cake in association with coiled tubing drilling fluid application.

In this study, it was intended to investigate if there is any contribution of the extensional viscosity on the formation of internal filter cake as suggested by Svendsen et al.15

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