Reversible, Crosslinkable Polymer for Fluid-Loss Control

A new, environmentally friendly polymer has been developed for use in well completion and stimulation treatments where a premium is placed on maximizing effectiveness while minimizing formation damage. This new polymer is finding uses in many operational areas such as high-permeability fracturing, gravel packing, zonal isolation pills, spacer pills, pipe line pigs, kill pills and the like. This presentation will center on its use as a low-damage, fluid loss control agent for completion operations, particularly well control during and after perforation of high-pressure, high-permeability wells.

The use of expensive, clean completion fluids is common on high-productivity, high-pressure wells to prevent the loss of productivity. High fluid loss can result in high completion cost, deep formation damage and, potentially, loss of well control; therefore, completion fluids must be effective in controlling fluid loss. A number of materials have been used for controlling fluid loss: sized particulates, viscous fluids and gelled fluids. Guar-based fluids and sized salts are commonly used due to their low cost; however, both types of systems can leave significant well damage.

Hydroxyethylcellulose (HEC) is accepted as a fluid affording minimal permeability damage during completion operations. Normally, HEC polymer solutions do not form rigid gels, but control fluid loss by a viscosity-regulated mechanism. Permeability damage has been shown to increase with increasing penetration of the viscous fluid.

Other natural polymers such as xanthan, guar gums and certain derivatives of cellulose such as sodium carboxymethyl-hydroxyethylcellulose (CMHEC), are easily crosslinked to form rigid gels by transition metal ions, and they control fluid loss by forming a polymer filter cake. Crosslinked polymers tend to have little penetration depth, however; the polymer filter cake is difficult to remove by normal means which can result in significant productivity damage.

Research has led to the development of a patented, double-derivatized cellulose polymer that can be crosslinked to provide the advantages of both fluid loss control mechanisms. This polymer (XLHEC) is prepared by grafting a crosslinkable moiety onto HEC that results in a polymer that has fluid properties similar to HEC but can be transformed into a rigid gel by adjusting pH to the basic range.

XLHEC is formulated into a non-flammable, non-hazardous environmentally safe liquid concentrate. Hydration is retarded allowing this material to readily disperse into most brine completion fluids. Hydration is initiated by lowering the pH to yield gels free of lumps and fisheyes.

Crosslinking is effected by the use of a slowly soluble, non-toxic metal oxide. This resulting crosslinked fluid demonstrates a shear-thinning/rehealing rheology that provides for easy pumping and good fluid loss control upon placement. (If preferred, conventional multivalent metal ions can be used to crosslink the fluid.)

Crosslinked XLHEC fluids have been effective in controlling fluid loss in both laboratory tests and field applications. Figure 1 compares the fluid loss control abilities of XLHEC with linear HEC. Table 1 exemplifies the ability of XLHEC at different concentrations to control fluid loss against high-permeability cores. It also shows how completely the fluids are broken.

In practice, two gel breaking mechanisms are employed. The first is a simple reversing of the crosslink reaction by lowering the pH to break the polymer filter cake. Continued exposure to low pH depolymerizes the backbone polymer. The pH can be dropped by either exposing the polymer to an external acid or by adding a slow release acid prior to crosslinking. Oxidants and enzymatic breakers can also be utilized as internal breakers.

XLHEC can be prepared in brines from 8.33 lb/Mgal to 15.2 lb/Mgal (CaCl₂/CaBr₂). These brines can contain any of the following salts: NaCl, KC1, NH₄Cl, NaBr, CaCI₂, CaBr₂, and sea salt. XLHEC fluids are used at pH values greater than 7. The high pH is in part responsible for the high temperature stability demonstrated by these fluids. In laboratory tests XLHEC fluids have remained rigid for 24 hr at 300 °F. In long-term tests, XLHEC fluids have remained stable for several months at 260 °F.

The advantages of XLHEC are summarized below:

• It is packaged as a environmentally safe, non-flammable, easily dispersed liquid gel concentrate.

• Crosslinked XLHEC features superior regained permeabilities.

• It creates a highly stable gel for temperatures up to 300°F (150°C).

• It can withstand high differential pressures.

• It can be completely broken with exposure to external acid washes.

• It can utilize internal breakers for gel removal.