Rheological Screening of Low-Molecular-Weight Polyacrylamide/Chromium(III) Acetate Water Shutoff Gels

Abstract

Abstract Low-molecular-weight polyacrylamides crosslinked with chromium(III) acetate are widely used in the oilfield industry to seal off watered out zones in matricial reservoirs. These products are usually mixed in surface facilities, then pumped down hole through coiled tubing and injected into the formation over a depth of several feet. For the operators, gelation time and gel consistency after well shut-in are the two most important parameters to control. The pumping time cannot exceed the gelation time and the maximum pressure drawdown sustainable by the gel is related to gel consistency (as measured, for instance, by the yield stress). Both are known to depend on temperature, polymer concentration and structure (molecular weight, hydrolysis degree), and crosslinker concentration. A systematic screening of these parameters has been undertaken by means of rheological measurements using a cone-and-plate rheometer. The procedure for determining gelation time consisted in monitoring the viscosity of the polymer and crosslinker solutions mixed at time t=0 and then submitted to shearing at a constant rate. The gelation time, which corresponds to a sudden rise in the viscosity, was observed to be independent of the applied shear rate but to strongly vary with temperature and, to a lesser extent, with polymer hydrolysis degree, concentration and molecular weight and crosslinker concentration. From the set of data obtained, simple correlations or rules of thumb for the gelation time variations are given, which can be used to optimize the formulation for a given field application. To evaluate gel consistency, some yield stress measurements were performed after a resting (shut-in) time of about 10 hours. The yield stress is related to the maximum drawdown sustainable by the gel in a formation through a simple relationship. Introduction The use of gels to seal off watered out layers has been increasing in recent years. The main advantages compared to cements area deeper penetration in the formation andan easy removal from the wellbore by water recirculation. However the control of the gelation process is often difficult due to the number of parameters affecting the process such as temperature, salinity, pH, polymer and crosslinker quality and concentration, making operational constraints delicate. Among the different systems, gels of partially hydrolyzed polyacrylamide crosslinked by chromium(III) acetate, hereafter referred to as PAMx (x is the hydrolysis degree or proportion of acrylate monomers in the polymer chain) and Ac3Cr, have been widely used recently.1–5 The system is fairly robust and overcomes the environmental restrictions of using chromium(VI)/reducing agent as a crosslinker. Two options are available. The first one, known under the trademark of MARCIT™, uses high-molecular-weight polyacrylamide and has been designed to treat fractured formations. The second one, known under the trademark of MARASEALT™, uses low-molecular-weight polyacrylamide and aims at sealing matricial formations.