Rheology of Sandy Polyacrylamide Gels: Application to Conformance in Cold Production
Authors: Bani E.S. , Tremblay B. , Polikar M. , Wiwchar B. , Huang H. 
Published: March 2010

In western Canada, many cold production wells must be prematurely abandoned due to watering-out. High permeability channels (wormholes) are known to develop during cold production. Tracer tests and wormhole growth experiments and simulations suggest that open (sand-free) channels form within the wormholes once they reach a certain length. Wormholes often break into water zones leading to premature abandonment of many wells.

Different sandy polyacrylamide gel systems, developed for blocking these open channels, have been used as treatments, and whose results are reviewed here. These systems combine good injectivity with higher strength, as tested in the lab. In addition, a series of viscosity, settling rate, and gel strength tests were performed for different sand concentrations, shear rates, polymer concentrations, molecular weight, and salinity. A promising sandy polymer gel system for water shut-off applications in cold production is described.

Introduction

Cold production of heavy oil is a non-thermal process in which sand and oil are produced simultaneously. The drive mechanism is generally thought to be solution gas drive. The oil is widely believed to be drained from the reservoir through highly permeable channels, called wormholes, which develop by erosion of the sand when the pressure gradient at the tip of the perforations reaches a critical value.

The development of high permeability channels can explain:the short travel time (within hours) of fluorescein dye tracers between wells up to 2 km apart(1);the caliper measurements of Elkins(2) showing wellbore enlargement over only a short length along the vertical well;the relatively low pressures observed during injectivity tracer tests(3);rapid steam communication between certain wells(4,5); and,radioactive tracer tests by Pan Canadian(6) showing tracer penetration over a thin (1 m) thick zone within a 7 m pay zone.

The development of wormholes can also explain the high sand cuts at the start of cold production [30% to 40%(7), 10% to 50%(2), 20% to 30%(8)] followed by a sharp decrease in sand cuts to nearly constant values in the range of 0.5% to 1%(7), 0.1 % to 2%(2), and less than 5%(8) after a period of six months to one year. Based on experiments(9) and numerical simulations(10) of wormhole growth, the high sand cuts would occur during the initial growth regime when the wormhole is completely filled with sand. The presence of an open channel within wormholes was strongly suggested by the fluorescein tracer dye study by Amoco(1) in which the dye was injected in one well and was produced from a neighbouring well 400 m away without any loss in concentration. Since the flourescein dye could be easily adsorbed on the surface of a porous medium, this indicated that the dye flowed through an open (sand-free) channel.

Numerical calculations(10) indicate that as the wormholes get longer, more oil is drained into the wormholes. Therefore, the flow rate along the wormhole would increase starting from the tip of the wormholes towards the well.