Decoding the Impact of Injection-Induced Fractures on the Sweep Efficiency of a Mature Polymer Flood through Pressure Falloff Analysis

Abstract

Polymer flooding is one of the most popular chemical enhanced oil recovery processes in the world, with many large-scale commercial field implementations. It improves mobility ratio and sweep efficiency over waterflooding. However, injection of high volumes of viscous fluid at high injection pressures almost inevitably induces fractures. Induced fractures are not necessarily negative for the flood, and they are documented to improve injectivity and reduce shear degradation of polymer. The big question is on their effect on areal and vertical sweep efficiency. Mangala is one of the largest and most commercially successful polymer floods in the world. The FM-1 unit has the highest stock tank oil initially in place among all the layers of Mangala. Polymer flood sweep efficiency in FM-1 is excellent despite the presence of injection-induced fractures and geological layering. Pressure falloff (PFO) studies are an important and inexpensive source of monitoring the growth of induced fractures in mature polymer floods. Extensive work has been done in the past to derive analytical solutions and numerically assisted solutions for polymer PFOs for non-Newtonian rheology and induced fractures with variable storage. There is also extensive and sometimes contradictory literature on polymer rheology in porous media at both high and low flux rates. Most of the work on polymer PFOs have published a few field examples or synthetic examples and addressed specific issues in depth. We have interpreted 40 PFOs carried out over a period of 1 year. All relevant data sources in the field on geomechanics, polymer rheology in core and rheometers, temperature logs, and saturation logs were integrated to get a consistent estimate for fracture half-length. The results provide insight into the role of induced fractures in improving the sweep efficiency of FM-1. The arguments are backed with literature survey. The work can be used by operators around the world to manage mature polymer floods. The observations mentioned in the work can be used by researchers to fine tune their work. Secondary hypothesis on polymer rheology in porous media is also derived out of the work, which may be of academic interest and merit further work. There are other important aspects of polymer flood PFOs, such as impact of suspended solids and rock dilation near wellbore, which have not been explored in the paper.