Positron-Emission Tomography Offers New Insight Into Wormhole Formation

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 180051, “New Insight Into Wormhole Formation in Polymer Gel During Water Chase Floods Using Positron-Emission Tomography,” by B. Brattekås, University of Stavanger; M. Steinsbø, A. Graue, M.A. Fernø, and H. Espedal, University of Bergen; and R.S. Seright, New Mexico Petroleum Recovery Research Center, prepared for the 2016 SPE Bergen One-Day Seminar, Bergen, Norway, 20 April. The paper has not been peer reviewed. Polymer gel is frequently used for conformance control in fractured reservoirs, where it is injected to reside in fractures or high-permeability streaks to reduce conductivity. Polymer-gel behavior is often studied in corefloods, where differential pressure and effluents from fracture and matrix outlets give information about gel deposition during placement and flow paths during chase floods. The work presented in this paper uses complementary positron-emission-tomography (PET)/computed-tomography (CT) imaging to quantify the behavior and blocking capacity of gel during chase waterflooding. Introduction Channeling of injected fluids through a highly permeable fracture network, and the following early fluid breakthrough, may be mitigated by placing a highly viscous polymer gel in the fracture. With polymer gel in place, higher differential pressures may be achieved during chase floods, which can contribute to increased sweep efficiency in the porous matrix adjacent to the fracture network. A polymer gel is formed when a gelant solution is exposed to elevated temperature for a given time, known as the gelation time. Previous work has investigated how the gel state during placement (gel or gelant) influenced the gel behavior during chase floods. The complete paper provides a detailed discussion of previous models for fluid leakoff. In this work, the authors study the extrusion of formed polymer gel through fractures, and its resistance to pressure during subsequent waterfloods. PET imaging is based on the decay of positron-emitting radionuclides. In this work, PET was used to visualize flow of radioactive water through a gel-filled fracture, to augment global measurements, and to increase the understanding of filter-cake formation during formed-gel placement and wormhole formation during chase floods. A discussion of the experimental setup, including limestone- and sandstone-core-plug preparation, polymer-gel placement, the chase-flood procedure, and the PET-imaging process, is provided in the complete paper.