Qualification of Biopolymer in an Offshore Single-Well Test

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 28019, “Qualification of Biopolymer in Offshore Single-Well Test,” by H. Urkedal, O.M. Selle, O.J. Seime, ø. Brandal, SPE, and T. Grøstad, Statoil; A. Todosijevic, SPE, M. Dillen, SPE, D. Prasad, SPE, and B. Ernst, SPE, Wintershall Holding; and F. Lehr and E. Mahler, BASF, prepared for the 2017 Offshore Technology Conference Brasil, Rio de Janeiro, 24–26 October. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission. A single-well polymer-injection and back-production test has been performed in an oil and gas field offshore Norway. The objective of the test was to verify at field conditions the properties measured in the laboratory for the biopolymer schizophyllan. The data confirm that the biopolymer is not biodegraded during the shut-in period because of efficient application of biocide injected simultaneously with the biopolymer. Rate and pressure data also support that the injectivity is maintained during the test compared with water injectivity. Introduction Polymer injection has been considered as a method to increase oil recovery from the Heidrun Field. The field is located at Haltenbanken in the Norwegian Sea, producing from a large concrete floating tension-leg platform with access to 56 well slots in addition to tie-in of five subsea templates and two production and three injection templates. The recovery factor for the different formations ranges from 60% for the homogeneous high-permeability zones to 10% for the most-heterogeneous low-permeability zones. Schizophyllan has a linear structure without charged functional groups, which results in virtually no loss of viscosity in high-salinity brine, especially in the presence of di- or multivalent ions, and comparably low adsorption on rock surfaces. Furthermore, schizophyllan shows short-term stability at temperatures up to 135°C and has shown half-life values of several years, exceeding 10 years at lower temperatures. Finally, schizophyllan provides high viscosity efficiency, with strong shear-thinning behavior but no shear-thickening behavior, at high shear rates. Laboratory Qualification A standard laboratory program was performed to qualify the biopolymer for Heidrun Field conditions. In addition, a special laboratory study was performed to determine long-term thermal and biological stability. The standard laboratory program included static bulk tests and coreflooding tests. The special laboratory program included long-term thermal-stability tests, long-term biological-stability tests using actual injection water, and microbial-degradation tests. The polymer had good rheological properties, was stable at high shear rates, demonstrated low adsorption and good filterability and injectivity, and was thermally stable for at least 10 months at 85°C. As expected, schizophyllan will biodegrade in the injection waters available at Heidrun unless protected by biocide. Glutaraldehyde and formaldehyde are both compatible with the biopolymer.