Abstract
Schizophyllan is a new biopolymer developed by Wintershall for its applicability particularly in high temperature and high salinity assets. Previous work on Schizophyllan (Leonhardt et al. 2011, 2014) have reported excellent bulk rheological properties and injectivity both in laboratory and field. The objective of this work is to characterize and quantify the limits of Schizophyllan flooding parameters and highlight the associated uncertainties.
Polymer filterability tests involving high pressure gradients are generally not representative of polymer transport deep inside the reservoir. Thus, a few standard coreflood tests were performed in-house for obtaining a better understanding on polymer injectivity and propagation in porous media. Since, corefloods are relatively expensive and time consuming, a new set-up comprising of a small core holder (1cm length) was assembled in Wintershall's laboratory. In comparison to normal corefloods, this assembly allowed a quicker assessment of polymer propagation using a narrower pore size distribution. Additionally, specialized corefloods were conducted at varying ranges of temperature (55 to 92 °C), salinity (up to > 200 g/L), permeability (94 to 5200mD), etc. to characterize polymer parameters such as retention, RF (resistance factor), RRF (residual resistance factor) and IPV (inaccessible pore volume).
Corefloods using consolidated outcrop (Gildehaus Sandstone) and reservoir cores usually showed good injectivity and propagation of Schizophyllan at permeabilities greater than 200 mD. In comparison, reduced injectivity was seen through synthetic cores of similar permeability but narrower pore size distribution (sintered Quartz). Based on these results, pore size limits for propagation of Schizophyllan can be estimated. Static adsorption values measured on both outcrop and reservoir sand reported values in the range of 560μg/g and 680μg/g, respectively. Dynamic retention values with and without residual oil saturation ranged from 9 to 21 μg/g for outcrop cores, depending on various polymer concentrations. For reservoir cores, dynamic retention values varied between 20 to 50 μg/g, which can be attributed to the poor handling of oxidized cores, mineralogical variation and core scale heterogeneity. The corresponding RRF values for reservoir cores ranged from 1.4 to 3.8 respectively, for different IPV values. Further analysis showed that calculated IPV values are very sensitive and have considerable uncertainty based on its method of interpretation. For the coreflood experiments conducted at higher temperatures (greater than 90°C), no loss of Schizophyllan solution viscosity was observed, confirming the good thermal stability of the biopolymer.
This paper focuses on quantifying the limits of related Schizophyllan parameters such as polymer retention, RF, RRF and IPV. It also elaborates on the qualification of Schizophyllan for porous media using a novel core plug set-up with regards to polymer injectivity and propagation. Considering sandstone reservoirs, such data have not been previously reported in the literature for biopolymer Schizophyllan.
