Water profile control, using indigenous nitrate reducing seawater bacteria in North Sea waters as blocking agents, has been investigated. Feasibility studies have been carried out, both in a micro (micromodels) and a meso scale (sand packs), aiming at identifying parameters that are critical for the process. The results show that there are several factors that are important for the penetration and distribution of a bioplug in a porous matrix. Although lab experiments have shown that bacteria preferentially block high permeable zones, it is questioned how relevant this information is for practical purposes. The long distances involved in transport of bacteria and nutrients into North Sea reservoirs implies problems that have to be resolved. Biofilms growing as a network in pores in the matrix, biowebs, are proposed to play a major role in the development of a bioplug. The filterability of the matrix is increased once the bioweb is established, and entrainment of fines and bacteria is dramatically impeded. pH alterations, due to acidic metabolic by-products, is believed to be rate limiting for the generation of biomass in the porous matrix. This problem, in conjunction with the problem created by the biowebs, is speculated to be main obstacles in a plugging scheme in large fields with distant target zones. The problem of scale seem to be extremely important. It is questioned if the current methodological approach in the study of biofilms is adequate for studying phenomena that are distant in time and space.
Introduction
A generic North Sea reservoir has sandstone petrology and is stratified, with high permeability contrasts in different zones of the reservoir. Many North Sea fields undergoing water flooding suffer of premature water break through due to fingering. Water diversion methods are attractive as a remedy of this phenomenon. Feasibility studies with the aim of developing a microbial method that serve the purpose of blocking such high permeable zones, was started in 1993. Major challenges for such a technique is high temperature (60-160 C) and pressure (170-760 bar), and long well spacing (1-2 km). The technique should also rely on bacteria growing anaerobically.
Problems with growth of sulfate reducing bacteria during implementation of such a technique is considered to be small. This is confirmed by our own experiments and by others. Nitrate is used to control sulfidogenic activity. The microbiological concerns relevant for the process has been presented earlier. The target for the process is near well zones with temperatures lower than 45 C, as this is the upper limit for growth of the indigenous seawater bacteria. Many reservoirs in the North Sea have temperatures significant lower than this limit, after a prolonged water flooding period. In several cases the temperature is around 20 C, and the gradient into the reservoir before the temperature reaches 45 C stretches for several hundred meters.
Indigenous, nitrate-reducing seawater bacteria serve the purpose as plugging agents in the process pursued. Problems with competition between near well bacteria and the introduced species is thus minimized. The matrix of the formation introduces several challenges for the transportation of bacteria. The size, morphology and mode of living of bacteria is crucial for their transport to the zones where the desired process preferentially should take place.
This paper deals with the problems connected to the injection and penetration of the bacteria, and the generation of a bioplug in a porous matrix. The plugging process has been examined at different scales, beginning with the micro scale, and ending with long sand packs. Some of the critical parameters pertinent to the plugging technique has been identified during these experiments.
