Abstract
This paper describes experimental work towards a new enzyme process for downhole cleanup of filtercakes from water-based reservoir drilling fluids. The objective was to develop a simple, effective enzyme cleanup procedure, using industrial enzymes customized for each drilling fluid. Our focus was on filtercakes from fluids based on modified starch and xanthan using thermostable α-amylases, and polyanionic cellulose (PAC)- based fluids using cellulase enzymes. Only enzymes supplied with a guaranteed activity vs. storage were used.
Pre-screening of enzyme efficiency was done by measuring the viscosity reduction of enzyme-treated muds, or polymer solutions in those cases where the filtration-control polymer is designed to have a low viscosity. A direct visual dissolution test on filtercakes deposited on porous discs was defined and used for screening. This test could be done rapidly on realistic fluids at temperatures up to 100 °C. Filtration tests on mud filtercakes deposited on porous discs were found to give useful quantitative information on filtercake breakdown. Direct and repeatable data for the relative efficiency of each enzyme on each filtercake were obtained. Filtration experiments on radioactively labelled enzymes were run to allow a separate quantification of enzyme in the solid and liquid fractions of the cake, in the filtrate, and in the remaining enzyme solution.
Results are given for a range of starch/xanthan-based reservoir drilling fluids built from various brines, using sized salt and calcium carbonate solids, and for a PAC mud with barite. An important result is that filtercakes from all the starch/xanthan-based fluids could be efficiently removed by α-amylases, and those from PAC/xanthan-based fluid by cellulases alone. In all starch cases studied, visual filtercake destruction was accompanied by complete starch removal, as confirmed by iodine test. The starch or PAC in these fluids appears to be solely responsible for bonding the filtercake. Hence, if required, filtercakes can be destroyed without affecting the xanthan viscosifier.
Flow through filtercakes was found to increase dramatically after enzyme treatment. The best enzyme soaks approach the efficiency of 15% HCl or 3% peroxide solutions in the lab. The mechanistic flow and adsorption analysis using labelled enzymes showed that enzyme rapidly penetrates the filtercake, adjusting its distribution in the system with changes in flow conditions. Enzyme accumulates in the filtercake only if intact polymer molecules still exist for enzyme attachment.
For cellulase treatment of PAC- and barite-containing muds, filtercake dissolution tests were inconclusive while filtration tests indicated very positive effects. Nevertheless, cellulases need a targeted development effort to reach oilfield application on any major scale, as the highest operating temperature with cellulases available commercially today is far less than typical downhole temperatures in cases where PAC would be favored over starch.
Ongoing work towards field trials in candidate reservoirs for which any use of acid in clean-up is undesirable is reviewed.
