DOWNHOLE NITRATE APPLICATIONS TO CONTROL SULFATE REDUCING BACTERIA ACTIVITY AND RESERVOIR SOURING

摘要

Reservoir souring commonly occurs in oilfields after waterflooding for secondary oil recovery. This is due to the activity of sulfate reducing bacteria (SRB) in the reservoir, which use nutrients from formation and/or injected water to generate sulfide. Conventional bactericide treatments may exert limited SRB control downhole. An alternative is the use of nitrate, encouraging the growth of nitrate utilizing bacteria to inhibit sulfide production by SRB. Laboratory tests were undertaken to determine the optimum nitrate treatment regime for a fractured chalk reservoir, including investigations into the potential for damaging effects such as corrosion, formation impairment, biofouling and solids removal or re-deposition. Upon completion of these tests, nitrate was dosed into the injection water on the Skjold field in the Danish sector of the North Sea. In one well pair with a short breakthrough time a reduction in H_2S production of some 80 percent was achieved. However, in less fractured regions with longer breakthrough times, the reduction in H_2S concentration in the production was much less pronounced.