The new Water Control Diagnostic Plots technique was found to be very effective for evaluation of a water conformance treatment conducted in Prudhoe Bay last year.
It determined the channeling as the main mechanism for poor conformance as seen in the diagnostic plots of offset production wells. Log-log plots of Water/Oil ratio vs time exhibited a characteristic behavior of channeling behavior. Post treatment well production history data of the offset production wells were plotted and compared with the pretreatment diagnostic plots. Change in slope or trend in Water/Oil ratio curves as used as a good indicator of successful problem correction and good treatments.
The thick Prudhoe Bay Sadlerochit sandstone formation, which is at a vertical depth of 8,800 ft' is bounded by a gas cap and a bottom water aquifer. Portions of the field have been under waterflood since 1980. The field is producing approximately 0.9 MM BPD oil, 1.5 MM BPD water and 7 MMM SCFPD gas. As reservoir depletion continues, water and gas production rates continue to increase.
Most of the produced water and gas are re-injected. The majority of gas is re-injected into the gas cap. Some is made into miscible injectant for a tertiary recovery process. The water is treated and filtered and incorporated into the ongoing waterflood program. However, the capabilities of water and gas handling surface facilities are limited even after recently completing a series of plant expansion projects. Increasing water and gas production drives up water and gas handling costs and decreases oil production. As reservoir depletion continues, gas cap and enriched gas as well as waterflood water breakthrough occurs frequently at the producing wells. The Gas/Oil and Water/Oil contacts get closer and closer to the perforated intervals. The potential of bottom water and gas cap gas coning and channeling increases. The gas and water produced by these adverse mechanisms by-pass reserves and impede normal production.
A new diagnostic plot technique has been developed to differentiate between mechanisms of excess water and gas production. The production data are used to plot in log-log the WOR, GOR or WGR (Water/Oil, Gas/Oil and Water/Gas Ratios respectively) and their derivatives versus time. Together with the completion and workover history, the excess water and gas production mechanism can be determined.
Figure 1 shows a comparison of WOR behavior for normal layered reservoir depletion and bottom water coning. This technique can be applied to oil or gas wells, vertical or horizontal wells. Figure 2 shows a case of rapid water entry occurring in a horizontal well. The drastic increase in WOR and its derivative indicate such a change after approximately 300 days. The mechanism is a near wellbore or cement channel problem.
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