Diagnostic Plots Evaluate Gas Shut-Off Gel Treatments at Prudhoe Bay, Alaska
Authors: Chan K.S. , Bond A.J. , Keese R.F. , Lai Q.J. 
Published: December 2007

The new diagnostic plots technique is proposed to evaluate four key gel gas shut-off treatments conducted at Prudhoe Bay last year.

The diagnostic plots determined that a permeability contrast or formation channeling was the main mechanism for large gas entries. A log-log plot of Gas/Oil ratio versus time exhibited the characteristic behavior of initial gas coning and quickly turned into a rapid formation channeling problem. The excess or free gas production during the coning to channeling evolution could be used to deduce a proper gel treatment volume.

Post treatment well production data were plotted and compared with the pre-treatment diagnostic plots. Changes in the slope or trend of Gas/Oil ratio curves were used as an indicator of a successful gas shut-off treatment. The technique will require further confirmation as more gel gas shut-off treatments are performed.

Introduction

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.

It is critical to develop an engineering approach for identifying and treating the excessive or free gas and water production wells. This entails first determining the gas/water inflow mechanisms and locating their entries at the wellbore. Then a viable but cost effective gel placement technique can be developed. Finally, a coherent production and reservoir engineering evaluation method for determining the treatment technical and financial success can be undertaken.

The Diagnostic Plots

New diagnostic plots have 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 WOR (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.

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