Conventional through-tubing isolation tools do not provide adequate isolation in wells completed with slotted liners, especially if there is an open annulus around the liner. As with vertical wells, selectivity is required to effectively place stimulation fluids or conformance fluids for gas or water shutoff.
A temporary annular gel-plug system has been developed for isolating zones in such completions. A fluid was designed to be placed in the annulus between the formation and the liner, which would resist the flow of subsequent treatment fluids and also clean up after the treatment.
Laboratory testing involved determining the rheological properties of candidate fluids, their chemical compatibility with the treating fluids and studying the placement of the systems in a typical wellbore geometry. The fluids developed in the laboratory are planned to be tested in the Rabi field in Gabon later in 1997.
Horizontal wells typically have long interval lengths, which means that they frequently suffer problems of uneven production from certain sections of the well, due to wellbore pressure losses or changes in geological and reservoir parameters.
As with vertical wells, differences in production can be reduced with (semi-) permanent mechanical shutoff or chemical treatment of selective intervals. Chemical treatments may include the use of conformance materials for water and gas shutoff, or acid for stimulating low productivity zones. Permanent mechanical zonal isolation for horizontal wells must be installed in the well at the completion stage using external casing packers or sliding side-doors. However, chemical treatment of selective intervals can be applied remedially. This requires temporary (remedial) zonal isolation after completion, which can be achieved either mechanically (using coiled tubing packers) or chemically (using gel plugs). In many cases, remedial treatment in this manner can be more advantageous than mechanical isolation in the completion stage, since the zones that require treatment may be better defined later in the well's life.
If a horizontal well is completed with slotted liners or prepacked screens, and the formation has not collapsed onto the screen, an open annulus will be present between the formation and the slotted liner. For these wells, sliding doors or coiled tubing straddle-packers will not seal externally in the annulus. Therefore, the flow in the annulus cannot be controlled. This problem can be solved with external completion packers for permanent isolation, or potentially temporary annular gel plugs for remedial isolation during the placement of treatment chemicals.
To date, temporary gel plugs have not been applied in slotted liners or prepacked screens with an open annulus, although they have been successfully used to plug openhole completions. It was therefore deemed advantageous to develop this technique for use in the Rabi field in Gabon, where slotted liner completions with an open annulus are common.
The Rabi Field in Gabon has an estimated STOIIP of 0.28 × 109 m3 with initial gas in place of 0.15 × 109 m3 situated in a reservoir at a depth of 1080 m with an oil column of around 45 m. The high porosity sand has horizontal permeabilities of around 5 darcy. The current reservoir pressure and temperature are around 110 bar and 47 C, respectively.
Following an initial vertical-well appraisal period, the field was developed with horizontal wells. Figure 1 illustrates a typical completion for horizontal wells in the Rabi Field, with horizontal interval lengths ranging from 800 to 1500 m.