This paper describes a water shut-off treatment on a horizontal well in a field operated by Mobil Producing Nigeria (MPN) offshore Nigeria. The well was initially completed with an uncemented slotted liner in a highly permeable formation. The field location and pre-treatment well completion schematic are shown in Figures 1 and 2 respectively. The intervention was carried out with the use of coiled tubing through the existing production tubing string.
On inception of production this well showed water cuts well above those normally expected. The high water cut, with the attendant reduction in produced oil, was not expected from a review of the geological and reservoir data of the formation. Therefore a coiled tubing conveyed production log was acquired to identify water entry points. Interpretation of the logs showed that the water was entering from a short section located approximately in the middle of the horizontal drain hole. It was observed that there was no contribution to (oil) flow below this zone, which was also found to be the deepest in the horizontal section and hence closest to the oil/water interface.
The section was therefore sealed off to increase oil production and maximize recovery from the section of the reservoir in which the well was drilled. The secondary objective was also to free the pipeline and other production facilities of this unwanted fluid (water).
A through tubing inflatable bridge plug and cement retainer were set respectively above and below the water entry section to provide a means to retard water production.
The water entry section of the horizontal wellbore was sealed off with a polyacrylamide gel. This gel was specifically tailored to seal off behind the slotted liner and prevent any flow back of polymer in the annular section that was contributing to oil flow. The treatment was performed in 3 days and the well was left closed-in for 2 days to allow the polymer to cure. The well was later reopened and it flowed spontaneously.
Post treatment production tests showed a decrease in water cut of approximately 70% while the corresponding oil gain was in excess of 25%. These figures have been maintained for at least 1 year after the treatment.
Based on these results two other similar treatments (designated as well Y and Z) have been performed for a minor operator in Nigeria. The results of these two treatments are also presented.
Many operators have considered remedial profile modifications in horizontal uncemented slotted liners difficult and even impossible. The main challenge when isolating a portion of a horizontal well is to provide the following:
isolation inside the liner across a particular section
isolation outside the liner (space between the liner and the formation face) across the same particular section
The first step can be easily accomplished by setting an inflatable through tubing bridge plug, while the second step is the key factor for ensuring a successful profile modification in a horizontal well completed in an openhole.
The second step requires that the annular space between the liner and the formation face must be filled circumferentially with an impermeable material in order to prevent any annular flow.