Until 1983, traditional cement squeeze techniques were used to shut off excess water production from wells in the Forties Field. The success rate was low, some 50% of workovers failing within 5 days of the well being returned to production.
Casing patches over 120 feet long are now being run to cover the squeezed off perforations and workover effectiveness has increased dramatically. Radioactive tracers are employed to ensure accurate depth control as the patches are run and set on drillpipe.
It is envisaged that continuous patches could be run when extremely long perforated intervals are encountered and alternative uses, e.g., to repair perforated intervals are encountered and alternative uses, e.g., to repair mechanically damaged casing, can be foreseen.
The Forties Field is situated some 150 miles North East of Aberdeen and production began from the first of four platforms in 1975 (Figure 1). production began from the first of four platforms in 1975 (Figure 1). Pressure maintenance, by water injection, was started almost immediately. Pressure maintenance, by water injection, was started almost immediately. By 1980, a workover schedule, primarily for water shut off, had been established. The use of bridge plugs had to be limited because access to lower portions of the reservoir was often required after water breakthrough to monitor flood sweep efficiency using the time lapse technique. A further limitation was that water breakthrough did not always occur from the lowest perforations first. Production logging occasionally showed that water breakthrough had occurred along the top of a shale break.
Hence water shut off workovers were performed by squeeze cementing the perforations and then cleaning out the casing to allow logging access. The perforations and then cleaning out the casing to allow logging access. The success rate was not particularly high and a number of techniques were introduced to try and improve the situation. These included the use of low water loss cements, hesitation squeezes, acidising prior to squeezing, the use of cement retainers to keep kill fluids above the perforations. None gave a dramatic improvement, although minor successes were noted.
Perhaps the most disturbing feature of this phase of the operation was Perhaps the most disturbing feature of this phase of the operation was that no acceptable test for squeezed off perforations could be found - other than returning the well to production. Pressure testing with completion fluid or mud was not indicative of success and it was felt that a 'dry' test (DST test) often shock loaded the squeezed off perforations causing premature breakdown.