Table 7 gives API free-water data for four MC slurries and settling-tube test data that correlate with downhole wellbore particle segregation along the vertical elliptical plane in a deviated wellbore. It is apparent that the higher-density slurries with a lower water/cement ratio have essentially no particle settling. Additives that add viscosity, control fluid loss, immobilize the water between particles, or give thixotropic properties to the slurry have been used to reduce or to prevent settling when lower water/cement ratios were not desirable.
Most of the preceding MC applications were developed in drilling operations before 1985. Relatively new applications have since been developed for workover operations. The first new application was novel in that MC lost-circulation pills were pumped down completion tubing attached to permanent completion packers to cure losses by temporarily plugging the perforations in injection zones. This strategy saved the expense of massive losses of workover brine and conventional LCM pills and the extra rig time needed to cure the losses or to keep the hole full. Although the cost of MC was four to seven times that of conventional cement or CaC03 LCM, it demonstrated several unique properties that made it cost effective. The resulting reduction in workover costs made it far more economical than other materials. A second new application was setting MC across producing perforations for a temporary abandonment plug. The plug was to be removed later by drilling and acidizing to resume production without formation damage. A third new application was to protect a downhole screen across a water-injection zone from being damaged by debris during a workover. The MC inside the screen was to be removed later by acidizing. A fourth new application was the temporary sealing of a 305-m perforated oil-producing zone to stop severe lost circulation during a workover to drill deeper and to set a liner across a new zone. Conventional LCM’S failed to stop the losses, and a 15.6-m3 MC squeeze was pumped to seal the perforations. The liner was cemented without circulation losses, and the MC was removed from the oil zone with no production decreases.
Of these four new applications, the first one has been used most frequently. This application was for a workover program in a field to introduce water-injection-zone selectivity in the form of multipacker completions. The technique of using MC lost-circulation pills, however, could have been applied to other workover to repair tubing, casing, or packer leaks or abandon old perforations and reperforate for better production.
Although the application was developed in a field where conditions justified a bullhead squeeze down the existing completion tubing, well conditions in other fields may warrant the use of concentric or coil tubing to circulate the slurry to the perforations and then to squeeze the slurry into the zone. The bullhead slurry placement method was selected by comparison of cost vs. effective placement results in eliminating or substantially reducing workover-fluid loss and zone permeability damage. The minimum acceptable reduction in the rate of losses was based on the cost of the lossses and the rig’s ability to keep the zone overbalanced by maintaining a fluid column with adequate hydrostatic pressure. MC has been the most cost-effective way of obtaining the minimum acceptable reduction in rate of losses. This included total elimination of losses by MC in many wells in the workover program. The volume of MC slurry used has varied between 1.59 and 15.9 m3, following the general guideline that slurry volume should equal three times the liner or casing volume from the top of the perforations to the lowest one plus the liner or casing volume from the top of perforations to 60m below the end of the packer tailpipe.
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