Low-permeability gas sands are often cased, perforated, and hydraulically fractured in stages. Commingled production is common, and coiled tubing (CT) has become the prevalent medium for communicating multiple productive intervals between fracture stages and for use as a remedial workover method.
High differential pressures in the wellbore complicate CT intervention considerably. Fluid and solid bridging agents are sometimes used to combat fluid losses and associated well control problems. Calcium carbonate may be the most common bridging agent because it is inexpensive and easy to mix; however, removing the damage caused by calcium carbonate slurries to the formation and proppant packs is difficult and costly. Rock salt is widely used because it causes less formation damage, but it is ineffective against fluid loss. Furthermore, to prevent dissolution of the pill, saturated brine must be used as a workover fluid, increasing costs and contributing to scaling problems.
A unique fluid-loss control agent has been gaining acceptance for temporarily isolating fractured sands. This double derivatized, crosslinkable, hydroxyethyl cellulose (DDHEC) polymer leaves less than 0.5% gel residue by weight, minimizing impact on production. It has demonstrated a regained permeability of 91 to 93% under laboratory conditions and can be completely removed by acid. The polymer has been used in conjunction with 1.25- and 1.75-in. CT with a high degree of success. It does not require expensive brines, is resistant to solvents, and can be used with foam.1,2
The polymer has been applied in eight coiled tubing operations in south Texas, none of which has negatively affected production. This paper provides case histories for several of these field applications.