Gelant or gel treatments have an extremely low probability of success when applied toward cusping or three-dimensional coning problems occurring in unfractured matrix reservoir rock. When treating coning problems, a common misconception is that the gelant will only enter the water zones at the bottom of the well. In reality, this situation will occur only if the oil is extremely viscous and/or the aqueous gelant is injected at an extremely low rate (to exploit gravity during gelant placement). In the majority of field applications to date, the crude oils were not particularly viscous, and gelant injection rates were relatively high. Consequently, one must be concerned about damage that polymer or gel treatments cause to hydrocarbon-productive zones.
Even if a polymer or gel reduces ko without affecting ko, gel treatments have limited utility in treating 3-D coning problems. Extensive numerical studies using a variety of coning models indicate that gel treatments can only provide improvement if the desired production rate is less than 1.5 to 5 times the pretreatment critical rate.47,52 This circumstance rarely occurs.
In contrast to the very limited potential of polymers and gels in successfully treating 3-D coning, these treatments have much greater potential for successfully treating “two-dimensional coning” where vertical fractures cause water from an underlying aquifer to be sucked up into a well. Whereas gel treatments will only raise the critical rate by factors from 1.5 to 5 in unfractured wells, they can raise the critical rate by more than a factor of 100 in fractured wells.47,52
A number of literature reports suggested that gel or foam treatments were effective in mitigating 3-D coning. A critical examination of these reports71 revealed that they fall into one of three categories:
Shell’s (PDO) experience in the Marmul field provides an interesting exception to the above observations.72 Five of fourteen gel treatments were quite successful in reducing the water cut—up to 45% in one case. Convincing evidence was presented that flow behind pipe and fracture-like features were not important. Gelant (0.4% to 0.5% cationic polyacrylamide with glyoxal as a crosslinker) was bullheaded into the wells, using 700 to 2,500 bbls per treatment (11 to 19 bbls per ft in gravel packed completions). The key question is, why were five of the treatments successful, when basic reservoir engineering calculations indicate a very low probability for success for gel treatments in three-dimensional coning applications? The answer may be tied to two special characteristics of this field. First, Shell’s simulation work suggests that effective barriers to vertical flow are present.72 These barriers were not recognized when the first treatments were applied. Second, the oil viscosity was about 80 cp. Thus, viscous fingers of water may have arrived at a given well much earlier in some of the discrete zones than others. Because the oil was much more viscous than the gelant (~10 cp), the gelant may have followed these water fingers and preferentially reduced flow in the water zones to a much greater extent than if a light oil was present. This scenario is consistent with basic reservoir engineering calculations.17,47 Of course, this scenario suggests that the real problem in this reservoir was not three-dimensional coning, but rather viscous fingering through discrete high permeability pathways. Thus, consistent with our original contention, gelant treatments are not likely to be effective against three-dimensional coning.
Gel treatments are also expected to be ineffective when treating cusping. In cusping, like three-dimensional coning, the well is produced so rapidly that viscous forces overcome gravity forces. For cusping in particular, water from an aquifer follows an inclined zone up to the well. The only practical method to stop water production from the zone (other than decreasing the production rate) is to plug the zone. Unless extraordinary circumstances exist (as in the Marmul case above), hydrocarbon-productive zones in radial flow must be protected during gelant placement. (For the Marmul treatments, one wonders whether the success rate might have been 14/14 instead of 5/14 if hydrocarbon zones had been protected during gelant placement.)