The success of treatments to control non-productive water and gas depends on knowledge of water entries at the wellbore and the selection of a proper placement technique. Often, the selection of the gel placement method is based on past field experience. There has been no engineering approach and guideline for field use.
This paper examines the effects of fluid and reservoir properties on gelant invasion and dispersive flow behavior in heterogeneous formations. The application of bullheading, mechanical wellbore isolation, and the dual-injection technique is evaluated with reservoir simulations. Key parameters dictating the choice of an appropriate gel placement technique are identified. Furthermore the need for a dual-injection method for complicated well completions (gravel-pack poor cement integrity, and near wellbore fissures) is elucidated. Case histories illustrate application of each injection method.
Simple bullhead injection is acceptable when very high permeability and saturation contrasts exist and a large pressure drop is available to breakdown gel damage in oil productive intervals or when reperforating the oil zone is an option. Bullheading is also appropriate when self-selective fluids are employed, although the use of self selective fluids has had mixed results with the exception of flowing gels that selectively invade fractures. More elaborate bullhead techniques utilizing overflushes or alternating stages of immiscible fluids may improve the placement selectivity under certain conditions. Mechanical isolation of the target interval is recommended when the wellbore has good casing and cement, there are no near wellbore fissures and if one or two water or gas entries have been identified. The simultaneous injection of gelant and a protective fluid, i.e. the dual injection technique, is recommended when there are no horizontal barriers, the vertical permeability is high, or the adjacent oil bearing zones are thin.
Crosslinked polymer gels are commonly used to treat production wells with excessive unproductive water or gas flows and injection wells with poor injection profiles. The decision to utilize a crosslinked polymer gel to partially or completely plug fractures or high permeability layers is made after the treatment has been designed; a process that requires verification of the water production mechanism and identification of the offending interval. Following the design step, one must carefully choose an injection technique to deliver the gelant to the intended location. A technique that minimizes the invasion of delayed crosslink polymer solutions (gelant) into adjacent productive intervals is of paramount importance for a successful treatment.
A common, but inappropriate field criterion used to choose among the various placement techniques has certainly led to numerous treatment failures. The criterion is that one bullheads gelant into the formation when the water entry is ill-defined and one employs a diverse combination of packers. bridge plugs, sand, and cement to limit gelant invasion to a specific interval when the water entry has been located.
Numerous papers have been published within the past decade that investigate the placement of gelants in the formation surrounding wellbores. The majority of the conclusions presented in these papers are for the case of non-communicating layers under the assumption that the gelant penetration in porous media can be simulated by incorporating non-Newtonian rheological models into standard reservoir simulators. Other work indicates that there are important phenomena, such as bridging-adsorption and the filtration of gel aggregates, that can preferentially inhibit gelant penetration into low permeability zones. P. 629^