Designing an In-Depth Chemical Conformance Gel System for a Mature Fractured Carbonate Field

Summary Mitigating excessive water production becomes a major issue in fractured carbonate reservoirs. Finding a suitable in-depth water control strategy to be applied in highly heterogeneous mature field by Polymer/gel injections is the key objective of the study presented in this paper. Targeted field suffers from severe conformance problems due to strong heterogeneities, fractures, vugs, with a water cut around 92%. The objective was to optimize a robust gel system that could be placed in-depth for a sustainable effect. A laboratory study has been performed to design a chemical gel system for in-depth conformance at the injector or near-well treatment at the producer. This requires optimization of chemical formulation, coreflood testing and overall behavior in reservoir conditions. Experimental program included gel rheology measurements, bottle tests for gelation time and gel stability assessments. Five gel systems were studied. Influence of polymer and crosslinker concentrations and brine composition was investigated to formulate and optimize gel properties. In a second step, coreflooding experiments conducted in fractured cores aimed at evaluating gelant injectivity, gel breakthrough pressure, permeability reduction and pore volume reduction induced by the gel. Viscosity measurements and gelation tests qualified two systems among the five tested ones. Both systems had a gelant viscosity meeting the maximum requirement of 200cP and lead to strong gels after a few days. Accelerated ageing tests were performed at 73, 80 and 90°C to mimic 5 years of ageing time at reservoir temperature (50°C). They confirmed an excellent gel stability for one system (called “Gelix”) achieving the target consistency whatever the temperature. Six corefloods showed that in-situ performance of Gelix was above those of the other systems. Gelix induces high gel parting pressure (70 bar/ft) and a residual permeability reduction (>500 000 ) in highly fractured core, making it suitable to block fractures and/or high permeability streaks for in-depth water control. Gelix system was thus qualified as the best gel to be used in this field. Finally, in order to protect the oil-bearing matrix, hybrid injection strategy was designed, using alternate slugs of microgel and Gelix. Such strategy can be applied in bullhead mode, optimizing gel placement in the fracture network, while minimizing the risk of damage by protecting matrix from gel leak-off. The paper describes the workflow to qualify in-depth water control and conformance products for fractured carbonate reservoirs. It highlights the benefits of a hybrid approach to protect oil bearing matrix while blocking the high-permeability streaks.