Water production control is a key issue in most mature reservoirs worldwide. Many techniques have been developed to achieve this purpose. Water Shut-off (WSO) treatments using crosslinked gels have proven to be an effective alternative. When appropriately designed and applied, these systems generate flow restrictions in high permeable or fractured formations that bypass water from highly saturated zones (injection water or natural brines). WSO treatments efficiency depends on several aspects as reservoir fluids flow patterns, rock petrophysics, formation heterogeneity, and WSO gel characteristics. This last aspect is adjusted to optimize the treatment based on experimental tests performed in the laboratory.
This paper presents an experimental methodology to evaluate and adjust WSO polymer systems according to operational and treatment performance requirements. These studies go from qualitative gel consistency and fluid compatibility bottle tests, to rheological characterizations to determine viscosity variations and gelation time (outside porous media tests), as well as flow tests performed on formation and Berea core plugs at reservoir conditions (inside porous media tests).
These properties are highly important to avoid early gelations and, at the same time, assure the appropriate WSO placement in the reservoir. Viscoelastic properties such as G* (complex modulus), G′ (storage modulus) and G″ (loss modulus) define the gel strength and provide structural information such as crosslinking density. These parameters are essential to design the gel formulations (polymer and crosslinker type and concentration) depending on the operational and reservoir requirements.
Finally, the flow tests performed on core plugs show the changes in water and oil permeability after injecting the treatment. This information is used to calculate the residual resistance factors to water (RRFw), oil (RRFo) and the gel resistance index (GRI). These parameters define the treatment blockage degree and allow estimate probable well production response as well as the best production regimes to extend the treatment life time. This methodology was applied on a new WSO gel system, developed for a wide range of applications. The effect of polymer concentration, temperature, salinity and flow rate are examined in detail.
Experimental tests are many times underestimated when planning a WSO job; however these tests will always provide valuable information that will increase the chances of successful water shut-off treatments. The connection between laboratory and field parameters and its influence in the oil and water productivity were analyzed.