New insight into wormhole formation in polymer gel during water chase floods with positron emission tomography

Polymer gel is frequently used for conformance control in fractured reservoirs, where it is injected to reside in fractures or high-permeability streaks to reduce conductivity. With successful polymer-gel conformance control in place, increased pressure gradients across matrix blocks may be achieved during chase floods, diverting water, gas, or enhanced oil recovery (EOR) chemicals into the matrix to displace oil. Knowledge of gel behavior during placement and chase floods is important because it largely controls the success of subsequent injections. Polymer-gel behavior is often studied in corefloods, where differential pressure and effluents from fracture and matrix outlets give information about gel deposition during placement and flow paths during chase floods. The work presented in this paper uses complementary positron emission tomography (PET) chromatographic tomography (CT) imaging to quantify the behavior and blocking capacity of Cr(III)- acetate hydrolyzed polyacrylamide (HPAM) gel during chase waterflooding. In-situ imaging provides information about changes that may not be extracted from pressure measurements and material balance only, such as changes in local fluid saturations and dynamic spatial flow within the fracture and within the structure of the gel network. Polymer gel was placed in core plugs with longitudinal fractures that connected the inlet and outlet, and chase water was subsequently injected to measure the gel blocking capacity. The water phase was labeled with a positron emitting radiopharmaceutical (F-18) to visualize and quantify local flows with PET during gel rupture and subsequent flooding. By use of PET, we study gel rupture and the development of wormholes during gel erosion after rupture as a function of flow rate. A particular strength with access to dynamic, local flow patterns is the direct comparison to global measurements, such as differential pressure and production rate, to verify existing gel-behavior models.