Low Salinity Chase Waterfloods Improve Performance of Cr(III)-Acetate HPAM Gel in Fractured Cores

Abstract

Polymer gels are frequently applied for conformance improvement in fractured reservoirs, where fluid channeling through fractures limits the success of waterflooding. Placement of polymer gel in fractures reduces fracture conductivity, thus increasing pressure gradients across matrix blocks during chase floods. A gel-filled fracture is re-opened to fluid flow if the injection pressure during chase floods exceeds the gel rupture pressure, thus channeling through the fractures resumes. The success of a polymer gel treatment therefore depends on the rupture pressure. Swelling of gels, e.g. pre-formed particle gels, due to salinity differences between the gel network and surrounding water phase has recently been observed, but the effect has been less studied in conjunction with conventional polymer gels. Using core floods, this study demonstrates that low-salinity water can swell conventional Cr(III)-acetate HPAM gels, thereby improving gel blocking performance after gel rupture. Formed polymer gel was placed in fractured core plugs and chase waterfloods were performed, using four different brine compositions of which three were low-salinity brines. The fluid flow rates through the matrix and differential pressures across the matrix and fracture were measured and shown to increase with decreasing salinity in the injected water phase. In some cores, the fractures were re-blocked during low-salinity waterfloods, and gel blocking capacity was increased above the initial level. Low-salinity water subsequently flooded the matrix during chase floods, which provided additional benefits to the waterflood. The improved blocking capacity of the gel was caused by a difference in salinity between the gel and injected water phase, which induced gel swelling. The results were reproducible through several experiments, and stable for long periods of time in both sandstone and carbonate outcrop core materials. Combining polymer gel placement in fractures with low-salinity chase floods is a promising approach in integrated EOR (IEOR).

Application of a Pickering Emulsified Polymeric Gel System as a Water Blocking Agent

“…25 The drawback associated with the gelant is that the rupture paths in the immature gel after gelation are wider in comparison to the mature gel. 26 To fill the comparable section of fracture, it requires a very high degree of swelling. Moreover, polymer adsorption is a common problem associated with gelant injection.…”

New Insight from Visualization of Mobility Control for Enhanced Oil Recovery Using Polymer Gels and Foams

“…After placement, the gel may undergo processes that change its volume and impact the success of a chaseflood. Several processes, such as syneresis [18,19], dehydration [20][21][22][23] and gel swelling [24,25], have been studied experimentally using bulk gel, core plugs and micromodels. During syneresis and dehydration, the gel volume is reduced due to loss of water caused by chemical reactions over time, changes in pressure gradients or matrix capillary forces adjacent to the gel network.…”

“…In strongly water-wet, low-permeable chalk rock samples, where a strong positive capillary pressure exists, residual oil saturations were reached during gel placement due to water leakoff [25]. The chalk core was placed in a Bruker 4.7T MRI, and a spin-echo based sequence was used to produce images during D 2 O-gel injection.…”

“…During gel placement in initially oil-filled fractured systems, it is assumed that the oil residing in the fracture is produced at gel breakthrough [25]. This assumption was based on the high differential pressure observed during chasefloods, comparable to conventional experiments using 100% water saturated cores.…”

“…The gel treatment may also be strengthened after placement by injecting a water phase with lower salinity relative to the water in the gel [66]. Both injection pressures and matrix production rates during chasefloods increased with reduced salinity compared with chasefloods where the salinity was matched with the gel [25], presumably due to swelling of the polymer gel network. The improvement of the polymer gel treatment was reversible, and gel blocking efficiency immediately decreased during chasefloods with matched salinity.…”