High-density potassium phosphate and potassium/cesium formate brines were tested in an advanced core flood test rig for their compatibility with low-permeability sandstone containing simulated formation water under HPHT conditions. The tests were carried out at a temperature of 175oC (347 oF) and pore pressure of 5,800 psi. Compatibility was determined by comparing the relative gas permeability of the cores before and after exposure to the brines under HPHT conditions.
The 10–20mD sandstone core plugs, containing simulated North Sea HPHT gas reservoir water at residual water saturation, were flooded with 10 pore volumes of test brine and subsequently equilibrated with the brine for 48 hours to allow chemical interaction. Drawdown pressures of up to 100 psi were then created on the wellbore side of the core to create gas flow. Gas permeability was measured after flowing 4,000 ml (approx. 2,000 pore volumes) of humidified gas through the core. The results showed that flooding this core with the potassium phosphate brine reduced the gas permeability by more than 90%. By comparison, flooding with cesium formate brine created a modest improvement in the permeability of the core to gas.
Examination by SEM of the core damaged by the phosphate brine showed that its pores were partially obstructed with a thick layer of scale. The scale was particularly plentiful in areas of the core where illite clays were present. The scale was examined in situ by EDS and found to contain potassium and phosphorus. It is known that phosphates in aqueous solution are strongly adsorbed onto mineral surfaces where they can form precipitates and complex insoluble salts from exposure to multivalent cations. Aqueous phosphate solutions are also known to react with clays to form solid precipitates. It is possible that the scale formed in the core as a result of interactions under hydrothermal conditions between the invading phosphate brine and, inter alia, the pore-lining illite clay deposits naturally present within the rock.
The scale modeling software, DownHoleSAT, predicted a significant risk of various phosphate scales being formed as a result of mixing of the formation water and phosphate brine inside the core under the experimental conditions. Such scales may have contributed to the formation damage observed in the core flooded with the phosphate brine.