The relative importance of buffering and brine inputs in controlling the abundance of Na and Ca in sedimentary formation waters

The concentration of Ca in the formation waters of petroleum reservoirs can play a major role in influencing the outcome of a number of processes that are of great significance to the oil industry. For example, formation water Ca concentration affects the risk of carbonate scale formation during production. In order to better understand the concentrations of Ca in formation waters, we have investigated the chemistries of formation waters from a range of onshore and offshore basins worldwide, using published sources, as well as unpublished data held by BP. Although calcium and sodium are the principal cations in almost all formation waters they vary enormously in their relative proportions. We have identified three distinct trends on a plot of XCa (Ca/(Na + Ca)) against Cl. Most data lie on a high-Ca trend, here termed Trend 1, and show an increase in XCa with salinity. We interpret this as tracking equilibration with Ca and Na-bearing minerals, with the ratio (mol Ca/mol Na2) remaining approximately constant irrespective of salinity for chloride-dominated fluids. At very high salinities, Br-enriched bittern brines that have taken part in dolomitisation lie at the Cl-rich end of this trend. Some brines remain Na-dominated up to very high salinities and define a distinct low-Ca trend, Trend 2. These are associated with dissolution of halite beds and are interpreted to arise when the amount of Na in the pore fluid greatly exceeds the amount of Ca available in minerals. We refer to such brines as mass-limited; the sparsity of Ca in the rock-fluid system constrains XCa to a low value. Remarkably few brines lie between these trends. Finally, dilute formation waters show very large variations in XCa and may have bicarbonate as the dominant anion. They define a distinct low-Cl trend, Trend 3. We conclude that the behaviour of Na and Ca in most formation waters reflects equilibration with minerals, and concentrations of Ca in solution are sensitive to pH and PCO2 as well as to chloride concentration. For some brines however, the amount of salts in solution is sufficient to overwhelm the buffering capacity of the wallrocks.

► There are 3 distinct trends for Ca and Na in oilfield waters globally. ► Many brines are rock-buffered, but some swamp the exchange capacity of the host. ► NaCl-dominated brines arise from dissolution of halite. ► The most CaCl2-rich brines are bittern brines that participated in dolomitisation. ► Variations in brine chemistry do not reflect changes in seawater through time.