**6. Fluid–Rock Interactions in Reservoirs**

The triaxial laboratory tests discussed in Section 5 cannot describe what takes place in a reservoir because they are carried out under isothermal conditions, whereas cold fluids are injected into a reservoir and reactions take place as fluids move through a thermal migration front. Figure 9 illustrates what is observed to happen in a reservoir. It plots the chemistry of fluids produced from a well 300–400 m from the nearest well into which seawater is being injected [32]. Chlorine does not react with the reservoir sediments. Seawater has a much lower chlorinity than Ekofisk formation water, and thus the production of low salinity water indicates the arrival of injected water. Following the changes in chlorinity in Figure 9, we see that during a period of 5–6 years, the chlorine concentration drops to 0.63 mol/L, which is close to the chlorine concentration in seawater of 0.525 mol/L. Thus, the water in the producer after 5–6 years is almost pure seawater.

Using the chlorine concentration as a tracer for seawater, we can predict what the concentration of all the other ions would be if no chemical reactions took place in the reservoir. The solid lines in Figure 9 represents the unreacted concentrations of calcium, magnesium, etc. The difference between the produced water chemistry (points) and these solid lines indicates the chemical exchange between pore waters and the reservoir sediments that has occurred as the injected seawater moved from the injector wells to the production well. It can be seen that a large fraction of Mg and SO4, and a bit of K, have been lost from the fluid, presumably going into dolomite or magnesite, anhydrite, and clay, respectively.

**Figure 9.** *Cont.*

**Figure 9.** (**<sup>a</sup>**–**h**) Chemistry of waters produced from an Ekofisk well 400–500 m from the nearest well where seawater is being injected. The chemistry of produced water (points) is compared to the predicted initial ion concentrations (solid lines), assuming that chlorine is a conservative (ideal) tracer for unreacted seawater and that the drop in chloride concentration is solely due to mixing between Ekofisk formation water and seawater. The difference between the lines and points indicates the chemical exchange that has occurred in the reservoir. Note that the Y-axes in the various graphs have different scales.

Figures 10 and 11 show models of alteration in a reservoir for a situation similar to that shown in Figure 9. These figures show streamlines between an injection and a production well 500 m away, assuming the reservoir is homogeneous. The streamlines are calculated using potential methods [32–34], injection rate is 30 Mbbl/day and production is constant at 5Mbbl/day; the flow is two phases and through a fixed depth interval of 70 m. The modelling methods are described in more detail in [32]. The snapshots after five years of seawater injection in Figures 9 and 10 show that the temperature gradient lags behind the injected waterfront. The lag results from the fact that water only moves in the pore space, whereas the temperature moves in the total volume (matrix and pore space). Due to the temperature dependence of the mineral reaction kinetics, the chemical alteration fronts travel at speeds different form both the water and the temperature fronts. Figure 9 shows the weight fractions of some of the minerals precipitated and dissolved. Much of the magnesium carbonate alteration occurs close to the injector. After 40 years of simulated seawater injection, about 1.5 wt% of calcite is dissolved and 1.5 wt% of dolomite is precipitated near the injection well, and the calcite to magnesium carbonate transition may extends about 400 m from the injection well [32]. The sulphate bearing mineral anhydrite precipitates at high temperatures and dissolves at low temperatures. The simulations predict a sulphate wave that travels at the same speed as the temperature front, where anhydrite is precipitated and dissolved again as the thermal front passes through the reservoir.

**Figure 10.** Streamlines approximately 5 years of seawater injection. (**a**) Water saturation (Sw), (**b**) temperature profile, (**c**) chlorine concentration, (**d**) magnesium concentration, (**e**) sulphate concentration, and (**f**) calcium concentration.

**Figure 11.** (**a–c**) Streamlines showing the weight fraction of alteration minerals in the reservoir at the time of water injection breakthrough (approximately 5 years of seawater injection). The wells are separated by 500 m.
