In order to study the influence of different wetting states on the mechanical properties of chalk, tests on both high porosity (45-49 %) and lower porosity (37-40 %) outcrop chalks had been conducted. The initial water saturation for the chalk cores was low. The cores were flooded with oil with different acid numbers and aged at reservoir temperature following a procedure designed to achieve a pre-determined wetting condition. The prepared cores were mounted in triaxial loading cells at reservoir temperatures in order to investigate mechanical parameters. Two types of fluids such as formation brine and synthetic seawater were flooded through the cores during the experiments. The results revealed that intermediate-wet cores were stronger than preferential and completely water-wet cores. The mechanical strength of chalk at various wetting conditions might be attributed to: (1) acid number of oils and the effect of oil films close to the intergranular contacts and/or (2) chalk dissolution and precipitation processes
Approximately 50 % of the global petroleum reservoirs are carbonates (limestone, chalk, and dolomite). The oil recovery from these is in average less than 30 %. Carbonates often contain considerable fractures, vugs, and dead-end pores [1]. In addition, most of the carbonate rocks have low permeability as well as low water wetness [2, 3].
During secondary recovery in naturally fractured reservoirs, water must imbibe into the matrix blocks to expel oil into the high permeable fractures. Therefore, the matrix permeability and wettability (the tendency of a fluid to adhere or spread on a solid surface, e.g., reservoir rock, in the presence of immiscible fluids [4, 5]) are important for production and oil recovery. However, the wetting state of most of carbonate oil reservoirs is reported to be intermediate to strongly oi-lwet [1, 3]. Obviously, the unfavorable surface wetting prevents spontaneous imbibition of water into the matrix. In order to enhance recovery from carbonate rocks, the matrix wetting state should be altered toward a more water-wet system.
On the other hand, significant increases in oil recovery by water flooding has been demonstrated in North Sea since early eighties, for example in the Ekofisk oil field [6]. It has been documented that the injection of seawater can increase spontaneous imbibition of water and eventually the oil recovery [7, 8]. The two main mechanisms of improved oil recovery by seawater injection are: (1) wetting alteration, and (2) compaction of the reservoir [9, 10, 11, 12, 13, 14, and 15]. Induced seabed subsidence and severe instability problems, as a consequence of compaction, have been encountered in many oil fields such as Ekofisk and Valhall in the Norwegian sector of the North Sea The compaction increased the oil recovery (which is an advantage), but it endangered offshore installations and required expensive lifting operations (which are considerable disadvantages). The influence of wettability on the mechanical stability of chalk has not received substantial attention in the past; but its understanding, is crucial to optimize field conditions.