Impact of Wettability on Rock Mechanics and Oil Recovery: A Comparative Study on Different Outcrop Chalks

Abstract

Chalk reservoirs are highly fractured and are known for their remarkable storage capacity as a reservoir rock for the petroleum industry. Chalk is a sedimentary carbonate rock primarily made up of calcium carbonate (calcite, CaCO3) and is a very important reservoir rock in the Norwegian and Danish Continental Shelfs. Water flooding has been used as a secondary production method in chalk reservoirs as it provides pressure maintenance to the reservoir and is a proven improved oil recovery (IOR) method to produce more oil from the production facilities. Ekofisk field in the Norwegian Continental Shelf is a successful example of production by water flooding method, where seawater is used as the injection fluid. The initial estimated oil recovery was 18% as mentioned in the initial development plan of the field and today’s estimates lie well above 50%. However, seawater has also shown to enhance reservoir compaction due to water weakening of the chalk formation. It can, for example, lead to buckling and loss of well pipes, arching of overburden rocks leading to stress redistributions, and the porosity/permeability decline of the producible formation, all physical effects that alter the ultimate recovery and recovery rates of oil-bearing chalk reservoirs. The detection of subsidence of the Ekofisk field in the Norwegian North Sea around 35 years ago has been linked to the compaction of chalk formations due to porosity reduction associated with field production. It attracted the attention of oil and gas researchers across the globe and since that time, considerable research has been carried out concerning chalk behavior, especially on how the pore fluid composition alters the mechanical properties of chalk. Research on chalk, so far, has primarily been carried out on water-wet systems. The studies have shown that when reactive brines are injected through chalk at elevated temperatures, chemical reactions occur between the rock and the injected brine, which affects chalk mechanical stability. It has been found that the divalent ions, such as magnesium (Mg2+), calcium (Ca2+) and sulfate (SO42-) play an important role in defining chalk mechanics. Mg2+ ions in the injected brines lead to dissolution of calcite, which in turn leads to precipitation of new mineral phases such as magnesite, huntite, talc, anhydrite etc. Both Mg2+ and SO42- ions also adsorb on the chalk surface leading to a reduction in the strength of the rock. [...]