Most hydrocarbon reservoirs – rocks containing voids and capable of bearing oil and gas – are either terrigenous or carbonate. Terrigenous reservoirs consist mainly of rock fragments and silicate minerals, including sands, silts and sandstones. As for carbonate reservoirs, they usually contain only two primary rock-forming minerals: calcite and dolomite. At the same time, terrigenous reservoirs are characterized by a low intensity of crack formation, whereas carbonate reservoirs are prone to oil and gas filtration due to an extensive system of cracks and caverns.
These differences play an important role in well repairs, which begin with well killing, when a special fluid is pumped into the well to temporarily block oil from seeping out. For reagents, aqueous solutions with various mineral additives are used. However, if the solution enters the bottomhole zone of the formation, it can negatively affect the porosity and permeability of the rock and reduce the productivity of the well after the repairs are completed. While this phenomenon has been extensively studied in terrigenous reservoirs, it remains practically unexplored in the case of carbonate reservoirs: it is believed that they do not contain clay minerals that can change wettability upon coming in contact with low-mineralized water, which also leads to changes in permeability.
However, in reality, the dissolution of carbonates can trigger the detachment of small particles filling the voids of the formation and thereby changing the permeability of the rock. This has been proven by the scientists from Perm Polytechnic University based on the results of a study of 10 samples from the productive part of two fields differing in the structure of the void space and the composition of their mineral inclusions. The scientists analyzed factors affecting the filtration and capacity properties of the samples: the chemical composition of the liquid filtrate, the mineralogical composition of the carbonate, formation pressure and temperature, as well as the duration of the filtrate’s contact with the rock.
By filtering oil through the samples, the scientists simulated the flow of feedstock from the formation into the well, and by pumping process water and holding it in the samples for seven days, they reproduced the process of killing the well for the duration of repairs. Experiments showed that the impact of process water causes the total number of voids in the core samples decrease by 32.4%. “This happens because mineral compounds dissolve the surface, releasing rock grains. Small particles fill the void space, which decreases the permeability of the formation,” Dmitry Martyushev, doctor of technical sciences, is quoted as saying by Perm Polytechnic University.
The scientists came to the conclusion that the penetration of filtrate into the rock disrupts the geochemical balance between formation fluids and rock minerals. This causes carbonate minerals to dissolve and clay components to swell and migrate. This risk can be curbed by changing the mineralization of water used to kill carbonate reservoirs, as well as by using nanoparticles that reduce the solubility of rocks.
