The main task when extracting oil from waterflooded reservoirs during late development stages is enhancing the mobility of the remaining reserves. In the context of declining oil export prices, the use of chemical and gas methods for enhancing oil recovery in mature fields is becoming increasingly unprofitable, given their associated technological and economic risks. To improve the efficiency of mature oilfield development, non-stationary (cyclic) waterflooding is an alternative to tertiary methods for enhancing oil recovery. The primary benefit of cyclic waterflooding is its simplicity of implementation and minimal extra costs, which have made this method very appealing in today's market conditions. The relevance of cyclic flooding within the context of this study is underpinned by the prevalence of mature fields in the region, including the asset under investigation. This article presents approaches for planning cyclic waterflooding, using an example of a complex carbonate reservoir from the Tournaisian stage (C₁t) oilfield of the Perm Krai, Russia. The novelty of this research lies in the presented laboratory-based studies of the hysteresis of relative phase permeabilities (HRPPs) in “water-oil” systems, which are then used to assess the effectiveness of cyclic waterflooding technology on an updated 3D hydrodynamic model via tNavigator software. Results from the multivariate calculations for cyclic waterflooding are presented, both with and without considering HRPP. The importance of incorporating models with inverse hysteresis loops into commercial simulators for carbonate porous media with mixed types of rock wettability is demonstrated and justified, and directions for future research are proposed. These findings emphasize the need for laboratory studies of carbonate reservoirs in imbibition and drainage modes to constrain the real reservoir processes occurring during cyclic waterflooding.

Laboratory filtration experiments are employed to investigate effective well killing while minimizing its impacts on surrounding rocks. The novelty of this experimental study lies in the prolonged exposure of rock samples to the killing fluid for seven days, corresponding to the average duration of well workovers in the oilfields in Perm Krai, Russia. Our findings indicate that critical factors influencing the interactions between rocks and the killing fluid include the chemical composition of the killing fluid, the mineralogical composition of the carbonate rocks, reservoir pressure and temperature, and the contact time. Petrophysical analyses using multi-scale X-ray computed tomography, field emission scanning electron microscopy, and X-ray diffraction were conducted on samples both before and after the well killing simulation. The experiments were performed using real samples of cores, crude oil, and the killing fluid. The results from this study indicate that low-mineralized water (practically fresh water) is a carbonate rock solvent. Such water causes the dissolution of rock components, the formation of new calcite crystals and amoeba-like secretions, and the migration of small particles (clay, quartz, and carbonates). The formation of deep channels was also recorded. The assessment reveals that the change in the pH of the killing fluid indicates that the observed mineral reactions were caused by carbonate dissolution. These combined phenomena led to a decrease in the total number of voids in the core samples, which was 25% on average, predominantly among voids measuring between 45 and 70 μm in size. The change in the pore distribution in the bulk of the samples resulted in decreases in porosity of 1.8% and permeability of 67.0% in the studied core samples. The results from this study indicate the unsuitability of low-mineralized water as a well killing fluid in carbonate reservoirs. The composition of the killing fluid should be optimized, for example, in terms of the ionic composition of water, which we intend to investigate in future research.

Many new and developed oilfields are confined to carbonate reservoirs with a complex void space structure. In this study, the behavior of carbonates as a function of decreased reservoir pressure was examined. The study was based on actual production data from wells rather than just on model data or results from computational experiments. Well-test data (170 well tests) and laboratory core samples (45 samples) results were used in the analysis. The problem of reservoir deformation has been comprehensively addressed at the micro and macro levels. The effect of reservoir permeability throughout the entire reservoir volume on reservoir pressure was determined. With a decline in reservoir pressure, the permeability of fractures and pores in carbonates decreases. The deformation coefficients, which characterize the actual rate of decrease in permeability with decreasing reservoir pressure, were also calculated. The deformation coefficients and data from core studies using μ-CT and SEM were compared. Based on micro-CT data, the effective diameters of the voids were calculated to be 93 μm, 109 μm, and 140 μm for the Vinnikovskoe, Sofinskoe, and Sukharev fields, respectively. Based on μ-CT and SEM data, the predominant geometric shapes of the voids were assessed. The deformation coefficient of voids was found to depend on their size and geometric shape. The study's findings revealed deformation coefficients of 0.172, 0.205, and 0.748 for the Vinnikovskoe, Sofinskoe, and Sukharev fields, respectively. Maximum deformation coefficients are typical for large voids with a predominant slot-like shape. However, even with the predominance of primary spherical voids in carbonates, the permeability of voids decreases with decreasing reservoir pressure.

Carbonate reservoirs develop many different types of microfractures that play an important role in increasing the effective reservoir space and permeability. Thus, the qualitative and quantitative characterisation of the effect of microfractures on permeability in rocks is essential. In this study, a quantitative method for evaluating the impact of different microfracture parameters on carbonate rock permeability was proposed. Lattice Boltzmann simulations were carried on two carbonate digital cores with different types of artificially added microfractures. Based on the simulation results, a partial least squares regression analysis was used to investigate the impact of microfractures on the permeability of the cores. Increases in the fracture length, aperture, and density were found to linearly increase the permeability of the carbonate rocks, and as the fracture length increased to penetrate the whole core, an exponential increase in permeability was observed. Additionally, the effect of microfractures on the digital core permeability was more significant in cores with high permeability compared to that in low-permeability cores. Although both fractures and matrix permeability contribute to the permeability of the digital cores, the former were found to have a greater effect on the permeability.