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The paper presents the methodology and results of coreflooding, taking into account the decrease in the permeability caused by the migration of natural colloids during fluid injection. As a porous medium, core samples of consolidated slightly clayey sandstone from one of the oil fields of the Komi Republic (Russia) were used. The coreflooding methodology implied a cyclic change in the injection rate and excluded the influence of other factors such as creep and chemical mobilisation of colloids on permeability. It has been established that a decrease in pore pressure leads to a more intense decrease in permeability during injection, which most likely indicates additional mobilisation of colloids. An analysis of the coreflooding results showed that porous media with lower permeability are more sensitive to colloid migration and pore-pressure changes.

During the development of oil and gas fields, the permeability of the reservoirs decreases due to a decrease in reservoir pressure and an increase in effective pressure, as a result of which significant reserves of oil and gas remain in the reservoir. To predict the rate of decrease in oil production rates during field development and to respond quickly, it is necessary to know the law of permeability decrease with an increase in effective pressure. Existing methods for describing the change in the permeability of rocks were analyzed in the paper. Numerical analysis of the results of core studies from previously published papers and the results of field well testing on the examples of the north Perm region oil fields showed that in both cases, regardless of the type of rock and the type of reservoir, the change in permeability can be described by the same equations (exponential and power-law). Obtained equations can be used to predict changes in the permeability of terrigenous reservoirs of the north Perm region oil fields. At the same time, according to the results of well testing, an intensive decrease in permeability is observed with an increase in effective pressure. Analysis of the nature of permeability changes using the Two-Part Hooke’s Model showed that significant irreversible deformations are currently taking place in the formations of the oil fields under consideration. Predicting the change in permeability from effective pressure can allow to optimize the development of oil deposits.

This paper presents an experimental investigation on the nonlinear nature of the dynamic geomechanical characteristics of a clastic rock (sandstone). Rock samples of 7.5 mm in diameter and 15.6 mm in length were prepared. Rock properties were identified. Firstly, the limits of the rock linear elasticity zone were defined during quasistatic loading and uniaxial compressive strength determination at the Tinius Hounsfield rig. Secondly, the small experimental custom-built rig was designed to study the nonlinear nature of the Young’s modulus in the zone of linear elasticity. At the rig the sample was stationary preloaded. The dynamic load was generated by a piezoelectric actuator powered with a signal generator. The displacement of rock sample surfaces was recorded by a laser sensor and an eddy current probe. The dynamic experiments were conducted at the load amplitude ranging from 50 to 250 N for each of frequencies of 25 Hz and 40 Hz. It was found that the dynamic Young’s modulus increased with amplitude for all the frequencies studied. The newly developed experimental rig allows to investigate elastic moduli dispersion of rocks at the strain up to 10-3 under vibrations with frequency up to 40 Hz.

Summary The efficiency of oil production is influenced by various factors, the main of which are the reservoir properties of oil-bearing formations - porosity and permeability. Porosity determines the storage capacity and permeability determines the filtration ability. Reservoir properties generally depend on the type of reservoir, the particle size of the reservoir rock, the presence of clay particles, etc. However, the formation depth has the greatest influence on the reservoir properties. For all types of rocks, permeability and porosity decrease with the bedding depth increasing. The volume of the pore space and the permeability of rocks also depends on the effective pressure. In this work, on the example of the northern Perm Region oil fields, confined to layers of the same geological age, the influence of the layer’s geological characteristics on their reservoir properties is assessed. When analyzing the reservoir properties of 40 oil layers, equations that describe the influence of the depth and effective pressure on the reservoir properties of oil layers were obtained. Analytical dependences of the influence of porosity on the permeability of rocks are also obtained.

Summary During the development of hydrocarbon fields geomechanical characteristics of rocks change under the effect of nonlinear dynamic loads. Elasticity modulus is one of the most important characteristics of mechanical properties of rocks used in engineering. It is known that the elasticity modulus of dry rocks does not depend on the frequency of the applied load. However, dispersion of elastic moduli in dry sedimentary rocks still exists. The mechanism, in accordance with which dispersion of the dynamic component of elastic modulus occurs, remains not fully disclosed. Studies on the dispersion of the dynamic component of the elasticity modulus under the influence of uniaxial dynamic loads are conducted on dry sandstone samples in the zone of linear elasticity. Physically substantiated relations are proposed. The nonlinear nature of the dynamic elasticity modulus under the influence of dynamic loads of given frequency and amplitude was confirmed. The obtained relations allow taking into account the influence of nonlinear dynamic loads on the elasticity modulus of the rock in the calculations when designing the development of hydrocarbon fields.

This paper considers the implementation of hydraulic fracturing in an oil field located in the Arkhangelsk region in Russia. During the exploitation, the production rates and injectivity of the injection wells in the field were intensively decreased. To increase well flow rates, hydraulic fracturing of the formation was carried out, and the evaluation of the efficiency was performed. Oil production rates after the fracturing increased 3.2 times and the productivity index increased twice. The influence of the geometrical sizes of fractures on the volume of injected proppant was investigated. An increase in the mass of the injected proppant from 2 to 3 tons per 1 meter of formation thickness leads to an average increase in the crack width by 0.5 mm, and the half-length by 40 m. Well work parameters after hydraulic fracturing of a reservoir were obtained as a function of the original parameters of the reservoir. It was observed that there is a sharp decrease in well production rates after fracturing in wells with low bottom hole pressures. When the pressure at the bottom of the well decreases from 60 to 20 MPa, it leads to an average decrease in the crack width by 2 mm, and the half-length of the crack by 50 m. Direct correlation between the well productivity coefficients after fracturing and the values of bottom hole pressures was observed. The optimal conditions for fracturing were identified, which made it possible to significantly increase the efficiency of the operation.

Development of fracture-porous reservoirs can be followed by breakthroughs of water injected into reservoirs through the system of cracks to production wells. This process can reduce the coverage of reservoir by sweeping, which will ultimately lead to a decrease in oil recovery factor. In order to perform a more effective fractured-porous reservoir flooding it is possible to use various methods associated with the injection of gel and sediment-forming agents, the use of wave technologies etc. The paper considers the formation with high-viscosity oil and fractured-porous reservoir. It is observed that water cut increase faster than oil recovery. The authors of the paper propose to use the wave effect associated with the stops of both injection and production wells. Shut-off and shut-in time of each well should be selected based on the parameters of the bottomhole zone. The specific value of the end time of the interaction of blocks and fractures when the pressure changes at the points of the reservoir can be roughly estimated from the beginning of the straight section of the pressure build-up curve in the fracture-porous reservoir. For the selected formation reservoir time of the end of the interaction between fractures and blocks with pressure changes was determined. Various options for implementation were proposed. Process impact modelling was performed using the Tempest More software package. According to the results of the simulation, it can be noted that the wave phenomenon is effective in terms of water cut reduction. Neverthelles, there are losses in oil production with long periods of shut-off of the wells. It should be noted that the time of interaction between fractures and blocks substantially depends on the permeability of the bottomhole zone; the higher the permeability the lower the time. It was also found that a variable frequency wave results in a greater effect.

In the article an account of extensive investigations of a well with carbonate deposits operating over a period of 44 months in Perm Region is given. Two well production enhancement techniques were used, namely abrasive jet perforation and acid treatment. A detailed analysis of abrasive jet perforation and acid treatment results was carried out, where changes of productivity index depending on reservoir and bottomhole pressures after operations were evaluated. It was shown, that an incremental oil production can significantly decrease with decreasing reservoir and bottomhole pressures. It was found that during the abrasive jet perforation with acid treatment, an incremental oil production has increased for abrasive jet perforation and acid treatment to about 65 % and 35 % respectively. It was also shown, that the abrasive jet perforation helps to slightly increase a well rate and decrease a reservoir energy consumption for fluid communications in bottomhole formation zone. After the acid treatment taking place two years later, oil production was increased and it is understood that 15 % of this increase is due to acid attack, and on 85 % due to growth in bottomhole pressure and fracture permeability. There was revealed a complete exclusion of reservoir energy consumptions to overcome the additional filtration resistance immediately after conducting operations. The reservoir has an excellent intrinsic fracturing property, so bottomhole formation permeability and energy consumption significantly depend on the reservoir and bottomhole pressures. It is recommended to maintain reservoir and bottomhole pressures higher than lateral rock pressure to increase effectiveness of well operations in reservoirs with advanced natural fracturing.

This paper presents a mathematical model that reflects the nature of the dynamic Young’s modulus of a dry sedimentary rock during nonstationary uniaxial loading. The model is based on an idealized model of a system suggested by Jaeger J.C. A rock sample is considered as a spring with stiffness, the bottom point of which is fixed, while the upper point carries a mass. A sample experiences dynamic load and the rock matrix response. Displacement of the mass from the equilibrium state sets the variation of the sample’s length. Displacement of all the sample’s points goes according to the same law regardless of the point location. The response of a rock to a disturbing nonstationary load is selected based on the combination of conditions of each experiment, such as the load frequency and amplitude and the mass, length, and diameter of a sample. The mathematical model is consistent with experimental data, according to which an increase in load frequency leads to an increase in the dynamic Young’s modulus for each value of the load. The accuracy of the models is evaluated. The relations underlying the model can be used as a basis to describe the Young’s modulus dispersion of sedimentary rocks under the influence of nonstationary loads.

Under dynamic loading, the geomechanical properties of porous clastic rocks differ from those in quasistatic loading. A small experimental rig was built to directly assess the influence of vibrations on the uniaxial compressive strength (UCS), Young modulus, and Poisson&rsquo, s ratio. A piezoelectric actuator powered by a signal from an oscillator was used in the rig as a generator of vibrations. A laser sensor and eddy current probe measured the longitudinal and transverse deformation. Tinius Hounsfield and Instron Series 4483 installations were used to determine the geomechanical properties of new red sandstone in a quasistatic regime. The boundaries of elastic deformations determined in the quasistatic loading were implemented in the dynamic loading. To perform the experiments in the elastic zone (on the graph of stress (&sigma, )&ndash, strain (&epsilon, )), small samples with diameters ranging between 7.5 and 24.7 mm were manufactured. The investigation demonstrated that the Young&rsquo, s modulus of the sandstone increased with increasing values of the dynamic load and frequency.