The paper summarizes the results of laboratory experiments performed on core samples of two carbonate reservoirs of the Republic of Tatarstan with modeling of real in-situ stresses to assess the dependence of permeability on a cyclic change in pore pressure. Each experiment consisted of 14 pressure stages with two changes in the direction: primary depletion, followed by increase in pressure, and then secondary depletion. At each stage, the permeability was measured for the saturating fluid, and the dynamic values of Poisson's ratio and Young's modulus were obtained by the acoustic method. The behavior of permeability depends both on the initial properties of the samples and saturating fluid. 'Looser' samples with the lowest initial values of Young's modulus showed the strongest permeability reduction during primary depletion. For samples with the highest Young's modulus and the poorest initial reservoir properties, the permeability changed slightly with a decrease in pore pressure. For water-saturated samples, reservoir compaction was recorded as the result of the cyclic change in pressure, most likely associated with plastic deformation of carbonate rock under the influence of water. For hydrocarbon-saturated samples, an increase in permeability during the cyclic geomechanical treatment (CGT) was obtained, ranging from 20% to 3.5 times. For a number of samples, during the increase in pore pressure, an evidence of the formation of a tensile fracture (equivalent to the hydraulic fracturing process) was noted. In all those cases, the value of the fracturing pressure was significantly lower than theoretically estimated which indicates a decrease in rock strength as a result of the previous pressure depletion.
The experimental results were used in flow simulations on sector models of two reservoir elements. Taking into account the dependence of permeability on pressure leads to more heterogeneous distributions of the reservoir pressure and oil recovery. It is shown that in order to assess the potential effect of CGT, appropriate consideration of reservoir compartmentalization and heterogeneity is important. For the sector model of the element with favorable parameters, an increase by 24.6% in cumulative oil production over 10 years was achieved due to the integrated use of CGT.
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