PSI - Issue 6

I. Bazyrov et al. / Procedia Structural Integrity 6 (2017) 228–235 Bazyrov ILdar et al// Structural Integrity Procedia 00 (2017) 000–000    − grad  = 0 ,  ∈  . 16 When pore pressure dynamics is initiated, there is a so called “pay zone” around the wellbore. Stresses around the wellbore (within the pay zone) are altered due to poroelastic stresses effects caused by pressure gradient in the reservoir (11)-(12). In case of hydrofracturing the rock is fractured by the pressure created by fluid injection in the wellbore. The main purpose of this operation is the stimulation of hydrocarbon production by initiation of the high permeability channel near the wellbore. To prevent frack closure small grains of proppant are injected into a vesicle. The initial fracture propagates in maximum horizontal stress direction. In the course of production the pore pressure reduces and the effective pressure on rock matrix and proppant grains increases. The orientation of the second fracture differs from the azimuth of the initial fracture in the pay zone (figure 2). Outside the pay zone, fracture direction takes the initial crack direction. The fracture initiation is described in [9], [10]. Nonlinear and nonequilibrium processes affect oil and gas field recovery, including hydrofracturing. [14, 15]. 231 4

Figure 2. Conceptual illustration of the hydraulic fracture reorientation.

Numerical simulation Over the last few years to solve the problems of field development planning and well stimulation, leading oil and gas companies apply the approach based on coupled geomechanics and reservoir simulation. In this work, 4D geomechanical model was constructed in order to establish the magnitude and the nature of change in the initial stress state due to the influence of field development. Subsequently, this model was used to evaluate the prospects of the wells A and B that were the candidates for repeated multistage hydraulic fracturing. For the well-candidates A and B 5 and 6 stages of hydraulic fracturing were planned respectively. Model construction features: in a real geological environment the investigated area is only a part of global geological system, which is affected by many processes of local and regional scale, up to the processes associated with movements of lithospheric plates. To take into account the influence of the surrounding geological volume and to minimize boundary-layer effect, cells (a host medium) with averaged mechanical properties were added into the model. The same principle was used to model the underlying stratum. Material properties: each cell of the model contains mechanical properties (Young's modulus, Poisson's ratio, strength, friction angle) and is assigned its density value. Stresses: the model is loaded by vertical stress (the weight of overlying cells) and regional horizontal stresses. Boundary conditions: On the ground surface condition of non-displacements is set. At the edges of the host medium an additional medium layer of cells with increased strength and a higher Young's modulus is created. To avoid horizontal deflections of a layer at a distance from the model gradients of existing regional stresses were assigned. In addition, for model construction rigid base was added by setting the highest value of Young's modulus to the deepest layer of the host medium. In addition, during the loading process the continuity condition at the boundaries of each element was set.