Mahmood Bataee

The Effects of Geomechanics on the WAG Injection Model

The main objective of this paper is to determine the effects of geomechanics on reservoir flow for the water alternating gas (WAG) process. Water alternating gas (WAG) injection has been a popular method for commercial gas injection projects worldwide. The WAG injection pressure, temperature and the initial in-situ stress is important to determine the stress after the injection. The change in stress changes the porosity, permeability and subsequently the flow pattern changes. There are some methods to investigate the effects of geomechanics on reservoir simulation, as implicit, explicit, iterative and psudo-coupling method. In this study we use the iterative coupling method. In this type of coupling, reservoir flow variables and geomechanics variables are solved by a reservoir simulator and a geomechanics module, and the coupling terms are iterated on at each time step. The results show the difference in the flow patterns, porosity and permeability with and without considering the geomechanics. Keywords— Geomechanics; WAG; reservoir simulation Introduction Geomechanics is the study of the rock elastic or plastic behavior and has direct impact on well integrity. Wellbore instability is a serious drilling problem that costs the oil industry over US

Wellbore Failure During Water-Alternating-Gas Injection by Use of Flow-Stress Coupling Method

500–1000 million each year. Over the past two decades the geo-mechanical analysis has made major changes in the petroleum industry by maximizing production and increasing the life of the well . Wellbore stability is dominated by the in situ stress system. When a well is drilled, the rock surrounding the hole must take the load that was previously taken by the removed rock. As a result, the in situ stresses are significantly modified near the borehole wall. This is presented by an increase in stress around the wall of the hole, that is, a stress concentration . Rocks are generally composite materials, and hence inhomogeneous on a microscopic scale. The way rocks behave, their elastic response, their failure stresses etc., depend, to a large extent on the non-solid part of the materials. In this section we will take into account the void space, which not only is essential for oil to be produced from a reservoir, but also play an important role in rock mechanical behavior. The theory of thermo-poroelasticity (or porothermoelasticity) is developed by combining the influence of thermal stress and differential solid/fluid expansion to rock stresses and fluid diffusion . Enhanced oil recovery (EOR) refers to a variety of processes to increase the amount of oil extracted from a reservoir after primary and secondary recoveries, typically by injecting water or gas . Water alternating gas (WAG) injection has been a popular method for commercial gas injection projects worldwide. The injection of water and gas alternatively offers better mobility control of gas and hence, improves the volumetric sweep efficiency. The injected fluids might push the oil in the reservoir or rather interact with the reservoir rock/oil system to create favorable conditions for oil recovery . Approach The general path of the study is sketched in the Fig. 1. This flow chart is a brief explanation of the methodology. First we have to define the parameters. The sample of the parameters used in this study is existed in the table 1. First, the initial state of the reservoir, pressures, saturations, the shape of the reservoir and etc. should be defined in the reservoir simulator. Table 1. The value of parameters used in the study. In the next step, the flow simulation should be run to prepare the results of WAG injection study. These results are different phases saturations, pressures, front movement, and production after injection. These results are obtained without considering the geomechanics . After enabling the geomechanic module in the simulator and defining the values for the geomechanical features, the stress state in the reservoir would be obtained . These changes will lead to the change in the porosity and permeability values in all parts of reservoir. The important result in this stage is the porosity and permeability distribution. This result is highly dependent of initial in-situ stresses, reservoir and injection pressures and temperatures . The change in porosity and permeability leads to the change in all results in the case of without geomechanics (as changes in different phases saturations, pressures, front movement, and production after injection) . As the scope of study it is mentioned that this paper had modeled the problem under following conditions: Parameter Value Density ρ103(kg/m3) 2.65 Young’s modulus E (GPa) 5 Poisson’s ratio ν 0.25 Unconf. compr. strength C0 (MPa) 100 P-wave velocity vp (km/s) 2.5 S-wave velocity vs (km/s) 2 Linear thermal exp. coeff.106 (K−1) 15 Formation Temperature (F) T0 125 Thermal conductivity Wm−1 K−1 1.7 Zero stress porosity, Φ0 (dimensionless) 0.1 Zero stress permeability, k0 (D) 1 Irreducible gas saturation for relative permeability function (dimensionless) 0.05 Irreducible liquid saturation for relative permeability function 0.3 Biot’s parameter, a 0.85 True vertical depth (ft) 3000 Maximum horizontal stress (psi/ft) H 0.82 Minimum horizontal stress (psi/ft) h 0.78 Vertical stress (psi/ft) v 1 Mobility (darcy/cp) k/μ 0.1 The volumetric thermal expansion coefficient of the porous matrix (1/F) m 2.70E-05 The volumetric thermal expansion coefficient of the pore fluid (1/F) f 2.78E-04 Reservoir pressure (psi) Po 1179 Wellbore radius (ft) rw 0.5 Injection temperature (F) Tw 70 Injection pressure (psi) Pw 5727 Oil viscosity (cp) 1.0 Oil compressibility (1/psi) 4.5*10-7 Oil density (lb/ft3) 62.4 Production rate (bbd) Injector 500 Reservoir Thickness (h) 30

Development of Stress Model near the Wellbore Using an Iterative Coupling Method

Accurate evaluation of failure pressure is crucial in the design of injection wells. Besides, in-situ stresses play an important role in obtaining the results. Pressure and rock stresses are related together as the role of effective-stress theorem. In fact, pressure changes with stress alteration caused by change in porosity and permeability. Therefore, it should be obtained with the coupling method. Moreover, to calculate pressure, temperature, and stress in the fully coupling method, a huge matrix should be solved, and it takes long processing time to implement it. Therefore, this study developed a wellbore geomechanical model for stability during injection by use of the iterative coupling method. The processing speed was enhanced in this study because the parameters were calculated separately. The parameters of pressure, temperature, saturation, and stress were obtained for the multiphase-flow condition with numerical modeling. Furthermore, in this study, the finite-difference method (FDM) had been used to solve pressure, temperature, and saturation, whereas the finite-volume method (FVM) was applied to solve the wellbore stress. On top of that, the iterative coupling method was used to improve the accuracy of the stress results. As a result, a correction of approximately 20 psi (0.14 MPa) was noted for pressure in relation to stress, which is 1 psi (6.89 kPa). Moreover, the Drucker-Prager failure criterion was used to model the fracturing on the basis of the stress results. Other than that, sensitivity analysis on horizontal maximum (o-H) and minimum (o-h) stresses showed that by increasing O-H, the maximum injection pressures to avoid fracturing had been reduced, whereas in the case for O-h, an increment was observed. Copyright

Numerical Simulation of Stress Change during Wellbore Injection

AbstractWater-alternating-gas (WAG) injection is increasingly applied globally as the effective enhanced oil recovery (EOR) method in oil wells. High injection pressure or low injection temperature could lead to wellbore failure. The rock stress around the wellbore is a function of the wellbore pressure and temperature, and it should be precisely determined to avoid wellbore failure. This study aimed to develop a wellbore geomechanical model for WAG injection using an iterative coupling method. The parameters of pressure, temperature, saturation, and stress were obtained for the multiphase flow condition using mathematical modeling. The finite-difference method was used to solve pressure, temperature, and saturation, and the finite-volume method was used to solve the rock stresses. Because the values for flow and rock stresses are related by the role of effective stress, the pressure is the key parameter to determine the stress. However, pressure changes with stress because of the change in porosity and p...

Wellbore stability model based on iterative coupling method in water alternating gas injection

The important matter during any process in the well is wellbore integrity. Regarding to the geomechanics of the well, high bottom-hole pressure and low temperature could lead to the fracturing. The stress near the wellbore is a function of the flow and temperature, and it precisely determined to avoid the wellbore failure. The objective of this paper is to simulate stress change in the well for the injection. In order to analyze the stress in the wellbore a finite volume analysis has done for the wellbore. The stress equation relates to flow equation with the equation of principle stress. It means that the pressure is a key parameter for determination of the stress. The procedure which has to be followed is transforming the equations to weak form, meshing the defined shape and programing to obtain the values for each node. The result for the stress is obtained for some meshed bodies. The accuracy enhanced by choosing smaller mesh sizing.

Review of Geomechanical Application in Reservoir Modeling

AbstractEnsuring wellbore integrity is the most important factor in injection well design. The water alternating gas (WAG) injection is increasingly applied globally as the effective enhanced oil recovery (EOR) method in oil wells. High injection pressure or low injection temperature could lead to compressive wellbore failure. The rock stress around the wellbore is a function of the wellbore fluid flow and it should be precisely determined to avoid the wellbore failure. The purpose of this study is to propose a method to ensure the stability of the wellbore for the WAG process using iterative coupling method. The parameters of pressures, temperature, saturations and stresses are obtained for the multiphase flow condition using mathematical modeling. In this study, finite difference method is used to solve pressure, temperature and saturations; and finite volume method is acquired to solve the rock stresses. Iterative coupling method is employed to improve the accuracy of the results. This study introduces improved iterative coupling method between flow and stress models to reduce the processing time of obtaining corrected stress and failure results. Wellbore stability model is developed to determine the maximum pressure values, which lead to wellbore failure in WAG injection process for some different boundary conditions.