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Sequential Methods for Coupled Geomechanics and Multiphase Flow

Sequential Methods for Coupled Geomechanics and Multiphase Flow

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196 CHAPTER 6. COUPLED MULTIPHASE FLOW AND GEOMECHANICS<br />

6.5.1 Case 6.1—Water injection <strong>and</strong> oil production in the 1D poroelas-<br />

ticity<br />

The schematic of this 1D problem is shown in the left plot of Figure 6.2. For Case 6.1,<br />

dilation <strong>and</strong> compaction occur around the injection <strong>and</strong> production wells, respectively. The<br />

water injection rate Qw,inj = 500 kg day −1 is the same as the production rate Qo,prod =<br />

500 kg day −1 . The domain is homogeneous with 15 grid blocks. The length of the domain<br />

is Lz = 150 m with grid spacing ∆z = 10 m. We have a constant overburden ¯σ =<br />

2 × 2.125 MPa. A no-displacement boundary condition is maintained at the bottom of<br />

the domain. The bulk density of the porous medium is ρb = 2400 kg m −1 . The initial<br />

oil pressure is Po,i = 2.125 MPa, where the <strong>for</strong>mation is fully saturated with oil initially.<br />

The fluid densities <strong>and</strong> viscosities are ρo,0 = ρw,0 = 1000 kg m −1 <strong>and</strong> µo = µw = 1.0 cp,<br />

respectively. Permeability is kp = 500 md, porosity is φ0 = 0.3, constrained modulus is<br />

Kdr = 1 GPa, <strong>and</strong> the Biot coefficient is b = 1.0. Capillarity is ignored. A monitoring<br />

well is located at the fifth grid block from the top (1, 5). No-flow boundary conditions are<br />

applied at the top <strong>and</strong> bottom. Gravity is neglected. The oil <strong>and</strong> water compressibilities<br />

are similar, co = 3.5 × 10 −9 Pa −1 <strong>and</strong> cw = 3.0 × 10 −9 Pa −1 , respectively. These values<br />

yield τ = 0.95 <strong>and</strong> τ = 1.05 <strong>for</strong> single-phase flow, respectively. The residual oil <strong>and</strong> water<br />

saturations are zero. The parameter values <strong>for</strong> Case 6.1 are also given in Table 6.1.<br />

Since the convergence behaviors of the drained <strong>and</strong> fixed-strain splits depend on the<br />

total fluid compressibility from Equations 6.31 <strong>and</strong> 6.44, water injection can cause non-<br />

convergence <strong>and</strong> instability during simulation.<br />

We use a linear relative permeability of phase J, kr,J, with respect to SJ, i.e.,<br />

where Sr,J is the residual saturation of J phase.<br />

kr,J = (SJ − Sr,J), (6.120)<br />

The top of Figure 6.3 shows the pressure history at the monitoring well during sim-<br />

ulation. The pressure jumps at the initial time because of the excessive overburden <strong>and</strong><br />

decreases with production. Then the pressure rises because the reservoir is being filled with

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