Define initial non-wetting pressure and capillary pressure in oil reservoir

Hi all,
I am trying to simulate the two-phase(water-oil) flow behavior in a 3D reservoir model during a water injection process using TH2M module. However, I am confused about the definition of gas pressure i.e. non-wetting phase(oil) pressure and capillary pressure. With a 600 meters in the depth direction, I think there should be a gradient distribution for the both pressures. Right now, considering an oil-saturated reservoir, I set the same value for gas pressure and capillary pressure so that the initial water pressure is zero. However, I found the saturation change(in an order of 0.0001 magnitude) is negligible in the whole 3600s process when a Vangenuchten saturation model is applied because of the large capillary pressure value in an order of 1e7Pa. So there must be some problems in my definition of initial gas pressure and capillary pressure.
I will appreciate it if anyone can give me some advice.
Best,
Yuhao

Hi all,
After some efforts, I guess my previous setting for my model is reasonable, that is, the same value for gas pressure and capillary pressure. The negligible change of saturation is because of the parameter settings of the VG model. Here are the details about it. https://doxygen.opengeosys.org/dc/dd5/classmaterialpropertylib_1_1saturationvangenuchten
For the exponent, 0.2915 is suitable as suggested by some articles concerning multiphase flow in porous rock.
Best,
Yuhao