Hydraulic fracturing is one of the methods for enhancing oil recovery in clastic reservoirs used all over the world, but there are not any fracturing operations provided by Oil and Gas operators in Sakhalin offshore. That is why care must be taken in fracturing operations planning and all associated risks should be accounted for. The 3D geomechanical model was built based on the geological information and coupled with hydrodynamic simulations. Deformations, total and effective stress values and faults characteristics were obtained. The results were compared to the 1D geomechanical models that were used as a basis.
The conducted studies showed that the reservoir is very heterogeneous due to depositional environment, bioturbation process and tectonic activity, so the areas of oil recovery and pore pressure reduction have an uneven shape as well. Under these circumstances the pore pressure and effective stresses values variation significantly impacts the final oil recovery. This variation should be considered during fracturing design and prior to well drilling, since the minimum horizontal stress and corresponding loss gradient will be different compared to the initial state. Due to the reservoir heterogeneity the 1D geomechanical model is not enough to properly support fracturing operations, though it can be used for drilling support.
As the result of the work, some major uncertainties were indicated. The regional stress regime is defined truly by the active Sakhalin-Hokkaido strike-slip, but the local stress regime is still questionable. It can be either strike-slip or normal. Nevertheless, the fracturing design is quite optimistic, because in both cases fractures will grow normally to the minimal stress, consequently vertically ensuring the maximum drainage area if the well is drilled in the minimal stress direction. The first fracturing operation data and extended logging data from future wells will be required to confirm the stress regime and the caprock properties.
This complex model will provide engineers with all the necessary data for safe well drilling, optimal fracturing design and for improving hydrocarbons recovery at any time of the field lifecycle. These data include porosity, permeability, saturation, pressure, deformations distributions as well as faults stability and stress values and directions.
References
1. Pavlov V., Korel’skiy E., Butula K. et al., 4D geomechnical model creation for estimation of field development effect on hydraulic fracture geometry (In Russ.), SPE 182020-RU, 2016.
2. Sim L.A., Bogomolov L.M., Bryantseva G.V. et al., Neotectonics and tectonic stresses of the Sakhalin Island (In Russ.), Geodinamika i tektonofizika = Geodynamics & Tectonophysics, 2017, V. 8, no. 1, pp. 181-202.
3. Twiss R.J., Moores E.M., Structural geology, New York: W.H. Freeman and Company, 2007, 736 р.
4. Zoback M., Reservoir geomechanics, Cambridge: Cambridge University Press, 2007, 505 р.
