Applied Uncertainty Analysis to Mitigate Subsurface Risks at Planning and Implementation Stages of the UV1 Sidetrack Drilling Campaign for One of the Fields in Western Siberia

This work focuses on an implementation of a procedure for assessing uncertainties using a reservoir model and subsequent sidetrack drilling program optimization for one of the fields in Western Siberia. We investigated the uncertainties of porosity and permeability models, net thickness, initial and residual water saturations, OWC level, PVT properties of oil, relative phase permeabilities, pore volume compressibility, aquifer parameters, vertical permeability anisotropy as well as hydraulic fracture height and minimum nonconductive thickness of otherwise impermeable layers. The performed sensitivity analysis showed that the uncertainty of the production profile is significantly influenced only by permeability, aquifer parameters, vertical permeability anisotropy and threshold conductivity thickness. An ensemble of reservoir model realizations was created for uncertainty estimation, which takes into account both the production history and the initial geological and geophysical data. The analysis using the technology of proxy models revealed that all realizations matching the production history can be divided into two clusters: with the prevalence of lateral and vertical fluid flow tendency. One realization from each cluster was taken for further investigation.

Optimal length and reservoir entry points were determined as a result of the joint analysis of remaining volume of oil reserves for both sidetrack realizations. Using automated optimization procedure for each realization separately we determined the optimal number of sidetracks, their lateral location and position of the sidetrack relative to formation top. Sidetracks which gave a positive forecast for both realizations were then chosen for drilling. The resulting drilling program consisted of two primary, one dependent and three sidetracks with high risks.

This work presents the results of practical application of uncertainty assessment methodology and successful implementation of sidetrack drilling program for UV₁ (Jurassic period deposits) formation of the Varingskoe oil field in Western Siberia. In total, five oil flowing horizontal sidetracks were drilled. According to the data obtained during the drilling program implementation stage it was concluded that lateral fluid flow is dominating during production (i.e. impermeable layers were indeed good flow barriers and hydraulic fracture length proved to be insufficient for significant vertical fluid flow away from producing wells).