The West Salym oil field is located in the West Siberian oil province (Russia). Its reservoir conditions are typical for the region: sandstone formation with temperatures as high as 83 °C, low crude oil viscosities of about 2 cP, and brine salinities in the range of 14,000-16,000 ppm. The field is waterflooded to maintain the reservoir pressure close to its initial level and to optimize oil recovery. Oil production from West Salym peaked in 2011 and since declined with increasing water cuts. The expected ultimate recovery factor due to waterflood, as reported in the field development plan, is between 35-40%. This recovery will be achieved through an evergreen waterflood optimization process consisting of infill/side-track drilling campaigns, pattern rebalancing, well workovers, etc. To increase the recovery factor, a tertiary oil recovery technique called Alkaline-Surfactant-Polymer (ASP) flooding was selected. Earlier studies indicated a potential of 15-20% incremental oil recovery due to ASP injection. Laboratory studies were started in 2008 with the surfactant/polymer screening and selection followed by core flooding experiments. In 2009, a successful single-well chemical tracer test was conducted to prove the efficiency of the developed ASP formulation at field conditions. In 2012 a final investment decision was taken to conduct a multi-well pilot to demonstrate oil recovery potential of the ASP flooding process and to collect sufficient information for decisions on subsequent commercial projects. Pilot operation was started in February 2016 with the start-up of the ASP mixing and injection plant. Active injection was completed in January 2018 and the end of production is expected in Q1 2018. The paper focuses on the pilot performance results, principal operational challenges and strategies to overcome them. The ASP injection resulted in the mobilization of a significant volume of oil in the confined 5-spot pattern. The water cut dropped from 98% prior to ASP injection to 88% due to oil mobilization by the ASP solution. The estimated incremental recovery is above 16% of the pilot STOIIP. Together with the mobilized oil a larger than predicted concentration of injected chemicals has been back-produced through the pilot producing wells. This has led to production issues, most notably failure of electric submersible pumps in the pilot producing wells due to carbonate scale and production of stable oil-water emulsions. Both issues required the use of methodologies and chemicals that were novel for West Salym field. The laboratory data and field observations collected during the pilot operation phase are presented to support our conclusions. Finally, actual vs. expected results of the ASP pilot and remaining uncertainties to further develop the chemical flooding technology are discussed.

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The paper considers borehole wall stability in a junction zone of coaxial wells where a borehole of bigger diameter connects with a smaller one. To determine the shapes and character of rock destruction, 3D poroelastic modeling of the stressed state of the rock around the coaxial junction with account for mudcake formation was performed. The geomechanical model considers the anisotropy of the medium’s deformation properties that are characteristic for the coastal-marine reservoirs of Western Siberia. The rock failure is estimated based on the Mohr-Coulomb criterion with account for tensile destruction condition. The paper considers cases of vertical and inclined junctions of a well drilled at a depth of 2 km in sandstone productive pay with known poroelastic anisotropic properties. The stress and pore pressure analysis has been performed for a mud pressure drop range from 1 to 70 atm and coaxial junctions with different combinations of borehole diameters. The safe mud pressure window has been determined for vertical and inclined junctions. It has been found that the rock failure pattern for junction of bigger diameters is, in general, similar to that for smaller diameters with some insignificant differences in the destruction areas shapes. It has also been demonstrated that in vertical junctions, the bottom holes of smaller diameter are more stable to reduced drilling-mud pressure than the mainboreholes, while in the inclined junction it is the mainwellbore that is more stable to increased drilling-mud pressure than the bottom hole.

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