Operation Features of Wells with an Extended Horizontal Wellbore and Multistage Hydraulic Fracturing Operation in Bazhenov Formation
Authors: Yushchenko Taras Sergeevich , Demin Evgeniy Viktorovich , Khabibullin Rinat Alfredovich , Sorokin Konstantin Sergeevich , Khachaturyan Mikhail Viktorovich , Baykov Iskander Vitalevich , Gatin Rustam Irekovich 
Published: October 2021

Wells with extended horizontal wellbore (HW) drilling with multistage hydraulic fracturing (MHF) is necessary for commercial oil production from Bazhenov formation (Vashkevich et al., 2015; Strizhnev, 2019). Today the maximum HW length for Bazhenov formation wells is 1500 m (Strizhnev, 2019, Korobitsyn et al., 2020). In international practice the maximum HW length for shale oil production is around 3000-400 m (Rodionova et al., 2019). Pump Down Perforator (PDP) technology is used for MHF: a liner is run in hole and cemented, then perforation and hydraulic fracturing (HF) are successively performed by stages at equal distances from the end to the beginning of HW to create a branched system of fractures in Bazhenov formation. Performed HF stages are isolated with special packer plugs (insoluble blind, dissolvable blind, insoluble with seat for dissolvable ball or dissolvable with seat for dissolvable ball)) (Mingazov et al., 2020). Consequently, the fluid inflow into the well is occurred along whole HW and the flow rate increases from monotonically from the end to the beginning of HW and has maximum value at last HF stage. The numbers of HF stages are about 24-30 (number of perforating clusters - 100) at one well in Russia and 50 in the world (Alzahabi et al., 2019).

One of important parameter during HF is the speed of HF fluid injection into the formation. Tubing outer diameters 114-140 mm. are used in HW to increase the injection rate and reduce friction losses in the well. The flow rate of HF fluid in this case reach to 14-16 m3/min (Ogneva et al., 2020; Astafiev et al., 2015). Monobore wells construction is planned to use with outer diameter 140 mm. A stinger is used as sealing element between tubing and liner to minimizing risk of HF liquids leaks into the annulus (Astafiev et al., 2015). As a result, the inner well diameter from wellhead to bottomhole is around constant in the process of MHF.

The pressure in the hydraulic fractures and the collector near fractures after MHF is highly exceeded the initial reservoir pressure. Hence wellhead pressure after MHF in water filled well is about 100-150 bar (Jing Wang et al., 2021). This fact significantly limits downhole well operations because of requires killing (tubing change, let down ESP, etc.). These works are required heavy well killing fluid because of high overpressure. It is undesirable because of it can reduce the fracture conductivity, worse well bottom zone properties and reduce well productivity. Therefore, the well is working at flowing mode in initial period usually until the reservoir pressure in the drainage area is decreased at the hydrostatic level or below (Jing Wang et al., 2021). After that the well can be killing using technical water with a density of 1.01 – 1.07 g/sm3 (the use of well-killing fluid with a density higher than 1.1 g/sm3 is undesirable). The possibility of well flowing working depends on properties of collector and reservoir fluid: High gas-oil ratio (GOR) and reservoir conductivity help well flowing until reservoir pressure drop off hydrostatic pressure.