More and more oil and gas fields are moving into the third stage of development - the stage of production decline. Oil and gas operating companies are looking for the most effective ways of production stabilization and extending the period of well exploitation. The most frequently used approach of improving exhausted reservoirs performance is reducing the pace of pressure and hydrocarbons production decline by well workovers and horizontal sidetracks drilling. The most widely used type of new producing wells trajectory in low - and medium-amplitude gas fields, which include most of the Western Siberia gas reserves, is horizontal completion. According to the analysis carried out by two major Rosneft scientific centers, in oil saturated reservoirs with thickness less than 20 meters, the efficiency of horizontal wells with 300 meters length is 1.6-4 times higher than for directional wells, depending on the reservoir thickness and permeability. In gas saturated formations, the efficiency of horizontal wells performance relative to the directionally drilled wells in similar geological conditions is 3-6 times higher. As the consequence of scientifically based well performance analysis the volume of horizontal wells drilling and horizontal side track completions at the assets of PJSC "NK "Rosneft" significanty increased as for the period from 2016 to 2021, Figure 1.

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3D hydrodynamic modeling is the standard tool for predicting the development of hydrocarbon (HC) fields. The relevance of the work is associated with the need to introduce fast and affordable optimization algorithms into engineering practice, which will reduce the cost of computer time to justify the best and most effective development solutions. The authors have proposed a new express method for finding the optimal option for the development of deposits. The method works with discrete sets of possible variations of the required development parameters (for example, the number of wells, the type of completion, the rate of hydrocarbon reserves withdrawal etc.) and minimizes the number of launches of the reservoir simulation simulator per forecast required for the feasibility of various combinations study of parameters and finding both local and global optimal combinations. Compared with other methods, its advantage is simplicity and realizability in the “manual” mode with a small number of variable parameters, which can be useful for practical problems. The method uses the principle of iterations and is tested on several examples, including the results of hydrodynamic modeling, a comparison is made with known optimization algorithms — in some problems the method allows finding the optimum faster. For example, in the problem of finding the optimal location of horizontal wells, the iterative search turned out to be faster than the “swarm of particles” method. On the other hand, the method does not allow one to reliably determine the optima of complex objective functions that have several local optima. Testing was carried out on the Himmelblau and Rosenbrock functions: in the first case, all five local optima were found, in the second case, seven out of twelve.

The task of choosing the sidetrack trajectory for drilling is one of the most common among specialists while workovers planning in oil and especially in gas fields. This task is solved by reservoir engineers, geologists and drilling engineers, both in a team and separately. Despite the widespread use of software for processing and analyzing the data and high automation of most routine tasks, many oil and gas companies lack a unified methodology for choosing sidetrack trajectory. Each oil and gas company has its own approach to this task. Usually, the process of analyzing candidates is based on the expert opinion of the well design engineer and has several limitations, such as: ▪Short deadlines - on average, it takes from two to three weeks to select the sidetrack drilling trajectory.▪Low automation of the process of creating and analyzing the trajectory - the trajectory is created by a specialist manually, this process takes, considering the time for analysis, from two to four days for one trajectory. As a result, about five candidates are considered for the allotted time.▪Analysis of a small number of influencing factors — two-dimensional maps of averaged permeability, residual reserves maps, a seismic map of average amplitudes, as well as the proximity of the candidate to the existing well stock are mainly considered during analysis of the trajectory.▪Prediction of the flow rates is not always based on the hydrodynamic model - when calculating the candidate's starting flow rate due to tight deadlines, not all trajectories are analyzed using the hydrodynamic model, for part of the trajectories flow rates are calculated only using analytical techniques, such as the Joshi equation (Joshi, 2018).▪The risk of human factor. Sidetrack drilling, like other workovers, is aimed at increasing the flow rate of the well and the cumulative production of the entire field. In gas fields with falling production and high drilling density, sidetrack drilling can help significantly extend production time with the same level or increase production. Drilling a sidetrack, rather than a new well, can significantly reduce drilling costs, since the main well has already been drilled. Potentially, almost any well, especially with falling production rate or high water cut, can be a candidate for sidetrack drilling. According to the data from the Federal Supervision of Natural Resources in 2017, almost a quarter of all wells in Russia are inactive or shut, which is almost 60,000 wells.