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The deterioration of the reservoir properties of potential oil and gas bearing areas on mature and green fields, as well as the increase in the volume of hard-to-recover reserves on low-permeable reservoirs set us new challenges in searching and using effective development technologies to maintain and even increase the oil production levels. Based on successful international experience, Russian oil and gas companies use horizontal wells (HW) with multi-stage hydraulic fracturing (MSHF) for the cost-effective development of low-permeable reservoirs. Thus, since the first pilot works of drilling technologies and completion of HW with MSHF in 2011, at the beginning of 2020, over 1,200 HW with MSHF were drilled and came on stream at the fields of LLC RN-Yuganskneftegaz, about half of which are at the exploitation play AS10-12 of the northern license territory (NLT) of the Priobskoye field. In searching the best technologies and engineering solutions, the company tested different lengths of horizontal section of HW, the number of hydraulic fracturing (HF) stages and distances between hydraulic fracturing ports, as well as different specific mass of the proppant per frac port. Recently, there has been a tendency in design solutions to increase the length of the HWs and the number of hydraulic fractures with a decreasing distance between the frac ports and a decreasing specific mass of the proppant per frac port. This work studies the actual and theoretical efficiency of HW with MSHF of various designs (different lengths of horizontal section of HW and the number of HF stages) and to assess the viability of increasing the technological complexity, as well as to analyze the actual impact of loading the proppant mass per port on performing HW with MSHF. The study is based on the results of the analysis of the factual experience accumulated over the entire history of the development of the exploitation play AS10-12 of the NLT of the Priobskoye field of the Rosneft Company. In studying the viability of increasing the technological complexity, especially, increasing the length of horizontal section of HW, increasing the number of HF stages, and reducing the distance between the frac ports: we discovered the typical methodological errors made in analyzing the efficiency of wells of various designs; we developed the methodology for analysis of the actual multiplicity of indicators of wells of various designs, in particular, HW with MSHF relative to deviated wells (DW) with HF; we carried out the statistical analysis of the actual values of the multiplicity of performance indicators and completion parameters of HW with MSHF of various designs relative to the surrounding DW with HF of the exploitation play AS10-12 of the NLT of the Priobskoye field; we performed the theoretical calculation of the multiplicity of the productivity coefficient for the HW with MSHF of various designs relative to DW with HF for the standard development system of the exploitation play AS10-12 of the NLT of the Priobskoye field; we compared the actual and theoretical results. The paper also presents the results of studying the actual effect of changes of proppant's mass per port on performance indicators of HW with MSHF of the same design and with an increase in the number of fractures of the hydraulic fracturing without changing the length of horizontal section of HW. As for performance indicators, being the basis for estimating the efficiency of HW with MSHF of various designs, we used the productivity index per meter of the effective reservoir thickness and the cumulative fluid production per meter of the effective reservoir thickness per a certain period of operation. And as the completion parameters, we used the length of the horizontal section of HW, the number of HF stages, the distance between the frac ports, and the specific mass of the proppant per meter of the effective reservoir thickness per frac port. The results of this work are the determining vector of development for future design decisions in improving the efficiency of HW with MSHF.
Summary In this study, unique field data analysis and modeling of operating wells with an extended horizontal wellbore (HW) and multistage hydraulic fracturing (MHF) in the Bazhenov formation were conducted. Moreover, a large amount of long horizontal well data obtained from the Bazhenov formation field was used. Wells with extended HW drilling and MHF are necessary for commercial oil production in the Bazhenov formation. Problems can occur in such wells when operating in the flowing mode and using an artificial lift at low flow rates. This study aimed to describe the field experiences of low-rate wells with extended HWs and MHF and the uniqueness of well operations and complexities. It was also focused on modeling various operation modes of such wells using specialized software and accordingly selecting the optimal downhole parameters and analyzing the sensitivity of fluid properties and well parameters to the well flow. The flow rates in wells with extended HW and MHF decrease in the first year by 70–80% when oil is produced from ultralow-permeability formations. Drainage occurs in a nonstationary mode in the entire life of a well, leading to complexities in operation. A comprehensive analysis of field data [downhole and wellhead pressure gauges, electric submersible pump (ESP) operation parameters, and phases’ flow rate measurements] and fluid sample laboratory studies was conducted to identify the difficulties in various operating modes. For an accurate description of the physical processes, various approaches were used for the numerical simulation of multiphase flows in a wellbore, considering the change in the inflow from the reservoir. The complexities that may arise during the operation of wells were demonstrated by analyzing the field data and the numerical simulation results. The formation of a slug flow in low flow rates in a wellbore was caused by a rapid decline in the production rate, a decrease in the water cut, and an increase in the gas/oil ratio (GOR) over time. Based on the results, proppant particles can be carried into the HW and thereby reduce the effective section of the well in case of high drawdowns in the initial period of well operation. Consequently, the pressure drops along the wellbore increased, and the drawdown on the formation decreased. Other difficulties were determined to be associated with the consequences and technologies of hydraulic fracturing (HF). These effects were shown based on the field data and the numerical simulation results of the flow processes in wells. In addition, corrective measures were established to address various complexities, and the applications of these recommendations in the field were conducted.
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