Fundamental Aspects of Oilfield Development with Water Breakthrough Risks - Time for a Paradigm Change
Authors: Nepop Roman , Smirnov Nikolay , Molodtsov Roman , Reyes Ahumada Vinicio , Nizametdinova Madina , Polushina Julia , Kudlaeva Nailia , Dolgushin Taras , Maltsev Andrey , Khazov Nikolay 
Published: October 2021

Abstract
The geological features of the reservoir structure are of great importance in the development of oil and gas fields in the West Siberian. A productive formation is usually considered as an integral system that includes two interrelated components: rocks and fluid. In this system, the appearance of water is usually associated with negative consequences - a decrease in the elastic-strength properties of geological substrate and the loss of reservoir rocks stability. How critical is the emergence of water from a well construction and / or operation point of view? The search for an answer to this question has become the main goal of the multidisciplinary investigations presented in this paper.
In the framework of these studies, an assessment of the in-time stability of an open borehole in case of presence or penetrating different reservoir fluids was carried out. The calculations were based on the high-resolution 4D modeling, which took into consideration the main dynamic and geomechanical effects stipulated by different production scenarios.
The concepts of the interrelations in the "rock - fluid" system were based on the results of special core studies. The main idea of these experiments was to reproduce the saturation conditions of reservoirs, initially related to transition zones and characterized by the presence of different types of fluids (formation water / oil). Testing was carried out on duplicate samples, which were characterized by very similar petrophysical properties, and were saturated with various models of formation fluid.
The research program included both standard geomechanical tests and experiments on a thick-walled cylinder, as well as experiments with brine of different chemical composition, salinity, temperature and affected by different methods of saturation. Obtained results confirmed the substantial influence of the formation fluid on the elastic-strength properties of the rocks. At the same time, a fundamental role is played not only by the type of saturating fluid, but also by the saturation technique. Several such techniques of core material saturation were analyzed. It provides critical knowledge for calculating the stability of the wellbore in conditions of incompatibility of fluids and rocks, modeling water breakthroughs and sand production, and also for calculating temporal effects. Further modeling made it possible to predict the change in open hole stability under different production scenarios. Finally, it was demonstrated that even with a significant water cut (up to 50%), it is possible to keep the wellbore open and to continue the well operation. The results obtained become critical for evaluating the performance of the field at a later stage of development, especially when there are risks of water breakthrough into the reservoir.
The main paradigm in which the industry is currently operating is the idea that the appearance (breakthrough) of formation water leads to a decrease in the strength of the rocks, sand production and, finally, to the destruction of the wellbore. The results of presented study change this paradigm. Integrated geomechanical modeling and core studies applying different saturation techniques make it possible to analyze various mechanisms of water penetration into the reservoir, which is not necessarily associated with both subsequent sand production and wellbore destruction.