Deep well as a facility for on-line hydraulic studies of the stress state of the rock mass in fluid-saturated fractured reservoirs

Abstract

Inthe southern areas of the Siberian platform, geological and mining conditions are highly complex for design, drilling and construction of deep wells for oil and gas. Complicating factors are related to specific features of deforming cavity-fractured carbonate reservoirs. Geological models of carbonate reservoirsshould be designedon a case-by-case basis, taking into consideration the conditions of each specific field, including its complex filtration field, confirmed deformation of filteringfractures with constant parameters of the cavern component, etc. If an incomprehensive geological model is used for production planning with a lack ofa well-based approach to the development of a carbonate reservoir, design well flow rates may not be achieved, the field reserve coverage will be reduced and, consequently, the oil recovery ratio will be lower. When developing oil, gas and brine deposits, it is required to consider the natural-technical system of‘reservoir – reservoir’ (mechanical impact) – ‘reservoir fluid system and well’ (hydrodynamic impact) on the basis of a comprehensive analysis of its interrelated and interdependent components. Our study shows that each part of the system contributes directly to the entire complex. The stress state of the rock mass in the fluid-permeable part of the natural fractured reservoir and fluid-saturated reservoirs is dynamically changeable during the fluctuations of the pressure field of the reservoir hydrocarbon system at the initial penetration of a well into the reservoir. The authors analyzed the natural-technical system ‘well – fluid-saturated bed’ as a real on-line model of leaking – hydraulic fracturing based on a standard pressure vs. time curve during hydraulic fracturing. Based on the knowledge of the on-going processes in carbonate cavern-fractured reservoirs, it becomes possible to predict the reaction of the rock mass during hydraulic repression (i.e., during the initial penetration of a well into the reservoir) and depression (during testing and operations of a production well). All pressure effects hydraulically applied to the reservoir fluid system can lead to changes in the stress state of the natural reservoir. Thestress state with significantly changing permeability parameters of filtering fractures is among the most complex states of stress [Belonin et al., 2005; Borevsky, 1986]. It is important to take into accountthat such stress state transition during the drilling process is uncontrollable and unpredictable, which means that the reservoir permeability and porosity may be irreversibly affected and considerably reducedand, consequently,the oil well productivity and the final profitability of developing the hydrocarbon field will be significantly decreased. Based on the actual deep drilling data, we continue comprehensive studies of deforming fractured reservoirs in the area of the stress state of the rock mass and investigation of pressure characteristics of fluid systems and their mutual influence during hydraulic operations in the well completion cycle. Wehave analyzed the stress state of the rock mass in various reservoirs. Indicators characterizing a particular state of the rock mass are substantiated. Geological and technological recommendations are specified for the initial penetration of wells into the reservoir, as well as for testing the wells with respect to the state of the natural-technical system ‘well – fluid-saturated formation’. New geological and technological solutions are proposed.

Keywords

• stress state
• geodynamics
• drilling
• horizontal well section
• drill-in technology of initial penetration
• riphean carbonate fractured reservoir