Development of well completion and reservoir stimulation technologies has fuzzified the well-established concepts that have been commonly used to identify and estimate productive natural reservoirs. As multi-stage hydraulic fracturing (MSHFC) is broadly applied, the petrophysical criteria in horizontal wells based on the definition of reservoir bottom interface turn out to be inefficient. This may be exemplified by quite a large-scale development of hard-to-recover oil limited to low-permeable rock ("tight oil") and shale rocks ("shale oil") whose reservoir properties are near cut-off points or even much lower. The current situation speaks for the necessity to review the existing petrophysical approaches to the estimating of the potential of natural hydrocarbon (HC) reservoirs. It is obvious that the downward revision of cut-off points for the reservoir will not be enough in such situation and we will need to improve the mere criteria applicable to estimating the productivity potential. Thus, it becomes a priority to develop a conceptually new approach to the estimating of rock productivity potential keeping in mind their future industry-related stimulation using state-of-the-art technologies. The said objective goes far beyond the investigation of standard parameters which are normally referred to as volumetric parameters (e.g., net pay porosity, permeability and oil saturation factor) and it has a much more complex and comprehensive nature. It is equally important to be able to determine the characteristics of the stimulated zone of the formation created as a result of technogenic impact of hydraulic fracturing methods.
Productivity of a contemporary well depends on a broad range of geological and technological parameters, and it is impossible to break them down by the degree of the effect that they have. Nevertheless, three main components, i.e., reservoir quality, drilling quality and completion quality are normally identified, which determine the productivity of horizontal wells with multifrac. In this case, the reservoir quality is understood as a set of parameters that characterizes the properties of a reservoir in its natural mode of occurrence. Drilling quality shows how the well design and its drilling in the reservoir comply with the key objectives for ensuring efficient reservoir stimulation and achieving maximum productivity in the natural mode of the available reservoir. Completion quality means a set of parameters that characterize the efficiency of the completed actions to prepare a well for HC production. Completion includes, inter alia, a package of HFC jobs.
Therefore, one can formulate the key objectives to estimate potential productivity of natural reservoirs in their development conditions using hydraulic fracturing; what should be done:
Estimate the resource base of a potential development target and identify the share of the reserves technologically available for the implemented production technology;
Characterize the susceptibility of the potential target to technology-related stimulation using HFC;
Justify the optimized position of a horizontal borehole in section based on the requirements applicable to the geometry of the stimulated reservoir volume (SRV);
Ensure that the horizontal borehole is drilled in the pre-set target interval;
Define the completion design and make HFC design calculations for it;
Carry out monitoring to identify the actual SRV geometry and analyze it for compliance with the planned design;
Carry out effiency analysis of the stimulation in terms of well operation and depletion of the resource base;
Based on the performed work, prepare a corrective action plan for efficiency improvement of the stimulation.
Following the logic of three main components that determine the productivity of horizontal wells with multifrac, the listed objectives may be broken into the following groups:
Nos. 1 and 2 refer to reservoir quality (RQ);
Nos. 2-4 are concerned with the assurance of relevant drilling quality (DQ);
Nos. 5-8 are directly related to completion quality (CQ).
The above objectives outline the agenda of a new area in petroleum engineering, which may be called the "Petrophysics of artificial reservoir". It is suggested in this manuscript to highlight possible methods to address those issues and to use cases to test the efficiency of those approaches.