Development of technology for evaluation of residual gas saturation in watered gas wells on the basis of multi-probe neutron logging

Determination of current reservoir gas saturations for the purposes of evaluation of the uniformity of reserve recovery across the reservoir body, and estimation of gas reserves remaining in the watered pay zone for a feasibility study on extraction thereof, constitutes one of the urgent tasks for gas fields, especially mature ones. This objective became particularly pressing due to introduction from January 1 2016 of a new Classification of reserves and Resources of Oil and Combustible Gases that mainly features an economic assessment of the development of hydrocarbon reserves, performed with varying degree of elaboration subject to the field exploration stage. The article describes the results of further development of the innovative technology of near-wellbore area logging at various depths with the use of SNGK and NNL logging methods under geological and technical conditions of mature gas wells.
Materials and methods
Procedural guidelines and software for MSNL (Multi-Sonde Neutron Logging) geological and geophysical interpretation technology were substantiated, developed and implemented on the basis of field and mathematical simulation with the use of basic petrophysical relationships among reservoir gas saturation and analytical parameters derived on the basis of neutron methods, field pilot/production logging (PPL) data.
Results
The presented results of conducted pilot/production and research/development works evidenced that the MSNL technology had proven to be very informative for determining reservoir residual gas saturation in wide range of varying geological and technical conditions of gas wells. The distribution of gas saturation (Kg and Kp*Kg) at various distance from bore hole walls shall be determined on the basis of MSNL logging data obtained in the reservoir near-wellbore area, i.e. through the logging of reservoir near-wellbore area at various depths; it provides a diagnostic criterion for identification of gas-saturated reservoirs in the watered portion of pay zones and may be used for subsequent evaluation of residual gas saturation.
Сonclusions
The MSNL technology has been developed in Russia and has no Russian of this foreign analogues. In this connection it was recommended by the Expert-Technical Council of the Federal State-Funded Institution «State Commission on Mineral Reserves» for deployment, and it may be used for achieving a wide range of geological-field objectives, specific for a mature production field, more specifically: identification and determination of gas saturation of shaly and hydrophilic reservoirs; determination of residual gas saturation of hydrophilic reservoirs; forecasting fluid dynamics in the reservoir near-wellbore area, including perforated interval. The possibility of reservoir near-wellbore area logging with determination of gas saturation values (Kg, Kp*Kg) at various distance from bore hole walls is the basic differential advantage of the technology. The magnitude and distribution of calculated values of gas saturation in the radial direction would serve as a basis for geological and geophysical interpretation in determination of reservoir saturation. The technology is informative at various backfilling in the context of multistring well program. The technology allows to determine calcium and silicium content, and, subject to sequential interpretation of the open hole logging suite data, enhance the quality of three-dimensional model of geological section, or, in the absence of a logging suite data, it would allow to develop a model on the basis of the results of processing of 3SNGK+ 2NNLt + NGL suite data.