A novel method for calculating the dynamic reserves of tight gas wells considering stress sensitivity is under consideration.
Authors: Chen Chang-Hao , Luo Jing , Cao Jian , Liang Jing , Cai Lin-Feng , Zhu Tan-Yin 
Published: June 2024

The accuracy of dynamic reserve calculation of gas storage is of great
significance for the rational and efficient development of oil and gas
fields and the system of gas well allocation. The reservoir in the Shaxi
Temple gas field in the ZT block of northwestern Sichuan is tight and
highly heterogeneous, with complex gas-water distribution and large
differences in gas well test production rates, resulting in rapid
decline rates. In order to accurately calculate the dynamic reserves of
such tight, water-bearing gas wells, this study takes the water-bearing
tight gas reservoirs in the ZT block of northwestern Sichuan as the
research object. Experimental methods such as rock X-ray diffraction,
constant rate mercury injection, and reservoir rock stress sensitivity
were conducted. Based on the experimental results, the porosity and
permeability of rock samples under effective stress conditions were
corrected using bivariate linear regression. These corrected parameters
were then incorporated into the water-gas material balance method to
establish a new method for calculating dynamic reserves of water-bearing
tight gas reservoirs under stress-sensitive conditions. The results show
that: (1) the rock porosity ranges from 6.08% to 10.22%, permeability
ranges from 0.035mD to 0.547mD, clay mineral content ranges from 6.58%
to 19.14%, pore radius distribution ranges from 90μm to 180μm, throat
radius distribution ranges from 0.61μm to 3.41μm, with significant
differences in throat distribution, indicating poor reservoir fluid flow
capacity and strong tightness; (2) after aging experiments, rock samples
exhibit plastic deformation, with porosity and permeability unable to
fully recover after pressure relief. The stress sensitivity curve of
rock samples shows a two-stage characteristic, with moderate to strong
stress sensitivity; (3) porosity stress sensitivity is mainly influenced
by pore radius and mineral composition - larger pore radius and higher
clay content lead to stronger stress sensitivity, with porosity loss
rates ranging from 8.26% to 23.69%. Permeability stress sensitivity is
mainly influenced by throat radius and mineral composition - smaller
throat radius and higher clay content result in stronger stress
sensitivity, with permeability loss rates ranging from 47.91% to
62.03%; (4) a comparative analysis between the traditional dynamic
reserve calculation method for gas wells and the new method considering
stress sensitivity shows a relative error between 0.90% and 2.41%,
with the new method demonstrating better accuracy. This study combines
physical experimental results with an effective stress model of
reservoir rocks to develop a new method for calculating dynamic reserves
of water-bearing tight gas reservoirs under effective stress conditions,
providing experimental data and example calculation results to support
subsequent dynamic evaluation of gas reservoirs and the establishment of
rational well allocation plans.