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Ying-Qiong Basin in the west of South China Sea contains plenty of abnormal high-pressure gas reservoirs, whose stress sensitivity is crucial for well productivity. To explore the influence of stress sensitivity on production, the variable outlet back pressure stress sensitivity experiments were applied to test core sample permeability under different burden pressure and obtain the relational expression of power function of core stress sensitivity. Afterwards, new productivity equation is deduced in consideration of reservoir stress sensitivity, and the affection of stress sensitivity on production is analyzed. The result demonstrates close link between stress sensitivity and productivity, since single well production decreases dramatically when reservoir stress sensitivity has been taken into account. This research is constructive for well-testing data interpretation in stress sensitive gas reservoirs.

Stress sensitivity refers to the phenomenon that permeability changes along with the effective stress in the production process of oil or gas pay. Abundant experiments showed that high-pressure gas reservoirs, especially those of low-per- meability have stress sensitivity [

Utilize HYKS-III high-temperature burden pressure porosity and permeability measuring instrument to conduct variable outlet back pressure stress sensitivity experiments and take core samples from the third phase of Meishan Formation in LS13A gas field.

To combine experimental result with theory, core stress sensitivity index is calculated to evaluate the stress sensitivity degree [

Integrate the expression above:

γ is the stress sensitivity index, M·Pa^{−1}.

Take natural logarithm of 1, the result is:

Treat

According to experiment data, stress sensitivity index-γ of each core shall be calculated by least square method.

Use stress sensitivity experiment data to calculate

The basic assumptions are: a vertical well in an isothermal, infinite, horizontal, isotropy gas reservoir with seal on top and bottom, that has stable thickness-h, initial pressure-p_{i}, slightly compressible fluid with comprehensive compress index-C_{t} and gas viscosity-μ. Single phase seepage is taken into consideration while gravity and capillary force are omitted.

From Darcy model:

In terms of deviated well productivity calculation, skin coefficient is introduced into the equation, which is:

Integrate Expression (3.1):

Expression (3.3) is deviated well productivity equation under Darcy model without consideration of stress sensitivity.

While in high-speed non-Darcy condition:

Expression (3.4) is deviated well productivity equation considering high-speed non-Darcy effect without consideration of stress sensitivity.

Reorganize Expression (3.4) to form the Darcy equation considering stress sensitivity:

Redefine pseudo-pressure function:

Integrate Expression (3.5):

Expression (3.6) is deviated well productivity equation considering stress sensitivity.

Take high-speed non-Darcy affection into consideration:

Expression (3.7) is deviated well productivity equation considering high-speed non-Darcy effect and stress sensitivity.

Productivity analysis considering stress sensitivity.

In

The geological parameters of LS13A gas field are formation pressure-65 MPa, average porosity-10%, permeability-0.5mD, effective thickness of gas pay-10 m and measured stress sensitivity index-0.0247. The non-Darcy flow index calculated by Chen Qianyuan formula is 0.039172 × 10^{−4} MPa^{−1}. ^{4} m^{3}/d, while that considering stress sensitivity is 8 × 10^{4} m^{3}/d. The deviation reaches 42.28%, which affects reservoir evaluation dramatically.

1) The variable outlet back pressure stress sensitivity experiment method can measure core stress sensitivity index effectively and appraise core stress sensitivity.

2) Stress sensitivity is non-negligible in abnormal high-pressure gas reservoir and has greatly affected the accuracy of single well productivity evaluation. The

deviation with and without consideration of stress sensitivity is as large as 42.28% in Well LS13A of Ying-Qiong Basin.

Zhu, Q. and Liang, H. (2017) Productivity Analysis Method of Abnormal High-Pressure Gas Reservoir in Ying-Qiong Basin. Natural Resources, 8, 410-415. https://doi.org/10.4236/nr.2017.86026