Abstract
There is high uncertainty in reserve estimation during the early development of deep ultrahigh pressure gas reservoirs, largely because it remains challenging in accurately determining the formation compressibility. To overcome this, starting from the definition of compressibility, a novel gas production of cumulative unit pressure drop analysis method was established, of which the effectiveness was proven by applications in calculating the reserves of three gas reservoirs. It has been found that, in the limiting case, i.e., when the formation pressure dropped to the normal atmospheric pressure, the dimensionless gas production of the cumulative unit pressure drop was the reciprocal of the initial formation pressure. Besides, the relationship curve of the dimensionless gas production of the cumulative unit pressure drop and pressure drop was a straight line in the medium term, extending the straight line and intersecting the vertical line passing through the original formation pressure point, and the reserves can be determined according to the intersection point and the initial formation pressure. However, due to the influence of natural gas properties, the value needs further correction, and the correction coefficient depends on the pseudocritical temperature of natural gas. Specifically, when the pseudocritical temperature is given, the correction coefficient would be close to the minimum value of the natural gas deviation factor. When the pseudocritical temperature is more than 1.9 and less than 3.0, the minimum deviation factor would be between 0.90 and 1.0, and the higher the pseudocritical temperature, the closer the ratio is to 1.0.
Highlights
The proportion of natural gas in primary energy consumption has been continuously increasing since the beginning of the 21st century, demonstrating its importance as a global strategic resource and a livelihood material
The relation curve of the cumulative pressure drop and the gas production of the cumulative unit pressure drop is given in Figure 4; linear regression is carried out for the data points, the straight line is prolonged to intersect with the vertical axis, and the intersection ordinate value is 0:31 × 108 m3/MPa, which is multiplied by the initial formation pressure 65.548 MPa, and the reserve is obtained as 20:29 × 108 m3
(1) Since it is difficult to accurately determine the formation compressibility in the material balance equation of high pressure gas reservoirs, the method without considering the formation compressibility is recommended for reserve estimation
Summary
The proportion of natural gas in primary energy consumption has been continuously increasing since the beginning of the 21st century, demonstrating its importance as a global strategic resource and a livelihood material. For high pressure and ultrahigh pressure gas reservoirs at the early stage of development, even if the production test time is up to 1 year and the drawdown scope reaches 3%-38% of the initial formation pressure or even higher, the starting point deviating from the early straight line segment would still not occur, and the starting condition of using the material balance method to calculate reserves cannot be met. This has provided new solutions to the reserve estimate problem of a high pressure gas reservoir when the starting condition is met in the middle and late stages of development. To bridge the gap, starting with the definition of formation compressibility and the material balance equation of volumetric gas reservoir, we established an analysis method for the reserve estimate based on gas production of cumulative unit pressure drop and evaluated its performance in three gas reservoirs
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