Tight Gas Reserve Calculation with Startup Pressure Gradient
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Solution Overview
Problem
Low-permeability and tight gas reservoirs with low porosity, high heterogeneity, and special seepage phenomena such as startup pressure gradient and stress sensitivity pose challenges in accurately calculating single-well controlled reserves and residual gas distribution, leading to reduced forecast accuracy and significant production decline in Block D1.
Innovation Solution
A method is developed to calculate single-well controlled reserves in low-permeability/tight gas reservoirs by considering the effects of startup pressure gradient and stress sensitivity, using a combination of material balance and modern production decline analysis methods, including Fetkovich, Blasingame, and Agarwal-Gardner type curves, along with a new differential equation that accounts for slippage effects and permeability stress sensitivity, to provide a more accurate dynamic reserve calculation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional reserve calculation methods are used in low-permeability/tight gas reservoirs, then calculation simplicity is maintained, but measurement precision and forecast accuracy deteriorate due to ignoring startup pressure gradient and stress sensitivity effects
Solution Approach 1:
The patent modifies the conventional material balance equation by introducing correction parameters for startup pressure gradient (parameter a) and stress sensitivity (parameter b). These parameters transform the standard equation into a corrected version that accounts for low-permeability reservoir characteristics, thereby improving calculation accuracy without completely redesigning the methodology
Solution Approach 2:
The patent introduces intermediate correction factors that mediate between conventional calculation methods and the complex physical reality of low-permeability reservoirs. These intermediary parameters allow the integration of startup pressure gradient and stress sensitivity effects into the existing material balance framework, bridging the gap between simplicity and accuracy
2Productivity
If formation pressure is reduced to increase gas production, then productivity improves, but loss of energy increases due to rapid pressure decline and formation energy loss
Solution Approach 1:
The patent establishes a feedback mechanism by using the corrected material balance equation to continuously monitor formation pressure changes and their impact on reserve calculations. This allows operators to assess the energy loss associated with pressure reduction and adjust production strategies to optimize the balance between productivity and energy conservation
Solution Approach 2:
The corrected reserve calculation method provides beforehand cushioning by accurately predicting the relationship between pressure decline and remaining reserves. This enables operators to plan production strategies that avoid excessive pressure reduction, thereby cushioning against unnecessary formation energy loss before it occurs
3Productivity
If production decline is accepted as inevitable in low-permeability reservoirs, then operational simplicity is maintained, but productivity deteriorates due to rapid entry into decline period
Solution Approach 1:
The patent applies preliminary action by using the corrected material balance equation to predict future production decline trends before they occur. This allows operators to take advance measures, such as optimizing well placement, adjusting production rates, or implementing pressure maintenance strategies, to mitigate the inevitable decline and extend the productive life of the reservoir
Solution Approach 2:
The patent introduces dynamics by making the reserve calculation method adaptive to changing production conditions. The corrected equation can dynamically adjust to different pressure regimes and production rates, enabling operators to optimize productivity at each stage of reservoir development rather than relying on static conventional methods
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method provides a more accurate calculation of single-well controlled dynamic reserves, with the reserve in Block D1 totaling 30.083×10^8 m3, demonstrating the significance of considering startup pressure gradient and stress sensitivity, and offering a practical approach for exploiting potential residual gas.
Implementation Method 1
Gas molecules at a tube wall are not completely stationary; instead, some of them are still active. Gas molecules between adjacent layers exchange momentum, and they directionally flow along the tube wall together with those at the tube wall, thus forming the so-called gas slippage.
Implementation Method 2
The strata have special seepage phenomena such as startup pressure gradient and stress sensitivity
Implementation Method 3
The strata have special seepage phenomena such as startup pressure gradient and stress sensitivity
Data Source
AI summary
The present invention belongs to the technical field of oil and gas field development, and discloses a method for calculating a single-well controlled reserve of a low-permeability/tight gas reservoir and analyzing residual gas thereof. The method includes: calculating a reserve controlled by each gas well in a block by using a gas reservoir dynamic reserve calculation method; establishing a new reserve calculation formula for solution and comparative analysis by an example; and quantitatively analyzing an effect of a startup pressure gradient and a stress sensitivity on a calculation result of the single-well controlled reserve, wherein the analysis of the factors affecting reserve calculation shows that, when the startup pressure gradient reaches 0.02 MPa/m, the calculated reserve is significantly reduced compared with a conventional method, but when the startup pressure gradient is greater than 0.1 MPa/m, the effect gradually stabilizes.