A Comprehensive Well Productivity Evaluation Approach and Sensitivity Analysis for a HPHT Ultra-deep Water Gas Reservoir

Well testing in ultra-deep water reservoirs is crucial for well productivity evaluation, whereas well testing time is very limited due to rig days concerns, safety and drillship well testing equipment constraint, leading to uncertainties in the well productivity evaluation. Generally, simplified methods such as the conventional binomial, exponential, and “one point method” are commonly applied to evaluate the well productivity. And the above-mentioned methods could bring in abnormal phenomena like poor correlation on regression curves, negative binomial coefficients, and indices greater than 1 when applied in a high-temperature and high-pressure (HPHT) ultra-deep water gas reservoir with trace condensate. Those phenomena lacking physical significance should be further investigated and mitigated to improve the well productivity estimation. First, the impact of different simplified forms of pseudo-pressure on the calculation of open flow rate of gas well was studied. The evaluation of gas well productivity was then conducted by comprehensively integrating test well interpretation, leveraging the pseudo-pressure method and well test parameter method. In addition, the impact of condensate oil content on gas well productivity was analyzed through sensitivity analysis. At present, the simplified calculation applicable standard of the “Technical Specification for Natural Gas Well Testing SY/T 5440-2009”, states that when the wellbore pressure is high (greater than 21 MPa), the pressure method can be used to calculate the open flow rate. The results indicate that the open flow rate obtained by the pressure square method is 7.2–18.8% smaller than that obtained by the pseudo pressure method, and the open flow rate obtained by the pressure method is 18.3–30.0% larger than that obtained by the pseudo-pressure method. Therefore, for ultra-deep water HPHT gas reservoirs with bottom hole pressure much greater than 21 MPa, the pseudo pressure method is currently the most suitable productivity evaluation method. After converting condensate oil to a corresponding amount of gas phase, the impact on open flow rate is small (less than 2%). Gas wells with large open flow rates are extremely sensitive to extrapolated pressure of well testing interpretation. Three-fold change in open flow rate can be caused by a 0.02 MPa pressure increase (upper pressure limit value). It is therefore concluded and recommended that future stable testing be conducted at even higher production rates if possible to evenly distribute pressure points in the deliverability curve to reduce pressure sensitivity. To sum up, the open flow rate obtained by the well testing parameter method is generally higher than that obtained by the pseudo-pressure method. The reasons for this could be attributed to the alternative interpretation characteristics of well test and the accuracy of production metering. The advantages of the well test parameter method are that it is less influenced by extrapolated pressure, and it can be used for calculation when there are no other available methods.