Reservoir safety, testing-string safety, and flow control are key factors that should be considered in deep-water unconsolidated sandstone gas well testing work system. Combined with the feature of testing reservoir, pipe string type and sea area, the required minimum testing flow rate during cleaning up process, as well as minimum test flow rate without hydrate generation, pipe string erosion critical production, the maximum testing flow rate without destroying sand formation and the minimum output of meeting the demand of development was analyzed; based on the above critical test flow rates, testing working system is designed. Field application showed that the designed work system effectively provided good guidance for field test operations; no sand production or hydrate generation happened during the test process; the test parameter evaluated the reservoir accurately; the safe and efficient test operation was achieved.
Oil and gas well testing is the most direct means to find and learn gas and oil reservoir in the process of oil and gas exploration and development, and also provide reliable data for oil and gas field development. This needs a very accurate, reliable test data of the oil and gas layer, so as to make the scientific evaluation of oil and gas reservoirs. Oil and gas well testing technology needs to optimize the open ways of the oil and gas layer and the test technology of the working system, and ultimately achieves the scientific understanding of reservoir and the optimization of oil and gas well production capacity [
Well LS-X located in Lingshui sag of the east in the deep water area of Qiongdongnan Basin, is made up of several relatively independent sand body structure and lithologic trap group (divided into A, B, C, D block), as shown in
only scattered in the high area of the structure from gas water distribution, having different gas water interface and the pressure system, and most are bottom water reservoir, a few are edge water gas reservoir, low water energy, the major drive type of gas reservoir is elastic drive, edge drive and bottom water drive.
Well LS-X-1 is located in the Block B of structure, all the gas horizon I, II, IV of Huangliu Formation in well LS-X-1 show good evidences of oil and gas with the depth of abnormal logging sandstone up to 64.0 m and the depth of logging interpretation gas horizons up to 53.4 m. (39.1 m in the gas group 1, did not drill in the gas water interface) For acquisition of the reservoir parameter of physical property, deliverability and liquid in the structure LS-X-1, and providing basis for the development of the tarp group and the next step exploration, testing operation is preceded in gas horizon Ibotton (3321.0 - 3351.0 m) which did not drill in the gas water interface.
To avoid damage on formations when testing, besides co-harmonization with different production purpose, producing method and supply and demand relations, proper testing flow should also combine with the feature of the reservoir to meet the requirements as follows: avoiding damage on down hole and reservoirs, deformation on reservoirs and mass sand production in testing wells; no gas hydrate generating when testing; testing flow with enough liquid carrying capacity.
In the formula:
Well cleanout should be done quickly and the liquid loading (testing liquid and cush) in bore holes should be blowing off in the initial test, and the minimum air-speed needed is:
In the formula: σ―surface tension of blew off liquid, mN/m;
Though the cross section area of test strings is A, the minimum flow needed to blow off the liquid loading is:
And the formula of surface tension is:
With the formation pressure of testing section being 39.08 Mpa, temperature being 77˚C, density of liquid loading being 1300 kg/m3, relative density of gas being 0.6636, radius of testing strings being 0.0428 m and gas deviation factor being 0.98, the minimum testing flow needed by carrier liquid is computed to be 1.93 × 104 m3/d.
The generation of hydrate has important influence on the success of the deep high permeability gas well test, it has been lots of research about the hydrate formation scale in the deep well testing, the hydrate formation area in the test string is forecasted by using the existing model.
The minimum critical testing pressure difference which leads to sand production in reservoir and reservoir damage is presented as follows:
In the formula:
As showed in
Using test string in the distribution of temperature, pressure, the velocity distribution within the wellbore and the critical erosion speed can be further calculated, as shown in Figures 1-5. In the 0.2 million∙m3/day - 1.6 million∙m3/day production circumstances, the wellbore flow are less than the critical erosion speed, erosion will not occur. When production reaches 2 million∙m3/day, the fluid velocity in reversing valve and pressure valve is greater than the critical speed of erosion, erosion occurs.
The velocity in the small string diameter area is high when testing, and it is in the risk of eroding, in no more than testing flow of 160 × 104 m3/d, there is no risk of erosion inside testing string, and safe range is larger, can satisfy the test requirement.
Production of meeting the requirement of the development in the internal evaluation is 120 - 160 × 104 m3/d.
Based on the analysis, the critical testing flow is 10.75 × 104 m3/d, the critical gas flow without hydrate is 25 × 104 m3/d, the critical gas flow with sanding production is 162 ×
Working system | Choke size (mm) | Gas flow (104 m3/d) | Yield time (h) |
---|---|---|---|
Initial open | 14.29 | 70 | 10 |
9.53 | 40 | 7 | |
19.05 | 110 | 7 | |
23.81 | 150 | 7 |
104 m3/d, the output that satisfies the development needs in the internal evaluation is 120 - 160 × 104 m3/d. Thus the production testing working system is designed as is shown in
After reservoirs in well LS-X-1 are perforated, hydrate proofing methanol is injected into the testing system before chock manifold respectively under the mud surface, on the mud surface and above the earth surface, and the adjustable bean is opened to put through quick well cleanout, which restrains the generation of hydrate effectively. Then beans with radius of 12.70 mm, 9.53 mm, 19.05 mm and 25.40 mm are adopted to compute production, and as showed in
Data interpretation indicates that the effective permeability of gas horizon is 565 mD, total skin factor is 0.78 and wellbore storage coefficient is 0.0488 m3/Mpa, combined with geological understanding, the gas and water boundary is about 1200 m and the lithologic boundary is about 880 m and 940 m, which can clearly block out the gas range of testing reservoir.
Combined with the feature of pay zone and testing pipe string, the required minimum testing flow rate during cleaning up process was analyzed; afterwards temperature- pressure field model was established for testing process; minimum test flow rate required to prevent hydrate generation was calculated; and then the maximum test flow rate that before sanding was determined according to logging data; finally working system for LS-X-1 was confirmed based on those critical flow rates. Field application shows that the designed work system effectively provides good guidance for field test operations and has reached the testing purpose.
Wu, M.W., Liang, H., Zhang, M.J., Sun, D.Q. and Zhong, P. (2016) Productivity Testing Design Method of Multi-Factor Control for Unconsolidated Sandstone Gas Reservoir. Engineering, 8, 815- 822. http://dx.doi.org/10.4236/eng.2016.811073