During deep-water gas wells testing period, predicting the generating zone of hydrate precisely in the whole flow range (bore holes and surface flow lines) is the key prerequisite of guarantee for testing flow. The unusual deep-water environment and low gas density make it easy to meet the conditions of hydrate generation during deep- water gas wells testing period, such as low temperature and high pressure, especially under the testing conditions, for instance, near mud line, surface chock line, low gas generating rate and surface shut-in. Wherefore, in view of all the operational modes during testing period (clean-out, variable rate flow, downhole and surface shut-in), based on temperature field of the whole flow range and phase equilibrium condition of hydrate, predicting method of hydrate generation zone is published; by taking enthalpy as the object of study, temperature calculation model increases the accuracy of temperature prediction; by integration of bore holes and surface lines, predicting plate of hydrate generation in the whole flow range is published. During flow period, the generating condition of hydrate is affected by rate of flow, and the lower the rate of flow is, the wider the hydrate generation zone is; during the stage of shut-in and initial flow, if pressure of strings is higher and temperature is lower, risk of hydrate generation will be greater and hydrate generation zone will be larger, so relevant actions should be taken to restrain hydrate generation.
During deep-water testing period, prediction of hydrate generation zone is the prerequisite of prevention for hydrate and flow guarantee for the whole flow range [
Foreign scholars have given a more comprehensive exposition [
Taking factors which affect temperature distribution in bore holes into comprehensive consideration, including environmental temperature, wellbore structure, structure of testing strings, job condition and formation fluid property, traditional method only based on heat transmission is changed, considering choking effect and work by volume change during flow behavior of high speed gas, and taking enthalpy of compactible gas during flow behavior as subject in the study, then temperature field equation of bore holes for deep-water wells during testing progress is established.
Hole section below mud line:
In the formula:
Hole section above mud line:
In the formula:
Computing formula of enthalpy is:
In the formula: V―specific volume, m3/kg;
The above formula can be transformed into differential form:
Adopting the thermodynamic computing method of hydrate generation condition, according to Vander Waals model [
If alcohol depressant added, formula (5) is changed into:
In these formulas:
According to the computing method of temperature in oil tubes for deep-water testing, temperature and pressure distribution of Well X1 in different testing period are predicted. Well X1 is with operating water depth of 1447.20 m, mud surface temperature of 3˚C - 4˚C, reservoir porosity of 25.6%, water saturation of 31.1%, and so on; component analysis indicates 0.4% of its natural gas component being CO2, 89.961% of that is C1, 4.843% of that being C2, 2.23% of that being C3 and the rest of that being other component. Gas component model is adopted to compute PVT parameters under reservoir condition: volume factor of natural gas being 3.283 × 10−3 m3/m3, viscosity being 0.031 mPa∙s and compressibility coefficient being 0.0137 1/MPa.
Generating zone of hydrate under different condition of gas production rate is predicted in
With deliverability of 20 × 104 m3/d, generating zone of hydrate at different downhole shut-in time is showed in
being 20 × 104 m3/d), if no preventive action is taken, there may be generating hydrate within the 600m range below the surface of the water (intersection range of temperature of strings curve and hydrate generation temperature (phase state) curve), and that after downhole shut-in, though temperature of strings decreases gradually, pressure in testing strings decreases fast and the risk of hydrate generation is low (there is no generating zone of hydrate of strings after shut-in).
During the period of deep-water gas well test, surface shut-in can lead to serious problem of hydrate generation. Over shut-in time the temperature of strings will come into contact with environment temperature gradually, but the pressure keeps a higher value, and the zone of hydrate generation has changed gradually from the 1000 m range below the surface of the water to the 1950 m range which is 600 m below mud line. Once there is hydrate in a relatively long string, the consequences will be serious causing new difficulties for deep-water test due to hydrate being non-decomposable, so surface shut-in should be avoided (
1) A new temperature and pressure calculation model during deep-water gas well
testing has built up, and prediction method of hydrate form region at different construction stages in testing is obtained combining with the formation condition of hydrate.
2) Result shows that the surface shut-in period has the risk of hydrate generation, in which gas hydrate is easy to generate as the temperature of and inside strings gradually decreases to the seawater temperature.
3) During flow period, the lower the yield is, the higher the risk of hydrate generation is. At the stage of initial well startup, the high pressure and low temperature make it easy for gas hydrate to generate.
Yang, H.J., Hao, L. and Wu, M.W. (2016) Risk Analysis of Gas Hydrate Formation during Deepwater Gas Well Testing. International Journal of Geo- sciences, 7, 1057-1063. http://dx.doi.org/10.4236/ijg.2016.79080