Safety Control Technology of Deepwater Perforated Gas Well Testing

Due to the high difficulties, high investment, and high risks in deepwater oil and gas well testing, major safety problems can occur easily. A key to prevent accidents is to conduct safety assessment and control on deepwater testing and to improve the testing technology. The deepwater of the South China Sea has some special environmental features: long distance from offshore, frequent typhoons in summer and constant monsoons in winter, and the presence of sandy slopes, sandy ridges and internal waves, coupled with the complex properties of oil and gas reserves which bring more challenges to deepwater well testing. In combination with deepwater well testing practice in the South China Sea, this paper analyzes the main potential risks in deepwater well testing and concludes that there are risks of failures of testing string, tools, and ground processes. Other risks are gas hydrate blockage, reservoir stratum sanding, and typhoon impacts. Specific precautions are also proposed in response to these risks in the paper.


Introduction
The deepwater zone in the South China Sea which is called "the second Person Gulf" is vast, rich of petroleum and natural gas hydrate reserves [1] [2].A number of significant discoveries of natural gas reserves have been achieved recently [3] [4].As an irreplaceable tool in deepwater oil and gas exploration and production, well testing not only provides valuable information for structure and trap evaluation, but also paves the way for effective oil and gas production.
However, deepwater well testing technology in China is in its infancy stage currently [5] [6] [7] [8].In order to meet the strategic needs of deepwater oil and H. Liang, M. W. Wu DOI: 10.4236/eng.2019.113011132 Engineering gas exploitation in the South China Sea, safety assessment and control technique of deepwater well testing is investigated.

Challenges of Deepwater Well Testing
1) Deepwater well testing should be accomplished with floating drilling platform.Affected by factors like wind, wave, and current, the platform is under constant complex movements such as rising, sinking, and drifting.Besides, the deepwater well testing string should also be constrained by the riser.As a result, the forces on the test string, especially that above the mud line, is extraordinarily complicated, which has brought with significant difficulty for its design and safety control.This challenge is even tougher as water depth increases.
2) The combination of low temperature at the mud line and the rapid drop of pressure once well shut-in is the major lure for natural gas hydrate, which will bring with not only failure to the test, but also dramatic risks to the well control, or even catastrophic accidents.
3) Under water facilities should work against the tough surroundings brought by the great water depth to ensure the entire well testing process is upon reliable foundations.Moreover, other factors including the limited space of the platform, the narrow window of formation pressure, high production rate and high formation pressure, will also add the challenges to well control and surface safety control.
4) As the test is conducted on a floating platform, unpredictable incidents like the breakdown of positioning system, undercurrents, and bad weather, will make the platform drift away from the well head.Under this situation, test string above the mud line should be dismissed from the rest in order to avoid a catastrophe.Consequently, the fast dismission of the string under emergency and the re-connection after that are other challenges in well testing.

Failure and Its Prevention of Testing String and Facilities
The test string, from which the underground fluid flow to the surface, is composed with three major components: bottom-hole test facilities, testing tubing, and under water facilities.For safety purpose, suitable bottom-hole test facilities should be selected.Besides, the test tubing should be optimized through specific mechanical analysis with the target of safety and high quality.It should not merely satisfy the demand of test process in the toughest environment, but also be convenient in use, suitable in material and economical.According to such principles, the optimization workflow of the tubing design in deepwater testing is shown as following (showing in Figure 1).

Failure and Its Prevention of Surface Process
A typical surface process of deepwater well testing includes: the flow head, safety chemicals is installed at the well head to inject methanol to prevent gas hydrate.
All the pressure vessels in deepwater well testing should be qualified with a third party qualification such as DNV, ABS or Lioyd according to their period of validity.Once the lectotype of facilities is finished, the workflow should be numerically simulated to check whether the temperature and pressure are in proper range, and the capability of the facilities is correct.Then the further optimization of the facilities and pipelines should be conducted.gas hydrate comes into being.Roughly, they can be classified as: graph approach, empirical equation approach, balance constant approach, and statistical thermodynamics approach which is the most accurate yet complicated method.On the basis of system theory, the statistical thermodynamics approach connects the macro phase behavior of gas hydrate with the inter-molecular reactions.By employing functions to describe the creation conditions of gas hydrate, this method is beneficial from the solid theoretical foundation and has a wide applicable range.With the help of computer, it can be applied to continuously calculate the temperature and pressure at which gas hydrate forms in a relatively wide range.

Prediction and Control of Natural Gas Hydrate
According to the classical absorption theory proposed by Vander Waals and Platteeuw, the phase balance conditions of gas hydrate can be applied to determine whether solid hydrate appears at given pressure, temperature and other conditions.As for well testing process, thermal or chemical methods are commonly used to prevent hydrate.The thermal method is applied to heat up the natural gas before choke.If the pressure drop at the choke keeps constant, to increase the temperature of the natural gas before choke means to increase the temperature of the gas after that.Once the temperature of the gas after choke is higher than the critical temperature at which hydrate appears, the target of hydrate prevention is achieved.
Some chemicals are helpful to decrease the balance temperature of gas hydrate.Under certain pressure, the balance pressure decreases as the chemical concentration rises.A lot of investigations have been conducted by researchers all over the world on the chemicals which can impede gas hydrate.Methanol and ethanediol are the most commonly used chemicals for this purpose, and the former is the recognized most effective gas hydrate inhibitor in deepwater well testing.It has the following advantages: 1) low viscosity, easy to injection and allocation; 2) high solubility and volatility, easy to contact with other fluids in the wellbore; 3) easy to be burned with the produced gas.But as methanol is toxic and highly flammable, the safety aspect should be paid more attention to.
The procedure of methanol injection should be clearly written in the Operation Procedure, and in the procedures of the testing company and the contractor.
Flow rate is another factor for gas hydrate, which is easier to appear at lower flow rate.Temperature change is violated at the mud line as the temperature at seabed is quite low.As is shown in Figure 2, No hydrate appears when the flow rate is 25 × 10 4 m 3 /d.

Sanding Risks and Control
Reservoirs in deepwater zones are generally shallow in buried depths.Furthermore, the compaction degree is decreased as a large interval of rock formations are substituted by sea water, whose density is less.As a result, sand production is common.In order for sand control, on one aspect, suitable well completion approaches should be adopted according to the specific property of the pay zone; on another aspect, sand control techniques should be applied whenever necessary.

Typhoon Prediction
Typhoon is a factor responsible for the excursion of the platform, which further affects the stress state of the pipe string above mud line.Once the excursion exceeds the limitation, the testing tree should be immediately dismissed to avoid the breaking of the string.According to the depth of water and the demands of the testing tree, the safety operation window of the floating platform can be calculated considering factors like the response time of the testing tree, response time of BOP, and drift analysis of the platform.When the platform drifts away

Figure 1 .
Figure 1.Optimization workflow of the string design in deepwater well testing.

Figure 2 .
Figure 2. Generated area of hydrate under different flow capacity.