High Pressure Gas Wells Research Articles

Predictions on temperatures of high-pressure gas/water/MEG mixtures flowing through wellhead chokes

The co-existence of gas, water, and monoethylene glycol (MEG) is common in produced fluids of high-pressure gas wells. Accurate predictions on the temperature changes during the choking process are essential for the design and operation of the choke valve. This paper presents an efficient multiphase isenthalpic flash method based on the cubic-plus-association equation of state (CPA EOS) to calculate the choke temperatures. In comparison with the traditional isenthalpic flash algorithm, this new method accounts for the self- and cross-association between polar water and MEG molecules, yielding more accurate enthalpy calculation results and multiphase component distributions for fluids containing water and MEG. The proposed model is validated by field test data with pressures from 8.68 MPa to 119.3 MPa. The average absolute deviations between the calculated choke temperatures and measured values are less than 1.6 °C even for vapor-liquid-aqueous three-phase mixtures at various pressures. Moreover, case studies show that accounting for the association between polar water/MEG molecules contributes to accurate predictions on choke temperatures. At high pressures, the CPA EOS tends to give higher choke temperatures in comparison with those calculated based on the traditional SRK-Peneloux EOS. In contrast, the CPA EOS tends to yield lower temperatures at low pressures.

Shelter-in-place risk assessment for high-pressure natural gas wells with hydrogen sulphide and its application in emergency management

This paper provides a risk assessment method of sheltering in-place for high-pressure natural gas wells with hydrogen sulphide. In this paper, the shelter-in-place risk is estimated by integrating the health consequences of an individual taking one kind of emergency response to the emergency orders of sheltering in place from the emergency decision makers and the probability of the corresponding emergency response action. The probability of the corresponding emergency response action in the proposed method is estimated through the accident probability analysis and the probability analysis of taking a certain response action. The health consequence estimation is based on air exchange rate test of the shelter buildings as well as accident consequence calculation. The evaluation of shelter-in-place risks based on “as low as reasonably practicable (ALARP)” guidelines was employed to provide suggestions for emergency management under both normal conditions and off normal conditions. A case study of risk assessment of sheltering in the local residential houses in Xuanhan County of Sichuan Province, China was taken as an example to illustrate the proposed risk assessment process of shelter-in-place and its application in the decision-making process for emergency management.

Simulated Calculation of Bullheading Method When the Well is Empty

In the case of drilling mud completely erupted out of wellbore in high pressure gas wells, a series of fluid flowing governing equations are established in the consideration of coupling relationship between gas in well bore and formation. The change in casing pressure and bottom hole pressure with time was numerically simulated during shut in and well killing process. The results show that casing pressure and bottom hole pressure can achieve stable value quickly after shut in. The casing pressure increases rapidly first and then decreases to zero in well killing process. The earlier a well killing is performed, the smaller the peak value of casing pressure will occur under the same kill rate. A high kill rate can generate a small peak value of casing pressure after the well killing starts.

Development of Keshen ultra-deep and ultra-high pressure gas reservoirs in the Kuqa foreland basin, Tarim Basin: Understanding and technical countermeasures

The Keshen Gas Field in the Kuqa foreland basin, Tarim Basin, is a rare fractured tight sandstone gas reservoir with ultra-depth and ultra-high pressure. During its pilot period of gas field development, the development effect is poor with a low development well success rate, a low utilization rate of production capacity and a rapid decline of gas well productivity. In view of these problems, development experiments and technological researches were carried out continuously after the geological characteristics of gas reservoirs, productivity control factors, reservoir connectivity, seepage characteristics, gas and water relations and water invasion laws were studied thoroughly. And consequently, the development countermeasures of “well placement in high position, moderate stimulation and early-stage drainage” were prepared, and five matching development technologies were formed, such as description technology of ultra-deep complex structures, well pattern optimization technology for fractured tight sandstone gas reservoirs, fracture net acid fracturing technology for fractured tight sandstone reservoirs, dynamic monitoring technology for ultra-deep and ultra-high pressure gas wells, wellbore integrity management and evaluation technology for high pressure gas wells. The following remarkable application results were achieved in the process of gas field development. First, the drilling depth error of the target formation drops from 125 m to less than 30 m. Second, the utilization rate of production capacity in the Keshen 8 Block reaches 100%. Third, the average absolute open flow rate is increased by 5 times to 273 × 104 m3/d from 50 × 104 m3/d before the stimulation. Fourth, safe and smooth production under high temperature and high pressure conditions is realized in the Keshen Gas Field. In conclusion, the successful and efficient development of the Keshen Gas Field provides experiences for the development of similar gas reservoirs at home and abroad, and its development countermeasures and matching technologies have important guidance and reference significance.

Dynamic risk analysis of hydrogen sulfide leakage for offshore natural gas wells in MPD phases

Leakage of high-pressure sour gas wells is one of many challenges for offshore drilling operations, which may cause serious consequences due to poisonous H2S gas diffusion in the platform with limited working space. This study presents a new method for dynamic risk analysis of H2S leakage in such sour gas fields during managed pressure drilling phases. This method can model the influence of uncertainty from accident probability and consequences, being reflected in failure rates and unmodeled factors. The accident cause-consequence analysis via BT modeling for H2S release is conducted, integrating dynamic characteristics with probability estimation based on the inference of dynamic Bayesian networks (DBNs). The individual risk under different consequence scenarios is performed by the DBN modeling as well as death probability prediction at key monitoring points dynamically. A case study focused on specific Chinese offshore wells is analyzed to demonstrate the feasibility of the proposed method. The results show that the vulnerable factors with higher values are worth being addressed for prevention. In addition, the tolerable duration in total risk with the upper bound is approximated from 4.5 to 15 min between individual risk values of 1.0E − 4 and 1.0E − 6, as well as the exposure time after 15 min deserves more attention in risk emergency management.

Stress corrosion crack evaluation of super 13Cr tubing in high-temperature and high-pressure gas wells

Abstract Super 13Cr is widely used for oil country tubular goods (OCTG) in wet CO2 environments with small amounts of H2S. This paper presents a case study of super 13Cr tubing that failed due to stress corrosion cracking (SCC). X-ray diffraction (XRD), energy dispersive spectrometry (EDS), scanning electron microscopy (SEM), optical microscopy (OM), and fluorescent magnetic particle inspection were used to investigate the failure mechanism. The results show that the inclusions, metallographic structure, yield strength, and tensile strength of the tubing meet the standard requirements. Furthermore, cracking analysis of the tubing shows that both secondary cracks propagating along grain boundaries and tree root branching were sunlight readable. Finally, failure was attributed to stress corrosion cracking (SCC). We concluded that the mechanism leading to stress corrosion cracking was corrosion product film breaking.

Prediction of sealed annular pressure between dual packers in HPHT deepwater wells

High-pressure and high-temperature (HPHT) deepwater oil and gas wells are under the influence of high-temperature formation fluid after putting into production. The thermal expansion pressure of sealed annular fluids caused by the annulus temperature change has large effect on the safety of strings among dual packers. This study presented a casing-cement sheath-formation model which take into account the effect of the sealed annulus temperature and volume changes between dual packers, established a displacement function of the sealed annular fluid, the free tubing and the cemented casing that based on the thermo-elastic mechanics, analysed the influence factors of sealed annular pressure, including the expansion coefficient and elasticity modulus of annular fluid, the wall thickness of casing and tubing stings. An example of a HPHT deepwater gas well was analysed. The results show that the sealed annular pressure has a positive linear correlation with the annulus temperature difference, and annular pressure buildup with the increase of thermal expansion coefficient and elasticity modulus of the annulus fluid, decreases with the casing wall thickness. To prevent the potential risk associated with annular pressure, high-compressibility and thermal-insulated materials could be developed to control sealed annulus thermal expansion pressure.

Application of precise MPD & pressure balance cementing technology

The precise managed pressure drilling (MPD) technology is mainly used to deal with the difficulties encountered when oil and gas open hole sections with multiple pressure systems and the strata with narrow safety density window are drilled through. If its liner cementing is carried out according to the conventional method, lost circulation is inevitable in the process of cementing while the displacement efficiency of small-clearance liner cementing is satisfied. If the positive and inverse injection technology is adopted, the cementing quality cannot meet the requirements of later well test engineering of ultradeep wells. In this paper, the cementing operation of Ø114.3 mm liner in Well Longgang 70 which was drilled in the Jiange structure of the Sichuan Basin was taken as an example to explore the application of the cementing technology based on the precise MPD and pressure balancing method to the cementing of long open-hole sections (as long as 859 m) with both high and low pressures running through multiple reservoirs. On the one hand, the technical measures were taken specifically to ensure the annulus filling efficiency of slurry and the pressure balance in the whole process of cementing. And on the other hand, the annulus pressure balance was precisely controlled by virtue of precise MPD devices and by injecting heavy weight drilling fluids through central pipes, and thus the wellbore pressure was kept steady in the whole process of cementing in the strata with narrow safety density window. It is indicated that Ø114.3 mm liner cementing in this well is good with qualified pressure tests and no channeling emerges at a funnel during the staged density reduction. It is concluded that this method can enhance the liner cementing quality of complex ultradeep gas wells and improve the wellbore conditions for the later safe well tests of high-pressure gas wells.

莺琼盆地高温高压井气测录井特征研究

莺琼盆地高温高压井气测录井特征研究

Analysis of the wellhead growth in HPHT gas wells considering the multiple annuli pressure during production

In high-temperature and high-pressure (HPHT) gas wells, the wellhead growth caused by temperature and pressure effects during production might damage the well integrity. A calculation model of the wellhead growth produced by temperature and pressure effects was built. For the case well, the maximum pressures of annulus A, B and C are 64 MPa, 48 MPa and 38 MPa, respectively. The maximum production and intervention time are 114.5 × 104 m3/d and 540 d, respectively. Based on the calculation process, the maximum wellhead growth is 412.7 mm. The axial load caused by the multiple annuli pressure is second only to that caused by the casing axial temperature difference. Wellhead growth increases with the annulus fluid thermal expansion coefficient and decreases with the annulus fluid isothermal compression coefficient. The increasing annulus temperature difference might aggravate the effect of annulus fluid thermal properties on the wellhead growth. Selecting the casing with greater wall thickness and lower thermal expansion coefficient can reduce the wellhead growth. The annulus width has little effect on the wellhead growth while the annulus length will significantly change the wellhead growth. The wellbore multiple annuli pressure can increase the wellhead growth prominently. The annulus pressure management shall be introduced into the production. Optimizing the well structure and production plan and installing the wellhead monitoring equipment contribute to mitigating the wellhead growth.

Ductile Rupture Pressure Prediction for Capped-Open Casing and Tubing Under Combined Loads

Casing and tubing is widely used as protective conduits during all the phases of operations and productions for the oil and gas industry. Recently, casing and tubing burst failure accidents often take place in high pressure and high temperature (HPHT) oil and gas wells during production. Therefore, it is very important to accurately predict casing and tubing bust strength in the casing and tubing design and operation process. PD CEN ISO-TR 10400 presents the ductile rupture model for capped-end conditions, but capped-end casing and tubing applied in oil fields is few. For this case, this document establishes the ductile rupture model for capped-open conditions under combined loads on the base of PD CEN ISO-TR 10400. Numerical and experimental comparisons show that the ductile rupture model for capped-open conditions under combined loads prediction values essentially coincides with burst data provided by PD CEN ISO-TR 10400.

Sealed annulus thermal expansion pressure mechanical calculation method and application among multiple packers in HPHT gas wells

For high-pressure and high-temperature (HPHT) gas wells with high production rate, the annulus expansion pressure caused by the change in annulus temperature and pressure creates large effect on the mechanical behavior and safety of strings among multiple packers. Based on the laws of the conservation of momentum and energy and the transient heat transfer property between the wellbore fluid and the annulus fluid, a calculation model of the temperature and pressure fields on single-layer and multi-layer annuli was established. For the completion strings with multiple packers, the calculation model of the sealed annulus thermal expansion pressure among multiple packers was established with an overall consideration of the effect of sealed annulus temperature and the effect of volume changing. An example of a HPHT gas well in Sichuan province of China was analyzed, The results show that the thermal expansion pressure of the tubing-production casing annulus is minimized and the thermal expansion pressure of the intermediate-surface casing annulus is maximized; The relationship between the sealed annulus thermal expansion pressure and the temperature difference is linear; Pressure increment caused by the temperature effect plays the leading role, and the construction rate of the volume changing effect for the sealed annulus thermal expansion pressure increases along with the increase in temperature; Based on the safety evaluation, the maximum production of the sample well is 286 × 104 m3/d, when the production is 175 × 104 m3/d, the maximum setting space between dual packers is 258 m. Releasing pressure of each annulus in the process of production and testing can reduce the annulus thermal expansion pressure, the analysis of the sealed annulus thermal expansion effect among multiple packers is of great significance to evaluate the safety of strings, optimize the completion plan and establish a reasonable production system.

An Entrained-Droplet Model for Prediction of Minimum Flow Rate for the Continuous Removal of Liquids From Gas Wells

Summary Experimental studies show that liquid drop is deformed from initial spherical shape into ellipsoid shape in annular-mist flow, and the available critical Weber number WeCrit determined by the experiment can vary from 2.2 to 60 for low-viscosity liquid. On the basis of the force equilibrium and the critical-Weber-number-calculation method proposed by Azzopardi (1985), this paper develops a new model to predict minimum gas rate. This model introduces a parameter Ck,Wecrit that describes the effect of liquid-drop deformation and the maximum drop-size difference on the minimum gas rate. The effect of liquid-droplet coalescence is also considered indirectly. A function to predict drop-deformation magnitude for different critical Weber numbers is developed on the basis of energy conservation. The function-prediction results are in good agreement with experimental data from the literature and the predicted result from the drop deformation/breakup model, and the average absolute deviation is 6.1%. The Ck,Wecrit calculated by the new model increases with the increase of the pressure and liquid amount and it varies from 3.99 to 7.3, which means the critical gas velocity increases with the increase of the pressure and liquid amount. Numerous gas-well data were used for the validation of these entrained models, including data from 33 low-pressure gas wells (wellhead pressure: 0.26–3.41 MPa) from Coleman et al. (1991) and 91 high-pressure gas wells (wellhead pressure: 0.7–56 MPa) from Turner et al. (1969). The result shows the new entrained model has a good comprehensive performance in judging liquid-loading status in both high- and low-pressure gas wells.

Choked Gas Flow at Pore-Scale and Its Implications to Production From High-Pressure Gas Wells

Production from a high-pressure gas well at a high production rate encounters the risk of wellbore tensile failure when the pressure gradient of the averaged gas flow becomes large. At the pore-scale, however, when flow in just one pore is choked, gas pressure gradient at the point of choking becomes singular, leading to an unbounded average of the pressure gradient. This study investigates the choking condition for compressible gas flow in a single pore. It is found that wellbore tensile failure can occur at a much lower inlet-to-outlet pressure ratio than predicted from the macroscopic theory of porous medium flow.

Prediction of sustained annular pressure and the pressure control measures for high pressure gas wells

Abstract The sustained annular pressure caused by channeling threatens the casing safety significantly. Cement mantle composite permeability and one-dimension unstable seepage were used to describe the channeling of high pressure gas based on the analysis of the structure of the cement mantle containing micro-fractures and the seepage process of high pressure gas. According to the mass conservation law and volume accordance principle, a model was built to predict and analyze the pressure rising process when gas invades into the trapped annulus, and then this model was used to study the impacts of different factors on the annular pressure. The pressure rising process can be divided into rapid rising stage and stable rising stage, and the factors affecting annular pressure include the gas solubility and compressibility of annular liquid, height of cement return, composite permeability of cement mantle and annular volume; the rising velocity of annular pressure declines with the increase of annular liquid compressibility; the ultimate value and rising velocity of annular pressure increase with the increase of cement return height; the higher the composite permeability of the cement mantle, the faster the annular pressure increase; the increase of annular volume can prolong the rising time of pressure. From the viewpoint of engineering, high pressure gas wells should take annular liquid with proper compressibility, self-repairing cement should be used in high pressure gas wells to improve cement quality, the composite permeability of cement mantle should be reduced by using proper measures if necessary, and annulus volume is properly increased to prevent the pressure from rising up too quickly.

A new approach to risk control of gas kick in high-pressure sour gas wells

During drilling in high-pressure sour gas well, the occurrence of gas kick may cause serious consequences. Generally, in order to prevent gas kick, higher density of drilling fluid is adopted to keep adequate positive pressure difference in hole bottom. The requirement of positive pressure difference in China is 5 MPa. However, due to the narrow mud weight window in most high-pressure sour gas well, this method could not keep both equivalent static density and equivalent circulation density within the safe window. The lower density of drilling fluid could not ensure hydrostatic pressure higher than a pore pressure of 5 MPa; while higher density may cause leakage and therefore trigger blowout. In this paper, an improved approach is proposed to solve this problem, which involves bottom-hole pressure control methods and necessary equipment. A case study shows that the conventional method cannot develop a reasonable mud density while the improved method can ensure a bottom-hole pressure within the narrow pressure window. The results show that the improved method not only solves the problem of circulation loss that occurs in conventional drilling, but also meets the Chinese standards of well control in high-pressure sour gas wells, providing a new technique for the risk control of gas kick.

A critical production model for deep HT/HP gas wells

Annular pressure buildup always occurs in the production process in deep high temperature high pressure (HT/HP) gas wells. The high annular pressure causes leakage and induces severe accidents, especially in high-sulfur gas reservoirs. Annular pressure increases with the gas production rate. Therefore, it is of great significance to ensure that the production rate is under sustainable rate for deep HT/HP gas wells. However, the critical production rate is not presented in previous research. In this paper, for the security of tubing and production casing, a new critical production model for deep HT/HP gas wells is built, in which the thermal expansion effect and annular volume changing effect are taken into consideration and an iterative method is employed to calculate the annular temperature and pressure distributions by considering the influences of well structure, tubular string combination and fluid composition. Finally, the model is applied to two deep HT/HP gas wells in China. It is indicated that this model provides alternative perspective for production strategy for deep HT/HP gas wells.

Burst strength of tubing and casing based on twin shear unified strength theory.

The internal pressure strength of tubing and casing often cannot satisfy the design requirements in high pressure, high temperature and high H2S gas wells. Also, the practical safety coefficient of some wells is lower than the design standard according to the current API 5C3 standard, which brings some perplexity to the design. The ISO 10400: 2007 provides the model which can calculate the burst strength of tubing and casing better than API 5C3 standard, but the calculation accuracy is not desirable because about 50 percent predictive values are remarkably higher than real burst values. So, for the sake of improving strength design of tubing and casing, this paper deduces the plastic limit pressure of tubing and casing under internal pressure by applying the twin shear unified strength theory. According to the research of the influence rule of yield-to-tensile strength ratio and mechanical properties on the burst strength of tubing and casing, the more precise calculation model of tubing-casing's burst strength has been established with material hardening and intermediate principal stress. Numerical and experimental comparisons show that the new burst strength model is much closer to the real burst values than that of other models. The research results provide an important reference to optimize the tubing and casing design of deep and ultra-deep wells.

An individual risk assessment framework for high-pressure natural gas wells with hydrogen sulphide, applied to a case study in China

Accidents of high-pressure sour gas wells at the well drilling and completion stages may cause serious consequences when poisonous H2S gas diffuses with the diversity of terrain and wind etc., especially in China, where most of sour gas wells are located in mountainous areas with complicated geological conditions. This paper focuses on an assessment framework to explicitly calculate the individual risk of high-pressure sour gas wells at the well drilling and completion stages. The method is based on accident probability analysis by fault tree simulation as well as accident consequence analysis through building reasonable accident scenarios for H2S gas diffusion related to death probability within a given distance. The death probability curves of different wind directions and corresponding minimum proximity distances to the surrounding buildings could be provided by probabilistic risk analysis. Finally the individual risk contours could be mapped by concentration interpolation, and be compared with acceptable risk level. The case study in Chinese Sichuan–Chongqing region shows that the minimum proximity distances of different directions range from 600 to 1200m locating between individual risk values of 10−4 and 10−6. The zones within 1.2-km radius and northwest area deserve more attentions in the design and deployment of the protective measures.

Prediction of Temperature and Pressure Distribution in HTHP Injection Gas Wells with Thermal Effect of Wellbore

In this article, a coupled system model of differential equations was derived in radial form to predict the pressure, temperature of formation, and temperature of wellbore in high-temperature–high-pressure gas wells. In the model, the thermal effect of formation was simplified as heat conduction, both the steady heat transmission model in the gas wellbore and the unsteady heat conduction model in the formation were considered, and the pressure effect due to variations in temperature was considered according to the gas equation of state, which links the temperature with the pressure. The finite difference method was used to simulate the solutions of the coupled models. The basic data for X Well (high-temperature–high-pressure gas well, 7,100 m deep) in China were used for case history calculations and a sensitivity analysis was performed for the models. Graphs of the curves for gas pressure and temperature along the depth of the well were plotted with different gradients, injection volumes, and the characteristic of heat transfer in the formation. The results provide technical reliability in well test design in high-temperature–high-pressure gas wells and the dynamic analysis of production.