Ultra-deep Gas Wells Research Articles

Factors Controlling Productivity – A Case Study on the Qixia Formation of the Shuangyushi Area, Sichuan Basin

The gas reservoir with high temperature and high pressure (HTHP) over 8,000 m is one of the carbonate gas reservoirs discovered in recent years in the Qixia Formation of the Shuangyushi area, NW Sichuan Basin, and the strong heterogeneity makes it difficult to deploy high-yield wells. The relationship between geology and productivity is given by using the research results of reservoir evaluation and statistical analysis of the dynamic analysis of trial production in the last 4 years in this paper. Then, the evaluation index system of high productivity controlling factors in the gas reservoir is given by using the fuzzy clustering analysis method. Finally, the relative importance of each index is quantitatively analyzed and evaluated by using the analytic hierarchy process to determine the key controlling factors affecting productivity. The evaluation results are highly consistent with the current characteristics of high-yield wells. The productivity of the newly completed Well 008 and Well 107 outside the trial mining area exceeded 1 million m3/day. The result shows that the high productivity of ultradeep gas wells in the Qixia Formation is mainly controlled by the degree of dolomitization, the distribution of high-energy shoal-mound complexes, and the degree of fracture development. It can be used to guide the deployment of productive wells in the southern area of the Shuangyushi-producing test area.

Transient prediction method for flow temperature at wellbore bottom

For the issue of an increase in the flow temperature at the bottom of the wellbore during the production in ultra-deep gas wells, the current completion design, the strength checks of tubing strings, the selection and combination of downhole tools, and theoretical prediction research do not take into account the micro-scale thermal effects of the formation. Thus, by coupling the temperature and pressure fields of the formation and the wellbore through source sink terms, a transient prediction model of the flow temperature at the bottom of the wellbore is established based on the Darcy seepage mechanism and the micro-scale thermal effects. Then, the influences of formation parameters on the flow temperature and the mechanical behavior of tubing strings are assessed. The results indicate that the gas production rate has the largest influence on the flow temperature, followed by the reservoir height, the initial formation pressure, and the initial formation temperature. Due to the influence of the micro-scale thermal effects, the flow temperature can be increased to 34 ℃ at a gas production rate of 400 × 104 m3/day, which is significantly higher than the rated working temperature of downhole tools. The safety factor of tubing strings is lower than the traditional design of tubing strings when the micro-scale thermal effects are taken into account. Finally, the research presented in this paper provides a theoretical prediction tool for a change in the flow temperature due to the influence of the micro-scale thermal effects.

Fluid–Structure Interaction Modeling and Characteristics of a Tubing String System for High-Pressure High-Production Ultra-deep Gas Wells

Fluid–Structure Interaction Modeling and Characteristics of a Tubing String System for High-Pressure High-Production Ultra-deep Gas Wells

Response analysis and safety control of tubing vibration in HPHT and ultra-deep gas wells

Under the condition of high pressure, high temperature(HPHT) and high yield in ultra-deep wells, the buckling deformed tubing will produce violent vibration when opening and closing the well or production fluctuation, which will make the tubing bear large periodic fluctuating load, cause the failure of tubing thread seal, form fluid leakage channel, and induce the problem of sustained casing pressure(SCP). Serious annulus pressure will lead to increased tubing leakage, casing failure or more serious consequences. In order to explore the failure process of well barrier caused by tubing vibration in HPHT and ultra-deep wells, a fluid solid coupling vibration model of tubing is established, and the coupling vibration equation is numerically solved by the characteristic line method. Finally, an example of a gas well in the west is analyzed. The calculation results show that there is an obvious Poisson coupling effect between the tubing and the gas in the pipe in well X. The axial stress of the tubing is always in an unstable pulsating alternating stress state, reciprocating vibration along the axis and gradually attenuated to stability; the closer to the wellhead, the more violent the gas fluctuation in the pipe, and the greater the axial vibration amplitude of the upper part of the tubing away from the wellhead; large vibration of tubing may induce seal failure, resulting in oil jacket connection and pressure on a annulus; finally, the allowable annulus pressure of the example well is calculated, and the annulus pressure control measures are put forward. The research results have good guiding significance for well barrier failure analysis and annulus pressure control caused by tubing vibration in HPHT ultra-deep wells.

Research on Prediction Methods of Wellbore Hydrate Formation of Ultra-Deep Gas Wells in Northwest Sichuan

According to the formation and handling situation of hydrate in ultra-deep high-pressure sulfurcontaining gas wells in northwest Sichuan, the formation conditions of natural gas hydrate was studied based on previous studies on hydrate, the molecular dynamics of natural gas hydrate and the multiphase flow law of high-temperature high-pressure high-sulfur-containing gas wellbore were combined, and the pressure prediction model with high-temperature high-pressure sulfur-containing gas wells as the target was built. The chemical and physical control methods of wellbore hydrate plugging were discussed to provide the scientific theoretical basis for the prediction and control of hydrate in high-temperature high-pressure high-sulfurcontaining gas wells.

Study on Fatigue of Tubing Joint Thread Induced by Vibration in HTHP Ultradeep Wells

Abstract According to the tubing string failure statistics in the oilfield, fatigue crack of tubing joint thread in high temperature high pressure (HTHP) ultradeep gas wells remains a problem because it can cause tubing strength degradation, tubing fracture failure, well workover, and even well abandonment. In order to obtain a better understanding of tubing thread fatigue and quantify the fatigue life of tubing joint thread in HTHP ultradeep wells, experimental study, elastic–plastic mechanical simulation, and multi-axial fatigue calculation are carried out in this paper. Based on the similarity theory, the vibration mechanical testing device of the tubing string is designed, and the multi-axial load of the tubing joint thread in the actual working condition is obtained. Meanwhile, tubing joint BX1 thread model is established with ansys software, and the stress distribution of tubing joint thread is analyzed on the boundary condition acquired from experiments of vibration test of tubing string. Finally, according to the multi-axial fatigue theory, the fatigue life prediction value of the tubing thread made of Super 13Cr110 and Super 15Cr125 is compared and analyzed. The work presented in this paper can provide theoretical method and technological basis for the study on the frequent failure mechanism of tubing joint thread in HTHP ultradeep gas wells.

Real-time determination of rheological properties of high over-balanced drilling fluid used for drilling ultra-deep gas wells using artificial neural network

Drilling depleted and ultra-deep gas reservoirs required special drilling fluids which should be capable of bridging along the walls of the well and withstands a high differential pressure. High-overbalanced water-based drilling fluid (HOBWBDF) is one of the most common drilling fluid used to drill the ultra-deep gas wells. Drilling fluids properties such as yield point (YP), plastic viscosity (PV), apparent viscosity (AV), flow behavior index (n) and consistency index (k) are very vital inputs in managing rig hydraulics, margins of surge and swab pressure, equivalent circulation density (ECD) and hole cleaning. There is a need to develop a new technique to estimate the rheological properties in real-time using the frequent measurements of mud density and Marsh funnel viscosity, which are carried out more frequently (every 15–20 min).The main goal beyond this work is to build novel empirical models that are capable of predicting the above-mentioned rheological properties of high over-balanced water-based drilling fluid (HOBWBDF) in real-time for the first time. The models are built based on only two inputs which are the mud weight from the mud balance test and Marsh funnel viscosity measured by the Marsh funnel test. More than 1200 field measurements of rheological properties were used to feed an artificial neural network in order to develop new novel empirical models.The developed models using the ANN technique showed high accuracy for predicting the different drilling fluid rheological properties. The obtained results showed a maximum average absolute percentage error (AAPE) less than 8% with a correlation coefficient higher than 93%. The main advantages of these models are their inexpensiveness as there is no need for additional equipment to be added to the rig site. Moreover, these models would greatly help the drilling engineers to manage the ECD, surge and swab pressure and hole cleaning which in return will be reflected positively on the drilling performance.

Reliability Analysis of Super 13Cr Tubing in Ultra-Deep Gas Well

Abnormal oil casing pressure appeared in the process of test production of multiple Ultra-Deep Gas Wells in Tarim Basin. The super 13Cr oil pipe string was used to analyze the causes of pipe string failure in view of the oil casing channeling well during the test and blowout period. The construction process of the well was analyzed in detail. Combined with the review of the operation flow and the detection of fracture string material and fracture morphology, the causes of pipe string fracture were analyzed and calculated in detail. Through field investigation, analysis and calculation, it was found that the main cause of cracking of super 13Cr tubing in this well is the decrease of vibration natural frequency caused by excessive fluid velocity in pipe and too long span of pipe string. At the same time, the mixed failure of stress corrosion cracking and stress load interaction occurred in Cl−1 environment and other corrosion environments.

Geochemical characteristics of ultra-deep natural gas in the Sichuan Basin, SW China

Abstract The natural gas components and geochemistry of 38 ultra-deep gas wells (burial depth greater than 6 000 m) in the Sichuan Basin were analyzed to determine the genesis of ultra-deep natural gas in the basin. The ultra-deep natural gas components of the basin have the following characteristics: Methane has an absolute advantage, which can be up to 99.56% with an average of 86.6%; ethane is low, with an average of 0.13%; there is nearly no propane and butane. So it is dry gas at over-mature thermal stage. The content of H2S can be up to 25.21%, with an average of 5.45%. The alkane gas isotopes are: the carbon isotope varies from −32.3‰ to −26.7‰ for methane and from −32.9‰ to −22.1‰ for ethane. There is nearly no carbon isotopic reversal among methane and its homologues. Hydrogen isotope varies from −156‰ to −114‰ for methane, and from −103‰ to −89‰ for some ethane. The carbon isotope of CO2 varies from −17.2‰ to 1.9‰ and most of them fall within the range of 0±3‰. According to the δ13C1-δ13C2-δ13C3 plot, except some wells, all other ultra-deep gas wells are dominated by coal-derived gas. Based on the CO2 origin distinguishing plot and δ13CCO2, except some individual wells, most of the ultra-deep CO2 are of carbonate metamorphic origin. H2S in the ultra-deep layer of Longgang and Yuanba gas fields belongs to thermochemical sulfate reduction (TSR), while H2S from Well Shuangtan belongs to thermal Geochemical characteristics of ultra-deep natural gas in the decomposition of sulfides (TDS).

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.

An optimal design of network-fracture acidification for ultra-deep gas wells in the Lower Permian strata of the western Sichuan Basin

The Lower Permian reservoirs in the western Sichuan Basin are ultra-deep with high temperature, high pressure and developed natural fractures. Leakage and contamination of drilling fluid is the main factor restricting reservoir stimulation effects, so the acidification will be the solution also as the first choice to enhance the gas recovery. In view of this, an acidification design was proposed to minimize the contamination skin factor to the highest degree. A model was first developed to calculate the critical pumping rate for opening natural fractures in deep beds. Then, the acidification model for the rock samples of natural fractures in the experimental scale was modified, and a model was established for predicting the effective penetration distance of acid and the fracture aperture in the conditions of wellbores. Accordingly, a skin factor calculation model for network-fracture acidification was developed. It is indicated that when the acid pumping rate is 5.0 m3/min, all natural fractures around Well S1-1 can be opened, regardless of their dip angles. Besides, the advantage of high-rate acid injection emerges gradually when the injected acid is more than 100 m3. Moreover, for minimizing the skin factor, the network-fracture acidification in Well S1-1 was optimized by pumping 210 m3 acid at the rate of 4.5 m3/min. According to the optimal design idea, network-fracture acidification has been successfully applied in Well S1-1, and a high-yield industrial gas flow was produced at the rate of 83.7 × 104 m3/d. It is concluded that network-fracture acidification technology is a safest, most economical and effective mode for the stimulation of such ultra-deep reservoirs in the study area.