Jin Gen Deng

Failure Analysis of Kelly Cock Valve Used in Drilling Process

Though statistically analyzing ten typical wells of kelly cock valve failure in certain oilfields, the main failure modes of the kelly cock valve were determined. Aiming at these modes and combining with mechanical analysis of knob, upper seat and main body of the kelly cock valve, its finite element calculation had been implemented and the weakness of all parts was found. By numerical modeling of the flow field of a kelly cock valve it was found that the velocity of fluid rapidly increased at upper and lower valve seats and this could easily prick out ball valves and valve seats, there was a annular low-velocity zone of minimum speed between the ball valve and the inner wall of the main body and this caused solid aggregation at this area. The research in this paper made the modes and causes of kelly failure to be recognized systematically. The results provided theoretical foundations of further improving the design and reasonable using of a kelly cock valve.

Investigation on Material Selection in Missan Oilfields Similar Environment

This paper analyzed the corrosion environment of Missan oilfields and investigated the oilfield country tubular goods used in other similar oilfields. Summarized the effect of partial pressure ratio of H2S/CO2 and Cl-to the corrosion behavior of OCTG. This paper concluded the service condition, test results and anti-corrosion mechanism of carbon steel, low-chrome steel, modified martensitic stainless steel and nickel alloy. Finally arrived at conclusion that the nickel alloy can meet the requirement of Missan oilfields, some literature reported that the modified martensitic stainless steel can apply in H2S/CO2 environment. In the condition that be easy to replace the tubular, carbon steel and low-chrome steel tubular can meet the requirement with corrosion inhibitor.

Wellbore Stability of Shale below Salt Formation of Faqui Oilfield in Iraq

Serious borehole collapse has occurred in shale below salt formation of Fauqi oilfield in Iraq. The Lots of bedding shale cavings returned to ground. The strength of bedding shale cores with different bedding dip angle are tested, which changes along in a high degree with the dip angle and reach the minimum in 50°-60°. The experimental results are regressed by different strength criterion, which demonstrate that McLamore strength criterion is more accurate. The collapse pressure of shale below salt formation is calculated by Mohr-Coulomb criterion and McLamore criterion respectively. The maximum collapse pressure by Mohr-Coulomb criterion occurs in the maximum in-stress orientations and 90° deviation angle, and that by McLamore criterion occurs in the maximum in-stress orientations and 50°-60° deviation angle. By analyzing the actual drilling situation, McLamore criterion is proved to be more accurate and can be used in Fauqi oilfield.

Wellbore Stability of Salt and Anhydrite Formation of Fauqi Oilfield in Iraq

Salt and anhydrite formation of Fauqi oilfield in Iraq contains salt, anhydrite and shale. Complex situations have occurred in drilling process, such as overflow and sticking. The cores of the three lithology rock are fetched and their strength and creep mechanical properties are tested. Anhydrite is with higher strength and lower creep properties, and shale and salt is with lower strength and higher creep properties. The collapse pressure and fracture pressure of the three lithology rock are calculated. The safe density window of anhydrite is the most widest and there is no risk of wellbore instability, and the safe density window of shale is the narrowest and wellbore instability easily occur. The low limit of mud density to prevent shale and salt creep are calculated by power law model. The safe drilling density window is determined and successfully applied in drilling field.

Analysis of the Formation Mechanism of Crack Network in Shale by CT

During the drilling process of shale gas horizontal well, wellbore instability problem happens frequently. Especially in the horizontal interval, shale which has high levels of gamma is more likely to collapse which lead to delays, and exist certainly collapse period. The development of shale gas is restricted seriously. Research on the formation process of cracks network around the wellbore by CT chromatographic technique. The result reveals that under the condition of uniaxial compression, the cracks initiated inside the shale formation around the wellbore, with the propagation of cracks, between the orientation of cracks and the maximum stress direction will emerge a certain deviated angle. The drilling fluid filtrates into the formation and reduce the strength of rock which will prompt crack propagation and form crack network. Finally the wellbore will collapse with drilling fluid continuous invasion, because the invasion have altered the stress intensity factor at fracture tip, and changed the propagating direction of crack and the friction coefficient of fracture plane, which will reduce the strength of rock and the effective stress of the rock around the wellbore. Crack network dominates the mechanism of instability; mud weight increases do not necessarily lead to a more stable borehole and can further destabilize the wellbore.

Abnormal High Pore Pressure Prediction and Cause Analysis of Salt and Anhydrite Formation of Fauqi Oilfield in Iraq

Abnormal high pressure exists in Lower Fars formation of Fauqi oil field in Iraq and lead to many complicated problems during drilling. There are salt, anhydrite and shale in Lower Fars formation through lithology analysis. Through the crossplot of sonic velocity vs. density data, it can be identified that the high pressure of Lower Fars is cause by the undercompaction of shale. Through composite analysis of different prediction methods of pore pressure, Eaton model is used to calculate the pore pressure of Lower Fars. The pore pressure begins to increase at the MB4 layer and declines to normal value at MB2 layer, and the peak is about 2.20 . The calculated results are in conformity with field situation, which can be applid to guide drilling practice in Fauqi oil field.

The Permeability Property and Borehole Stability in Bedding Shale

Wellbore collapse is one of the often encountered problems when drilling through shale formations. The instability mechanism and effect factors should be made sure to adopt suitable countermeasures. The bedding shales permeability was measured by experiment, and with this foundation, the effects of drilling fluid seepage on formation strength and wellbore stability were discussed. Results showed that laminar shales permeability is anisotropic and random. The permeability is extremely low in the direction perpendicular to the bedding plane, and it is very high parallel to the bedding plane when there are interlayer microcracks. The drilling fluid permeating along the bedding plane leads to great decrease on cohesion and internal friction angle, and then decreases the shale formations strength, finally leading to borehole shrinkage, pipe sticking, and borehole collapse. Increasing mud weight only is good for borehole stability in short term. However, it intensifies seepage finally and cant control borehole shrinkage. In proper mud density, increasing the mud sealing capacity to control the seepage speed is an effective means to prevent borehole instability.

The Critical Pressure Difference Prediction of Sand Production in Deepwater Sandstone Gas Reservoirs

Sand production in deepwater gas fields can cause great harm, so it is vital to determine a reasonable production system. At present the prediction models of critical pressure difference of sand production in oil-gas wells are numerous, but among them there is no prediction model that aims at deepwater gas fields that the mining environment of high risks. The authors carried out a lot of experiments with real rock cores to simulate sand production in gas wells by using a set of self-developed radial displacement experimental devices. During experimenting they measured critical flow rates and critical pressure differences when sand production happened, and calculated the critical pressure differences of sand production under the condition of actual reservoirs. Comparing the results of the experiments with that of multiple sanding prediction models, the most suitable model for gas reservoirs was determined. Through doing error analysis and coefficient modification on the model, a simple and effective method was ultimately formed to predict the critical production pressure difference of gas wells. The research results showed that for gas fields if the production pressure difference was controlled within 0.4 times of the uniaxial compressive strength, sand production could be effectively prevented, and the prediction error could be controlled within 10%. This technique has been successfully applied in one deepwater gas field in the South China Sea areas, which guided the selection of sand control method and the design of production pressure difference, thereby ensuring reasonable and efficient mining of this gas field.

A New Calculation Method for Micro-Hole Collapse Pressure of Shale Gas Reservoirs in Thrust Fault

With the ever increasing energy demands, unconventional gas becomes the new force in energy field. However, shale gas horizontal drillings are prone to instability problems because of the brittleness of gas shale. The casing programs of shale gas wells in China and North America demonstrate that the more drilling problems are encountered with the larger borehole sizes. In the thrust faults, based on vutukuris size effect theory and Mclamores failure criteria, a new prediction model of collapse pressure in transversely isotropic rock mass is proposed to analyze the critical mud weight of micro-hole in gas shale formations. The trajectory sensitivity analysis is also carried out. The results show that micro-hole drilling technique in thrust faults can significantly reduce the collapse pressure of directional wells by 12% approximately.