New development of theories in gas drilling

Theories established from engineering fundamentals have been of great value in supporting the design and execution of drilling operations in gas drilling where gas is used as a drilling fluid.This work presents an overview of new theories developed in recent years for special gas drilling operations including horizontal wells.These new theories are found in the areas of gas-mixture flow hydraulics in deviated and horizontal boreholes,hole cleaning of solids accumulation,hole cleaning of formation water,flow diverging for washout control,bit orifice optimization,and depression of formation water influx.This paper provides drilling engineers with updated mathematical models and methods for optimizing design to improve gas drilling performance.

Analysis of drilling difficulty of extended-reach wells based on drilling limit theory

With the continuous increase in vertical depths and horizontal displacements of directional wells,the difficulties of drilling operations continue to increase,and more accurate methods of drilling difficulty evaluation are needed.In this paper,a drilling difficulty evaluation method is built by combining drilling limit model and expert evaluation.Firstly,the concept of drilling difficulty index is introduced,and the method to calculate drilling difficulty index is established.Next,the meanings of five drilling difficulty levels are explained and the optimization design method with drilling difficulty as the target is built.At last,the theoretical model is applied to the extended-reach drilling of the Liuhua oilfield in the South China Sea,in which drilling difficulties are evaluated and the relationship between drilling difficulty and development control radius is revealed.The results indicate that extended-reach drilling in the Liuhua oilfield is on the“normal”difficulty level on average,rotary drilling in 8_(1/2)-in.section is the most difficult,and the main constraint conditions are excessive torque and high friction.Through technology upgradation,the drilling difficulties are decreased,the development control radius increases from 6.6 to 11.4 km,and the maximum horizontalto-vertical ratio increases from 5.3 to 8.7.Then,the development wells in marginal oilfields and adjustment wells in old oilfields can be drilled on“normal”difficulty level.Therefore,technology upgradation,especially drilling rig upgradation,is the most important development direction for extended-reach drilling in the South China Sea.

A drilling technology guided by well-seismic information integration

The predictions by drilling-related mechanical and geological models are in some degree inaccurate due to non-unique solution of seismic velocity model.To address this problem,a new drilling technology guided by well-seismic information integration is proposed which consists of seismic velocity update of drilled formations,seismic velocity prediction of the formation ahead of drilling bit,and the prediction of geological feature and drilling geological environmental factors ahead of bit.In this technology,real information(velocity,formation and depth)behind the drilling bit and local pre-stack seismic data around the wellbore being drilled are used to correct the primitive seismic velocity field for a re-migration of seismic data and to update geological features and drilling geological environmental factors ahead of the drilling bit.Field application shows that this technology can describe and predict the geological features,drilling geological environmental factors and complex drilling problems ahead of the bit timely and improve the prediction efficiency and accuracy greatly.These new updated results are able to provide scientific basis for optimizing drilling decisions.

STUDY ON THE ON－LINE OPTIMAL CONTROL OF PARAMETERS OF THREE－CONE BLAST HOLE DRILLS

A model recognition method for the on-line optimal control of the parameters ofthree-cone blast drills is developed. It takes a few of on-line measurements and has a rapidoptimization speed. The mathematic model for on-line optimal control of the parameters and thedetermination of the parameters in the model are also presented.

Drill string dynamic characteristics simulation for the ultra-deep well drilling on the south margins of Junggar Basin

Improper drilling parameters may cause severe vibration of drill string which leads to reduce the rate of penetration and drilling tool premature failure accidents in the drilling process of ultra-deep well.The study on dynamic characteristics of drill string plays an important role in increasing the safety of drilling tool and optimizing the drilling parameters.Considering the influences of real borehole trajectory,interaction between bit and formation,contact between drill string and borehole wall,stiction of drilling fluid and other factors,a comprehensive drill string dynamic model was established to simulate the changes of wellhead hook load,torque,equivalent stress of drill string and BHA(bottomhole assembly)section acceleration and motion trajectory with time at different WOBs(weights on bit)and rotary speeds.The safety factor and overpull margin of wellhead drill string were calculated and the strength of drilling tool in ultra-deep well was checked using the fourth strength theory.The analysis results show that,in the drilling process of ultra-deep well,the transverse motion amplitude of the drill string near the wellhead is relatively small and vibration of drill string mainly occurs in the lower well section.As the rotary speed increases,the number of collision between lower drilling tool and borehole wall increases,wellhead transverse stress increases,change in torque is not large and change in wellhead equivalent stress is relatively small.As the WOB increases,wellhead torque will increase,axial load and equivalent stress will decrease and vibration acceleration of BHA will increase sharply.Wellhead overpull margin and safety factor will decrease with the increase of rotary speed and increase with the increase of WOB.Wellhead safety factor of S135 drilling tool in an F190.5 mm ultra-deep well on the south margins of Junggar basin changes around 1.8.The drilling tool is safe and has relatively sufficient ability to deal with the downhole accidents if a large size high steel grade drill string(F139.7 mm S135)is used.However,in view of BHA safety,neither rotary speed shall be too high nor WOB shall be too large.

Selection Optimal Drill Rigs by Grey Relational Analysis Method

Based upon the main properties of the relational analysis in grey system theory, a new method to select optimal drill rigs is proposed in this paper. Under the unified technique economic index, with the practical data of four kinds of drill rigs in the mining district, east of Sichuan, China, an optimal reference number series was constructed. Through calculating the relational grade between the technique economic indices of various drill rigs and the reference number series and making a relational sequence in accordance with priority, the results achieved are in conformity with that of reference by Fuzzy Set Theory, and grey situation decision making and yet this method is simple, easy and intuitive in analysis etc..

Confined compressive strength model of rock for drilling optimization

The confined compressive strength(CCS)plays a vital role in drilling optimization.On the basis of Jizba's experimental results,a new CCS model considering the effects of the porosity and nonlinear characteristics with increasing confining pressure has been developed.Because the confining pressure plays a fundamental role in determining the CCS of bottom-hole rock and because the theory of Terzaghi's effective stress principle is founded upon soil mechanics,which is not suitable for calculating the confining pressure in rock mechanics,the double effective stress theory,which treats the porosity as a weighting factor of the formation pore pressure,is adopted in this study.The new CCS model combined with the mechanical specific energy equation is employed to optimize the drilling parameters in two practical wells located in Sichuan basin,China,and the calculated results show that they can be used to identify the inefficient drilling situations of underbalanced drilling(UBD)and overbalanced drilling(OBD).

Hydrocarbon accumulation conditions and key technologies for exploration and development of Yuanba gas field

This paper discusses the exploration and development history and lessons of the large ultra-deep biogenic reef gas field in Yuanba area,where the previous“Kaijiang-Liangping shelf”model provides an insight for the discovery of the biological reef and bank on the platform margin.Systematic analysis of key geological conditions for hydrocarbon accumulation shows that the reef and bank reservoirs in Yuanba area are characterized by row and zonal distribution along“Kaijiang-Liangping shelf”,the bank in early and the reef in late,the reef in the front and the bank in the back,and dominated by lateral accretion and progradation.The major exploration target of high-quality reef-bank reservoirs are developed in Changxing Formation instead of in Feixianguan Formation,Three stages of fractures and dissolution,and dolomitization control the development of the high-quality reservoirs.Two sets of effective source rocks(Permian Dalong Formation and Wujiaping Formation)provide sufficient gas source for the large gas field.Three dimensional migration pathway system composed of micro-faults,micro-fractures and interlayer fissures facilitates the hydrocarbon migration and accumulation.The accumulation model of Yuanba gas field is characterized by the near-source accumulation,three-micro migration pathway system,lithological-stratigraphic reservoirs and tectonics controlling enrichment.According to the complex geological conditions such as ultra-deep reservoirs and multiple pressure systems in Yuanba gas field,several key technologies are developed during hydrocarbon exploration and development,including fine prediction of ultra-deep reef-bank reservoirs and identification of gas and water,fine-scale reservoir description and characterization of complex thin and small reef gas reservoirs,optimal&fast drilling of ultra-deep horizontal well,geo-steering of ultra-deep horizontal well for complex reef and bank.All these technologies resulted in the efficient exploration and development of Yuanba gas field and accelerated the innovation of theoretical technology and methods for ultra-deep reservoirs.