Drilling Fluid Experiment Study On Wellbore Stability In The Co-exploitation Of Coalbed Methane, Shale Gas, And Tight Sand Gas In Coal Measures Strata Of Southwest China

The co-exploitation and development of “coalbed methane,shale gas,and tight sand gas(three gas)” in coal measures strata has expanded the exploration area of coalbed methane.It is an important way to improve the single well output and benefit of CBM.Therefore,it is becoming the key exploration direction at present.Because of the differences in occurrence mechanism,reservoir reconstruction technology,gas recovery,and gathering technology,“three gas” in coal strata is difficult to produce generally.Generally,wellbore instability does not likely happen in sandstone.Therefore,higher requirements for water-based drilling fluids are required,including solving the problem of wellbore stability when drilling into coal seams and sandstone formations.The reasons lie in that the drilling fluid system is not likely to be converted frequently during the actual drilling process.It is necessary to take the reservoir protection into consideration because of the reservoir drilling.The thesis analyses the physical and chemical composition of coal strata in Zhijin and Bijie areas of Guizhou of Southwest China.The law and mechanism of electric property and wettability on the stability of the coal and shale wellbore are investigated.Apart from that,the water-based drilling fluid system is also studied which can achieve the wellbore stability of coal system formation under the co-exploitation of “three gas”.The main work completed in this paper is as follows:The first chapter introduced the source,purpose,and significance of the topic analyzed the current research status at both home and abroad,and introduced the research contents,research objectives,and research methods.In the second chapter,the physical and chemical characteristics of coal formation rock samples(coal and shale)in terms of mineral composition,micro-structure,specific surface area and porosity were studied.It is found that the coal samples of Zhijin Longtan Formation have more clay minerals,higher non-crystalline content,and the development of fractured pores and larynx.The coal samples of the Longtan Formation in Bijie have obvious stratiform structure and the fracture is more developed.It is easy to break.The content of the dolomite and quartz is higher and the hardness is larger.Shale in Xiushan area has higher shale content,higher brittleness index,and secondary clay mineral content.They contain some chlorite hydrated expansive rock mineral and the effect of hydration expansion is moderate.In the third chapter,the electrical properties change of drilling fluid on the stability of the coal and rock wellbore are studied.It is found that the inhibition effect of inorganic positive gel MMH-1 on the coal samples of Longtan group in Zhijin area is better.The positive gel solution can effectively slow down the pore pressure transmission in coal samples,thus enhance the stability of the coal-bearing stratum wellbore.The MMH additive mainly embodies the inhibition by electrical neutralization and competing with clay particles for water molecules.In the fourth chapter,the influence of wettability on the stability of coal formation wellbore is studied.The preferred surfactants can effectively reduce the interfacial tension between the drilling fluid and rock samples(coal and shale)and improve contact angle with rock samples for Zhijin and Bijie area.The composite surfactants can effectively prevent the transmission of pore pressure in coal strata and enhance the stability of coalbearing strata by enhancing chemical adsorption between shale and cationic surfactant,it can increase the contact angle between drilling fluid and shale,so as to improve shale hydrophobicity and enhance shale wellbore stability.In the fifth chapter,aiming at the coal strata in Zhijin and Bijie area,we put forward with a set of water-based drilling fluid system suitable for simultaneous drilling of “three gas”.It is found that the compatibility of inorganic positive MMH-1 and composite surfactants with water-based drilling fluid is good for coal strata in Zhijin area.The contact angle between the optimized drilling fluid and coal rock and shale is 77.5° and 66.5° respectively,which has good inhibition,anti-moderate and anti-pollution(inorganic salt and drilling cuttings)ability,and the damage degree to the original coal and rock core is only 3.8%.For the coal strata in Bijie area,drilling fluid has moderate viscosity and API filtration loss is the only 7mL.It has a strong inhibitive effect on coal and shale hydration,low reservoir damage,strong anti-pollution ability and is friendly to the environment.The main innovations of the dissertation are concluded as follows.First,the mechanism of improving the wellbore stability in coal measure strata by controlling the wettability was explained in detail.Cationic surfactant can increase the contact angle between drilling fluid and shale by the chemical adsorption on shale surface,so as to improve the hydrophobicity of shale and enhance shale wellbore stability.Second,a kind of water-based drilling fluid system was proposed separately for Zhijin city and Bijie city in Southwest China which can improve the wellbore stability in coal measure strata.Compared with existing similar drilling fluid systems,the proposed drilling fluid systems not only can inhibit the hydration of coal rock and shale,have good temperature endurance and anti-pollution(inorganic salt and drilling cuttings)capability,and low reservoir damage,but also can obviously improve the hydrophobicity of coal rock and shale,and have low surface tension,which is beneficial to mitigate the resistance during the exploitation of coal-series gas.The next step is to carry out research work on the selection of surfactants with better capabilities and strive to the field trial of the drilling fluid technology.