The thermal stress near the wellbore occurs because the temperature near the wellbore is changed by mud in well. The stress distribution near the wellbore is changed by the thermal stress, Eventually, the wellbore stability is affected. So, in this paper, first the integral equations of the thermal stress caused by varied temperature near the wellbore are derived from the thermoelastic theory. Then, the numerical computation is conducted for the thermal stress equations with the numerical results simulated for the temperature field near the hole. The plots of radius thermal stress distribution is obtained as a function of time. Further, the thermal stress is stacked with the pure mechanical stress near wellbore. Finally, the effect of the thermal stress on the wellbore stability is analyzed by use of the Mohr-Coulumb criterion for the any drilling condition and any time. The simulated result shows that the tangential and axial thermal stress on the holel formation interface may make the total effective stress to decrease on it below some point of section, and make the total effective stress to increase up some point. It is an important influence on the stress distribution near the wellbore. Therefore, the theories and simulated results in this paper have a guiding signification for the wellbore stability in the high temperature well and for the recovery of the production, such as the fracturing, injection, and thermal production. P. 85
In this paper, the effects of drilling fluids on shale strength have been tackled, and a case study has been presented to study the instability mechanism and the stability methodology in the shale formation. A program has been undertaken to develop a better understanding of the effects of the using polymer drilling fluid on the shale mechanical properties and how to modify its performance. Over 20 different drilling fluids were examined for their effects on shale strength. Emphasis here was on the mechanical properties of shale samples after exposed to different drilling fluids for 24 hours. All shale samples selected from similar depth and all tri-axial rock mechanic tests were performed under the conditions of 45MPa confining pressure. The mud which maintains high compressive strength of the shale is the optimum selection. The all shale samples exposed to using polymer drilling fluid exhibit the lowest strength, from the stress-strain curve, almost lost all strength. The use of KCl brine in drilling fluids is based on its shale-inhibition characteristics as it is thought that the k+ ion provides additional attraction among the clay platelets due to its size and charge. However, in Quka shale, the shale samples also lose most part of strength when exposed to the modified using polymer drilling muds only adding 5% KCl and 10% KCl respectively. The one of main possible reasons is Kaolinite in the shale. The shale samples will keep more original strength when exposed to the modified using polymer drilling muds with chemical agents with plugging ability to pore throat and micro-fracture. The all shale samples exposed to using polymer drilling fluid with 10%KCl and 3%A and B% exhibit the highest strength, then it was used to be a new system in drilling 2 new wells. The results show that the drilling fluid density has been decreased largely, and the wellbore stability has been improved remarkably while shale formation is stable. Introduction Wellbore instability is a serious drilling problem. It is reported that shale account for 75% of all formations1 drilled by the oil and gas industry, and 90% of wellbore stability problems1,2 occur in shale formations. The main factors that influence stability of shale formation are related to mechanical properties and physicochemical factors. Shale hydration is the main factor that causes instability problem3. All drilling fluids were found to cause instability problems due to water phase. Gas drilling fluid, oil based drilling fluid or all-oil drilling fluid can minimize the instability problem which is caused by shale hydration. However, we must be sure that there won't meet water zone in gas drilling process, and gas drilling isn't suitable to brittle shale formation, as well as the gas drilling equipment is expensive, so massive compatible analysis should be made in the drilling process. It is no doubt that oil based drilling fluid and all-oil drilling fluid will minimize the instability in shale formation. But oil based drilling fluid can't replace water based drilling fluid due to the environmental restrictions and expensive disposal costs4–6.
This paper presents the design and the field application of amphoteric polymer mud system to solve the problems conventional polymer caused, when drilling high reactive shale formation. It has been studied relationship of the structural features and mechanisms of polymer molecules with the performance of polymer mud. Amphoteric polymers have been developed with cationic and anionic groups in the one polymer molecule chain to be the dual function to improve the inhibition and to maintain excellent rheology of mud. This new kind of polymer mud system has been utilized in most oilfields of China, in diverse geographic locations, formation lithologies and hostile environments such as high density, high salinity and high temperature. It has been observed in the fields that this new drilling fluid has the desired optimum shale and borehole stabilization, improved rheological properties, high rates of penetration and benefit for oil formation protection. Introduction Unlike other mud systems, polymer mud properties are controlled by water soluble linear type polymer (acrylate polymer and partially hydrolysed polyacrylamide). Clay particles can be adsorbed and bridged by this type of polymer to form a special structure in mud, which shows the shear thinning property. Even at low solid content, high viscosity can be obtain at low shear rate and low viscosity at high shear rate. High drill rate, solids-carrying capacity, hole clean and reasonable distribution of drilling hydraulic power can also be obtained. On the other hand, high molecular polymer can adsorb on shale formation, encapsulate drilling cuttings to inhibit the dispersion of the shale formation and the disintegration of cuttings, which improve the borehole stability, as a result, the low solid content and low clay dispersion can be obtained in the mud. The two kinds of characteristic of polymer in mud are closed relative each other, so that how to obtain the better rheological and inhibitive properties is an important polymer mud technology. Laboratory tests and field application shown that enhance inhibitive properties of a polymer mud can improve shale formation stability, reduce cuttings desperation, limit formation damage and more important action is to control a excellent rheological properties and maintain low solid content in the mud. The research and application in China and abroad focus on enhancing mud inhibitive property and improving mud rheological properties to suit hostile drilling conditions. P. 577
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