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Wellbore instability caused by the failure of shale fonnations represents a major challenge to drilling in the oil and gas industry. Incompatibilities of drilling fluids and shale fonnations are often the root cause of shale instability or wellbore instability. The most common and effective solution to shale instability is through drilling fluids design and selection, although drilling fluids-shale interaction and shale stability are complex and not well understood. There is no single testing or modeling method to solve this complex shale instability problem. Addressing drilling fluids and shale interaction requires a holistic approach.
Geomechanics engineers and mud engineers have the same goal of minimizing shale instability by selecting the optimum mud weight and chemistry, but their methods of addressing shale instability are quite different. This paper highlights and shares with geomechanics engineers some of the important laboratory and well-site testing techniques that are often used by mud engineers for characterizing and remediating drilling fluids and shale interaction. Each testing technique is effective and useful in eliminating or confmning one single attribute of fluids-shale interactions. When these test results are put together, they give a more complete picture and the root cause of shale instability and thus a potential solution options.
Wellbore instability arguably is the most prevalent underlying cause of non-productive time during well construction. While a number of parameters affect wellbore instability, the mud density and chemistry invariably play the major role. The optimum mud weights are often based on geomechanics wellbore stability modeling studies by geomechanics engineers, while the mud type and chemistry are selected based on fluids performance lab testing and environmental compliance considerations by mud engineers. Geomechanical wellbore stability studies generate boundaries for operating mud weight windows and recommended mud weight strategies, based on analyses of offset log data, well histories, and rock mechanics models. Drilling fluids or mud engineers perform a suite of lab tests to evaluate drilling fluids performance, mostly to evaluate shale-fluids interaction and shale stability, and to recommend the optimum drilling fluids chemistry and formulations for the mud program. Many of the problems associated with the use of drilling fluids type and chemistry are caused by incompatibilities between the drilling fluids and the shale formations encountered.
These incompatibilities can result in washouts, poor penetration rates, increased drilling costs due to solids handling, rig time, dilution requirements, shale sloughing, borehole encroachment and other wellbore instability events. Drilling fluids-shale interaction and shale stability are complex and not well understood, although there are many published model studies and laboratory techniques and studies. Various lab testing techniques have been developed in the literature to address or screen the above stated issues. Each testing technique is developed to characterize or evaluate a single attribute about the shale-fluid interactions, and many of these tests are often qualitative, rather than quantitative. There is no single testing or modeling method to solve this complex shale instability problem. Addressing drilling fluids and shale interaction requires a holistic approach.
Abstract 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 formations drilled by the oil and gas industry, and 90% of wellbore stability problems1, 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 problem. 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 costs.