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The practice of using hollow glass microspheres to reduce the density of drilling fluids and cement slurries has become more prevalent in recent years due to increasing demand for "hydrostatic pressure management" with high performance low density cement and fluids. The use of hollow glass microspheres in drilling fluids presents unique benefits and challenges. This paper addresses a case study of a recent offshore managed pressure drilling project which ONGC conducted using glass bubbles in the Mumbai High. The study demonstrated the value of bubbles as a mud density reducer in a challenging drilling environment and also showed the importance of specific operational parameters in the project's success. A subsequent field study was conducted to quantify, under controlled conditions, the impact of these parameters on the bubbles performance while drilling. Mud density and mud solids content were monitored as flow rate (FR), weight on bit (WOB) and drill string rotation speed (RPM) were changed. The tests were conducted at the GTI Catoosa Test Facility rig under the co-direction of 3M and Newpark Drilling Fluids. A new generation of glass bubble, with a density of 0.42 g/cm3, an isostatic compressive strength of 8000 psi, and a smaller particle size was selected for this work. Solids control equipment settings were shown to be one of the key components to success in this experiment. A comprehensive mud sample collection plan enabled the monitoring of mud density, solids content and rheology parameters versus time. This study confirmed that glass bubbles can be used to satisfactorily maintain the mud density within an acceptable range in an economical way. The glass bubbles, specifically the new generation with a higher strength to density ratio, were shown to be an efficient, convenient, and sustainable way to reduce the density of a drilling fluid. 1- Field Application of Hollow Glass Microspheres in Offshore Drilling Operations, Mumbai High, India: 1a - Project Rationale The reservoirs of "Miocene L-III" in Mumbai High predominantly consist of limestone interbedded with shale. This reservoir is the largest oil producing reservoir in the Mumbai High (see Figure 1). However, the pressures in the L-III are mostly sub hydrostatic and present challenges commonly associated with low pressure zones such as enhanced fluid loss, loss of circulation and formation damage. Additionally, the use of conventional fluid loss control pills was not suitable to L-III as the fractures are the main route for oil migration from the formation to the well. It was necessary to design a drilling fluid which would enable ONGC to re-drill into the L-III formation and perform workover activities while preventing fluid losses and formation damage. These factors were the key elements that led to the design of the unique glass microspheres based drilling fluid system to drill the reservoir L-III.

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This paper relates severe lost circulation problems often encountered when the well is drilled overbalanced through high-permeability, fractured, or easily fractured formations. As numbers of LCM pills, neat cement plugs were placed in damaging and non-damaging reservoirs, resultant failed to provide an adequate solution.Recently, severe mud loss problems were encountered during drilling in Mumbai Region from depleted reservoirs, unconsolidated, highly permeable, fractured and fragile limestone formations, both from nonproducing and producing reservoirs (L-III, L-II, L-I, H1A, H3B, etc.). The losses range from 100 BPH to total loss. Attempts are made to stop the losses with the placement of advanced lost circulation treatments. To mitigate these types of losses, an innovative technique developed with a temperatureactivated Rapid Setting Fluid (RSF) Pill from indigenously available chemicals and used as an alternative to LCM / cement plugs for severe loss control problems in Mumbai Offshore. The RSF system has proven successful for curing severe dynamic and static losses by pumping treatments through the drill string bottom hole assembly (BHA) (no trip out required) and taking advantage of its highly controllable fast setting properties.The RSF pill is a stoichiometric mixture of magnesium compound, forming hard and a series of high mol. wt. magnesium inorganic poly-hydrates, displaying characteristics of conventional cement slurry in pumpability, but very different chemically. As the RSF Pill enters the lost zone, it sets rapidly (a rapid transition from liquid to solid) and builds sufficient compressive strength within 5-6 hrs. The reaction causing solidification is wholly a function of time and temperature and unaffected by hydrostatic pressure. The pill is non-damaging as it is non-invasive, sets at the lost zone mouth and 90-100 % soluble in 15 % Hydrochloric acid. The pill can be successfully used up to 140°C BHST.The field trial has been carried out successfully in two wells of Mumbai Offshore: N_12#3Z (Mumbai High L-III reservoir) in partial loss conditions and HG#6H (Miocene Bandra Limestone formation) with total losses. It has also established that the new Loss Control Solution, RSF Pill can be placed successfully in all types of lost zones (like L-III, L-II, L-I, H1A, H3B) to mitigate the mud losses.The paper also details the specific chemical and mechanical characteristics and field performance of the RSF pill. It concludes that the combination of unique fluid properties and best practices in fluid placement procedures plays a vital role in the successful implementation and development in the treatment of loss circulation in limestone formations. This technique is unique, easy, very effective and provides cost benefits.