High degree of depletion and overbalanced drilling have caused ideal conditions for fracturing and lost circulation. Thief zone introduces severe lost circulation and well control problems, often resulting into loss of the interval or the entire well. In most cases, operators choose to drill such depleted sections with SBM or OBM systems rather than WBM. Dumping mica, nut hulls and cross-linked polymer pills, certainly still work, for some applications, but have narrow operating limits. Very few lost circulation remedies have been successful, especially when using invert emulsion drilling fluids. Controlling losses in this zone has proved to be extremely difficult. A novel alternative LCM pill has been designed to seal temporarily or indefinitely critical intervals where drilling and cementing operations are very difficult to successfully accomplish. This formulation is prepared with invert emulsion, cross-linked with magnesia, up to 90% soluble, especially designed to be used in pay zones, a low cost alternative and is appropriate for sealing non-productive intervals. The phase change of invert emulsion and addition of light burned magnesia form the viscous fluid and remains stable till activator is not added. By addition of activator, control process of static gelation starts and forms the magnesia cement. Such formulations have sp gr 1.10–1.45 and stable at high temperature (< 120deg C). The apparent viscosity of this pill is kept in the range of 45–60cp to avoid any settling of magnesia. This pill can be retarded for desired time of placement and gives its vertical setting. The compressive strength of the molds is observed 300-1000psi within a short time and also soluble in 15% HCl. The paper presents various formulations of innovative pills which can be used effectively and economically to combat circulation loss in depleted reservoir formations. INTRODUCTION Loss circulation is an age-old problem normally encountered during drilling. It is one of the most time-consuming and costly problem faced by oil well drilling industry. Loss circulation or loss of returns is defined as the drilling fluids into the formations which are usually lost to natural or induced formation fractures. After the lost circulation occurs, the level of the drilling fluid in the annulus may drop and then stabilizes at a particular level depending upon the formation pressure. The loss of drilling mud results in increased drilling time, loss of expensive mud, plugging of productive formations, and / or loss of well control. Normally, bridging agents and plugging materials are used to combat loss circulation. And another approach is used to reduce the density of the circulating fluid. In some instances, both approaches are combined to establish circulation. In many cases the lost circulation zones have fractures of different sizes which must be sealed of with bridging or plugging materials [Eric, Davidson, 2000]. Wide assortments of bridging or plugging materials, commercially available for reducing lost circulation during drilling, are classified as granular, flake and fibrous.
This paper presents a new process for development of a novel cementing composition to provide ductility and thermal stability to the cement sheath in thermal recovery wells subjected to high temperature cycling during ISC process. In this novel composition a carbon-containing natural mineral fiber and an aluminum silicate additive are introduced along with Portland cement to perform effectively in wellbores having conditions prone to stress-induced cement failure. ONGC's heavy crude oil-fields in north Gujarat, India necessitate the inroduction of thermal recovery process for improving the oil recovery. However the durability of the cement sheath consisting of API Class G cement admixed with silica is endangering the success of EOR process. The developed composition with carbon-containing natural mineral fiber (graphite) and an aluminum silicate additive (metakaolin) because of its durability at high temperature thermal cycling (<450°C) and compatibality with of Portland cement is an ideal composition for application in thermal recovery wells. The formulated thermally stable cement slurry with graphite and metakaolin has no significant effect on compressive strength and liquid permeability at high temperature thermal cycling (<450°C), thereby making it suitable for placement against combustion zone. The field evaluation of this cement system in ISC wells of Balol & Santhal fields of ONGC in Mehsana asset is under-way. The developed novel cement system will solve the problem of charging of the subsurface shallow layers because of cement sheath failured during the in-situ combustion process. Introduction The heavy oil narrow belt of North Gujarat covered an area of about 45 km2 is marked by the western boundary of Mehsana horst and eastern boundary by Jotana field has in place oil reserves of 12–18° API with viscosity ranging between 60 cp to more than 550 cp. Primary recovery produces at most 15 –18 per cent of such in-place oil due to adverse mobility contrast between oil and water. This necessitated the need to identify an appropriate Enhanced Oil Recovery (EOR) technique, to recover more residual oil. The screening of EOR techniques by the Institute of Reservoir Studies (IRS) of ONGC favors the application of thermal recovery processes such as steam flooding and in-situ combustion which is based on the principle of improving oil-mobility by reducing the viscosity of oil by heating it within the reservoir. Based on encouraging laboratory results, the pilot project was started in 1990. The flue gas analysis indicated the high concentration of nitrogen (70%) and carbondioxde(14%) with low concentration of oxygen(around 0.2%), establish the utilization of air in combustion process. During the wet combustion phase the concentration of these gases persisted indicating sustenance of fire- front. In-situ combustion process was successfully commercialized in Balol and Santhal fields in mid- 1997 and, commercial scheme extended for Lanwa and Becheraji field1. In Situ Combustion Process The in-situ combustion recovery requires the injection of sufficient air to support and sustain the combustion front in the reservoirs. The high temperature combustion front proceeds slowly through the reservoir to the production wells. The reservoir temperature may normally fluctuated upto 300–450°C and the fire front may reach up to 650–800°C. The combustion drive is converted into wet combustion by the alternate injection of water along with air. Wet combustion offers the advantages over the dry combustion by higher oil recovery2. During in situ process periodical quenching combustion front by water causes temperature falls to normal as a result of which the cement behind the casing and the casing are subjected to thermal cycling. The success of in-situ combustion process depends on the reservoir and geological considerations. Since in situ combustion is an inter-well drive process of good horizontal continuity and is critical for the success of the project. Lack of good sand continuity, fractures and joint trends may create preferential flow channels causes of failure of in-situ process. Gaps in formation overburden or leaky inter-zonal seals in stratified reservoirs can allow fluid to leak into overlying strata and reduce the effectiveness of the injectant.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractLost circulation problems encountered during drilling is caused by the differential pressure of the hydrostatic column which is generally greater than the formation pressure, and is especially through the openings in the rock in low pressure or depleted zones. Combating circulation loss is often achieved by using variety of LCM formulations or placing the cementing plug. The quick set sealing cementitious composition is formulated that produces cuttings compatible to drilling fluid. An engineering approach applied with the cementitious compositions admixed with synthetic fiberous LCM placed at the thief zone in the well set into a cementitious plug. The fiber based slurry allows setting in high permeable or fracture zone and caused strong impermeable net to prevent the losses.Results from a laboratory study on cementitious formulations are discussed and compared with Portland cement plugs. The cementitious formulation designed with GGBS and silica fume admixed with synthetic LCM formed a slurry which is activated with calcium hydroxide. The sealing effect of the formulation has been evaluated against the sieves and slots of different sizes. The results revealed the substantial improvement in sealing the thief zone. The study of thickening time and compressive strength of formulations having sp gr 1.4--1.6 has been carried out at 60 deg C to get sufficient pumping time and strength to resume drilling within 3-4hrs. The contamination of drilling fluid with cementitious compositions has no significant changes in rheology that saves the extra expenses for conditioning the drilling fluid.The paper presents various formulations of cementitious compositions which can be used effectively and economically to combat circulation loss during drilling.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCement slurry designing using conventional cement additives for primary casing cementation of wells with low / moderately low BHCT (40 -70 Deg. C) generally encountered in shallow wells or in wells located in mature / depleted reservoirs is an industry challenge. Cement slurries for field implementation needs to possess adequate placement time, minimum fluid loss, good rheological behaviour, minimum free water separation, short transition time, good stability and early strength development capability under operational conditions.However, even with all the recent
Total loss of circulation is a common problem during drilling through the fractured limestone in western offshore fields and coal seams in the CBM fields of eastern India. Most of the the times the wells are completed in total loss conditions either by drilling blind or by use of sub hydrostatic drilling fluid as conventional loss control measures were not effective. Moreover requirement of long cement column to cover multiple coal seams with excellent zonal isolation for fracturing in CBM wells during the production stage makes the cementation a big challenge in these wellls. To overcome these challenges, an inert dispersible fiber based cement system along with an improved placement techniques have been tested in the field with great success the details of which has been presented in this paper. An innovative inert fiber based cement system has been formulated and evaluated in the laboratory for plugging fractured formations under simulated conditions prior to its application in total loss condition. Computer simulation studies were performed with field data and permissible slurry density and displacement rate were determined to minimize losses and formation damage during cementing. Limiting friction pressure by adjusting the rheological properties & displacement rate of the slurry during slurry placement also minimizes the lost circulation problems. Using this combination of improved cement system and placement techniques, circulation of the slurry was regained while cementing and the designed cement top could be achieved thereby avoiding anticipated repair jobs. Case histories of wells drilled with total loss conditions where successful implementation of this cementation technique prevented cement invasion into the production zone and lead to improved zonal isolation are presented in this paper. This paper established the significance of application of these innovative cement system and cementing techniques in terms of better well completion and improved productivity in cementing across formations having total loss condition during drilling. Introduction Lost circulation is the partial or complete loss of drilling fluid or cement slurries into formation voids during drilling, circulation, running casing, or cementing operations. It was reported that lost circulation occurs during drilling on approximately 20 to 25% of wells drilled worldwide. It is one of the most troublesome and costly problems encountered in drilling a well. Numerous lost-circulation treatments are available in the oil industry, but due to the mixed results of such treatments, the loss circulation problem is still a major issue of concern. Lost circulation has also been blamed for reduced production because it resulted in failure to secure production tests and samples, while the plugging of production zones have led to decreased productivity. Lost circulation problems may be encountered at any depth when the total pressure exerted against the formation exceeds the formation breakdown pressure. Generally, four types of formation are responsible for lost circulation such as natural or induced fractured formations, vugular or cavernous formations, highly permeable formations, and unconsolidated formations. Lost circulation in naturally fractured, cavernous/vugular and unconsolidated formations cannot be avoided completely but can be minimized with proper treatment/techniques.
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