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Fluid-Loss-Control Additives (FLAs) are used to maintain a consistent fluid volume within a cement slurry to ensure that the slurry performance properties remain within an acceptable range. The variability of each of these parameters (slurry performance properties) is dependent upon the water content of the slurry. If the water content is less than intended, the opposite will normally occur. The magnitude of change is directly related to the amount of fluid lost from the slurry. Because predictability of performance is typically the most important parameter in a cementing operation, considerable attention has been paid to mechanical control of slurry density during the mixing of the slurry to assure reproducibility.
A challenge in many permeable, water-sensitive, subhydrostatic reservoirs is avoiding the loss of completion fluid when completing or working over wells. To overcome the limitation of conventional fluid-loss-control pills, a low-viscosity system was developed. The system is composed of a viscous disproportionate permeability modifier (VDPM) with sized synthetic polymer particles and fibers, which degrade into organic acids. The VDPM reduces the effective permeability to water-based fluids, and the sized particles create an impermeable filter cake. When the particles degrade, the organic acid acts to break any remaining polymer. The limitations of many conventional fluid-loss-control pills have resulted in the development of a number of solids-free fluid-loss-control pills.
Abstract Workover operations in shallow low pressure heavy oil unconsolidated sandstone reservoir in Kuwait presents a major challenge due to significant killing fluid loss which causes wellbore plugging, incremental operational costs due to more rig days, excess brine volumes, and more importantly the impact of deferred production due to formation damage. This paper presents an innovative fluid-loss control pill added to killing fluids, which has resulted in significant cost savings and well productivity improvements. The subject heavy oil reservoir have formation pressure equivalent to 6.3 PPG versus 9.3 PPG Potassium Chloride brine used as killing fluid. This overbalance condition is a requirement as safety barrier but conversely it leads to hundreds of barrels of killing fluid losses with the consequently invasion and formation damage. Kuwait Oil Company recently added a new customized fluid loss control pill of high purity vacuumed dried evaporated salt to the well killing procedure. Using this fluid loss control pill both drilling and reservoir engineers achieved their aim in terms of safety operation and no formation damage. To test this new pill, two shallow wells with 220 psi reservoir pressure and perforation set at 630 ft were selected to record the losses. The first well had undergone workover including recordings from caliper, cement, and ultrasonic logs, which measured the positive impact of the new control pill on logs quality by excluding fluid pumping while logging and having constant fluid level at surface, which saved cable head from unnecessary tensions. In a second well, there was hanged standalone screen on a packer against the perforation and there is no direct access to the perforation. The control pill was customized to be pumped into the screen, which sealed the screen itself perfectly. The control pill flowed back easily in both wells and same loss rate was observed after removing the pill, which confirmed no negative impact on reservoir permeability. KOC confirmed that the two jobs were successful and the pill to be approved for full field implement in other operations. The achieved success criteria summarized as follows: Hydrostatic column is a safety barrier that assuring fluid level at surface during workover is safety requirement especially in high Gas oil ratio wells. Full circulation enhances sand cleanup operation. Fluid level at surface results in accurate logging by eliminating invasion into reservoir and support improved operations.
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 174169, “Controlling Losses When Recompleting Low-Pressure Reservoirs,” by G. Uguna, Petroamazonas, and R. Rachid, SPE, A. Milne, SPE, and S. Ali, SPE, Schlumberger, prepared for the 2015 SPE European Formation Damage Conference, Budapest, Hungary, 3–5 June. The paper has not been peer reviewed.
A challenge in many permeable, water-sensitive, subhydrostatic reservoirs is avoiding the loss of completion fluid when completing or working over wells. To overcome the limitation of conventional fluid-loss-control pills, a low-viscosity system was developed. The system is composed of a viscous disproportionate permeability modifier (VDPM) with sized synthetic polymer particles and fibers, which degrade into organic acids. The VDPM reduces the effective permeability to water-based fluids, and the sized particles create an impermeable filter cake. When the particles degrade, the organic acid acts to break any remaining polymer.
Traditional Polymer-Gel Systems
The limitations of many conventional fluid-loss-control pills have resulted in the development of a number of solids-free fluid-loss-control pills. In high- permeability reservoirs, a highly crosslinked gel is needed to achieve good fluid-loss control. Polysaccharides, such as guar, have been widely used for this application because of their low cost and availability. These guar-based fluids are typically crosslinked with borate or organometallic crosslinkers. The viscosity of crosslinked guar decreases significantly at temperatures greater than 200°F because of the limited thermal stability of the polymer. For higher-temperature applications, polyacrylamides can be used to form crosslinked gels.
A limitation of crosslinked polysaccharide and polyacrylamide polymers is that they require an internal or external breaker. The breaker is required to break the crosslinked polymer and lower the viscosity of the fluid so the broken gel can flow out of the formation matrix. Even when using an internal breaker, some polymer remains in the pore spaces, effectively reducing and damaging the permeability of the formation.
To overcome the limitation of residual-polymer damage, hydroxyethylcellulose (HEC) has been used extensively because of its low residual-solids content. However, linear HEC polymer solutions do not form rigid gels but control fluid loss through viscosity and gradual filtration. This means that, as the linear fluid penetrates deeper into the formation, the shear rate decreases and the apparent viscosity increases. Permeability damage has been shown to increase with increasing penetration of viscous fluids, not only with HEC.
Despite all the advances made regarding the design of fluid-loss-control pills, the greatest challenge remains the same, which is to have a fluid that prevents the loss of water-based fluids into the reservoir but does not limit the production of crude out of the reservoir when the well is put on production.
Abstract The challenge in many permeable water sensitive subhydrostatic reservoirs is to avoid the loss of completion fluid when completing or working over wells. It is not unusual to lose up to 1,000 bbl of completion fluid, resulting in a 20% to 50% reduction in production. Several different approaches have been used to prevent these losses. The most common is viscous fluid-loss control pills with sized particulates, such as calcium carbonate, which often requires a further treatment to remove the solids. An alternative is a solids-free crosslinked viscous polymer pill, which must be spotted and kept in position across the reservoir prior to it crosslinking. To overcome the limitation of conventional fluid-loss control pills, a low-viscosity system was developed. The system is composed of a viscous disproportionate permeability modifier (VDPM) with sized synthetic polymer particles and fibers, which, with time and temperature, degrade into organic acids. The VDPM reduces the effective permeability to the water-based fluids, and the sized particles create an impermeable filter cake. When the particles degrade, the organic acid acts to break any remaining polymer. The system can either be bullheaded or spotted in the wellbore prior to perforating and effectively seals the formation for several days. The fluid-loss pill has been used successfully in more than 25 wells. The volume of fluid lost into the formation is often less than 1 bbl, and production is maintained after the workover. The technique has made it possible to selectively stimulate new intervals or recomplete wells with an average increase of more than 240 BOPD, whereas, historically, a decline in production was not uncommon.
Abstract Oil well cementing uses a variety of organic additives such as dispersing agents, retarders or fluid loss control additives. The later, which prevent interstitial water from filtering into the formation during cement placement, are generally polymer based. A widely used class of fluid loss control additive are the high molecular weight Sulfonated copolymers, generally comprising AMPS (2-Acrylamido-2-methylpropane sulfonic acid) copolymerized with Acrylamide (Am) or N, N′ Dimethylacrylamide (DMA). The mechanism of action of these polymers has been studied recently and it was demonstrated that adsorption onto the cement surface is crucial to achieve the required product performance. It was also shown that other solutes and admixtures present in the cement interstitial solution can hinder adsorption resulting in performance losses. Thus it has been recommended to incorporate an additional monomer containing strongly adsorbing units in the copolymer to enhance the interaction with the cement surfaces hence limiting competitive adsorption issues. In this study we investigated the use of diblock copolymers comprising a short but strongly adsorbing block and a long second block of DMA-AMPS as a potential new class of filtration control agent. We showed that diblock copolymers with much lower molecular weights than statistical polymers can provide satisfactory fluid loss control performance. Furthermore, it was demonstrated that these structured polymers show good formulation flexibility and deliver more robust performance in the presence of a wide range of admixtures and solutes. Finally we focused on the analysis of the adsorption on cement of various formulation admixtures and how it affected the adsorption of our diblock copolymers. With the aid of an analytical method utilising size exclusion chromatography of collected filtrate from HPHT filtration cells, it was possible to have a direct access to a fluid loss polymer concentration in the filtrate even in the case of complex formulations. Based on these studies, the mechanism of action of the diblock copolymers as fluid loss control agents is discussed with reference to that evoked for statistical polymers.
Abstract Hydrolytically degradable polymers (generally aliphatic polyesters) have been used in a variety of applications in the oil field, such as fluid diversion, fluid-loss control, and filter cake removal applications. In general, diverting agents and fluid- loss-control materials are only necessary to perform the intended function for a finite amount of time. Once the well is completed or placed on production, it is desirable that the degradable materials be removed so that they no longer have any influence on subsequent fluid flow. With time and temperature, the degradable polymers will break down by forming water- soluble byproducts, leaving behind limited, if any, residual formation damage. The effective formation sealing by these materials while in place, and eventually during cleanup, has made them sought after for a growing number of applications. However, for cooler temperatures, and applications where the well must be placed on production more quickly, there have not been many options to controllably increase the rates of polymer degradation. This has considerably limited the realization of the full potential of the technology. There is an increasing demand for faster cleanup of the diverting agents and fluid-loss- control agents for applications at cooler temperatures. Strong acids and bases are known to accelerate the degradation of these polymers. Use of such materials can present several disadvantages, such as corrosion and/or undesirable reactions with the formation. Although enzymes are used for the activation of ester hydrolysis, the use of enzymes has not demonstrated effectiveness in all situations. There is a need for chemical activators that can controllably accelerate the degradation of polymers at wellbore temperatures without the detrimental effects of using strong acids or bases. This paper discusses specific activators that can degrade various types of polymers and polymer blends containing degradable functional groups in the polymer backbone. The polymers tested in this study had variable crystallinities and chemical compositions. This paper also presents a new approach to degradation activation that should be more desirable in oilfield applications where the degradation times can be controlled for wide range of temperatures and wellbore conditions.
Abstract In Ecuador, the principal reservoirs are subhydrostatic, with permeability ranging from 100 to 2,000 mD and significant clay content. The crude oils are prone to form emulsions in contact with completion fluids. After workover operations—pulling electric submersible pumps (ESP) or recompleting a well—it is common to lose more than 1,000 bbls of completion fluid, resulting in a 20% to 50% reduction in production. Fluid loss control pills containing sized particulate, such as calcium carbonate or sized salt, are frequently used to control the fluid loss; however, a further treatment is required to remove the solids. Using this technique, 25 wells producing a total of 8,000 BPD were worked over, after which the production decreased to 5,000 BPD. The main challenges in developing a solids-free fluid loss control pill to control losses of completion fluid during a well intervention are: a) the low, subhydrostatic (0.16 to 0.36 psi/ft.) reservoir pressure, b) high matrix permeability, and c) cleanup when well is put on production. To overcome this, a highly viscous, polymer crosslinked fluid with an internal breaker was developed to temporarily isolate the reservoirs. Using this fluid, it is possible to work over a well without losing fluid into the reservoirs and having the associated loss of production. In 15 wells worked over using the fluid loss control fluid combined with modified workover procedures, fluid losses were controlled and production was maintained after the workovers. This technique also made it possible to selectively stimulate certain intervals, increasing production. The total production prior to the interventions was 7,300 bpd. Afterward, it increased 27% to 9,250 bpd. The fluid-loss control fluid, together with the new workover procedures, has now been adopted as a standard. It has proved an effective means to protect the subhydrostatic reservoirs in mature fields during workover interventions.