鹽層有濱海相和內陸相鹽湖沉積兩種成巖環境，基本物質以氯化鈉為主，也含有其他復合鹽類，如：氯化鈣、氯化鉀、芒硝等，其在構造應力、上覆巖石壓力、溫度、傾角、厚度等因素作用下會發生溶解、蠕變、滑移或塑性流動，對套管產生相當大的外擠載荷，會導致套管擠扁、彎曲、變形甚至錯斷。因此，這類鹽層給鉆井或固井操作帶來了嚴峻的技術挑戰，甚至成為了增加單井生命周期中的“痼疾”。
在鉆井或固井作業過程中，礦物鹽溶解會影響泥漿性能甚至造成套管擠毀，這無疑會增加運營商的經營成本并增加非生產時間，甚至會導致難以預料的施工事故，使作業成本額外增加幾千到數百萬美元，這種情況對于深水鹽下油氣藏開發領域尤其是巨大的技術挑戰。
在鹽層鉆井作業時，固井水泥漿體系除要滿足正常施工所需的稠化時間、失水控制和抗壓強度要求外，還要充分考慮到泥漿體系的沉降穩定性和流動性。礦物鹽溶解會造成水泥漿凝固、流動性變差、稠化時間縮短等問題，嚴重影響頂替效率，進而影響固井質量。
哈里伯頓公司針對上述技術難題推出了SaltShield固井水泥漿體系，該技術將一套先進的決策建模軟件和一套先進的固井系統結合在一起，能夠有效解決巖溶層鉆進問題。
鹽層固井建模工具石油圈原創www.h29736.cn
WellLife 是哈里伯頓公司研發出的高端有限元分析軟件，該軟件不僅可以建模研究單井生命周期內（鉆井、試井、完井及生產階段）壓力與溫度變化對水泥環周期應力變化的影響，還可以研究鹽層對水泥漿固井體系的動態影響。
鹽層拉力或擠壓力的變化會導致水泥環所受載荷發生變化，哈里伯頓的WellLife 能夠對這種情形進行了建模研究。固井失效建模包括幾個方向：套管擠毀引起的居中度較差；鹽層在鉆井過程中的縮徑、井壁垮塌；水泥環和鹽層間形成的徑向裂紋；鹽層塑性流動對水泥環形成的剪切破壞。
WellLife?通過建模研究井深結構、地層物性以及單井長期井況來預測水泥環的穩固性，此外，軟件通過模型分析能夠得出水泥環在井下復雜應力環境下所必需的熱學特性與機械性能，模型中所考慮的系列特性參數包括楊氏模量、泊松比、體積收縮率、熱膨脹、抗壓強度、抗拉強度、熱導率和比熱。
軟件會以直方圖的形式表示固井水泥漿體系所能承受的井下應力狀態，同時也會顯示水泥環發生應力失效前所能承受的應力余量。當水泥環失效風險較高時，WellLife 軟件相關模塊就會針對水泥漿體系的系列參量給出調整值，從而增加水泥環體系強度、降低失效風險。
鹽層固井泥漿體系石油圈原創www.h29736.cn
SaltShield 能夠適應一般巖鹽環境和“侵略性”很強的光鹵石、溢晶石環境。強侵略性的礦物鹽一般都含有氯化鎂組分，該成分在溶液中的移動速度比氯化鈉要快100倍以上并可與水泥漿發生化學反應，影響泥漿體系的使用性能。該體系可以在鹽層段達到很高的頂替效率，快速穩固井筒，抗壓強度的快速變化使水泥漿侯凝時間最小化，膨脹性還能削弱鹽層蠕變的消極影響?，F場實踐表明，SaltShield體系可以幫助運營商有效縮短非生產時間并降低鹽層鉆完井等施工環節的作業費用。
鹽層的塑性流動對井筒完整性是巨大的威脅，會對水泥環、套管造成破壞。SaltShield 體系可使地層應力載荷分布更加均勻，從而防止發生套管變形或擠毀等作業事故。該體系在泥漿流動階段（泵送頂替）和固化階段（水泥環形態）都有著獨特的屬性，例如，即便在鹽侵比例達到了12%的情況下，水泥漿體系流變學性能適應性微小變化也有助于有效提高頂替效率。
一旦停止泵送，該體系抗壓強度就會迅速提高，從而有效承受鹽層蠕變載荷，在SaltShield 的設計過程中會嚴格控制一些參量，其中包括失水量、水化過程中體積損失、抗壓及抗拉強度、彈塑性等，從而使其能適應巖鹽地層，避免失效事故發生。即便井下體系鹽侵比例達到12%，水泥漿稠化時間也不會受到影響，最初與最終的抗拉、抗壓強度也都可以滿足固井施工要求，從而保障該體系可以迅速穩固套管與鹽層，在套管與地層之間形成一個完整的水泥環，使套管負荷均布，消除點載荷與不均勻載荷對套管的損壞。
傳統型水泥漿體系在水化過程中會發生體積收縮，進而造成對套管的載荷不均勻，甚至引起套管擠毀；SaltShield體系在固井過程中會發生膨脹，彌補了傳統型體系的不足。
強“侵略性”礦物鹽中的鎂離子會溶解于水泥漿并發生化學反應導致體系促凝，從而降低頂替效率。SaltShield 與礦物鹽接觸時能夠抑制化學反應的發生，抑制促凝現象，因而該新型體系在鹽層“重災區”（在巴西、西非以及北海深水開發區域，鹽層厚度可達2000米）固井施工中可以達到理想的頂替效率。
SaltShield體系還能夠抑制鹽層溶解，維持鹽層穩固。早期靜膠凝強度發展與水泥漿向水泥環轉變的固化時間縮短有助于減少地層流體入侵，尤其是在開發鹽下油氣藏的過程中意義更加顯著。
在腐蝕性環境中，溶進地層液體中的二氧化碳會形成碳酸，碳酸會與水泥環發生反應，降低其機械性能并增加其滲透性，從而對水泥環造成破壞。當鹽下油氣藏含有二氧化碳氣體時，例如巴西和安哥拉相應區塊，SaltShield體系會抑制該氣體的溶解。
技術優勢石油圈原創www.h29736.cn
- 當發生鹽侵時（包括含有鎂離子的強“侵略性”礦物鹽所發生的鹽侵），SaltShield體系可以減緩促凝現象的發生；
- 抗壓強度的快速建立與早期靜膠凝強度的發展有助于實現快速井筒穩定，還可以在套管與地層之間形成一個完整的水泥環，使套管負荷分布均布，消除鹽層蠕變對套管的損壞；
- 該體系對任意濃度的鹽溶液均具有兼容性，可以解決鹽侵問題，抑制鹽層溶解，維持鹽層穩固；
- SaltShield體系的各項參數在鹽侵前后保持穩定；
- 傳統水泥漿體系在水化過程中體積會縮小0.5%~4%，但是SaltShield 體系在水化過程中會發生體積膨脹；石油圈原創www.h29736.cn
- 可以承受鹽層塑性流動引起的載荷變化；
- 實驗表明，SaltShield體系與碳酸經過長達一年的“接觸”后，所形成的水泥環具有低滲透性，抗壓強度依舊很大；
- SaltShield體系對鹽溶腐蝕性和二氧化碳溶解流體具有抗性，可應用于鹽層或鹽下油氣藏開發；
- 該體系所適應井底循環溫度（BHCT）不可高于88℃，更高的溫度下需要添加緩凝劑；
- 不會對污染周圍環境。
如需獲取關于該技術的更多資料，請聯系石油圈技術小編：驚蟄，2582825239。
來自/Halliburton ?譯者/姚園 ?編輯/Lemon Zhang
Salt-zone properties are widely variable in composition, aggression of attacks on wellbore fluids, plasticity and pressures, thicknesses and boundaries, solubility and contamination. As such, salt sections are particularly unpredictable, making them very challenging not only during drilling and cementing operations, but throughout the producing life of the well.
Salt zones are notorious for causing problems such as wash out from dissolution of salt into the cement slurry or casing collapse, making it difficult for operators to stay within the authorization for expenditure and avoid non-productive time. Unpredicted issues can range in costs from a few thousand dollars to multiple millions of dollars. This is particularly true in deepwater fields where these zones are known as pre-salt zones and where rig costs are high and materials more constrained. Reactive salts can gel conventional cement slurries during placement to the point of no longer being pumpable. Formation salts can dissolve into the slurry and adversely affect curing and/or weaken the formation.
Halliburton offers the SaltShieldSM cementing service, a combination of an advanced modeling tool for planning along with an advanced cement system.
Modeling Tools for Planning Salt Operations
Halliburton has developed an advanced finite element analysis (FEA) tool, WellLife? service. This tool models the cyclic stresses to the cement sheath induced by pressure and temperature changes throughout the life of the well. While modeling the cyclic stresses from planned operations during drilling, testing, completions and production operations, this tool can also account for the dynamic influence of the salt zone.
Halliburton’s modeling includes analysis of pressure-induced changes involving tensile and compressive forces from the salt zone. The cement failure phenomena that can be modeled include de-bonding from casing, de-bonding from the formation, radial cracking, and shear failure. The model simulates the well structure, properties of formation, and long-term critical well conditions
to predict the competence of a cement sheath. Going further, the model then generates thermal and mechanical properties required to survive downhole stress conditions. The properties estimated in the modeling include Young’s Modulus, Poisson’s Ratio, shrinkage, thermal expansion, compressive strength, tensile strength, thermal conductivity, and specific heat.
The ability to withstand the downhole stresses is presented via histograms. These charts show the percent of remaining capacity available before succumbing to stress-related cement failure phenomena. If the sheath is shown to have a high risk of failure, WellLife service has a powerful module that indicates modification to the various cement-property options for increased capacity and decreased failure risk.
Resilient Salt-zone Slurry
SaltShieldTM cement delivers properties that overcome simple salts like halite, and even the aggressive salts such as carnallite and tachyhydrite. The aggressive salts contain Magnesium Chloride, which can move at rates 100 times faster than Halite and could chemically react with cement. SaltShield cement can enable placement of full columns of cement across the salt zone, quickly stabilizing the wellbore, minimizing wait-on-cement time due to rapid compressive strength development, and expanding while curing to help mitigate effects of salt creep. SaltShield cement is field-proven to help operators more predictably mitigate the unexpected non-productive time and operational expenses from salt zone issues.
One unique threat to well integrity from salt zones is plastic flow. Plastic flow, or salt creep, can damage the cement sheath, the casing or both. SaltShield cement helps distribute the formation load more evenly to alleviate stress loading even the stress loading from aggressive salts, thus helping prevent casing deformation and/or casing collapse. This is accomplished with specific properties for both the slurry state (during placement) and the
set state (as a cement sheath). For instance, minimal changes in rheology, even after 12% salt contamination (by weight of water), helps facilitate full coverage and efficient displacement during pumping. Once pumping stops, this system quickly develops compressive strength, withstanding load from moving salt. It is designed with tight control of properties such as fluid loss, volume reduction during hydration, compressive and tensile strength, and elasticity, helping avoid compromise to the cement system as well as the salt formation. Even at 12% salt contamination, thickening time, as well as initial and final strength development are not compromised, which can enable this cement to quickly stabilize the casing and the formation, while reducing non-uniform load points that can cause casing failure.
SaltShield cement expands while curing. Conventional slurry systems shrink during hydration and can thus invite irregular stress loading that can then induce casing failure.
SaltShield cement withstands chemical effects when in contact with salts. Magnesium ions present in harsh salts can dissolve in slurry systems and the reaction results in gelation, which can impede pumping. This slurry system is resistant to gelation from salt contamination, helping facilitate successful slurry placement even across long and chemically-aggressive salt sections such as zones found in offshore Brazil, West Africa and the North sea presenting as much as 2000 meters of salt rock involving aggressive salts.
SaltShield cement also helps prevent formation weakening or washout from dissolution of formation salts. Early static gel strength development and shortened transition time to a set sheath helps minimize formation fluid influxes, particularly important when exiting the salt zone where the casing could be set in the pre-salt reservoir or zones with potential for fluid flow.
In corrosive environments, CO2 gas dissolves into formation fluids and forms carbonic acid. This acid can damage a conventional cement sheath with a reactive mechanism that lowers the mechanical properties and increases permeability of the cement sheath. SaltShield cement is resistant to CO2 for seamless use when the reservoir contains CO2 and is below the salt section, such as in Brazil and Angola.
Benefits
- Helps mitigate gelation of cement slurry upon salt contamination, including aggressive salts containing magnesium chloride.
- Rapid compressive strength and early static gel strength development helps quickly stabilize the wellbore and facilitate further drilling while mitigating uneven point loading on casing that could otherwise cause collapse or failure due to salt creep.
- Compatibility with all concentrations of salt overcomes salt dissolution issues helping mitigate washouts or weakening of the salt formation.
- No significant variation in slurry properties before and after dissolution of formation salt in the slurry.
- Expands during hydration in contrast to conventional cement that shrinks 0.5% – 4%.
- Can withstand loads induced by plastic salt flow.
- Demonstrates low permeability and good compressive strength after one year of exposure to carbonic acid.
- Single system resistant to corrosive fluids from salt and CO2 for use across salt and reservoir section.
- Demonstrates low permeability and good compressive strength after extended exposure to carbonic acid.
- Can be used at bottomhole circulating temperatures (BHCTs) ranging from ambient to 190oF (88oC); higher temperatures would require a retarder in the slurry.
- Poses little or no risk to the environment (PLONOR).
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