電纜爬行器，是一種用于水平井或大斜度井的測井過程中，將測井儀器輸送到預定位置的特殊工具，具有操作方便、工作效率高、使用成本相對較低的特點。本文對比了其與CT的優劣，介紹了其在高溫高壓水平氣井修井作業中的成功應用。
來自 | Muhammad H. Al-Buali等
編譯 | 白小明
在過去的幾十年里，水平井鉆井在油氣工業中的地位正變得越來越重要，水平井可以有效鉆達復雜的地質結構，開發邊際油田，增加儲層接觸面積。在此期間，出現了一些用以解決水平井鉆后修井問題的新技術。通常，當最大井斜為70°~80°時，重力輔助傳輸方法往往受到限制，而且該方法不適用于曲折的井眼軌跡。最終，在復雜井的大井斜段，需要使用連續油管（CT）爬行器和電纜（e-line）爬行器來將修井鉆具組合下到位。
電纜爬行器技術已成功應用于沙特成熟油田的油藏監測以及向井內送入生產測井管串組合等作業。相比其他形式的傳輸方法，如CT，爬行器具有特別的優勢。具體來說，爬行器可以用于具有復雜井眼軌跡的、采用裸眼完井的水平井，可以獲得生產井和注水井的高質量流動剖面數據。
高溫高壓（HPHT）井鉆井技術在沙特氣田的應用，帶來了水平井修井新的難題，包括但不限于使用爬行器下入射孔槍，傳輸較重的底部鉆具組合（BHAs），以及在井底溫度（BHT）大于300°F（149℃）的條件下進行作業。為了解決上述難題，需要改進技術并改變工具下入方式，以成功使用爬行器進行修井作業。
沙特致密氣的開發
沙特的Ghawar油田富含大量的伴生氣
Ghawar油田的Khuff地層為晚二疊紀、埋藏較深、非均質的白云質碳酸鹽巖氣藏，有兩個主要的產層，即Khuff-B和Khuff-C。自20世紀70年代以來，已在Khuff-C產層開展了大量的鉆井和開發工作，而更致密的Khuff-B產層近期才開始采用水平井鉆井和完井工藝進行重點大規模開發。Khuff-B產層滲透率較低，氣井需要采取酸化增產措施，使用多級橋塞-射孔方法，或者使用多級壓裂尾管系統。關于該油藏的主要特點以及完井方式已有大量的描述資料。
該氣藏地層的非均質性以及甜點位置的不確定性，意味著需要使用射孔的方式將長井段與儲層連通，并采用增產的措施提高成功的可能性。種種因素表明，開發該氣藏時所鉆的這些井，在有很大挑戰性的同時，非常適合采用爬行器進行修井作業，不過要想成功進行作業，有必要進行一些技術和操作方面的優化。
適用于沙特高溫高壓氣田的爬行器技術
針對這些井，在設計使用電纜爬行器作業時，需要解決如下問題：
腐蝕環境
這些井高含硫化氫（H2S）和二氧化碳（CO2），必須采用特殊設計的由高鎳合金制成的抗腐蝕電纜，H2S和CO2中等含量水平環境下需要耐溫高達425°F（218℃）。與非酸性環境中所使用的電纜相比，這些電纜的一個重要缺點是其具有高的電阻抗。高阻抗不利于對于向井下設備傳輸電力，尤其是交流電（AC）。
更高的溫度
隨著油藏目標越來越深，爬行器必須能夠承受300℉（149℃）至330℉（166℃）的高溫，而且將來溫度很可能會超過這一范圍。
更長且更重的BHAs
與傳統的油藏監測生產測井工具相比，使用的BHA（尤其是帶有射孔槍的BHA）將更長更重。
從成功下入爬行器的角度來說，其必須滿足以下目標：
1.能夠在高溫條件下可靠操作；
2.能夠使用超長高阻抗電纜操作；
3.能夠承受爆炸性射孔槍帶來的高沖擊載荷。
爬行器的核心是電機，其驅動液壓泵為輪組提供動力，電纜爬行器可以使用AC或DC電機。雖然AC電機爬行器在高溫環境下有著良好的使用記錄，但是如前所述，它們不適用于腐蝕環境下使用的超長高阻抗電纜。
DC電機是用于爬行器更可行、更具操作性的選擇，盡管其在高溫環境下的使用記錄有限；然而，在經過近期一些技術改進后，主要生產商將DC電機引入到爬行器的設計中，DC電機驅動的爬行器可以在更高的井眼溫度環境下維持更長的操作時間，本文將介紹使用DC電機驅動的爬行器在井下的使用情況。
1.將抗沖擊性引入到爬行器的工具設計和測試功效中；
2.在爬行器和射孔槍之間加入減震工具，以減小沖擊；
3.根據使用狀況進行維護，在爬行器返回實驗室進行全面拆卸、維護和翻新之前，限定其傳輸射孔槍的趟數。
作業的可靠性
與其他井下電子技術的實施過程一樣，使用爬行器時有三個主要因素影響其作業的可靠性：
1.工具的操作范圍；
2.工具的有效性；
3.根據使用狀況進行工具的維護。
Successful Utilization of E-line Tractor in Horizontal High-Pressure / High Temperature Gas Wells
Over the past few decades, horizontal drilling has become increasingly important to the oil and gas industry. It enables efficient access to complex structures and marginal fields, and horizontal wells increase the reservoir contact area. New technologies have emerged during this time to address post-drilling intervention challenges presented by such wells. Gravity assisted conveyance methods are generally limited to wells with maximum deviations of 70° to 80°, and they are unable to negotiate tortuous well paths. As a result, coiled tubing (CT) and electric line (e-line) tractors are needed to deploy bottom-hole intervention assemblies in complex wells with greater deviations.
E-line tractor technology has been successfully deployed in Saudi Arabia for reservoir surveillance, deploying production logging assemblies in wells in mature fields1. Tractors provide specific advantages as compared to other forms of conveyance, such as CT. Specifically, they can successfully negotiate complex well trajectories in horizontal open hole well completions, enabling acquisition of good quality flow profiles in both producers and injectors.
The application of horizontal drilling to high-pressure/high temperature (HPHT) gas fields in Saudi Arabia has introduced newer challenges to horizontal well intervention. These include, but are not limited to, operating tractors with perforating guns, Fig. 1, conveying heavyweight bottom-hole assemblies (BHAs) and performing jobs at bottom-hole temperatures (BHTs) greater than 300 °F. In response, the technology and deployment strategies have had to be enhanced to perform successful tractor interventions in these wells.
TIGHT GAS OPERATIONS IN SAUDI ARABIA
Ghawar field in Saudi Arabia is one of the world’s largest oil producing fields, Fig. 2, and has large associated gas reserves.
The Khuff formation in Ghawar field is a Late Permian age, deep, heterogeneous carbonate dolomitic gas reservoir with two major producing zones — Khuff-B and Khuff-C. The Khuff-C zone has been extensively drilled and exploited since the 1970s, while the tighter Khuff-B zone has only recently been the focus of an extensive development program with horizontal drilling and completion. Wells with low permeability in the Khuff-B undergo acid stimulation operations using a multistage plug and perforation method or using multistage fracture liner systems. Key features of the reservoirs and completions have been previously described.
The heterogeneity of the formation and the uncertainty of locating the sweet spots sometimes mean that long intervals must be connected to the reservoir with perforating charges to improve the likelihood of success with the stimulation operation. A combination of factors makes these wells a unique and challenging setting for performing well intervention with tractors, and the optimization of several technical and operational factors is necessary for interventions to be successful.
TRACTOR TECHNOLOGY FOR SAUDI ARABIAN HPHT GAS FIELDS
Planning e-line tractor operations in such wells requires addressing the following concerns:
- Corrosive environment: The levels of hydrogen sulfide (H2S) and carbon dioxide (CO2) in these wells mean that specially designed corrosion resistant e-line cables must be constructed from a high nickel alloy8 and then rated to 425 °F for medium levels of H2S and CO2. One important downside of these lines is their high electrical impedance, as compared to non-sour service wirelines. The high impedance can be quite obstructive to the transmission of electrical power — especially AC power — to downhole assemblies.
- Higher temperature: With deeper reservoir targets, tractor deployment must withstand 300 °F to 330 °F, and temperatures could possibly increase beyond that range in the future.
- Longer and heavier BHAs: The BHAs being conveyed — especially those configured with perforating guns — are longer and heavier than the more typical reservoir surveillance production logging tools.
From a tractor deployment standpoint, the tractor must be able to satisfy the following objectives:
- Perform reliably under high temperature conditions.
- Operate on long high impedance wirelines.
- Withstand the high shock loads that come with the explosive perforating guns.
At the heart of the tractor is the electric motor, which drives the hydraulic pump that supplies power to the wheels. Both AC and DC powered motors are used in e-line tractors. Although AC powered tractors have a good track record with high temperature operations, they are not suitable for operating on long high impedance lines like the lines required by corrosive environments, as previously described.
As shown in Table 1, the DC powered motors are a much more feasible and operationally viable option for tractor deployment, albeit with some concerns over their limited track record in high temperature operations; however, after recent technological advancements in these motors are incorporated into the design of tractors provided by leading providers, DC powered tractors can sustain extended operations at higher borehole temperatures9, and they were selected for operations in the wells described in this article.
AC Tractors DC Tractors
Pros Highly reliable
- Proven track record with high temperature operations ? Usable with high impedance cables.
- Simultaneous communication and power transmission, as well as capacity for logging down with the e-line tools below the tractor.
- Better operational control for the tractor operator, reducing deployment risk.
Cons Limited usability on long high impedance S77 corrosion resistant wirelines.
- Heavy surface equipment.
- Lack of fine control for tractor operator. ? Limited experience with high temperature(>300℉) operations before 2013(this article provides information about recent experience to demonstrate that the improvements have extended their range).
- More complex design as compared to AC powered motors.
- Incorporating shock tolerance into the tractor tool design and testing efficacy.
- Placing a shock absorption tool between the tractor and the perforating guns to reduce impact.
- Performing condition-based maintenance to ensure that tractors are exposed to a finite number of perforating runs before they are returned to the lab for comprehensive disassembly, maintenance and refurbishment.
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