A few horizontal wells were drilled in Kuwait, heavy oil field, as a part of cold production testing. Various workover interventions were performed on these wells. However, some of the wells showed sharp production decline and were producing below expectations. It was suspected that formation damage may have occurred in these ultra-low reservoir pressure wellbores due to the overbalance of the fluids used during interventions.
Concentric coiled tubing (CCT) technology comprising of a downhole jet pump, was recently employed for the first time in Kuwait and was determined to be an effective method to clean the horizontal sections and investigate the reasons for the production problems. The single phase cleanout fluid is circulated down the inner string to power the jet pump, creating a localized drawdown that vacuums the formation solids or fluids out of the wellbore and the combined sand/fluid stream returns via the CCT annulus. The multiple operating modes provided the benefit of cleaning and treating the wellbore in the same run.
This specialized system was successfully utilized to remove sand, evaluate the formation damage and enhance production; meeting all objectives in a single well intervention. Pressure and temperature gauges run below the tool on two wells recorded bottomhole pressure of only 150 psi. Post-production results has pushed the boundaries of the well interventions in heavy oil field in Kuwait and has unlocked several wellbore cleanout and formation damage evaluation opportunities using the CCT technology.
This paper reviews the benefits of the concentric coiled tubing technology and provides a comprehensive case study of the first three horizontal wells. The analysis of the sand and fluid influx profiles obtained during the vacuuming process assisted in to evaluating well production provided crucial data in formulating a management strategy.
Al-Muhailan, Mohannad (Kuwait Oil Company) | Patil, Dipak (Kuwait Oil Company) | Aljarki, J. (Kuwait Oil Company) | Mahesh, V. S. (Kuwait Oil Company) | Shehab, A. (Kuwait Oil Company) | Al-Azmi, Salah (Kuwait Oil Company) | Alshammari, Faisal (Baker Hughes) | Al-Jaber, Mohammed (Baker Hughes) | Ababou, Mounir (Baker Hughes)
This paper highlights the design, planning, challenges, operational complications and successful execution of coil tubing application in active deep well in West Kuwait. The aim of coil tubing job is to clear the pipe from inside to recover the stuck pipe to eliminate the sidetrack in highly pressurized complicated Salt/Anhydrite sequence.
In one of the West Kuwait wells, during drilling the well got a kick with high gain rate. During shutting in and at starting of killing the well, it was observed that the pipe & annulus were plugged. Pipe puncture job was carried out & the well was killed off bottom with 19.7 ppg mud. Throughout running in hole with the free point locator tool prior to back off job, the held up was observed at 12,490 ft i.e. 1,300 ft above the bit. It was then selected to clean inside drill pipe to avoid sidetracking.
The well conditions presented challenges to the design and operation of coil tubing in this well. Challenging factors included: Use of high weight and yield strength, 15 ksi coil tubing, high mud density of 19.7 ppg, high pumping pressures, deep well, ID restriction 3 ½ in DP with 2 in ID, active well, deviated well of around 57 degrees. The coil tubing job design was critical for success of the operation. It included selection and analysis of coil tubing material, size, wall thickness; managing potential coil tubing burst and collapse pressures, calculation of coil tubing stretch, circulation pressure with high density mud, coil tubing force analysis, and wellbore solids removal with very minimum clearance & minimum pumping rate.
Initial simulations with 1.5 in coil tubing showed that circulation pressures would go above the 15 ksi rating. It was then decided to switch to high pressure 1.75 in coil tubing with which simulation showed that pressures at the rotating joint would be at 8,000 psi, using a jetting nozzle. While lowering with jetting nozzle, held up was observed at overshot due to the deviation. After changing jetting nozzle with the 1 11/16 in kick off tool, the coil tubing was able to pass through the gelled mud with circulation. To keep under check, high circulating pressure with aid of hydraulics analysis, related to dynamic circulation rate to 0.2 bpm at 7,500 psi & static rate to 0.35 bpm with 8,500 psi. Resulting in successfully clearing the drill pipe from inside to 12,974 ft, below the observed settling of hard barite.
Coil Tubing intervention with the restricted pipe diameter and heavy mud at high inclination well using a kick off tool was done for the first time in Kuwait. It achieved the purpose of cleaning the pipe to definite depth enabling back off operation below the jar & enhanced the chance of pipe recovery.
Historically, logging sub-hydrostatic wells to determine water entry points was restricted to a limited number of wells of a particular construction. These wells had to be dual-completion wells or 9 5/8-in. cased wells, able to accommodate dual-string completions. One string would be used for running the logging tool via coiled tubing or wireline, and the other would be used for artificial lift and produce the well while logging.
Single-completion wells were a challenge for production logging runs because two strings of tubing and dual wellhead would be required, necessitating workover rig operations. However, for smaller cased holes such as 7- or 4 ½-in. casings, running a dual completion string is typically too complex or impossible. When operators had to determine water-entry points in fields with high water cuts, well intervention costs could become prohibitive to contemplate.
Instead, a conventional concentric coiled tubing (CCT) string can be transformed into a specialty CCT by adding an encapsulated single-conductor wire inside. The string can then be run with a modified vacuum bottomhole assembly and a logging tool to achieve production analysis and logging simultaneously.
Combining these two proven technologies into one solution is an economical way to produce the wells via the CCT and its vacuum BHA while logging the well in one run. It has economic benefits to operators by eliminating the high cost associated with installing dual completions and dual wellheads. It also enables logging in 7- and 4 ½-in. cased holes that were not accessible for production logging using prior technologies.
This paper will describe the revolutionary technology and case histories along with lessons learned. It will also describe in particular the applications of the technology for high-producing water wells.
Concentric Coiled Tubing Vacuum Technology (CCTVT) was developed in the mid 1990s in Canada, and since then has spread worldwide. The technology was initially focused on sand cleanouts in heavy-oil, low-pressure, deviated wells, where
other cleanout methods, including conventional CT interventions, were inefficient. Under such conditions, CCTVT provided a simple but very effective solution.
In general terms, the technology comprises a downhole jet pump run on a concentric coiled tubing (CCT), which is a coiled tubing string inside another coiled tubing string. A single-phase fluid is pumped through the inner string to power the
downhole jet pump, creating a localized drawdown that vacuums well fill (fluids and/or solids), increases the return pressure, and circulates fluids and solids back to surface via the CCT annulus.
Since the introduction of this technology, it has gone through several updates in order to face new challenges. The latest BHA versions include multiple operational modes to improve cleanout efficiency, minimize runs, function with supplementary tools
and most recently, to accommodate a specialized electric conductor to run a real-time logging tool in conjunction with the jet pump.
Currently, the application of the system has extended beyond solids removal on heavy oil, onshore wells. The current work scope ranges from very simple operations such as drilling fluid and filter cake removal, liquid unloading, inflow profiling,
evaluating completion integrity, matrix stimulation, pressure and temperature logging, etc.; to more complex and challenging operations such as memory production logging, H2S inhibition, multilateral wells, chemical sand consolidation, hydrate removal and realtime production logging. These operations are occurring in the full range of oilfield locations: onshore and offshore wells, from fixed installations to mobile rigs, in jungle to arctic conditions.
This paper will summarize the new techniques that are being applied in many global locations with CCTVT. Each technique will be illustrated with case histories detailing time and cost savings and, where relevant, production improvements.
Recovering acid after a stimulation job can prove to be very challenging in sandstone reservoirs, especially if the well has a potential to produce H2S. When the customer's gathering station is unequipped to handle acid or water, then it becomes impossible to flow the acid via the flow lines. The acid must be disposed at the surface in a separate facility as fast as possible. Allowing the acid to remain downhole for long time carries the risk of damaging the sandstone reservoir due to secondary precipitations. The customer, as per standard operating policy, does not allow flowing back fluids into their pits. Only return tanks are allowed, adding to the difficulty of flowing back stimulation fluids.
With the help of concentric coiled tubing, an innovative technique to produce back acid from H2S wells became the sole solution to a major flowback challenge in South Oman wells. The technology successfully enabled the recovery of stimulation acid from a customer's well that had high H2S content, without producing any H2S at surface. The unique approach consisted of mixing an H2S scavenger with a scale inhibitor. This mixture was used as the power fluid for the concentric coiled tubing tool. With the help of the good shearing energy at the WellVac BHA, combined with the small concentric coiled tubing return annulus, the H2S scavenger solution was perfectly mixed downhole with the return fluids, and no H2S was produced or recorded at the surface return tanks. This new technique will certainly open new possibilities to stimulate many more wells that could not be stimulated previously due to high H2S content. The technique would also eliminate the great costs of a portable flare system.