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This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 179090, “Optimization of Single-Trip Milling Using 2-in. Coiled Tubing,” by Elizabeth Snyder, SPE, and Justin Noland, SPE, Sanjel, prepared for the 2016 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, Houston, 22–23 March. The paper has not been peer reviewed.
Larger-diameter coiled tubing (CT) recently has been used to perform millouts because of its improved setdown force and increased annular velocities (AVs) for cleanout purposes. Service companies and operators have reduced the number of wiper trips when using larger-diameter CT, to save time and money. Milling efficiency using 2-in. CT can be dramatically improved by maintaining proper fluid rheology throughout the operation. By doing so, 2-in. CT has been used to perform single-trip millouts, reducing operational time by 40%.
The main objective of CT operations in horizontal wells is to clean the lateral completely of any debris without compromising the well’s integrity. The first and most important component of single-trip-millout operations is correct fluid rheology. This system comprises the AV, Reynolds number (RE), and fluid viscosity. With correct use of chemicals and proper AVs, the fluid system allows sand and debris to travel out of the wellbore.
The RE determines the flow regime as laminar, transitional, or turbulent. Turbulent flow is characterized by swirling of the water (i.e., presence of eddies), which agitates the settling bed, enabling sand and debris to flow out of the lateral and, in turn, out of the well. RE can be broken into three components: fluid velocity, hydraulic diameter (flow area between casing and coil), and kinematic viscosity (funnel viscosity). In this case, the hydraulic diameter is predetermined by the 2-in. CT working in 7- to 4.5-in. cased wells, leaving the velocity and viscosity dependent on each operation.
When pumping any treatment fluid through CT, a fluid friction reducer (FR) must be pumped continuously to reduce friction pressure generated from pumping fluid through a restricted area. The FR decreases the circulating pressure, enabling higher pump rates to be achieved. A rheology-control unit uses electric variable-frequency-drive pumps to add FR and other chemicals to the fluid and to provide the ability to change dosage on the fly. These pumps are accurate down to thousandths of a gallon, enabling higher chemical usage in smaller doses. The chemicals are injected directly into the flow, then inline mixers enable shearing of the chemical to attain consistent mixing. Direct injection and inline mixers eliminate the unnecessary waste and oversaturation of chemicals seen when using conventional mixing tubs. This optimized fluid increases the longevity of the recirculated water while maintaining a high RE, in turn decreasing the cost to the operator.
Another common practice of conventional millouts was to send multiple linear-gel sweeps throughout the lateral. The objective was to increase the viscosity of the fluid to lift sand and debris out of the well. However, it has been discovered that the more-viscous-gel sweeps lead to laminar flow, which is not conducive to moving sand and debris in the lateral. The laminar fluid flows over debris, enabling it to settle and stay in the lateral section of the well. Gel sweeps have viscosities of 20 cp or greater, compromising the RE down to below 20,000. Because the viscosity is high and the RE is too low to transport sand and debris, gel sweeps were limited during optimized single-trip millouts.
Abstract Conventionally, the same best practices from coiled tubing cleanout interventions in vertical wells have been applied to horizontal wells; pumping a high volume of viscous gel sweeps and performing a number of wiper trips in an effort to remove debris from the wellbore. Very often this practice leaves a significant amount of solids behind as a result of inadequate fluid rheology, increasing not only the risks of the operation but the overall completion cost of the well, while falling short from accomplishing the main objective of the Coiled Tubing (CT) intervention. With the added pressure of reduced oil prices, operators are looking for ways to increase efficiency and reduce cost. Larger diameter CT has recently been used to perform millouts due to its improved set-down force and increased annular velocities for cleanout purposes. Service companies and operators have reduced the amount of wiper trips when using larger diameter coil to save time and money. Milling efficiency using 2 inch CT can be drastically improved by maintaining proper fluid rheology throughout the operation. By doing so, 2 inch CT has been used to successfully perform single trip millouts reducing operational time by 40%. This paper will provide an analysis of the theoretical process and time comparison of multiple verse single trip millouts using 2 inch CT. The fluid rheology required to adequately clean out horizontal wells relies on the effective utilization of chemicals and correct fluid control throughout a given operation.
Larger-diameter coiled tubing (CT) recently has been used to perform millouts because of its improved set-down force and increased annular velocities (AVs) for cleanout purposes. Service companies and operators have reduced the number of wiper trips when using larger-diameter CT, to save time and money. CT can be dramatically improved by maintaining proper fluid rheology throughout the operation. CT has been used to perform single-trip millouts, reducing operational time by 40%. The main objective of CT operations in horizontal wells is to clean the lateral completely of any debris without compromising the well's integrity.
Abstract Development of unconventional resource plays traditionally were completed using the "plug and perforate" the method (plug-n-perf). In recent years, however, multi-stage fracturing sleeves have seen growing industry acceptance as an alternative completion method to plug-n-perf and is now being employed with increasing frequency with both cement and openhole isolation methods in unconventional resource plays. This type of system is operated by dropping a ball from the surface that seats in a landing baffle to actuate the sleeve and allow for fracturing of the formation. These balls and baffles often can be removed from the ID of the casing string by milling, post frac to remove possible restrictions. However, there are situations that can affect the successful milling of the balls and baffles. This paper explores the conditions that can affect the ball and baffle millout process of multi-stage fracturing sleeves. Different aspects of the milling process will be reviewed to determine the critical elements that must be taken into consideration when milling the balls and baffles. Specific factors include multi-stage fracturing sleeve dimensions, wellbore trajectory, torque and drag, depth location, mill design, weight-on-bit (WOB), viscous pill sweep frequency, and other milling procedures. The investigation of the millout of 185 multi-stage fracturing sleeves in Eagle Ford Shale well completions will analyze these factors, which then will be contrasted with surface millout testing on over 100 multi-stage fracturing sleeves performed on a custom millout testing machine. The surface testing allowed visual observation of millout processes and real-time changing of millout variables that reduced risk and lowered operating cost. Both sets of data will then be analyzed to illustrate the critical factors for successful millout operations and discuss the solutions to the millout challenges.
Stolte, Chris (Shell China Exploration and Production Co. Ltd.) | Wu, Chris (Shell China Exploration and Production Co. Ltd.) | Carroll, Darryrl (Shell China Exploration and Production Co. Ltd.) | Shiqian, Wang (PetroChina Southwest Oil & Gas Co.)
Abstract Shell China, in partnership with PetroChina Southwest began horizontal completions in its Sichuan shale gas project in 2012. By the end of 2012, two vertical and three horizontal wells were drilled and completed in the Lower Longmaxi Hot Shale interval. Performance has continuously improved throughout the implementation of the completions program and significant technical and operational challenges were overcome. The three horizontal wells were drilled with progressive horizontal lengths. On the third and longest well in the sequence, difficulties were encountered when attempting to mill plugs in the horizontal section. Well pressures were in excess of coiled tubing limits and the plugs could not be milled. As a result, the well was produced with flow-through plugs in place until the well pressure could be reduced and a higher pressure coiled tubing reel was available. Due to the high well pressures observed, additional measures were taken to reduce operational risk during the coiled tubing millout and ensure a successful operation. The flowback package design was improved by utilizing hydraulic chokes in place of manual chokes and a dual-plug catcher system was installed in the flowback package. In addition to improving the flowback package for millout operations changes were made to the flare system. After the coiled tubing millout was completed, difficulties were encountered during the full phase well test requiring the horizontal flare to be replaced with a vertical flare due to HSE concerns. The challenges experienced during this operation and the improvement aspects gained will be utilized in the completions planning and execution of the next phase of horizontal wells. This paper highlights the coil tubing millout, flowback, and well test learning experiences gained during the completion operations of the second and third horizontal wells.