Lost circulation, while cementing, compromises the objectives of cementing an oil or gas well. Losses encountered during cementingcan cause a weak casing shoe, poor zonal isolation, early water breakthrough for an oil producer, as well as increasing the possibility of costly intervention work. Execution of primary cementing operations can be subject to unplanned circumstances; when a slurry is being pumped or displaced and losses are recorded, in most circumstances the operation switches to damage limitation by slowing down the pumping rate. The Nong Yao field (Figure 1)is characterized with an interbedded unconsolidated sand / clay lithology within a highly compartmentalized structure, and as such, well construction operations have encountered unpredictable lost circulation during 7-in. casing cementation (but rarely during the drilling phase). Over 60% of the wells recorded losses during 7-in. cementing; it became evident that a proper loss mitigation plan was necessary to combat lost circulation and improve the probability of successful cementation execution. Although the primary objective is to achieve zonal isolation, equally as important for Nong Yao drilling operations are provision of annulus barriers, slurry compressive strength development, "gas tight" qualities, optimum slurry Thickening Time (TT) to allow for safe batch drilling operations. Figure 1 Nong Yao field localization To overcome the challenges, an "out of the box" approach was essentialwhich yielded two innovative solutions: i) a combination of advanced lightweight cementwith engineered reticular fiber (ERF) systems, which allows safer placement of the cement in the annulus, while minimizing the potential losses; ii) a combination of several lost circulation materials (LCM) in an optimized ratio in an engineered fiber-basedlost circulation weighted spacer package, which has an additional function of preventing and mitigating risk of losses during cementing. This approach was intended to reinforce the loss zones by using the four-step methodology; disperse, bridge, plug and sustain. The severity of lost circulation while cementing was significantly reduced without compromising the abovementioned objectives. This paper will discuss the successful implementation of the new approach solution by integrating different technologies to overcome the challenges of unpredictable losses during cementation. Two case histories from numerous jobs will be discussed with cement post-job evaluation via playback simulations and standard cement bond logs, which validates that the new approach increases the chance of achieving well objectives. Consequently, the risk of unplanned (UNP) operations and costly remedial operations are substantially reduced.
Effective zonal isolation in wellbores with a challenging mud removal environment is well known to be very difficult to achieve. In wells at the technical limits of Non- Aqueous Fluid (NAF) removal prior to cement placement, cement bond quality and hydraulic isolation can be compromised by leaving channels behind the casing, which can result in several long-term well integrity issues. An Interactive Cementing System (ICS) is developed through special experimental methodologies to mitigate mud channeling issues and improve zonal isolation, by immediately interacting with any residual mud channels left in the well after cement is in place, hence reducing the permeability of mud channels and sealing off microannulus gaps. Casing centralization is considered to have the greatest influence on mud removal efficiency because it directly affects the flow movement on each side of the wellbore. Mud removal has been studied from numerical simulations, laboratory experiments, and field results, and these show that good mud removal can be achieved only when adequate casing standoff is achieved during cementation. In modern wells where there are many operational restrictions and limitations, especially in highly deviated and horizontal wellbores, final cement designs may not allow good casing standoff and thus not all of the best practices for effective mud removal can be applied. The objective of the innovative cement system is to have a design that interacts with residual mud in the annulus to "fix" the channels, thereby enhancing cement bond quality and zonal isolation. Two detailed case histories of the application of this technology in the development campaign showed visible improvement in cement bond logs using the ultrasonic imaging tool as compared to offset well that was cemented using a conventional cement system. After two successful implementations, the ICS was selected as the cement system of choice for wells with challenging mud removal.
Exploration drilling obviously requires a robust drilling fluid system to be a key factor in overcoming both the known and unexpected challenges of a structure that consists of reactive clay and lost circulation zones. Extra consideration has to be given to regulatory environmental requirements and complications resulting from regional politics. A High-Performance Water Based Mud (HPWBM) system was selected to address the aforementioned issues. The HPWBM was customized to respond to the subsurface conditions with the main requirement to provide maximum shale inhibition through a non-dispersed environment. Polyamine was utilized to stabilize all types of clay; an encapsulation polymer and a non-ionic polymer were included to prevent dispersion and to seal micro-fractures. A complete shale study was performed to determine the optimum concentration of the base fluid and each shale inhibitor. Then hydraulic behaviour of the mud was simulated with contractor proprietary software to understand the parameters for optimal hole cleaning as well as Equivalent Circulating Density (ECD) simulation. The HPWBM system successfully facilitated the execution of the exploration well and provided highly effective clay stabilization. No Non-Productive Time (NPT) was recorded as a result of reactive clay issues. The mud system also facilitated a good rate of penetration (ROP), formation stability, and lubricity. Waste cuttings transportation was not required. In addition, there is also no requirement for costly base oil including its associated transportation costs. The successful implementation of the HPWBM yielded an estimating saving of 25% compared to invert emulsion fluids, prior to considering costs associated with an expensive Liquid Mud Plant (LMP), environmental, and freight costs. Significant cost savings were achieved by eliminating the need for LMP rental, mobilization and demobilization. Another notable saving was realized from the reduced system maintenance of the HPWBM as less dilution was required compared to a regular Water Based Mud. Thinking outside of the box and embracing the departure from the default consideration of an invert system with a thorough risk assessment augmented value to wellbore construction. A smartly designed HPWBM system provided performance comparable to an invert emulsion system but with superior benefits with respect to environmental protection, simplified logistics and lower costs.
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