Summary Offshore wells drilled in the central and northern North Sea have historically suffered from borehole-instability problems when intersecting the Upper/Lower Lark and Horda Shale formations using either water-based mud (WBM) or oil-based mud (OBM). A wellbore-stability investigation was performed that focused primarily on improving shale/fluid compatibility. It was augmented by a look-back analysis of historical drilling operations to help identify practical solutions to the borehole-instability problems. An experimental rock-mechanics and shale/fluid-compatibility investigation was performed featuring X-ray-diffraction (XRD) and cation-exchange-capacity (CEC) characterizations, shale accretion, cuttings dispersion, mud-pressure transmission, and a new type of borehole-collapse test for 10 different mud systems [WBM, OBM, and high-performance WBM (HP-WBM)]. The results of this investigation were then combined with the results of a well look-back study. The integrated study clearly identified the root cause(s) of historical well problems and highlighted practical solutions that were subsequently implemented in the field. The borehole-instability problems in the Lark and Horda Shales have a characteristic time dependency, with wellbore cavings occurring after 3 to 5 days of openhole time. The problems were not related to mud-weight selection but were instead caused by mud-pressure invasion into the shales, which destabilizes them over time. An experimental testing program revealed that this effect occurs in both WBM and OBM to an equal extent, which explains why nonoptimal field performance has historically been obtained with both types of mud systems. New HP-WBM formulations were identified that improve upon the mud-pressure invasion and borehole-collapse behavior of conventional OBM and WBM systems, yielding extended openhole time that allows the hole sections in the Lark and Horda Shales to be drilled, cased, and cemented without triggering large-scale instability. Look-back analysis also indicated that secondary causes of wellbore instability, such as barite sag, backreaming, and associated drillstring vibrations, should be minimized for optimal drilling performance. A new HP-WBM system, together with improved operational guidelines, was successfully implemented in the field, and the results are reported here.
U.S.A., Khuff gas reservoir is one of the most challenging gas fields in the Middle East; in the past several wells suffered from sustained casing pressure due to the downhole stresses and other contributing factors. The severity of the leaking annulus can range from the most hazardous blowouts, to less severe cases of residual gas pressure at the wellhead; recent zonal isolation solution consisted in using Flexible and Expanding cement system in the 12 ¼" open hole to case off the 9 ⅝" Liner, this system is good when the well operating envelope is well known, outside this envelope the Flexible and Expanding cement do not perform as well, this leads the cement to fail (A.S. Al-Suwaidi et al 2008).To further ensure a robust cement design and long term well integrity; Self Healing Cement is introduced in the 9 5/8" tie back section as an added assurance to provide a secondary barrier for the 12 ¼" open hole. Self healing cement is based on a responsive material with intrinsic self-healing properties automatically activated upon hydrocarbon exposure to rapidly seal by healing the damage; within hours the downhole well integrity is restored, reducing the health, safety and environmental risks and the extra costs associated to remedy to these problems including loss of production. This paper will cover the qualification and field implementation of the self-healing cement. It also includes case history covering Khuff gas wells.
Tor/Ekofisk wells drilled in the Danish sector of the North Sea have historically suffered from borehole instability problems when intersecting the Upper/Lower Lark and Horda shale formations using either water-based mud (WBM) or oil-based mud (OBM). An extensive wellbore stability investigation was carried out, focused primarily on improving shale-fluid compatibility. It was augmented by a lookback analysis of historical drilling operations in order to identify practical solutions to the borehole instability problems. A state-of-the-art experimental rock mechanics and shale-fluids compatibility investigation was carried out featuring X-ray diffraction and cation exchange capacity characterizations, shale accretion, cuttings dispersion, mud pressure transmission and a new type of borehole collapse test for 11 different mud systems (WBM, OBM and high-performance WBM). The results of this investigation were then combined with the results of a comprehensive well lookback study. The integrated study clearly identified the root cause(s) of the Tor/Ekofisk well problems and highlighted comprehensive practical solutions, which were subsequently implemented in the field. The borehole instability problems at Tor/Ekofisk in the Lark/Horda shales have a characteristic time-dependency, with wellbore cavings occurring after 3-5 days of open-hole time. The problems were not related to mud weight selection, but were instead caused by mud pressure invasion into the shales, which destabilizes them over time. An extensive experimental testing program revealed that this effect occurs in both WBM and OBM to equal extent, which explains why non-optimum field performance has historically been obtained with both types of mud systems. New high-performance WBM (HP-WBM) formulations were identified that significantly improve upon the mud pressure invasion and borehole collapse behavior of conventional OBM and WBM systems, yielding extended open hole time that would allow the hole sections in the Lark/Horda shales to be drilled, cased and cemented without triggering large-scale instability. Lookback review also indicated that secondary causes of wellbore instability, such as barite sag, extensive backreaming and associate drillstring vibrations should be minimized for optimum drilling performance. A new HPWBM system, together with improved operational guidelines, was successfully implemented in the field.
For the first time in a Danish field, micro-fine cement was used to abandon a multi-zone horizontal well by plugging each sand fractured zone sequentially. Lab testing was performed to investigate the depth of penetration of different conformance systems in a 20/40 sand pack. The micro-fine cement showed the highest penetration and was considered the best option for abandoning sand propped fracs in horizontal wells. The first field application was unsuccessful due to pre-setting of the micro-fine cement in the Coiled Tubing. Extensive lab tests were performed to understand the causes of the pre-mature setting of the cement. A new cement recipe was selected and subjected to modified lab test procedures to successfully abandon the horizontal well. To economically develop chalk reservoirs in the North Sea, long horizontal wells are drilled, hydraulically fractured and waterflooding is applied to the field. To ensure conformance, flow from each fracture is controlled by a Sliding Side Door (SSD). Well A suffered injection water breakthrough which could not be controlled by closing the SSDs. To enable the full abandonment of each zone the decision was made to isolate each zone sequentially from the toe of the well up with cement retainers and to use Coiled Tubing to squeeze a pre-determined volume of special micro fine cement into the zone in order to plug the tubing, the tubing-liner annulus, any cement channels and a limited section of the sand propped fracture. In this manner any potential water channeling between zones and with other wells in the pattern would be prevented. This paper discusses the results of initial lab tests, causes of failure of the first job, testing of new micro-fine cement and the successful application. This paper also discusses the results of computational fluid dynamics simulations which were performed to understand the behavior of cement in a fracture and enable further improvements to similar abandonments in the future.
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