Complete to intermittent dynamic losses during drilling operations in the Early Permian reservoirs of the Bowen Basin are common because of depleted zones, particularly in the Bandana Coal Measures. The losses encountered while attempting to drill to total depth (TD) result in notable setbacks in time, remediation costs, and zonal isolation integrity. Operators have access to a large portfolio of loss circulation (LC) treatments depending on the loss type, rate, and nature of losses. This paper presents a low-solids, highly thixotropic (LSHT) cement system that can help cure losses without incurring costly nonproductive time (NPT). During the recent Springwater campaign, which consisted of 12 wells targeting the Cattle Creek Coal Measures, three of the wells experienced significant to total losses. A number of conventional lost circulation materials (LCMs), including particulate materials, reticulated foams, and fibers, were unsuccessful in curing the losses. As an alternative to a conventional LC cement plug, an LSHT cement system was deployed. The low-solids content and avoidance of any traditional LCMsenable the LSHT system to be pumped through the bottomhole assembly (BHA), which allows for continued drilling. The LSHT cement system is shear-rate dependent, yielding to thin rheologies on application of shear to easily penetrate into the loss zone. When shear is reduced, the slurry gels rapidly to arrest the rate of flow. Additionally, the quick early compressive strength development at shallow depths allows operations to continue without incurring significant loss of operating hours. The solutions provided were pumped through open-ended pipe in one case and through the BHA with a drilling bit in another; both performed in the same manner without hampering equipment capability. Successful application of the LSHT cement technology helps minimize time lost using conventional cement plugs because the LSHT cement can be pumped through the BHA, saving time associated with tripping in and out of the hole to change the BHA. Additionally, the system's early compressive strength development yields time savings, along with savings in otherwise lost fluids, and achieves drilling efficiencies to help manage drilling operations costs.
The development of Early Permian reservoirs in the Fairview Field is limited by highly depleted shallower Bandanna coal seams and naturally fractured formations that would not allow to reach the original geological goals. Offset wells in the area had been executed as three string well design due to long open hole section across depleted Bandanna and multiple loss zones. Multi-stage cementing was required to ensure there is proper zonal isolation. These resulted in significant incremental cost to achieve 5-1/2″ production casing. Through the depletion of upper coals, fracture pressure in these zones has decreased due to reduction in pore pressure. Drilling in two string design carries higher execution risk due to long open hole section, depleted formations and differential sticking, highly fractured formations, narrow window between fracture gradient and pore pressure, and multiple loss zones which demands special techniques for treatment of loss zones and hole stabilisation while drilling. Considering the downhole conditions, which pore pressure ranging from 2 to 9 ppg in the different formations, the importance of zonal isolation, establishing barriers in aquifers and achieving overall cement coverage is also extremely important. With introduction of various new LCM products, cement plugs, foam cementing, RCD for drilling with no returns (blind drilling), Santos has achieved major success developing Early Permian fields. This Paper describes how two string design was successfully implemented to drill Early Permian wells. This paper serves as an operational guide to ensure challenging depleted and highly fractured zones can be successfully drilled while minimising risks.
The Australian coal seam gas (CSG)/coal bed methane (CBM) fields present multiple challenges (i.e., losses, naturally fractured formations, and highly depleted zones) typically addressed using various well design and cementing techniques (i.e., additional casing strings, stage cementers/tools, lost-circulation materials, lightweight designs, and additional cement volume) to manage the potential risk of losses. Foamed cement is a viable option to address these challenges; however, despite the documented benefits, its application within Australia has been minimal at best. The lack of application can be primarily attributed to misconceptions concerning cost, potential risk, excessive equipment footprint, and a limited proven track record, which increases the potential risk of execution. A recent 10-well campaign conducted within the Bowen Basin, Queensland, Australia, demonstrated that foamed cement can be used as an alternative to traditional lightweight and/or elastomeric cement slurries. Further, the benefits of foamed cement make it an economically and technically advantageous solution compared to conventional cementing techniques used to overcome CSG challenges. This case study details the campaign challenges, cement design, and detailed results of the 10 wells with the intention of providing other engineers a road map to better understand foamed cementing with a specific Australian context and how it could be applied to their projects.
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