Application of Thixotropic Cement Slurry to Cure Losses in Depleted Zones in Coal Seam Gas Wells

Abstract 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.