Shallow Water Flow Planning and Operations: Titan No. 1 Exploration Well, Deepwater Gulf of Mexico

Abstract

Summary Shallow water flows (SWF) from overpressured aquifers have been a persistent problem in the deepwater Gulf of Mexico (GOM) and can create significant financial and operational risks for exploration and development drilling programs. For the GOM, SWF intervals typically occur between 300 and 2,500 ft below mudline (BML) and in water depths greater than 1,500 ft. If left unchecked, the disturbance from the water flow can cause loss of soil strength surrounding the wellbore, thereby compromising the structural integrity of the well. In industry, extreme cases have led to collapsed casing and/or total loss of wellbores. This paper details the up-front planning efforts implemented and the operational procedures utilized to successfully mitigate a SWF event experienced at the Titan No. 1 exploration well drilled in Garden Banks Block 785 (GOM), in 4,640 ft of water. Operations at Titan were based on the philosophy that performing a shallow hazards assessment, preparing and implementing SWF contingency plans, drilling a 9 7/8-in. pilot hole (for dynamic kill), identifying a SWF zone quickly, controlling a water-flow zone with weighted mud (while drilling riserless), and using nitrified foam cement to seal off a water flow behind 20-in. conductor pipe, were all critical in successfully containing and drilling through a SWF occurrence. Introduction Although it is generally believed that overpressured shallow sands are the result of rapid sedimentation, they can also be caused by other mechanisms including sand collapse, gas charging, and salt tectonics. SWF incidents are Gulf-wide and have been experienced most notably in areas such as Garden Banks, Green Canyon, Mississippi Canyon, and Viosca Knoll (Fig. 1). SWF sands are most troublesome in the 24- or 26-in. conductor hole section of the wellbore since this section is usually drilled with seawater (while taking returns on the seafloor) and prior to the installation of the subsea blowout prevention (BOP) stack and marine riser. Once penetrated, the overpressured sand causes an influx of water, sand and/or sediment into the wellbore. If left uncontrolled, this flow can lead to large washouts resulting in inadequate zonal isolation when cementing 20-in. conductor casing. Additionally, an uncontrolled flow of water, sand, and/or sediment has the ability to remove the vertical support surrounding the wellbore, thereby allowing the conductor casing to buckle and, if there is a localized group of wells present, could allow crossflow between wells behind pipe. In other cases, a SWF may channel up behind casing becoming evident only later in the drilling operation. This scenario may also lead to the loss of the well. Risk Mitigation for Shallow Water Flow In general, there are six main methods currently being used by operators to mitigate the effects of SWF, and operators may use one or a combination of these methods. The first option is implementing a drilling plan that controls the SWF with weighted mud. The philosophy behind this scenario is to quickly identify the SWF and to immediately stop the flow with weighted mud. Drilling of the conductor hole interval may then be resumed with sufficiently weighted mud to keep the SWF contained while taking returns to the seafloor (since the subsea BOP stack and marine riser are not yet run). To identify the flow, frequent flow checks through observation by the remotely operated vehicle (ROV) are used along with constant monitoring of downhole conditions via logging-while-drilling (LWD) and annulus pressure-while-drilling (PWD) tools. Once encountered, the flow is killed with weighted mud and weighted mud is then used for all subsequent drilling of the section. This method requires the maintenance of an adequate supply of weighted mud should it become necessary to kill and drill beyond a shallow pressured sand. The major advantages to this first option are that it provides the opportunity to successfully drill through even the shallowest SWF zones to the planned 20-in. conductor casing point, and it affords an excellent chance to minimize hole washouts (which is critical when attempting to obtain zonal isolation of the SWF when cementing). This method was used at Titan and will be discussed in greater detail in the remainder of this paper. Another option may be to top set 20-in. conductor casing above the suspected SWF zone (provided the casing setting depth is sufficient to ensure adequate formation strength at the casing shoe to allow mud returns back to the rig). This scenario allows the operator to install the subsea BOP stack and marine riser and begin circulating mud returns to surface when drilling the subsequent hole section. Although this method has the advantage of conserving the weighted mud, this method depends on the depth of the shallow sand and may require at least one additional casing string if the flow zone is very shallow. Additionally, the weak formation integrity, typical in the shallow sections of the Gulf of Mexico (GOM), may lead to lost returns. The third method is to drill a larger 30-in. hole from below 36-in. structural casing and set 26-in. pipe across the shallow flow zone where formation integrity is still too weak to run the subsea BOP stack and marine riser. While this method may seal off the very shallow flow zones, well planning must include the installation of a hanger inside the 36-in. structural casing to allow the hang off of the 26-in. pipe. Also, additional time will be spent running and cementing the additional string of pipe and drilling out the cement from within the 26-in. casing. Further, the SWF may have washed out the hole to such an extent that it could lead to zonal isolation problems when cementing the 20-in. casing. The fourth option is to drill riserless with seawater allowing the overpressured sand to flow until reaching conductor casing depth. The well is then killed with weighted mud, and the 20-in. conductor casing is run and cemented. This technique allows the operator to quickly put any SWF zones behind pipe, but it may also allow the hole to become enlarged (once again leading to zonal isolation problems during cementing operations). The fifth and sixth methods are drilling the shallow hole section from start to finish with weighted mud, and drilling the shallow hole section with a top-set 26-in. riser. Although the fifth method may prevent a SWF from occurring, it requires the up-front commitment to use and discharge weighted mud even if a shallow flow zone is not present. As for the sixth method, an additional casing string is required (to top set the potential water flow zone). This method also necessitates the up-front purchase of the 26-in. riser, further adding to cost. In this scenario, weak formation integrity, typical in the shallow sections of the GOM, could also lead to lost returns.

Salinity-Based Pump-and-Dump Strategy for Drilling Salt With Supersaturated Fluids

“…Pump-and-dump operations were developed principally to deal with shallow water flows or shallow gas. Turner and Morales (2000), Schuberth and Walker (2000), and Roller et al (2001) present case histories of using a pump-and-dump strategy to overcome shallow water flows, and they provide excellent information regarding logistics and equipment setup. Rohleder et al (2003) discuss the use of an undersaturated pump-and-dump fluid to drill into salt followed by a saturated "spotting" drilling fluid in the riserless hole.…”