API RP 65 Standard for Exploration & Production
This document is the compilation of technology and practices used by many operators drilling wells in deep water. In a number of cases, there is not a single way of performing a specific operation. In some cases, several options may be listed, but in others there may be practices which are successful, but which are not listed in this document. This document is not meant to limit innovation.
In wells drilled in deep ocean waters, water flows from shallow formations can compromise the hydraulic integrity of the tophole section. Modes of failure include: (1) poor isolation by cement resulting in casing buckling/shear; (2) pressure communication to other shallow formations causing them to be overpressured; and (3) disturbance of the seafloor due to breakthrough of the shallow flow to the mudline. Such damage can and has resulted in the complete loss of drilling templates containing previously cased wells. Additionally, such shallow flow can result in changes in the state of stress in the tophole section, possibly resulting to damage to existing casings in the present or adjacent wells later in the life of the well.
Flows from these shallow formations are frequently a result of abnormally high pore pressure resulting from under-compacted and over-pressured sands caused by rapid deposition. Not all flows are the result of these naturally developed formation geo-pressures. Hydraulic communication with deeper, higher pressure formations is another cause for abnormal shallow pressures. Some of the observed shallow flow problems have been due to destabilization of gas hydrates or induced storage during drilling and casing and cementing operations. Although minor compared to geo-pressured sands, flows due to induced storage may still cause damage from sediment erosion or mining, breakthrough to adjacent wells and damage to the cement before it sets. These problems can worsen with each additional well when batch setting shallow casings. Although most of the discussion in this text is focused on shallow water flow (SWF), shallow flows can be mixtures of water, gas and formation fines. In most cases the concepts are similar and can be employed with minor modifications, depending on the type of flow.
Flows allow production of sand and sediments resulting in hole enlargement which can increase the flow potential and make it more difficult to control. The enlargement may also cause caving of formations above the flow interval. The flow of water and formation material from these zones can result in damage to the wells including foundation failure, formation compaction, damaged casing (wear and buckling), reentry and control problems and sea floor craters, mounds and crevasses (OTC 11972, IADC/SPE 52780).
The Gulf of Mexico has been divided into areas by the severity of the hazard based on data from geotechnical wells (SPE/IADC 67772). The Minerals Management Service (MMS) also maintains a map showing the location of flow incidents on a web site at http://www.gomr.mms.gov/homepg/offshore/safety/wtrflow.html.
The following factors make drilling in deep water with SWF potential unique:
- Temperatures at the mud line and through the shallow sediments are quite low and may approach 40°F.
- Pore and fracturing pressures are very close, making the drilling window very narrow.
- The hole is drilled riserless, with returns taken to the sea floor.
- Seawater is used for drilling.
- There is no means to control flow at the wellhead.
- Returns and flows are observed only remotely through video from a remotely operated vehicle (ROV).
- In development projects, conductor and surface casing are batch set.
The shallow water flow conditions described in this document exist in wells drilled in water depths greater than about 500 ft and more commonly at water depths greater than 1000 ft. These wells are commonly drilled from floating drilling rigs such as drill ships, semi-submersibles, spars and tension leg platforms.
Shallow water flow sands are typically encountered at depths of 600 ft - 2500 ft below mud line (BML). The conditions favoring the formation of shallow water flow sands include:
- High rate of deposition (> 1500 ft/million years) sedimentary basins of current or ancestral river complexes, such as the Mississippi River depocenter.
- Areas with substantial regional uplift, in which once deeply buried sediments are encountered at shallow depths - North Sea, Norwegian Sea.
- Continental slope regions subject to large scale subsea slides - Storegga Slide area, Norwegian North Sea.
Abnormal pressures may be present in the top hole section of a deepwater well. Abnormal pressure can be trapped below the impermeable layers found above the SWF sands, or may begin at or near the mud line and increase more-or-less linearly with depth. In general, the degree of over-pressurization is consistent with the rate of deposition. Pore pressures equating to 8.6 lbm/gal to 9.5 lbm/gal equivalent mud weight (EMW) may be encountered in the SWF zones. When abnormal pressures are trapped below impermeable barriers, the pore pressure can be very close to the fracture gradient of the sediment. This results in a very narrow pressure margin within which drilling operations must be conducted to maintain well control and prevent induced fracturing of formations. (See SPE/IADC 67772.) The margin between pore pressure and fracture gradient becomes more narrow as water depth increases.
Temperatures at the mud line of a deepwater wellbore are quite low, in the range of 35°F - 55°F depending on water depth, latitude, and presence of warm/cold ocean currents. The low temperatures result in slow hydration of the cement making special slurries and/or additives necessary. The geothermal gradients found in deepwater areas may be sequestered as a result of the water depth effect and may suppress wellbore temperatures throughout the entire stratigraphic column. In other areas the geothermal gradient may rise quickly to normal values as depth increases.
Because of such problems and to form an effective seal while preventing flow, careful attention must be paid to the cementation of wells having the potential for shallow flow. This document addresses the drilling and cementing process and makes recommendations for such wells. Appendix F gives a matrix for this process with values for each step. The resultant score provides the user with a factor of the relative chance of success of the cementation process. This process and matrix are based on known industry practices and are meant to be used to apply the process within the constraints of the well conditions with the greatest degree of risk minimization.
The process includes:
- Site selection.
- Fluid properties.
- Wellbore preparation and conditioning.
- Operational procedures and good cementing practices.
- Mud removal and placement technique.
- Cement slurry design.
- Pre-job preparation.
- Cement job execution.
- Additional considerations.
- Post cementing operations.
- Remediation of flows.
A number of 'best practices' have been developed for drilling and cementing in the deepwater, shallow water flow environment. Generally, these have been developed from lessons learned while drilling deepwater wells. These practices are applied to minimize the risk of shallow water flow and to aid in successfully drilling and cementing the casing through the SWF zones. These practices include the following, which are discussed in more detail throughout the document.
- Site selection to minimize the risk for and severity of shallow water flow.
- Use of pressure while drilling and resistivity tools to identify permeable sands and flow events.
- Use of ROV to check for flow with each connection.
- Rapid action to contain flows.
- Switching to mud to control flow as soon as it is encountered.
- Selection of casing seats/casing program to facilitate control and to reach the well objectives.
- Low fluid loss and gel strengths of pad mud spotted in the hole just prior to running casing.
- Use of foamed cement and/or special slurries to maintain control across the SWF zones.
- Batch setting conductor and surface casings.
A list of 'lessons learned' in successfully isolating the top hole section in the presence of SWF include the following:
- The pore pressure of SWF sand(s) must be hydrostatically contained at the first indication of flow.
- SWF zones that are drilled underbalanced while flowing will not likely be isolated with cement.
- Flows that are not contained soon after beginning can jeopardize the success of the project.
- Wells in which the SWF sands have been hydrostatically controlled must still be cemented with flow mitigating cement systems.
- Mechanical isolation devices, when used without flow mitigating cement systems, may not provide zonal isolation over the life of the well.
Note that this document is not meant to be a training manual. Although fairly comprehensive, there are still many details which are not discussed and which must be addressed when drilling and cementing wells in deep water. It is meant to highlight key parameters for increasing the chance of successfully drilling and cementing casings where there is a risk of shallow water flow and to discuss options that are available. Many more details can be gleaned from the references listed in the Bibliography. Most of the information in this document is from U.S. Gulf of Mexico experience. The concepts can be applied in other deep water environments with appropriate modifications. The user should consult experts within the industry for specific details of the cementing process relating to the technology being employed by a specific company for a specific scenario. The construction of the casings through the SWF zones must be a team effort to be successful. All parties involved must participate in the planning and execution of all phases of the process to ensure successful construction of the conductor and surface casings.
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