API RP 65-1
Cementing Shallow-water Flow Zones in Deepwater Wells
|Publication Date:||1 June 2018|
This standard describes practices designed to prevent shallow-water flow (SWF) during and following the cementing of wells located in deepwater. It is the compilation of technology and practices developed and used by many operators around the world. Although most of the discussion in this standard is focused on SWF, shallow flows can be mixtures of water, gas, gas hydrates, and formation fines. There is no single method of preventing shallow-water flow, and many of the activities described may require customization to fit individual well conditions. While this standard is extensive, it is not meant to limit innovation and new technology.
The content of this document is not all inclusive, and guidance from other sources may apply. Note that this standard is not meant to be a stand-alone training manual or well design standard.
Although fairly comprehensive, there are still many details that are not discussed and that should be addressed when drilling and cementing wells in deepwater. It is meant to highlight key parameters for increasing the chance of successfully drilling and cementing casings where there is a risk of shallowwater flow and to discuss options that are available. 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 deepwater environments with appropriate modifications. The user should consult experts within the industry for specific details of the cementing process relating to the technology being used by a specific company for a specific scenario. The construction of the casings through the SWF zones should be a team effort to be successful. All parties involved shall participate in the planning and execution of all phases of the process to ensure successful construction of the conductor and surface casings.
In this standard, where practical, U.S. customary units (USC) are included in parentheses for information. The units do not necessarily represent a direct conversion of metric units (SI) to USC units, or USC to SI. Consideration has been given to the precision of the instrument making the measurement. For example, thermometers are typically marked in one-degree increments; thus, temperature values have been rounded to the nearest degree.
The objective of this standard is to communicate methods to prevent shallow-water flows in the tophole sections of deepwater wells during and after cementing operations.
Conditions of Applicability
This document is applicable to the following well conditions:
- tophole section(s) of the well;
- shallow hazard assessment concludes a risk/potential for shallow flow.
This does not include isolating potential flow zones beyond the tophole section(s) of the well; see API Standard 65-2.
SWF are water flows from permeable formations at shallow depths below the mudline in wells drilled in deep ocean waters. The water flows can compromise the structural or hydraulic integrity, or both, of the tophole section(s). The tophole section is a part of the well often drilled without a marine riser where drilling and cementing fluids exit the well annulus at the seafloor. SWF can occur in sections drilled with a riser where fluid returns are at the rig. Modes of failure include:
- poor isolation by cement resulting in casing buckling/shear;
- pressure communication to other shallow formations, causing them to be overpressured;
- disturbance of the seafloor due to breakthrough of the shallow flow to the mudline.
NOTE Such damage can result in the complete loss of drilling templates containing previously cased wells.
Flows from these shallow formations are typically a result of abnormally high pore pressure in undercompacted and usually unconsolidated sediments formed in a rapid depositional environment. Not all flows are the result of these naturally developed formation geopressures. Other causes for abnormal shallow pressures include hydraulic communication from deeper, higher-pressure formations, tectonic uplift, and induced storage during drilling, casing, and cementing operations.
Flow of sediments results in hole enlargement, which can increase the rate of flow 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), re-entry and control problems, sea floor craters, mounds, and crevasses (R.M. Ostermeier et al. 2000, Eaton, L.F. 1999). Flow channels through or around the cement sheath may be formed as a result of one or more of the following:
- poorly designed or executed primary cementing;
- flow occurring due to dilution or channeling while the cement is being placed;
- flow occurring after the cement slurry is placed, but before it has attained a structure sufficient to prevent formation fluid flow, will create channels through which flow can continue;
- poor hole conditions.
Geopressure can be transmitted through channels in the cement sheath and, if trapped by a seal (mechanical isolation), can charge or fracture a formation of lower pressure or strength. If the fracture extends beyond the wellbore, it could eventually reach the surface, breaching at the seafloor around the well. Fractures can also extend to neighboring wells and create a flow path to the seafloor if those wells are not properly cemented.
Since the structural, conductor, and surface casing strings are the foundation of the well, obtaining a quality cement sheath is critical to achieving well objectives.
Challenges for Controlling SWF
The following factors make drilling and cementing sections with SWF potential challenging:
a) Temperatures at the mudline are low, usually in the range of 1.5 °C to 13 °C (35 °F to 55 °F). 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 recover quickly with depth. The low temperatures result in slow hydration of the cement, often making special slurries or additives, or both, necessary.
b) Pore and fracturing pressures are close due to the depositional environment, making the drilling and cementing pressure margins narrow.
c) The hole section is usually drilled riserless, with no means to control flow at the wellhead, and returns are observed from a remotely operated vehicle (ROV).
d) In development projects, tophole casing strings may be in close proximity to each other through the SWF zone.
The shallow-water flow conditions described in this document exist in wells drilled in water depths greater than 150 m (500 ft) and more commonly at water depths greater than 300 m (1000 ft). Shallow-water flow sands are typically encountered at depths of 180 m to 760 m (600 ft to 2500 ft) below the mud line (BML). The conditions favoring the formation of shallow-water flow zones include:
- high rate of deposition [>450 m per million years (>1500 ft per million years)] in 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, such as the Caspian Sea;
- continental slope regions subject to large-scale subsea slides, such as the Storegga Slide area in the Norwegian Sea.
Summary of Considerations
Isolating a shallow flow zone with cement is an interdependent process. Individual process elements, such as slurry design and testing, applied engineering, and job execution all impact the ability to successfully isolate the wellbore and establish structural integrity. Superimposed upon these elements are the conditions found in the well at the time of cementing.
Certain cementing process elements contained in Annex D may be individually critical to isolating a potential flow zone or may be of minor consequence until made critical by a separate (sometimes unrelated) event or past well engineering decisions. Conversely, certain elements may not be dominant factors in the success of one cementing operation, yet are vitally important in another.
Collectively, the elements described in Annex D produce the design, engineering, and operational framework for successfully isolating a potential flow zone.