Shallow water flows have been observed in the deepwater Gulf of Mexico since 1985. Over the past 14 years, understanding of the mechanics of shallow water flows (SWF) and of the methods to control them have evolved. Operators take a variety of approaches and use different techniques for the prevention and control of flow from shallow geopressured sands.
A subset of the BP Amoco SWF database has been studied to determine the relative cost and value of some of these techniques. One hundred and six wells drilled by a cross-section of deepwater operators, where detailed information on operations and costs was available, were analyzed. Of these wells, 57 were exploration and appraisal (E&A) wells and 49 were development wells.
Expenses related to shallow water flows were divided into two categories - preventative or remedial.
- Preventative expenses include pilot hole costs, special cement (foam or microfine), measurement while drilling (MWD) costs, external casing packers (ECP), mud expenses, extra casing runs, and associated rig time. The test for this expense was: "Would we have made the expenditure if we knew there would be no SWF?" These expenses were estimated from daily drilling reports and customary charges.
- Remedial expenses include squeeze costs, special flow logs, re-running of casing, loss of well, re-drilling the hole, and any associated rig time with these activities. The test for these expenses: "Was the expenditure incurred because the primary plan to contain the SWF did not work?" The expenses were also estimated from daily drilling reports and customary charges.
Analysis of these wells determined that a total of $175 million was spent on both prevention and remediation, or approximately $1.6 million per well. Approximately 34% ($59 million) was spent on prevention, and 66% ($116 million) was spent on remediation. The ratio of prevention to remediation expenses is approximately the same for both exploration and appraisal wells, as well as development wells. However, the typical E&A well is only a total of $1.3 million per well while the typical development well's SWF related expenses is $2.1 million (50% above E&A wells).
Six categories relative to preventative expense were identified in the analysis of the 105 wells: MWD, annular pressure while drilling (PWD), 24-in. casing, pilot holes, special SWF cement, use of a marine riser, and SWF planning. Average preventative expenses were found to be: MWD/PWD: $20,000/well; 24-in. casing: $500,000/well; Pilot holes: $300,000/well; SWF cement: $200,000/well; Riser: $500,000 per well (est.); and Planning: unknown.
These expenses can vary depending upon water depth, rig rates, volume contracts, vendor, and methods applied to optimize expenses. The numbers shown are typical.
The merits of each of these methods employed to prevent shallow water flows needs to be weighed relative to the expense.
- MWD/PWD: MWD/PWD typically costs $20,000 per well and helps determine where to perform flow checks. PWD can determine if the sand is flowing brine and eroding sand. They both will determine if geopressured sands will be confined to the tail cement, as well as the optimum centralizer placement. These technologies identify optimum casing seats. PWD is a critical tool in template patterns for monitoring sand erosion, but does nothing to actually prevent flow.
- 24-in. casing: The 24-in. casing program typically costs $500,000 per well, but is critical when using a riser or subsea diverter in narrow pore pressure/fracture gradient environments. This is one logical alternative, when sands are too shallow for 20-in. tail cement. However, the approach is generally over-rated and over-used.
- Pilot hole: Pilot holes typically cost $300,000 per well. Some operators claim these holes aid in killing flows and reducing friction pressure. However, once the pilot is drilled, the operator still must drill the large diameter hole to run casing. Pilot holes offer a better logging environment, which may be critical to pressure measurements. Such holes may lead to excessive erosion and loading of nearby wells. Another risk is that the extended erosion that results from drilling the hole twice leads to larger surface area at the eroded sand face which, in turn, causes higher flow rates, making the well actually harder to kill. For these reasons, pilot holes should be avoided in template situations.
- SWF cements: SWF cements cost about $200,000 per well. These cements help operators maintain a hydrostatic head through transition zones, preventing geopressure-induced channels. The cements cannot compensate for the absence of centralizers, cement contamination, or eccentered casing. Even with SWF cements, geopressured sands need to be confined to the tail.
- Risers: A riser system costs $500,000 per well. The SWF riser offers definitive flow checks, quantitative understanding of flow rate, but results in high equivalent circulating density (ECD) while transporting cuttings. There is additional rig-up time needed to run and retrieve the riser, although this up time at development sites can be reduced through batch operations. Such a riser results in higher hydrostatic pressure at the previous shoe and may require additional casing. This solution becomes more problematic as water depth increases.
- Planning: The cost of planning has not been estimated. Good planning for SWFs facilitates having the right equipment on location. To plan properly, an operator needs sand prediction - analog and seismic based. Proper planning may also require seismic reprocessing, which can be enhanced with shallow hazard data. It is also advantageous to have a shallow pore pressure fracture gradient profile.
Should the primary plan to contain pressure in shallow sands fail to meet the objective, consideration will need to be given to remediation. History has shown that remediation is possible, although expensive. Consideration needs to be given to current level of investment, probability of avoiding the problem in a re-spud, depth of operations in the current well, risk of further damage to the current well, expected life of the current well, probability of repairing the current well, and other factors.
In exploration and appraisal wells, minor flows from the annulus at the wellhead have been tolerated in a number of deepwater wells for several months. In many cases, they bridge off by themselves. Remediation is not warranted if the flow is minor, there are no other nearby wells, and there is no intention of retaining the well for future use. If sediment is being transported, the flow probably should not be considered minor.
In development wells, minor flows can lead to subsurface compaction and subsequent casing loading and collapse. The amount of flow required to collapse casing can be calculated with current understanding of rock mechanics and materials strength. The ability of the casing to resist collapse will be a function of the degree of washout around the casing at the flowing geopressured sand. This risk should be quantitatively evaluated.
Failure of an exploration or appraisal well in an isolated location can result in the loss of the well, reduction in evaluation of objective horizons, hydrocarbons to surface, disturbance of seabed integrity, and reduction in shallow fracture gradients for future drilling. Failure of a development site can result in all of these potential problems as well as the loss of nearby wells.
The probability of the occurrence of these events can be quantitatively calculated using current fluid mechanics, petrophysics, geomechanics, and material strength understanding.
- Fluid mechanics can be used to determine downhole pressures for any of the drilling techniques that might be employed to control shallow water flows (PWD can be used under actual operating conditions).
- Petrophysics can be used to determine brine flow rates from shallow geopressured sands, given the downhole wellbore pressure.
- Geomechanics can be used to determine sand erosion rates, given the brine flow rates and petrophysical properties of the formation (rates can also be determined from PWD data under actual operating conditions).
Given expected drilling constraints, total sand erosion can be estimated and distributions of stresses in the well area estimated, quantitatively. Stress distributions can then be used to determine stress loads on the drilling or adjacent wells. This would allow material strength relationships to be used to assess the degree of loading on wells and the probability of collapse, buckling, or failure.
BP Amoco is in the process of assembling this model into a software package that will allow simulation of the drilling of a SWF site and to determine stress loading on casings for a variety of drilling techniques. A variety of operational drilling proposals can then be simulated and the stress loads evaluated and relative risks quantitatively assessed. This will be used to determine a plan of action that provides the desired risk/cost ratio.
This same model will be used to monitor stresses while operations are conducted. If stresses are building at a higher rate due to drilling conditions or changes in formation properties, operations can then be altered or well placement changed to mitigate the risk of a failure.
Some additional steps can be taken to aid in reducing the risk of well failure due to SWFs. These include:
- Running baseline casing caliper logs and directional surveys to serve as a baseline for detecting early signs of buckling
- Running ditch magnets to monitor metal shavings for detecting early signs of casing wear that might be caused by early stages of buckling.
During the appraisal process, enhancing the ability to model stress behavior at a potential template site should be included. Critical data to obtain are sand strength, shallow LOT data, formation pore pressure, seismic properties of the geopressured sand packages, and sand permeability and porosity.
Riserless, weighted mud
The problems associated with shallow water flows have lead to a number of innovative applications to help reduce the risk of well loss due to shallow water flows. A number of these services have already become part of routine SWF drilling operations. The more notable are foam cement, PWD, remotely operated vehicle (ROV) monitoring, leak-off test (LOT) measurement without a riser, and LOT prediction at shallow depths.
Some of the more promising techniques that are under development include riserless drilling with weighted mud. In at least five cases, operators have successfully drilled shallow geopressured sands with weighted mud and returns to the seafloor. These limited applications have shown promise in preventing significant erosion of sand, which should result in little or no stress accumulation in nearby wells.
Applications so far have been hampered by storage capacity of bulk fluids on the rigs, weather problems in transporting the large quantities of fluid to the rig, cost burden of the fluids used, and concern for environmental implications. Mud companies, drilling companies, and operators have a variety of efforts aimed at solving the logistic problems, reducing mud costs, and delivering environmental assurance. Implementation of this technology is feasible now.
With experience, this could prove to be a method to both overcome the erosion problem associated with shallow water flows and a method to push the conductor casing many thousands of feet below the mudline. This technique would eliminate one or more strings of casing (potentially, 24-in. and 16-in. in a typical deepwater well design). Successful implementation could provide significant well cost reduction and significantly reduce the risk of casing collapse.
Riserless, mud lift
Another promising technology is riserless drilling with a mud lift system. Several efforts exist to develop a true riserless drilling system for deepwater applications. Under this system, a rotating blowout preventer (BOP) would be placed on the ocean floor and mud and cuttings pumped from the seafloor back to the rig in a hydraulically isolated mud lift system. This system promises to significantly reduce casing needs through the application of a dual gradient mud system.
This same system could be applied to the drilling of shallow water flow sands. This technique would offer all the benefits of the riserless drilling with weighted mud and returns to the seafloor without the concerns for logistics, mud costs, or environmental implications. Although progress is going well, first commercial deployment is still many years away. Weight of the BOP stack may have some implications for surface casing setting depths. As in the previous case, successful implementation could provide significant well cost reduction and significantly reduced risk for casing collapse.
Diverter, mud lift
A subsea diverter combined with mud lift is another potential solution. Several efforts have existed to develop a subsea diverter to apply backpressure at the wellhead to prevent flow from shallow water flow sands.
The system raised concerns over the distribution of pressure (required tighter casing points), consistency of backpressure (due to solids interfering with choke performance), and lack of mudcake at the sand interface to provide fluid and solids control.
However, the coupling of the diverter with the mud lift system could provide the benefits of the riserless drilling system in a more timely fashion with a significantly reduced concern for weight on the surface conductor.
Analysis of the 106 deepwater wells studied indicates that the industry spends an average of $1.6 million per well for the prevention or remediation of SWF induced problems.
Typically, the addition of MWD/PWD for SWF purposes adds $20,000 to the cost of a well. The use of 24-in./26-in. casings adds $500,000 to the well cost. Pilot holes add $300,000 to the cost of a well, the use of SWF cement adds $200,000 to the cost of a well. The use of a riser for drilling the shallow section adds $500,000 to the cost of a well, and planning for the purposes of identifying shallow geopressured sands prior to drilling typically adds $20,000 to the cost of a well.
Minor flows in exploration wells can typically be tolerated, but minor flows in development wells need to be quantitatively assessed. Stress induced in nearby wells from planned or executed drilling operations can be modeled using current geomechanics understanding.
Appraisal wells should be designed to collect data on SWF sand strength, porosity, permeability, pore pressure, and seismic properties and overlying fracture gradients to reduce risk associated with development design.
Techniques to drill SWF sands using dual gradient mud systems hold high promise to significantly reduce risk associated with developments in areas of SWFs and provide a means to set shallow casings significantly deeper; thereby, significantly reducing well costs.
Mark Alberty is a petrophysicist with BP Amoco in Sunbury, UK. He works on a wide variety of petrophysical problems including drilling operations, shallow water flows, petrophysical interpretation techniques, seismic calibration, fluid substitution, geological facies prediction, and formation evaluation.