PART I: This is the first of a two-part series dealing with the shallow water flows (SWF) problem in deepwater. Part I deals with early detection and sharing of information. Part II, which will appear in January, 2000 will focus on SWF encounters and remediation.
In the ongoing battle to identify and control shallow water flows, industry experts agree that more time and effort should be spent on early detection and evaluation. In the past, the focus has been on mechanical remediation equipment and procedures, early identification and well-site evaluation may offer greater long-term returns for the industry.
Shallow water flows (SWF) are abnormally pressured zones that occur between 500 ft and 4000 ft below the mudline in deepwater areas, and have been encountered primarily in the Gulf of Mexico in areas of rapid recent sedimentation. SWF is one of the greatest threats to deepwater exploration and the integrity of the drilling operation.
As a wrap-up to the recent International Shallow Water Flows Conference (PennWell-sponsored), an industry panel was asked to consider the major questions surrounding this elusive and destructive phenomenon.
When deposition of sediments occurs faster than the sediment's ability to compact and de-water, the pore pressures in the formation can remain abnormally high. These unconsolidated sand zones contain large volumes of water that are released into the wellbore when penetrated by the drill bit.
SWF developments
When a drilling program encounters such a zone there are a number of potential problems that can develop:
- Large volume fluid flows
- Massive sand production
- Severe washout and caving of the hole
- Massive seabed failure.
Because these zones have very narrow drilling margins (the difference between the pore pressure and fracture gradient), it is difficult to maintain over-balanced well pressure without fracturing the formation. Attempts to increase mud weight to control SWF can result in formation failure that causes the loss of the wellbore. These factors work in tandem to complicate adjustments in the mud weight, and leave the driller little room to maneuver to mitigate a SWF event.
Cementing these zones is a challenge in itself because of the unstable formation, irregular hole gauge, and potential for communication with the mud line. Typically, SWF zones are handled with a combination of drilling techniques designed to get through the zone quickly and installing a contingency string of casing that can be sealed off to isolate the flow zone. The issues of casing programs and well spacing are also of central concern when drilling in an area prone to SWF.
Central issue of SWF
To effectively deal with SWFs, it is essential to have advanced notice that the potential exists for the presence of such flows. Modeling pore pressures from seismic and logging data can be useful not just in detecting the presence of SWFs, but accessing their severity prior to encountering them. Pressure prediction is widely held to be a key technology needed for dealing with SWFs.
As the industry has so far focused its resources on isolating a SWF once it is encountered, the preferred solution would be to detect the presence and severity of these flows so they could be avoided, or accounted for in the drilling program.
To this end, the panel suggested that better sharing of well data from areas where strong SWFs were encountered might help the industry as a whole to understand what signs in logging and shallow seismic show the presence of SWFs.
There are basically two methodologies that the industry uses currently in dealing with SWFs, according to Alan Huffman of Conoco, who chaired the forum and panel discussion. These are pre-drill identification/characterization, and mitigation after the fact. There is some debate about which of these should take precedent in future research efforts.
In the area of prior planning and identification, John Pelletier with Shell E&P Technology said his company spends a lot of money planning to mitigate SWF in areas where they think the problem might be present. Such steps as designing a big-bore casing program and carrying extra mud volumes can be very expensive if it turns out the SWF is not present.
On the other hand, not being prepared for a SWF contingency can cost a well site. The key is planning for SWF, and this can be streamlined with better identification and characterization. If an operator knows not only that the SWF exists on the well site, but also the magnitude of the problem, the operator can make a decision to either move off to an alternative site, or build remediation contingencies into his well plan. The current practice of building contingencies into every program that shows the potential for SWF is too expensive to continue over the long term.
Spotting SWF
Early wellbore monitoring and detection technologies such as measurement while drilling (MWD) and pressure while drilling (PWD) tools are essential to any program of identification. As these technologies advance further ahead of the drill bit and return real-time data to the drill floor, it will be less likely an operator will drill through such a zone blind. Currently, such technology is not sophisticated enough to show definitively that a SWF event has occurred. There is too much room for interpretation of the data and too many possible causes for data that could be read as a SWF.
Mike Smith with the Gulf Coast Region of the US Minerals Management Service (MMS) said he would like to see the focus of research shift from mitigation, to identification using innovative techniques. These techniques include the multi-component seismic technology that was presented in a keynote presentation by Huffman and John Castagna of The University of Oklahoma.
This technique, which uses the latest methods in ocean bottom cable (OBC) technology along with state-of-the-art seismic analysis techniques being pioneered by Conoco, would allow SWFs to be detected and characterized in 3D before wells are drilled.
Smith said he hopes the MMS database of SWF events might help with the understanding of where these flows are likely to occur. He called for standardization in the reporting of SWFs. Also, standardizing the reported severity of the flows encountered would be a positive step in providing a uniform profile of the type flows encountered in different blocks of the Gulf of Mexico.
Regardless of the amount of planning done ahead of time, Richard Vernotzy (panelist with Aker Maritime) said there would always be unforeseen events. To deal with these events, the industry must continue moving toward a solution that allows an operator to control, as well as contain, these flows.
Pelletier (Shell) said that civil engineering has developed tools to measure sand pore pressure. He said he would like to see these tools used to measure pressure in the shallow sands further ahead of the drill bit. Using this procedure, an instrument probe is pushed into shallow formations to measure the push and friction, which can then be used to determine pore pressure.
It has been Shell's experience that a controlled, low volume leak off test can help avoid formation damage in SWF zones, he said. Such damage often occurs when too much fluid is pumped into the formation.
Geotechnical holes
There seems to be little argument that drilling small-bore geotechnical holes prior to drilling larger-gauge exploration wells can provide a wide range of valuable data on the presence and intensity of SWFs. There are two major considerations for such an approach:
- These programs are not cheap. Pelletier said it is necessary to set some type of pipe in the hole to avoid side-tracking off vertical. This adds substantially to the cost of the pilot hole.
- What must be done when an SWF is encountered. If the operator is planning a drilling program nearby, it must be decided whether or not to drill a pilot hole. Such a flow would possibly wash out the drill site or destabilize the seafloor. The pilot hole may disturb the formation sufficiently to initiate a SWF event act, which could trigger a formation failure over a large area and damage a potential drilling site.
