William FurlowCoping with narrow pressure differentials
Technology Editor
Subsalt drilling in deepwater is already a problem for many operators as they move exploration drilling operations further off the continental shelf in the Gulf of Mexico. Ronnie Faul, a technical specialist with Halliburton, said virtually all deepwater fields in the Gulf include salt sheets or some other salt formation.
Traditionally, in on-shelf drilling, salt was simply avoided by operators who chose to drill around it. The problem is, some of the same attributes that make subsalt drilling difficult, also make it a good place to look for hydrocarbon accumulations. Most operators split the difference, drilling stepouts close to the salt. Faul said there was a time when a driller who encountered a salt sheet would abandon the well convinced he would never get through the dense layer of salt.
Drilling through salt formations is slow going, but not particularly difficult. The salt is very dense which means it's hard on bits and the density is also translated into vibrations, which in turn, can damage bottom hole assemblies. Salt is also hard to image through. In fact, for quite some time it was believed that one could not collect seismic images through salt. With the growing implementation of 3D seismic and related technologies operators gained confidence that they could see what was below the salt. The trick remaining was to access these suspected hydrocarbons economically.
With close to 50 subsalt wells drilled in the Gulf of Mexico, experience is growing. Many of the techniques that have proved successful in drilling subsalt on the shelf will translate to deepwater fields. Faul said there also are a number of innovations that will help companies address other subsalt problems unique to deepwater.
The most problematic aspect of the subsalt drilling, in deep and conventional water, is the disturbed zone below the salt shelf. The salt is very dense and lighter than the surrounding formation so that it is actually floating and moving through the surrounding rock. As the salt moves, it leaves behind overpressured areas of pressurized water and gumbo soil that make for difficult drilling. The pore pressure and fracture gradient of these zones are very close together so that it is difficult to maintain circulation and control the well.
While the phenomenon is present in most subsalt plays, it is exacerbated by other limitations of deepwater drilling. Because there is such a long and heavy hydrostatic head in a deepwater well, it is difficult to accurately control the mud weight to deal with these narrow drilling margins. In addition, many of the deepwater fields in the Gulf also have shallow water flow zone near the mudline.
The solutions to these two problems are often similar, that is, isolating the zone with a string of casing to keep the hole clean and maintain circulation. In subsalt wells, where there has been the need for shallow water flow remediation, a driller may sacrifice so many casing strings that it is no longer profitable to try and produce the well.
Although the Gulf of Mexico deepwater has a large percentage of salt formations, it could be worse. In some areas of the world, the North Sea for example, the salt is "creeping" or plastic. That means it is actually in motion, albeit very slow. This motion pushes against tubulars, causing wear, fatigue and failure. Plastic salts can collapse casing strings, destroying a well.
This requires the setting of extra strings cemented and centralized to bolster and protect the annulus from the pressure of the surrounding salt. To date, most operators in deepwater have faced subsalt in the Gulf of Mexico, including Chevron, Shell, and Texaco.
Solutions
Sperry Sun says deepwater drilling has been a boom for pressure while drilling tools (PWD). This technology, offered by a number of companies, gives the driller real-time information to be used to navigate through the rubble zone without losing circulation or losing control of the well. The PWD is used in addition to the hydraulic model a driller follows in determining his mud weight.The models work well under most situations, but there is such a narrow margin for error in these affected zones that only real-time information can give the driller the kind of minute detail needed to regulate equivalent circulating density (ECD). If there is a pressure loss, the sensors will pick it up and the mud weight can be adjusted accordingly.
Steve Hodge, technology manager for Sperry-Sun, said it is critical to get the rubble zones cased off as soon as possible.
The subsalt wells he has worked on have magnificently narrow pore pressure fracture gradient differentials, less than a pound. This means the driller must have a clear idea of the ECD at all times to maintain well control, but not lose circulation.
There are so many factors that go into calculating the ECD that Hodge said it is often the techniques of the individual driller that make the difference in getting through these zones without incident. Such things as Shell's solid shell expandable casing offer a potential solution. This casing can be run in a reduced diameter and then expanded to virtually the same size as the previous string. This w
ould allow an operator to isolate the rubble zone without sacrificing an entire casing string. It could mean the difference between success and failure.
This zonal isolation gets the trouble zone behind steel so the well can continue. Hodge emphasized the need to do a good job in the drilling and casing of this section of the well. If mistakes are made in this zone they can come back to haunt the driller.
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