Core stabilisation using the Lithotarge foam injection process was initially undertaken in 1992. The initial fieldwork using the method was undertaken on behalf of Elf Nigeria. Elf Nigeria also sponsored the additional research work that underpins the development of the method.
The research work investigated the use of different foam types, foams with different densities and expansion characteristics. The pressures and temperatures associated with use of these foams were recorded using sensors placed along the surface of the cores being tested. Different Injection methods were recorded visually through transparent tubing in which the sediments were held.
The data from these tests, and the input derived from continued laboratory monitoring of work completed in the field, provide the basis for the development of the most effective and successful core stabilisation process available.
To date over seven thousand five hundred meters of core have been successfully stabilised. The majority of core has originated in West Africa although cores from as far afield as Malaysia, the North Sea and Venezuela have been stabilised using the Lithotarge process.
Many of the cores stabilised with the Lithotarge method have been CT Scanned. The results from the scans have never given any indication that the process has resulted in damage to the sediments. Nevertheless Kirk Petrophysics felt that there would be benefit in undertaking additional work which dealt specifically with the possibility of the stabilisation process moving or damaging sediments, and also the possibility of creating or opening fractures. These aspects were not isolated as specific tasks during the original work and as such questions concerning this area remain rhetorical.
In order to investigate these areas a programme of testing was undertaken at the Corpro Laboratory in Norway. This laboratory has an in house CT scanner, the facilities to prepare the samples and document the process. All the CT scanning work and slabbing were undertaken by Corpro personnel. J. Donald Kirk and Cary Tamm of Kirk Petrophysics undertook the Lithotarge foam injection process. The testing was undertaken over a period of two weeks in June 2002.
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Testing Methods
1.Initially the test samples were created by mixing loose sand with oil and water and allowing the material to settle into shape in a cylindrical tube (section of core inner barrel). The initial tests utilised heavy crude but this was later changed to a Mineral Oil. Only the samples made with the heavy crude could be handled, and then only with great care as the samples became unstable when moved.
2.The test samples were formed inside the section of tube into which they were subsequently stabilised. Some attempts were made to prepare a sample in a tube with a smaller diameter, with the idea of transferring the samples to the larger tubes before stabilising. This route proved to be inappropriate as the samples were damaged during the transfer. The annular space in the tubes used for undertaking the stabilising was created by settling the sand onto the bottom of the tube long axis.
3.Further test samples were made from sand mixed with mineral oil only. After preparing the samples they were CT scanned. They were then stabilised without removing the tubes from scanner.
4.In addition to the above some tests were made using sand without adding oil or water to assist cohesion. These samples were used to investigate the efficiency of the process and to provide a pictorial (qualitative) indication of the lack of stability of these samples.
The foam injection methods were varied, utilising the Lithotarge standard method and several variations. Two types of foam and three different nozzle types were used. The tubes were injected from the top end and through one or two injection points along the length of the tubes. The tube lengths were kept below fifty centimetres to allow for the complete length to be CT scanned without removal of the tube from the scanner.
After completing the comparison CT scans the samples were slabbed (vertically), the cut being made through the top injection points. This allowed an inspection of the core directly at the point at which the foam impacts the core and facilitates a comparison with the CT scans.
A total of eleven separate tests were undertaken for evaluation and correlative measurements.
Results
The attached photographs provide a qualitative guide as to the integrity of the samples tested. Although no quantitative measurements of rock strength were made, we consider that these samples represent sands that are representative of core that would require stabilisation before shipment.
The initial tests, in which the samples were removed from the scanner before stabilising and then returned for further scanning did not have a depth correlation as exact as those undertaken when the tubes were not moved. However, these initial tests demonstrate effectively that the test samples are unaltered at the injection points, in the top cap or along the tube length. See photographs on page 1 of the photographic section.
In addition this series of tests highlighted that when unconsolidated sands are moved without stabilising careful handling is required. This point is important as although the lithotarge process requires the cores to be cut into one metre sections and laid out horizontally, the samples do not have to be inclined to allow the removal of mud or gravitational fill of stabilising medium. The results from samples that were inclined in this way exhibit gross changes in the movement of sample.
Further tests to investigate potential damage were made after undertaking the stabilisation in situ. The samples were CT scanned, stabilised in situ and then re scanned. The results clearly show that the Lithotarge process has not damaged the core.
The test samples prepared from the loose dry sand represent the most challenging scenario. As described earlier the samples were prepared using an inner liner and the core was not moved from the CT scanner in order to stabilise the core.
The CT scans indicate that, although grains can be dislodged at the point of injection, there is no movement or disturbance of the sand throughout the length of the tube section. The grain disturbance at the injection points was restricted to an area of one centimetre from the point of injection. To place this result in perspective, it was possible to create the same degree of grain movement resulting from the stabilisation process, by blowing on the injection port to remove drill burrs.
Using the same samples we did not record any movement of grains at the top injection cap or in the main section of core. However, some collapse of the loose sand at the bottom end cap was recorded. It maybe that this area is a weak point in the sample construction and exacerbated by the space available for the sand to collapse into. Different end caps were tried and the best solution was effected when a foam spacer was used to partly support the unconsolidated sand.
One test was undertaken using a core that had been fractured during handling (phase one). The result shows that the Lithotarge process does not propagate the fractures.
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An Investigative Study Aimed at Assessing Some Aspects of Stabilising Unconsolidated and Fragile Sediments using The 'Lithotarge' Procedure
Conclusions
The study established that there is the possibility for a loss of core integrity at the bottom end cap area of the tube. A specially shaped spacer will significantly reduce this effect while promoting better stabilisation. This will be recommended for all future stabilisation projects.
Lithotarge Foam Injection did not result in damage to the unconsolidated test samples.
Lithotarge Foam Injection did not create fractures or propagate open fractures in unconsolidated sands.
The study reinforced earlier study conclusions:
Lithotarge Foam Injection has a low propensity for damage to core due to minimal handling requirements and tube positioning.
Lithotarge Foam Injection retains a small overburden pressure on the sediments assisting in the retention of core integrity. The pressure generated is insufficient to damage the core.
Lithotarge Foam Injection allows for unconsolidated/fragile sediments to be slabbed without freezing.
Lithotarge is adaptable and efficient in all well site environments. Core Integrity is maximised by the ability of Lithotarge to fill and protect even unseen voids without impacting on the core Integrity.
It should be stressed that all the tests were undertaken using established Lithotarge Foam Injection products and methods developed during the initial research. This previous work has shown that it is indeed possible to damage core and generate large pressures, dependant on the injection method, and types of chemicals used. It is therefore important to recognise that the Lithotarge foam injection is tried and tested over time with a wide range of sediments. Foam injection per se does not mean non-damaged stabilised core.
These tests were undertaken as an adjunct to existing work and are specific to the Lithotarge process. As such these conclusions should not be taken to be inclusive of any other foam based method for core stabilisation.
Initially the test samples were created by mixing loose sand with oil and water and allowing the material to settle into shape in a cylindrical tube (section of core inner barrel). The initial tests utilised heavy crude but this was later changed to a Mineral Oil. Only the samples made with the heavy crude could be handled, and then only with great care as the samples became unstable when moved.
